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Lex Fridman Podcast · · 187 min

Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions | Lex Fridman Podcast #468

Lex FridmanJanna Levin

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TL;DR

  • Levin’s foundational call is that a black hole is “more of a place than it is a thing”: the event horizon, not the collapsed star, is the essential object. A massive stellar core may create that horizon after shrinking to city scale, but the matter continues inward and disappears from causal contact; what remains externally is a flawless system described only by mass, charge, and spin. For investors in frontier science, that distinction matters: the next breakthroughs are likely to come from interrogating information, geometry, and quantum structure—not merely finding denser astrophysical objects.

  • The black-hole information paradox is physics’ highest-leverage stress test because general relativity permits information to vanish while quantum mechanics treats its preservation as “as sacred as conservation of energy.” Hawking radiation makes a black hole lose mass and eventually evaporate, yet its apparently thermal, featureless particles seem unable to return what fell inside. Fuzzballs, soft hair, firewalls, holography, and ER=EPR each preserve different assumptions, but Levin’s favored direction sacrifices locality—allowing quantum entanglement through non-traversable wormholes—rather than unitarity.

  • The most consequential theoretical possibility is not simply that gravity can be quantized, but that gravity and smooth spacetime may emerge from entanglement: “maybe it’s just quantum mechanics all the way down.” Maldacena’s holographic construction shows a gravity-filled universe in a box can be equivalent to a boundary theory with no gravity and no information loss, though not yet for our observed universe. If that direction holds, quantum information becomes underlying infrastructure while spacetime resembles temperature: a clean macroscopic quantity produced by vast collective behavior.

  • Extra dimensions, dark matter, and dark energy could be one connected “dark sector,” but Levin repeatedly marks that as hope rather than result. Extra dimensions might be tightly folded, selectively constrained while three dimensions expanded, or large dimensions through which our three-dimensional membrane moves; gravity could potentially travel through that higher-dimensional “bulk.” The empirical base remains stronger than the theory: familiar observable matter is less than 5% of the inferred universe, dark matter clumps around galaxies, and dark energy accelerates expansion while resisting natural numerical explanation.

  • Wormholes and warp drives remain within plausible mathematical physics, but they are nowhere near ordinary engineering projects. Researchers can specify a desired spacetime architecture, reverse Einstein’s equations, and infer the required matter; the recurring obstacle is negative energy or violations of assumed energy conditions, although quantum effects such as the Casimir effect show that negative energy is not automatically forbidden. Levin’s calibrated verdict is crucial: “I’m not saying it’s simply an engineering problem,” but neither does she see an obvious abuse of physical law.

  • LIGO is the episode’s strongest capital-allocation case study: a 50-year, multi-generation program built two four-kilometer instruments capable of sensing less than one ten-thousandth of a proton’s width. Its first generation, built after 2000, detected nothing; the collaboration nevertheless kept going and built a second generation, which recorded a black-hole merger on September 14, 2015, before its formal science run. Ray Weiss had warned only a month earlier that failure to detect black holes would mean “we’ve led this country down this wrong path”—the kind of accountability and duration frontier infrastructure demands.

  • The broader innovation thesis is that open intellectual systems compound because talent migrates toward freedom, while discovery still depends on individuals refusing to pretend they understand what they do not. Levin rejects Lex’s simple one-third allocation of America, Germany, and the Soviet Union as equally likely first builders of the atomic bomb: European scientists disproportionately fled to America, not Nazi Germany, and intellectual freedom made the odds asymmetric. Her research ethic is equally unsentimental—curiosity should remain childlike, but “any self-respecting physicist” must kill an idea when the argument fails.

Digest · the substance, structured for research

1. A black hole is a causal boundary, not a dead star

  • Levin separates formation from definition: massive stars are one way to make black holes, but a dead star is not synonymous with one. “The black hole is the event horizon,” a surface separating events that can still affect the outside from events that cannot.

  • Matter and probes can cross inward, but nothing inside can send a causal influence back out. That one-way structure is the profound feature: at the horizon itself there need be no material, signpost, or dense shell—just an empty region of spacetime.

  • Her signature formulation is deliberately unsettling: black holes are “no thing” and “nothing,” or “more of a place than it is a thing.” Their gravitational presence survives even after the material that formed them has fallen beyond external knowledge.

2. Schwarzschild’s thought experiment found both a prediction and a failure signal

  • In 1915–16, amid World War I, Karl Schwarzschild solved Einstein’s newly completed equations while serving on the Eastern Front. He imagined crushing a star’s entire mass to a point—not as plausible astrophysics, but as an intentionally extreme mathematical setup.

  • The resulting solution both describes ordinary gravity around objects such as the Sun and predicts an event horizon. Lex’s framing holds: an extreme thought experiment designed to expose a theory’s limitations instead uncovered something nature may actually build.

  • The same mathematics also points toward its own breakdown at the singularity. Levin likens the theory to “a dying man marking in the dirt that something’s gone wrong here”: extreme curvature requires quantum mechanics, though nobody yet knows what replaces the classical singularity.

3. Stellar collapse leaves the horizon behind like a Cheshire-cat grin

  • A star roughly 20–30 times the Sun’s mass can exhaust its fusion fuel after reaching iron, where further fusion becomes energetically costly. Its collapse generates a shock and supernova; much of the expelled carbon, oxygen, and heavier material later becomes ingredients for planets and life.

  • A surviving core may stop as a neutron star, “an actual thing” resembling a giant nucleus. If the core remains heavier than roughly twice the Sun’s mass, collapse continues; Levin’s example puts about 10 solar masses inside roughly 60 kilometers, compared with the Sun’s 1.5 million-kilometer diameter.

  • Once curvature traps even outward-moving light, the horizon forms—but the star cannot remain parked there. It falls onward, leaving only gravitational attraction, in John Wheeler’s Cheshire-cat analogy: “One leaves behind only its grin, the other only its gravitational attraction.”

4. Oppenheimer predicted collapse before Wheeler accepted it

  • Oppenheimer and his student opened their compact 1939 paper with the categorical line, “When all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse.” It appeared the same day Nazi Germany advanced into Poland, burying an extraordinary prediction beneath an epochal crisis.

  • Wheeler initially considered Oppenheimer’s model too simplified and spent decades opposing its conclusion. Early computer work eventually pushed him to reverse himself: realistic massive stars did not make collapse disappear; black holes were likely end states.

  • The name arrived only in 1967, reportedly after someone interrupted Wheeler’s cumbersome phrase “the catastrophic end state of gravitational collapse” with “How about black hole?” Oppenheimer, by then disengaged, reportedly answered Wheeler’s eventual conversion with, “I’ve moved on to other things.”

5. The bomb exposed science’s geopolitical neutrality and institutional dependence

  • Levin calls the nuclear era “an excruciating moment” because physicists pursuing quantum mechanics and stellar energy were pressured into weapons work. Niels Bohr had supposedly remarked that nobody had found a way to kill people with quantum mechanics; fission and the atomic bomb ended that innocence.

  • Lex argues that America, Nazi Germany, or the Soviet Union might each have had roughly a one-third chance of building the bomb first, and that the American outcome was the least harmful game-theoretic branch. Levin accepts there was no plausible branch with no bomb, but disputes those equal odds.

  • Her reason is institutional: scientists fled Europe to America, while Americans did not flee toward Nazi Germany. The Bronx produced disproportionate particle-physics Nobel laureates because European immigrants found intellectual freedom there; if that freedom erodes, she warns, “the world” will no longer flock to the United States.

6. Crossing the horizon is locally uneventful and globally irreversible

  • Levin’s waterfall analogy makes the horizon concrete: spacetime rains inward while an outward photon swims like a fish against Niagara. Exactly at the horizon, that photon can appear externally to hover, even while a falling astronaut sails past it without noticing any physical boundary.

  • Lex presses the apparent contradiction that an outside observer sees the astronaut’s clock slow nearly to a stop. Levin concedes the idealized hovering picture is unstable: the astronaut’s own mass perturbs the horizon, which “bobbles” and absorbs them in finite external time; quantum perturbations also push only inward.

  • Locally, however, little time passes and no marker announces the point of no return. Rockets might rescue someone just outside, but once inside, “all the energy in the universe” cannot reverse the causal direction.

7. Inside, the singularity behaves like a future time rather than a location

  • Levin’s most striking interior description is that space and time effectively exchange roles. An exterior observer calls the singularity the sphere’s spatial center; the falling astronaut experiences it as an unavoidable moment in the future.

  • That rules out ordinary evasions: no orbit around the singularity, rocket burn, or sideways maneuver can avoid it, any more than propulsion can prevent the next moment from arriving. For a stellar-mass black hole, the interval from horizon to classical singularity may be microseconds.

  • Size reverses intuition. A larger black hole has gentler horizon-scale curvature—Earth looks flatter than a basketball—so crossing a supermassive horizon could be less noticeable. Lex suggests that, with a sufficiently large black hole, the astronaut could hang around for some months, even though the ultimate causal destination remains unchanged.

8. A black-hole interior could be bright enough to show cosmic history at once

  • Darkness characterizes the exterior shadow, not necessarily the falling observer’s view. Light from the galaxy can follow inward and focus behind the astronaut, producing an increasingly concentrated flash as the singularity approaches.

  • External clocks then appear extraordinarily fast relative to the astronaut’s. Levin says the observer might see millennia, perhaps the galaxy’s evolution, compressed into one bright beam: “a near-death experience,” except that it is “definitely a total death experience.”

  • Lex notes that human senses could not process a microsecond-scale torrent. Levin’s hedge depends on mass: a sufficiently large black hole offers more proper time, but no claim that a human could decode the accumulated information.

9. Supermassive black holes probably did not grow one stellar merger at a time

  • Stellar black holes may be abundant: Levin estimates hundreds of millions, perhaps a billion, in the Milky Way, even though only about 1% of stars end that way. Black holes nevertheless have multiple possible formation channels.

  • Galactic nuclei contain black holes from millions to tens of billions—and perhaps hundreds of billions—of solar masses. Levin says the universe likely lacked enough time to assemble the largest solely through successive mergers of stellar remnants.

  • A leading possibility is direct collapse from primordial material within a few hundred million years of the Big Bang. The chicken-and-egg ordering remains open: early stars, gaseous protogalaxies, black-hole jets, and feedback that curbed galactic growth may all have co-evolved.

10. Einstein traded absolute space and time for an absolute light speed

  • Levin says nobody genuinely visualizes four-dimensional spacetime. Physicists suppress dimensions and draw imperfect maps, much as a flat map misrepresents spherical Earth while retaining correction rules that recover the right distances.

  • Einstein’s enormous abstraction placed matter and energy on one side of his equations and spacetime deformation on the other. Different distributions then generate specific solutions: orbital geometry, black holes, gravitational lenses, or an expanding universe.

  • His decisive wager was to preserve the strange constancy of light speed for every observer. Since speed is distance divided by time, he abandoned absolute space and time instead—an audacious move while other researchers tried to make the experimental result look conventional.

11. Free fall is the purest experience of gravity

  • Einstein’s “happiest thought,” the equivalence principle, begins with a falling elevator. Cut its cable and both passenger and floor fall together; absent outside clues, the passenger cannot distinguish falling around Earth from floating in empty space.

  • Levin therefore inverts everyday language: pressure from a chair or floor is not pure gravity but electromagnetic matter preventing free fall. “The experience of gravity unfettered, uninterrupted by atoms is weightlessness.”

  • A thrown pen follows a natural spacetime curve until the ground interferes. The International Space Station similarly turns off its engines and continually falls around Earth.

12. Great science begins where experts refuse a comfortable explanation

  • Newton unified a falling apple with Earth’s orbit around the Sun, yet remained disturbed by action at a distance. General relativity answered by making Earth curve spacetime: the apple needs no invisible pull across empty space; the tree is what prevents its natural fall.

  • Levin’s ideal scientist remains childlike enough to ask simple questions and confident enough to admit ignorance. “They’re never going to lie to themselves that they understand something that they don’t understand.”

  • Lex extends that tension to the origin of life and consciousness: researchers understand evolution, biological machinery, and information inheritance, yet remain bothered by the unexplained beginning. Levin agrees discovery often comes because somebody “couldn’t sleep at night and couldn’t rest.”

13. Black holes erase every distinguishing feature except three numbers

  • A settled black hole is completely characterized by electrical charge, mass, and spin. Every black hole sharing those values is identical; nobody can identify one as “mine” through a ridge, scar, or other macroscopic feature.

  • Add a Mount Everest-like imperfection and the hole shakes it off through gravitational waves until it returns to a silent, featureless state. That makes it resemble a fundamental particle more than an ordinary astronomical body.

  • This “no-hair” quality is why black holes are unusually productive thought-experiment terrain. Their simplicity forces general relativity, thermodynamics, quantum mechanics, information theory, and causality into the same sharply constrained system.

14. Hawking radiation turns empty space into an information crisis

  • Hawking added only “a little smidge of quantum mechanics” to the vacuum near an event horizon. Quantum uncertainty prevents empty space from possessing exactly zero activity; transient, entangled particle pairs can arise with properties that cancel together.

  • If the horizon separates a pair, one particle may fall inward while the other can no longer recombine and disappear. The escaping particle looks like radiation, while its inward partner contributes negative energy in the exterior bookkeeping, reducing the black hole’s mass.

  • The crucial point is that the emitted particle did not travel outward from the interior. “The black hole steals one of these virtual particles and forces the other to live,” gradually converting the hole’s mass into Hawking radiation.

15. Featureless evaporation seems to destroy quantum information

  • Hawking’s radiation is thermal: its temperature reveals only the black hole’s mass, already measurable outside, and appears to carry no fine-grained record of what fell through the horizon. Larger black holes are colder; as evaporation shrinks one, it becomes progressively hotter.

  • The process takes far longer than the universe’s current age for ordinary black holes, but its endpoint is conceptually violent: the remaining horizon disappears, leaving radiation without the original information. “Everything’s gone. Poof.”

  • Quantum mechanics forbids that disappearance through unitarity. The resulting “black hole wars” therefore posed a stark choice: quantum theory is incomplete, the classical horizon is wrong, or some missing framework preserves information in a way neither description reveals.

16. Fuzzballs, soft hair, and firewalls diagnose the paradox without settling it

  • String-theory fuzzballs replace the smooth horizon and interior with a horizon-sized tangle of strings and branes. Nothing truly crosses, so information never becomes trapped; Levin finds the proposal interesting but does not think it is the answer.

  • Soft-hair proposals add low-energy quantum excitations—soft photons or gravitons—that could encode what fell inward. They challenge classical no-hair results without abandoning quantum field theory, but Levin again remains unconvinced that the mechanism can store and release the vast required information.

  • Firewalls propose an energetic horizon that incinerates anything crossing it. Levin treats the AMPS argument as intentionally provocative rather than a literal prediction: it exposed flaws hidden by earlier complementarity arguments, prevented the field from stalling, and motivated more serious entanglement-based resolutions.

17. Holography makes information loss impossible in a solvable universe

  • Black-hole entropy scales with surface area, not interior volume. Try to place more information in a region than its boundary can support and the region forms a black hole, suggesting that the surface supplies the fundamental information capacity.

  • Susskind called this a hologram; the radical extrapolation is that an apparent three-dimensional interior may be a projection from lower-dimensional data. Levin notes that extending the idea to our entire universe remains unresolved.

  • Juan Maldacena’s late-1990s construction made the intuition precise for anti-de Sitter space: a gravity-filled universe “in a box,” complete with black holes, is equivalent to a boundary quantum theory with no gravity and no information loss. It strongly indicates that unitarity wins, without yet showing the detailed escape route.

18. ER=EPR trades locality for quantum-mechanical consistency

  • ER=EPR links Einstein–Rosen bridges with Einstein–Podolsky–Rosen entanglement. Interior degrees of freedom could remain entangled with outward Hawking radiation through tiny, non-traversable wormholes, connecting the two sides without ordinary information crossing the horizon.

  • Levin pictures the horizon as “embroidered” from quantum wormholes. Seen coarsely, it remains a solid black shadow; magnified to the relevant scale, the fundamental structure may be entanglement rather than a perfectly smooth surface.

  • Her preference is explicit: she will entertain the loss of locality before accepting lost quantum information. In principle, Hawking radiation would contain subtle correlations encoding the interior, although recovering them might take longer than the universe’s age—like reconstructing a burned page from every smoke current and molecule.

19. Gravity itself may be an emergent collective variable

  • ER=EPR raises a deeper possibility than reconciling two independent theories: “Does that mean that gravity is fundamentally quantum mechanics?” Smooth spacetime may emerge from quantum connections rather than exist as a fundamental continuum.

  • Levin’s analogy is temperature. No individual particle carries temperature as an intrinsic property; one macroscopic number emerges from the collective motion of enormous numbers of particles.

  • A theory of everything might therefore lack a single direct equation connecting familiar gravity to microscopic ingredients. Black holes remain the best trail, but Levin refuses a timetable and calls quantum gravity technically harder than the origin-of-life problem without claiming it will necessarily take longer.

20. Extra dimensions could hide both matter and entire neighboring worlds

  • Physics observes three large spatial dimensions but does not explain why only three expanded. Additional dimensions could form a tiny “origami,” with strings or branes constricting some directions while preferentially allowing three to unravel.

  • Other models place observable matter on a three-dimensional membrane moving through larger dimensions. Humans would miss those directions because ordinary matter is glued to the membrane, not because the higher-dimensional “bulk” is necessarily small.

  • Mathematically, other membranes and civilizations could exist in that shadow space. Levin first says communication would be impossible, then accepts Lex’s correction: gravity may cross the bulk, so a sufficiently capable civilization could send gravitational waves—though generating and localizing them would demand immense energy.

21. Alien life may be abundant yet unlike anything that broadcasts

  • Exoplanet discoveries over roughly 30 years imply the Milky Way probably contains more planets than stars. Levin finds it increasingly difficult to imagine life never arising elsewhere, perhaps even within a few hundred light-years, though no clear civilization has appeared.

  • Lex rejects familiar great-filter answers, arguing that at least some technological species should survive their self-created dangers. Levin emphasizes the alarming datum that humanity reached potentially self-destructive technology within only a few hundred thousand years, while dinosaurs persisted for roughly 250 million years without engineering extinction.

  • Her deeper pushback is anthropocentric: life need not seek conquest, groups, records, or environmental control. Jellyfish lack localized brains and some can revert to an earlier state after injury; alien life might be solitary, nonviolent, effectively immortal, or informationally active without producing signals humans recognize.

22. Life may be an energetics transition with no bright dividing line

  • Levin’s minimum commonality is movement of electrons: nervous systems, metabolism, and organisms that extract energy even from rocks or minerals all exploit energetic flows. Life locally organizes matter and lowers entropy while increasing entropy in the larger system.

  • The slowest major terrestrial transition may have been multicellularity, which she frames as an energy problem. If cooperation among cells is energetically favorable it can spread; if too costly, evolution will not sustain it.

  • She expects the boundary between inanimate and animate matter to be gray, as disputes over viruses already suggest. Carbon is valuable because it combines into complex structures, but she leaves chemistry open: “I don’t know, maybe sulfur would do.”

23. Wormholes are valid geometries blocked by exotic material requirements

  • Einstein’s equations determine local curvature but do not fully dictate global topology. Space could be multiply connected: travel straight long enough, Levin says, and one might eventually return past the Virgo Cluster, Milky Way, and Earth.

  • For a desired wormhole, theorists reverse-engineer Einstein’s equations: first specify the architecture, then calculate what matter and energy would support it. Kip Thorne’s work exposed the recurring problem—traversable wormholes seem to require negative energy unlike ordinary stable matter.

  • Quantum mechanics weakens the prohibition. Casimir configurations between nearby metal plates can produce negative energies, suggesting quantum effects might prop a throat open. Still, enlarging microscopic structure to human scale remains entirely theoretical: “I’m not saying that quite yet” when Lex asks whether only engineering remains.

24. Warp-drive mathematics identifies requirements before it identifies fuel

  • Levin assigns advanced students to invent warp drives by contracting spacetime ahead of a traveler, crossing the shortened distance, then expanding it again. General relativity permits writing such geometries even when no known material supplies the required stress and energy.

  • Her analogy is dark energy: physicists observe accelerated expansion and can infer the responsible component’s pressure without knowing what it is. Dark energy has positive energy and negative pressure, so it is not the required wormhole material, but nature has already demonstrated unexpectedly exotic macroscopic behavior.

  • The vacuum-energy explanation looks structurally attractive yet fails numerically: calculations tend to produce either an enormous value or zero, not today’s tiny observed acceleration. That mismatch signals “absolutely a phenomenon” physics does not understand, perhaps tied to the size or topology of extra dimensions.

25. Precision cosmology made the missing 95% visible as a problem

  • Levin concedes that “dark matter” and “dark energy” are proxy names for unknown causes, but rejects the claim that they are arbitrary inventions. Only extraordinary measurement precision made it possible to infer that familiar observable matter contributes less than 5% of the cosmic inventory.

  • Dark matter has especially visual evidence: colliding galaxy clusters separate interacting luminous gas from the gravitationally reconstructed mass. The bright material collides and stalls while the non-interacting mass passes through, leaving lensing and light distributed differently.

  • Neutrinos prove that genuinely dark particles exist—they stream through bodies and Earth without interacting with light—but known neutrinos have insufficient mass to supply the missing component. Levin hopes dark matter, dark energy, and extra dimensions share one mechanism, while clearly labeling that unity as a desired result.

26. Gravitational waves are sound-like ripples in spacetime, not light

  • Orbiting black holes drag their curvature patterns around, but those changes cannot propagate faster than light. The moving geometry therefore launches waves in the shape of spacetime; after merger, the remnant radiates imperfections away and settles into a “quiescent, perfectly silent” spinning black hole.

  • The final mass is lower than the two starting masses because E=MC² energy leaves as gravitational radiation. No electromagnetic band—radio, infrared, optical, X-ray, or gamma ray—is required; an entirely dark collision can announce itself only through spacetime.

  • Levin’s analogy is a drum struck by black-hole mallets. Close enough, the squeezing and stretching could occur in the human auditory frequency range and directly move an eardrum even in vacuum: one could “literally hear these waves ringing.”

27. LIGO turned four kilometers of vacuum into a cosmic instrument

  • Gravitational waves interact so weakly that they can travel for billions of years largely preserved, aside from dilution and cosmological stretching. LIGO was designed like a gigantic musical instrument: record the changing shape of the ringing “drum” of spacetime and replay it as time-dependent sound.

  • Each L-shaped instrument spans four kilometers through enormous evacuated tunnels. The target displacement was less than one ten-thousandth of the variation across a proton—precision Levin calls so extreme that she remains amazed the project worked at all.

  • The effort consumed about 50 years: people who began in their 30s or 40s reached their 80s before success. Its first post-2000 generation returned “crickets,” yet the collaboration preserved funding, leadership, and technical confidence long enough to build again.

28. LIGO succeeded in the narrow gap between calibration and operation

  • On September 14, 2015, teams in Louisiana and Washington were still stress-testing the second-generation machines—driving trucks and braking nearby to characterize noise—before the formal science run. They eventually left for the night but kept the instruments locked.

  • Within roughly an hour, a wave reached Louisiana and then Washington. Levin says the source began more than 1.5 billion years earlier, before multicellular organisms had emerged on Earth, and arrived during the centenary year of Einstein’s general relativity.

  • A month earlier, Ray Weiss had told her that missing black holes would make the whole project a failure and mean “we’ve led this country down this wrong path.” Levin sees the detection as both scientific confirmation and an almost unbelievable triumph of collective engineering.

29. Gödel and Turing found hard boundaries around proof and computation

  • Kurt Gödel overturned the expectation that every true mathematical statement should be formally provable. His incompleteness result made mathematics itself contain truths unreachable as theorems—effectively, Levin says, “no theory of everything for mathematics.”

  • Alan Turing approached the same boundary through uncomputable numbers and mechanized reasoning. Asking what a proof or thought process is led him to the universal machine: one hardware system capable of executing different instructions, the conceptual computer.

  • Turing was recruited to help break Enigma, and the work is widely believed to have helped turn the war in favor of the Allies. His philosophical move was broader: perhaps humans are biological machines whose thought can be replicated. Gödel instead treated mathematical objects as more real than the physical world, making their intellectual connection a study in opposite metaphysics.

30. Genius does not require madness, but character traits can become tragic flaws

  • Levin rejects the “mad genius” cliché: insanity neither creates brilliance nor follows from it. Yet people who abandon conventional rewards, social attachment, or creature comfort for elusive problems can become isolated and vulnerable to runaway instability.

  • Gödel was probably a paranoid schizophrenic, feared poisoning, and ultimately starved himself. Turing, prosecuted after honestly reporting a theft involving a homosexual encounter, was chemically castrated despite his war service. The belief that he later died after biting a cyanide-poisoned apple is disputed; Levin says she does not know whether that account is apocryphal.

  • Their stories fit Levin’s belief in the Greek tragic flaw: “What makes us great is ultimately our downfall.” She does not romanticize suffering; her narrower claim is that the same unusual disposition can produce both an extraordinary contribution and personal destruction.

31. Sustainable research requires knowing when obsession is no longer evidence

  • Lex contrasts solitary, years-long bets by Andrew Wiles and Grigori Perelman with Terence Tao’s ability to set aside an intractable problem and redirect toward one he can solve. Wiles spent about seven years on Fermat’s Last Theorem, announced a proof, endured discovery of a flaw, then repaired it.

  • Levin acknowledges that a correct path can approach a result without ever reaching it, making abandonment difficult. Her discipline is to kill most of her own ideas: “Any self-respecting physicist should be able to do that.”

  • She still cannot imagine withholding a discovery merely because it might later be misused; Bohr could not foresee quantum mechanics killing people either. Her current work on black holes and extra dimensions feels safe, but history makes categorical assurances impossible.

32. Art and science belong to one culture of making

  • At Pioneer Works, founder Dustin Yellin transformed an old Brooklyn ironworks into a collision of serious artists and scientists. Levin rejects “outreach” as the governing label: a scientist bringing insight back from a summit is extending culture, just as an artist does by exhibiting completed work.

  • The institution hosts live scientific conversations and publishes Broadcast, placing disciplines beside one another without requiring artists to imitate science or scientists to make art. Levin values it as genuine collaboration because removing one participant changes the project’s shape.

  • Her reading follows the same instinct: Kazuo Ishiguro’s Never Let Me Go, Martin Amis’s Time’s Arrow, Orwell, and Cormac McCarthy’s The Road use speculative structures to expose human relationships. Science supplies architecture; language and character remain the point.

33. Mystery survives every answer, while every human record eventually disappears

  • Asked what she would demand from an oracle, Levin chooses quantum gravity or whether gravity is emergent, then hesitates: a scientific life partly depends on unanswered questions. Lex answers that every definitive reply would generate another “wait—why?”

  • Relativity produced black holes; black holes produced the information paradox; cosmology produced questions about what came before the Big Bang and whether other universes exist. Knowledge raises the next level of abstraction rather than closing inquiry.

  • Cosmically, Levin expects every prize, proof, conversation, influence, and even AI eventually to expire. Her response is neither nihilism nor permanence-seeking: judge whether one contributed a near-term “net positive” and focus on “drawing something beautiful in the sand” before it is washed away.

Speaker 1

Black holes curve space and time around them in the way that we've been describing. Things follow along the curves in space. If black holes move around, the curves have to follow them, right? But they can't travel faster than the speed of light either.

So what happens is, let's say, black holes move around. Maybe I've got 2 black holes in orbit around each other. That can happen. It takes a while. A wave is created in the actual shape of space, and that wave follows the black holes. The black holes are undulating.

Eventually, those 2 black holes will merge. As we were talking about, it doesn't take an infinite amount of time, even though there's time dilation because they're both so big. They're really deforming spacetime a lot. I don't have a little tiny marble falling across an event horizon. I have 2 event horizons.

In the simulations, you can see them bobble, and they merge together. They make 1 bigger black hole, and then it radiates in gravitational waves. It radiates away all those imperfections, and it settles down to 1 quiescent, perfectly silent black hole that's spinning. Beautiful stuff.

It emits E = mc² worth of energy, so the mass of the final black hole will be less than the sum of the 2 initial black holes. That energy is radiated away in this ringing of spacetime. It's really important to emphasize that it's not light. None of this has to do literally with light that we can detect with normal detectors.

X-rays are a form of light. Gamma rays are a form of light. Infrared, optical—all of this, the whole electromagnetic spectrum. None of it is emitted as light. It's completely dark. It's only emitted in the rippling of the shape of space.

A lot of times, it's likened more closely to sound. Technically, we've kind of argued—I haven't done an actual calculation—but if you're near enough to 2 colliding black holes, they actually ring spacetime in the human auditory range. The frequency is actually in the human auditory range, so the shape of space could squeeze and stretch your eardrum even in a vacuum. You could literally hear these waves ringing.

The following is a conversation with Jenna Levan, a theoretical physicist and cosmologist specializing in black holes, cosmology of extra dimensions, topology of the universe, and gravitational waves in spacetime. She has also written some incredible books, including How the Universe Got Its Spots on the topic of the shape and the size of the universe; A Madman Dreams of Turing Machines on the topic of genius, madness, and the limits of knowledge; Black Hole Blues and Other Songs from Outer Space on the topic of LIGO and the detection of gravitational waves; and The Black Hole Survival Guide, all about black holes. This was a fun and fascinating conversation. This is a Lexman podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Jenna Levin.

Lex Fridman

I should say that you sent me a message about not starting early in the morning, and that made me feel like we're kindred spirits. You wrote to me, “When the great physicist Sidney Coleman was asked to attend a 9:00 a.m. meeting, his reply was, ‘I can't stay up that late.’” Yeah, so classic.

Speaker 1

Sidney was beloved. I think all the best thoughts, honestly—maybe the worst thoughts, too—all come at night. There's something about the night. Maybe it's the silence. Maybe it's the peace all around. Maybe it's the darkness, and you can be with yourself and think deeply.

I feel like there are stolen hours in the middle of the night because it's not busy. Your gadgets aren't pinging. There's really no pressure to do anything, but I'm often awake in the middle of the night, and so it's sort of like these extra hours of the day. I think we were exchanging messages at 4 in the morning.

So in that way, and in many other ways, we're kindred spirits. Let's go into one of the coolest objects in the universe: black holes. What are they? And maybe even a good way to start is to talk about how they're formed.

Speaker 1

In a way, people often confuse how they're formed with the concept of the black hole in the first place. So when black holes were first proposed, Einstein was very surprised that such a solution could be found so quickly, but really thought nature would protect us from their formation. Then nature thinks of a way to make these crazy objects, which is to kill off a few stars.

But I think there's a confusion that dead stars—these very massive stars that die—are synonymous with the phenomenon of black holes. It's really not the case. Black holes are more general and more fundamental than just the death state of a star.

Even the history of how people realized that stars could form black holes is fascinating, because the entire idea really just started as a thought experiment. If you think of it, it's 1915, 1916, when Einstein fully describes relativity in what is the canonical formulation. There was a lot of changing back and forth before then.

It's World War I, and he gets a message from the Eastern Front from a friend of his, Karl Schwarzschild, who solved Einstein's equations while sitting in the trenches amid cannon fire. It was joked that he was calculating ballistic trajectories. He's also perusing the proceedings of the Prussian Academy of Sciences, as you do.

He was an astronomer who had enlisted in his 40s, and he found this really remarkable solution to Einstein's equations. It's the first exact solution. He doesn't call it a black hole. It's not called a black hole for decades.

What I love about what Schwarzschild did is that it's a thought experiment. It's not about observations. It's not about making these things in nature. It's really just about the idea.

He sets up this completely untenable situation, and he says, “Imagine I crush all the mass of a star to a point. Don't ask how that's done, because that's really absurd. But let's just pretend and imagine that's a scenario.” Then he wants to decide what happens to spacetime if I set up this confounding but somehow very simple scenario.

Really, what Einstein's equations were telling everybody at the time was that matter and energy curve space and time, and then curved spacetime tells matter and energy how to fall once spacetime is shaped. He finds this beautiful solution, and the most amazing thing about the solution is that he finds this demarcation, which is the event horizon—the region beyond which not even light can escape.

If you were to ask me today, with all this mass crushed to a point, the black hole is the event horizon. The event horizon is really just a point in spacetime, or a region in spacetime. It's actually, in this case, a surface in spacetime.

It marks a separation in events, which is why it's called an event horizon. Everything outside is causally separated from the inside, insofar as what's inside the event horizon can't affect events outside. What's outside can affect events inside.

I can throw a probe into a black hole and cause something to happen on the inside, but the opposite isn't true. Somebody who fell in can't send a probe out. This one-way aspect really is what's profound about the black hole.

Sometimes we talk about black holes being nothing, because at the event horizon there's really nothing there. Sometimes when we think about black holes, we want to imagine a really dense dead star. But if you go up to the event horizon, it's an empty region of spacetime. It's more of a place than it is a thing.

Einstein found this fascinating. He helped get the work published, but he really didn't think these would form in nature. I doubt Karl Schwarzschild did either. I think they thought they were solving theoretical mathematical problems, but not describing what turned out to be the end state of gravitational collapse.

Maybe the purpose of the thought experiment was to find the limitations of the theory. You find the most extreme versions in order to understand where it breaks down.

Speaker 1

Yeah, and it just so happens in this case that it might actually predict these extreme kinds of objects. It does both. It also describes the Sun from far away. The same solution does a great job helping us understand the Earth's orbit around the Sun.

It's incredible. It does a great job. It's almost overkill. You don't really need to be that precise with relativity. Yes, it predicts the phenomenon of black holes, but it doesn't really explain how nature would form them.

Then, on top of that, it does signal the breakdown of the theory. You're quite right about that. It actually says, “Oh, man.” You go all the way toward the center, and, yeah, this doesn't sound right anymore.

Sometimes I liken it to a dying man marking in the dirt that something's gone wrong here, right? It's signaling that there's some culprit, that there's something wrong in the theory. Even Roger Penrose, who did seminal work trying to understand the formation of black holes from gravitational collapse, thought, “Oh, yeah, there's a singularity that's inevitable. There's no way around it once you form a black hole.”

But he said this is probably just a shortcoming of the fact that we've forgotten to include quantum mechanics, and that when we do, we'll understand this differently. According to him, the closer you get to the singularity, the more quantum mechanics comes into play, and therefore there is no singularity; there's something else.

I think everybody would say that. I think everybody would say the closer you get to the singularity, for sure, you have to include quantum mechanics. You just can't consistently talk about magnifying such small scales, having such enormous ruptures and curvatures and energy scales, and not include quantum mechanics. That's just inconsistent with the world as we understand it.

So you've described the brain-breaking idea that a black hole is not so much super-dense matter, as it's sometimes described, but more akin to a region of spacetime—but even more so, just nothing.

Yeah, it's nothing. That's a thing you seem to like to say.

Speaker 1

I do like to say that black holes are no thing. They're nothing.

Okay. So what does that mean?

Speaker 1

That's what I mean. That's the more profound aspect of the black hole. You asked originally, how do they form? I think that even when you try to form them in messy astrophysical systems, there's still nothing at the end of the day left behind.

This was a very big surprise. Even though Einstein accepted that this was a true prediction, he didn't think that they would be made. It was quite astounding that people like Oppenheimer—it's probably Oppenheimer's most important theoretical work—were thinking about nuclear physics and quantum mechanics in the context of these utopian questions: Why do stars shine? Why is the sun radiant and hot, this amazing source of light?

People like Oppenheimer began to ask the question: Could stars collapse to form black holes? Could they become so dense that eventually not even light would escape? That's why I think people think that black holes are these dense objects. That's often how they're described.

But what happens is that these very massive stars are burning thermonuclear fuel. They're burning Earthfuls of thermonuclear fuel and emitting energy through E = mc². They're fusing. It's a fusion bomb. It's a constantly going thermonuclear bomb.

Eventually, they're going to run out of fuel. They're going to run out of hydrogen, helium, and stuff to fuse. They hit an iron core. Going past iron with fusion is actually energetically expensive, so they're no longer going to do that so easily. Suddenly, they've run out of fuel.

If the star is very, very massive—much more massive than our sun, maybe 20 or 30 times the mass of our sun—it'll collapse under its own weight. That collapse is incredibly fast and dramatic, and it creates a shock wave. That's the supernova explosion.

A lot of these stars rebound because, once they crunch, they've reached a new critical capacity where they can reignite to higher elements, heavier elements, and that sets off a bomb, essentially. The star explodes, helpfully, because that's why you and I are here: Stars send their material back out into space, and you and I get to be made of carbon and oxygen and all this good stuff. We're not just hydrogen. The suns do that for us.

What's left sometimes ends up as a neutron star, which is a very cool, fascinating object. It's superdense but bigger than a black hole, meaning it's not compact enough to become a black hole. It's an actual thing. A neutron star is a real thing. It's like a giant neutron. Literally, electrons get jammed into protons and make this giant nucleus and this superconducting matter. Very strange, amazing objects.

But if the core is heavier than that—and that's heavier than twice the mass of the sun—it will become a black hole. Oppenheimer wrote this beautiful paper in 1939 with his student, saying that they believed the end state of gravitational collapse is actually a black hole. This is a stunning and visionary conclusion.

The paper was published on the same day the Nazis advanced on Poland, so it did not get a lot of fanfare in the newspapers.

Yeah, we think there's a lot of drama today on social media. Imagine that. Here's a guy who predicts how, actually, in nature, this most radical of objects would form—an object that broke even Einstein's brain—while one of the most evil, if not the most evil, humans in history was starting the first steps of a global war.

What I also love about that lesson is how agnostic science is, because he was asking these utopian questions, as were other people of the time, about nuclear physics and stars. You might know the play Copenhagen by Michael Frayn. There's this line that he attributes to Bohr. Bohr was the great thinker of the early foundations of quantum mechanics, a Danish physicist. In the play, Bohr says to his wife, “Nobody's thought of a way to kill people using quantum mechanics.”

Now, of course, then there's the nuclear bomb. What I love about this was the pressure scientists were under to do something with this nuclear physics and to enter this race over a nuclear weapon. But at the same time, in 1939, Oppenheimer was thinking about black holes.

There's even a small line in Christopher Nolan's film. It's very hard to catch. There's a reference to it in the film where they're sort of joking: “Well, I guess nobody's going to pay attention to your paper now,” because of the Nazi advance on Poland.

That's the other remarkable thing about Oppenheimer: He's also a central figure in the construction of the bomb, right? So it's theory and experiment clashing together with geopolitics.

#468

Exactly. So, of course, Oppenheimer, now known as the father of the atomic bomb, talks about destroyers of worlds. But it's the same technology, and that's what I mean by science being agnostic, right? It's the same technology: overcoming a critical mass and igniting thermonuclear fusion.

Eventually, there was fission. The original bomb was a fission bomb, and fission was first shown by Lise Meitner, who showed that a certain uranium, when bombarded with protons, broke into smaller pieces that were less than the uranium. Some of that mass, through E = mc², had escaped, and it was the first concrete demonstration of Einstein's most famous equation.

All of this comes together, but the story is that they still weren't called black holes. This was 1939, and they had these very long-winded ways of describing the end state, the catastrophic end state, of gravitational collapse.

What you have to imagine is the star collapsing. What's the sun? The sun's 1.5 million kilometers across. Imagine a star much bigger than the sun, with a much bigger radius. It's so heavy that it collapses. It goes supernova. What's left is still maybe 10 times the mass of the sun, just what's left in that core.

It continues to collapse. When that reaches about 60 kilometers across, imagine 10 times the mass of the sun, city-sized. That is a really dense object, and now the black hole essentially has begun to form. The curvature in spacetime is so tremendous that not even light can escape. The event horizon forms.

But the event horizon is almost imprinted on spacetime, because the star can't sit there in that dense state any more than it can race outward at the speed of light. Even light is forced to fall inwards. The star continues to fall, and that's the magic part. The star leaves the event horizon behind and continues to fall into the interior of the black hole.

Where it goes, nobody really knows. But it's gone from sight. It goes dark. There's this quote by John Wheeler, who's like the granddaddy of American relativity. He has a line that's something to the effect: “The star, like the Cheshire cat, fades from view. One leaves behind only its grin; the other, only its gravitational attraction.”

He was giving a lecture. It's actually above Tom's Restaurant, from Seinfeld, near Columbia in New York.

Nice.

#468

There was a place—or there still is a place—there where people were giving lectures about astrophysics. It was 1967. Wheeler was exhaustively saying this loaded term, “the end state of catastrophic gravitational collapse.” Rumor has it that someone shouted from the back row, “Well, how about black hole?”

Apparently, he then foisted this term on the world. Wheeler had a way of doing that.

Lex Fridman

Well, I love terms like that: Big Bang, black hole. There's something about them. It's just pointing out the elephant in the room and calling it an elephant. It is a black hole. That's a pretty accurate and deep description.

I just wanted to point out that I'm looking for the first time at a 1939 paper from Oppenheimer. It's like 3 pages.

#468

Oh yeah, it's gorgeous. The simplicity of some of these—that's so gangster. Just revolutionize all of physics with this. Einstein did that multiple times in a single year.

Mhm. “When all thermonuclear sources of energy are exhausted, a sufficiently heavy star will collapse.”

That's Oppenheimer.

#468

Mhm.

Lex Fridman

“Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation reduce the star's mass to orders of that of the sun, this contraction will continue indefinitely.”

#468

Now, I have to say that Wheeler, who actually coined the term black hole, gives Oppenheimer quite a terrible time about this. He thinks he's wrong, and they entered what has sometimes been described as a kind of bitter—I don't know if you would actually say feud—but there were bad feelings.

Wheeler actually spent decades saying Oppenheimer was wrong. Eventually, with his computer work—that early work Wheeler was doing with computers, when he was also trying to understand nuclear weapons in a peacetime world—he found himself returning again to these astrophysical questions and decided that Oppenheimer had been right.

He thought it was too simplistic, too idealized a setup that they had used, and that if you looked at something that was more realistic and more complicated, it would simply go away. In fact, he drew the opposite conclusion.

There's a story that Oppenheimer was sitting outside of the auditorium when Wheeler was coming forth with his declaration that, in fact, black holes were the likely end state of gravitational collapse for very, very heavy stars. When asked about it, Oppenheimer sort of said, “Well, I've moved on to other things.”

Because you've written in many places about the human beings behind the science, I have to ask you about this: nuclear weapons, where the greatest physicists are coming together to create this most terrifying and powerful technology.

And now I get to talk to world leaders for whom this technology is part of the tools used, perhaps implicitly, on the chessboard of geopolitics. What can you say, as a person who's a physicist and who has studied physicists and written about the physicists—the humans behind this—about this moment in human history, when physicists came together and created this weapon that's powerful enough to destroy all of human civilization?

#468

I think it's an excruciating moment in the history of science. People talk about Heisenberg, who stayed in Germany and worked for the Nazis in their own attempt to build the bomb. There was this kind of hopeful talk that maybe Heisenberg had intentionally derailed the nuclear weapons program, but I think that's been largely discredited.

He could have made the bomb had he not made some really simple errors in his original estimates about how much material would be required or how they would get over the energy barriers. That's a terrifying thought. I don't know that any of us can really put ourselves in that position of imagining that we're faced with that quandary—having to take the initiative to participate in thinking of a way that quantum mechanics can kill people, and then making the bomb.

I think overwhelmingly physicists today feel we should not continue in the proliferation of nuclear weapons. Very few theoretical physicists want to see this continue. At that moment in history, the Soviet Union had incredible scientists, Nazi Germany had incredible scientists, and the United States had incredible scientists. It's very easy to imagine that one of those 3 would have created the bomb first, not the United States. How different would the world be?

The game theory of that, I think, says the probability is 33% that it was the United States.

#468

If the Soviet Union had the bomb, I think they would have used it in a much more terrifying way in the European theater and maybe turned on the United States.

And obviously, with Hitler, he would have used it.

#468

I think there's no question he would have used it to kill hundreds of millions of people.

In the game theory version, this was the least harmful outcome.

#468

Yes. Yes. But there is no outcome with no bomb that any game theorist would play. But I think if we just remove the geopolitics and the ideology and the evil dictators, all of those people are just scientists.

I think they don't necessarily even think about the ideology. It's a deep lesson about the connection between great science and the annoying, sometimes evil politicians that use that science for means that are either good or bad.

#468

Mhm.

Lex Fridman

And the scientists perhaps don't—boy, do they even have control of how that science is used?

#468

It's hard. They don't have control, right? Once it's made, it's no longer scientific reasoning that dictates the use or its restraint. But I will say that I do believe that it wasn't 33% down the line, because America was different, and I think that's something we have to think about right now in this particular climate.

So many scientists fled here. They fled here. Americans weren't fleeing to Nazi Germany. They came here, and they were motivated by—it's more than a patriotism. It was a patriotism, obviously, but it was more than that. It was really understanding the threat of Europe, what was going on in Europe, and how quickly that life turned—how quickly this free-spirited Berlin culture was suddenly in this repressive and terrifying regime.

So I think that there was a much higher chance that it happened here in America.

Yeah. And there's something about the American system—the, you know, it's cliché to say it, but the freedom, all the different individual freedoms that enable, at its best, a very vibrant scientific community. That's really exciting.

#468

Absolutely, to scientists. And it's very valuable to maintain that, right? The vibrancy of the debate, of the funding, those mechanisms. Absolutely, the world flocked here, and that won't be the case if we no longer have intellectual freedom.

Yeah. There's something interesting to think about—the tension in the Cold War between China and the United States in the 21st century. Some of those same questions, some of those ideas, will rise up again, and we want to make sure that there's a vibrant, free exchange of scientific ideas. I believe most Nobel Prizes come from the United States, right?

#468

Oh, yeah. I don't have the number, but disproportionately so. Disproportionately so. In fact, a lot of them from particle physics came from the Bronx.

[Laughs] And they were European immigrants. How do you explain this?

#468

They fled Europe precisely because of the geopolitics we're describing.

Yeah. And so instead of being Nobel Prize winners from the Soviet Union or from the Eastern Bloc, they were from the Bronx. And that's the thing you write about, and we'll return to time and time again: science is done by humans.

Some of those humans are fascinating. There are tensions and battles. Some are loners, some are great collaborators, some are tormented, some are easygoing—all this kind of stuff. That's the beautiful thing about it that we forget sometimes: it's humans, and humans are messy and complicated and beautiful and all of that.

So what were we talking about? Oh, the star is collapsing. Okay. Can we just return to the collapse of a star that forms a black hole? At which point does the super-dense thing become nothing? If we can just linger on this concept.

#468

Yeah. So if I were falling into a black hole and I timed it really right, right as I crossed this invisible region—this demarcation—I happened to know where it was. I calculated it, because there's no line there. There's no sign that it's there. There's no signpost.

I could emit a little light pulse and try to send it outward exactly at the event horizon. It's racing outward at the speed of light. It can hover there because, from my perspective, it's very strange. Spacetime is like a waterfall raining in, and I'm being dragged in with that waterfall. I can't stop at the event horizon. It comes, it goes. It's behind me really quickly.

That light beam can try to sit there because it's like a fish swimming against Niagara, swimming against the waterfall. It's stuck there. That's one way you could have a little signpost. If you fly by, you think it's moving at the speed of light—it flies past you at the speed of light—but it's sitting right there at the event horizon.

So you're falling back across the event horizon. Right at that point, you shoot outwards a photon.

#468

Yes. And it's just stuck there. It just gets stuck there. Now, it's very unstable, so the star can't sit there. That's the point. It just rains inward with this waterfall.

But from the outside, all we should ever really care about is the event horizon, because I can't know what happens on the inside. It could be pure matter and antimatter thrown together, which annihilates into photons on the inside and loses all its mass into the energy of light. It won't matter to me, because I can't know anything about what happened on the inside.

Okay. Can we just linger on this? What models do we have about what happens on the inside of the black hole at that moment? I guess one of the intuitions—one of the big reminders that you're giving to us—is, “Hey, we know very little about what can happen on the inside of a black hole.” That's why we have to be careful about making claims. It's better to think about the black hole as an event horizon.

What can we know, and what do we know, about the physics of spacetime inside the black hole?

#468

I don't mind being incautious about thinking about what the math tells us. I'm not such an observer. I'm very theoretical in my work. It's really pen on paper a lot. These are thought experiments that I think we can perform and contemplate. Whether or not we'll ever know is another question.

One of the most beautiful things that we suspect happens on the inside of a black hole is that space and time, in some sense, swap places. While I'm on the outside of the black hole, let's say I'm in a nice, comfortable space station. This black hole is maybe 10 times the mass of the Sun, 60 kilometers across. I could be 100 kilometers out. That's very, very close, orbiting quite safely. No big deal. Hanging out. I don't bug the black hole; the black hole doesn't bug me. It won't suck me up like a vacuum or anything crazy.

But my astronaut friend jumps in. As they cross the event horizon, what I'm calling space—I'm looking on the outside at this spherical shadow of the black hole, cast by the light around it—it's a shadow because everything gets too close and falls in. It's just this contrast against a bright sky.

I think, “Oh, there's the center of a sphere, and in the center of the sphere is the singularity.” It's a point in space from my perspective, but from the perspective of the astronaut who falls in, it's actually a point in time.

Their notions of space and time have rotated so completely that what I'm calling a direction in space toward the center of the black hole, like the center of a physical sphere, they're going to tell me—well, they can't tell me, but they're going to come to the conclusion—“Oh, no, that's not a location in space. That's a location in time.”

In other words, the singularity ends up in their future, and they can no more avoid the singularity than they can avoid time coming their way. There's no shenanigans you can do once you're inside the black hole to try to skirt the singularity. You can't set yourself up in orbit around it. You can't try to fire rockets and stay away from it, because it's in your future, and there's an inevitable moment when you will hit it.

#468

Usually for a stellar-mass black hole, we think it's microseconds—microseconds to get from the event horizon to the singularity.

To the singularity. Oh boy. So that's describing this from your astronaut friend's perspective.

#468

Yes.

Lex Fridman

But from your perspective, what do you see when your friend falls into the black hole and you're chilling outside and watching?

#468

One way to think about this is that, as you're approaching the black hole, the astronaut's spacetime is rotating relative to your spacetime. Let's say right now my left is your right. We're not shocked by the fact that there's this relativity in left and right. It's completely understood, and I can perform a spatial rotation to align my left with your left. Right now, I've completely rotated left out.

If I just want to draw a kind of diagram, at the top of maps there's north, south, east, and west. But now time is up and down, and one direction of space is, let's say, east-west. As you approach the black hole, it's as though you're rotating in spacetime.

So what is the effect of that? As this astronaut gets closer and closer to the event horizon, part of their space is rotated into my time, and part of their time is rotated into my space. In other words, their clocks seem to be less aligned with my time. The overall effect is that their time seems to dilate.

The spacing between ticks on the clock of their watch, on the face of their watch, is elongated—dilated—relative to mine. It seems to me that their watches are running slowly, even though they were made in the same factory as mine. They were both synchronized beautifully, and they're excellent Swiss watches. It seems as though time is elapsing more slowly for my companion, and likewise, for them, it seems like mine's going really fast.

Years could elapse in my space station. My plants come and go. They die. I age faster. I've got gray hair, and they're falling in, and it's been minutes in their frame of reference. Flowers in their little rocket ship haven't rotted. They don't have gray hair. Their biological clocks have slowed down relative to ours.

Eventually, at the event horizon, it's so extreme, it's so slow, it's as though their clocks have stopped altogether from my point of view. That's to say that it's as though their time is completely rotated into my space. This is connected with the idea that, inside the black hole, space and time have switched places.

I might see them hover there for millennia. Other astronauts could be born on my space station. Generations could populate it, watching this poor astronaut never fall in.

So basically, time almost comes to a standstill, but they still fall in, right?

#468

They do fall in eventually. That's because they have some mass of their own. They're not a massless particle, and so they deform the event horizon a little bit. You'll actually see the event horizon wobble and absorb the astronaut. So, in some finite time, the astronaut will actually fall in.

So it's like this weird spacetime bubble that we have around us. And then there's a very big spacetime-curvature bubble thing from the black hole, and there's a nice swirly-type situation going on. That's how you get sucked up.

#468

Yeah.

Lex Fridman

So if you're a perfect, infinitely small particle, you would just take longer and longer and probably just be stuck there or something.

#468

But no, there's quantum mechanics. Eventually, you'll fall in there. Any perturbation will only go one way. It's unstable in one direction—in one direction only.

It's really important to remember that, from the point of view of the astronaut, not much time has passed at all. You just sail right across, as far as you're concerned, and nothing dramatic happens here. You might not even realize you've come to the event horizon. You might not even realize you've crossed the event horizon because there's nothing there.

This is an empty region of spacetime. There's no marker to tell you you've reached this very dangerous point of no return. You can fire your rockets like hell when you're on the outside and maybe even escape, right? But once you get to that point, there's no amount of energy—all the energy in the universe will not save you from this demise.

You know, there are different-size black holes. Maybe we can talk about the experience that you have falling into a black hole, depending on the size of the black hole?

#468

Yeah, that might surprise people. The bigger it is, the less noticeable it is that you've crossed the event horizon. One way to think about it is that curvature is less noticeable the bigger it is.

If I'm standing on a basketball, I'm very aware I'm balancing on a curved surface. My 2 feet are in different locations, and I really notice. But on Earth, you actually have to be kind of clever to deduce that the Earth is curved. The bigger the planet, the less you're going to notice the curvature—the global curvature.

It's the same thing with a black hole. A huge, huge black hole just kind of feels flat. You don't really notice.

I'm trying to figure out how the physics works, because if you don't notice and there's nothing there, the physics is weird in your frame of reference.

#468

No. Well, another cool thing—I'd like to dispel myths.

Yeah. Do you need a minute? You're holding your head.

#468

There's a sense that you should be able to know when you're inside a black hole, when you've crossed the event horizon. But no, from your frame of reference, you might not be able to know. At first, at least, you might not realize what's happened.

There are some hints. For instance, black holes are dark from the outside, but they're not necessarily dark on the inside. This is fascinating: your experience could be that it's quite bright inside the black hole because all the light from the galaxy can be shining in behind you, and it's focusing down because you're all approaching this really focused region in the interior.

You actually see a bright white flash of light as you approach the singularity. I joke that it's like a near-death experience: you see the light at the end of the tunnel. You would see millennia pass on Earth. You could see the evolution of the entire galaxy—one big, bright flash of light.

It's like a near-death experience, but it's definitely a total-death experience. It goes pretty fast.

But looking out, everything's going super fast.

#468

Yeah. The clocks on Earth, on the space station, seem to be progressing very rapidly relative to yours. The light can catch up to you, and you get this bright beam of light as you see the evolution of the galaxy unfold. It depends on the size of the black hole and how long you have to hang around. The bigger the black hole, the longer it takes you to expire in the center.

Obviously, the human sensory system—we're not able to process that information correctly, right? It would be a microsecond, right?

#468

That would be too fast.

Yeah, but it would be so cool to get that information. But with a big black hole, you could actually hang around for some months.

So, how are small black holes versus supermassive black holes formed, just so people can kind of load that in? Are they all—is it always a star?

#468

No. This is also why it's important to think of black holes more abstractly. They are something very profound in the universe, and there are probably multiple ways to make black holes. Making them with stars is most plentiful.

There could be hundreds of millions, maybe even a billion, black holes in our Milky Way galaxy alone. There are that many stars. It's only about 1% of stars that will end their lives in a death state that is a black hole.

But we now see—and this was really quite a surprise—that there are supermassive black holes. They're billions or even hundreds of billions of times the mass of the Sun—millions to tens of billions, maybe even hundreds of billions. So, extremely massive.

We don't think that the universe has had enough time to make them from stars that just merge. We know that 2 black holes can merge and make a bigger black hole, and then those can merge and make a bigger black hole. We don't think there's been enough time for that.

So it's suspected that they're formed very early, maybe even 100 or a few hundred million years after the Big Bang, and that they're formed directly by collapsing out of primordial stuff. There's a direct collapse right into the black hole.

So, in the very early universe, these are primordial black holes from the stars?

#468

Not quite.

Wait, how do you get from that soup to black holes right away?

#468

It's odd, but it's weirdly easier to make a big black hole out of something that's just the density of air if it's really, really as big as what we're talking about. In some sense, if they're just allowed to directly collapse very early in the universe's history, they can do that more easily.

It's so much so that we think there's one of these supermassive black holes in the center of every galaxy. So they're not rare, and we know where they are: in the nuclei of galaxies. They're bound to the very early formation of entire galaxies in a really surprising and deeply connected way.

I wonder if it's the chicken or the egg. Which came first? How critical, how essential, are the supermassive black holes to the formation of galaxies?

#468

Yeah, I mean, it's ongoing, right? It's ongoing. Which came first, the black hole or the galaxy? Probably big early stars, which were just made out of hydrogen and helium from the Big Bang.

#468

There wasn't anything else, not much of anything else. Those early stars were forming, and then maybe the black holes and the galaxies were kind of these gassy clouds around them. But there's probably a deep relationship between the black hole powering jets, these jets blowing material out of the galaxy, that shaped galaxies, maybe kind of curbed their growth. And so I think the mechanisms are still ongoing attempts to understand exactly the ordering of these things.

Can we get back to spacetime, just going back to the beginning of the 20th century? How do you imagine spacetime? How are we, as human beings, supposed to visualize and think about spacetime, where time is just another dimension in this 4D space that combines space and time? Because we've been talking about morphing in all kinds of different ways, is the curvature of spacetime like that? How are we supposed to conceive of it? How do you think of it?

#468

Yeah, time is just another dimension. There are different ways we can think about it. We can imagine drawing a map of space and treating time as another direction in that map. But we're limited because, as 3D beings, we can't really draw 4 dimensions, which is what I'd require.

Three spatial dimensions, because I'm pretty sure there's at least 3. I think there's probably more, but I'm happy just talking about the large dimensions: the 3 we see—up, down; right, east, west; north, south—and time is the 4th. Nobody can really visualize it, but we know mathematically how to unpack it on paper. I can mathematically suppress 1 of the spatial dimensions, and then I can draw it pretty well.

Now, the problem is that we'd call it a Euclidean spacetime. A Euclidean spacetime is when all the dimensions are orthogonal and are treated equally. Time is not another Euclidean dimension. It's actually a Minkowski spacetime. That means that we're misrepresenting spacetime when we draw it, but we're misrepresenting it in a way that we deeply understand.

I can give you an example. The Earth, I can project it onto a flat sheet of paper. I am now misrepresenting a map of the Earth, and I know that, but I understand the rules for how to add distances on this misrepresentation because the Earth is not a flat sheet of paper; it's a sphere. As long as I understand the rules for how I get from the North Pole to the South Pole, that I'm moving along really a great arc, and I understand that the distance is not the distance I would measure on a flat sheet of paper, then I can do a really great job with a map.

I can do the same thing with spacetime. I can draw it on a flat sheet of paper, but I know that it's not actually a flat Euclidean space. My rules for measuring distances are different than the rules I would use—for instance, Cartesian rules of geometry. I would know to use the correct rules for Minkowski spacetime, and that will allow me to calculate how much time has elapsed, which is now a kind of length, a spacetime length on my map, between 2 relative observers. And I will get the correct answer, but only if I use these different rules.

So then, according to general relativity, what do objects with mass do to spacetime?

Speaker 1

Right, exactly. So Einstein struggled for this completely general theory, not a specific solution like a black hole, or an expanding spacetime, or galaxies making lenses. Those are all solutions. That's why what he did was so enormous: it's an entire paradigm that says, over here is matter and energy. I'm going to call that the right-hand side of the equation. Everything on the right-hand side of Einstein's equations is how matter and energy are distributed in spacetime. Everything on the left-hand side tells you how space and time deform in response to that matter and energy. And it can be impossible to solve some of those equations.

What was so amazing about what Schwarzschild did is that he found this very elegant, simple solution within about a month of reading this final formulation. But Einstein didn't go through and try to find all the solutions. He sort of gave it to us, right? He shared this, and then lots of people since have been scrambling to try to say, “I can predict the curvature of spacetime if I tell you how the matter and energy is laid out.”

If it's all compact in a spherical system like the Sun or even a black hole, I can understand the curvature in the spacetime around it. I can solve for the shape of spacetime. I can also say, well, what if the universe is full of gas or light and it's all kind of uniform everywhere? I'll find a different and equally surprising solution, which is that the universe would expand in response to that, that it's not static, that the distances between galaxies would grow.

This was a huge surprise to Einstein. All of these consequences of his theory came with revelations that were not at all obvious when he first wrote down the general theory. He was afraid to take the consequences of that theory seriously because the theory itself, in its scope and grandeur and power, is scary. So I can understand. Then there's the edges of the theory where it falls apart. The consequences of the theory that are extreme—it's hard to take seriously. So you can sort of empathize.

Yeah, he very much resisted the expansion. So if you think about 1905, when he's writing this sequence of unbelievable papers as a 25-year-old who can't get a job as a physicist, and he writes all of these remarkable papers on relativity and quantum mechanics. Then, even in 1915–16, he does not know that there are other galaxies out there. This was not known. People had mused about it. There were these kind of smudges on the sky that people contemplated: What if there are other island universes? Going back to Kant, he thought about this. But it wasn't until Hubble—it really wasn't until the late 1920s—that it's confirmed that there are other galaxies.

Wow. Yeah, he didn't—obviously, there's so much we think of now that he didn't think of. So there's no Big Bang, static universe. But these are all connected. Wow. Yeah. So he's operating on very little information.

Speaker 1

Very little information. That's absolutely true. Actually, one of the things I like to point out is the idea of relativity was foisted on people in this kind of cultural way, but there are many ways in which you could call it a theory of absolutism. And the way Einstein got there with so little information is by adhering to certain very strict absolutes, like the absolute limit of the speed of light and the absolute constancy of the speed of light, which was completely bizarre when it was first discovered.

Really, only massless particles have this property: they have an absolute speed. And if you think about it, it's incredibly strange.

Yeah, it's really strange.

Speaker 1

Incredibly strange. And so, from a theoretical perspective, he takes that seriously. He takes it very seriously, and everyone else is trying to come up with models to make it go away, to make the speed of light be a little bit more reasonable, like everything else in the universe.

If I run at a car, or if 2 cars are coming at each other, they're coming at each other faster than if 1 of them stops. It's a really basic observation of reality. This is saying that if I'm racing at a light beam and you're standing still relative to the source, we'll measure the same exact speed of light. Very strange.

And he gets to relativity by saying, well, what's speed? Speed is distance. It's space over time. It's how far you travel. It's the space you travel in a certain duration of time. And he said, “Well, I bet something must be wrong, then, with space and time.” So this is an enormous leap. He's willing to give up the absolute character of space and time in favor of keeping the speed of light constant.

How was he able to intuit a world of curved spacetime? I think it's one of the most special leaps in human history, right? It's amazing. It's very, very, very difficult to make that kind of leap.

Speaker 1

I'll tell you, I can't say this is how he got there exactly. It's not as though I studied the historical accounts or his descriptions of his internal states. This is more about having learned the subject and how I try to tell people to get there in a few short steps.

One is to start with the equivalence principle, which he called the happiest thought of his life. The equivalence principle comes pretty early on in his thinking. And it starts with something like this: right now, I think I'm feeling gravity because I'm sitting in this chair and I feel the pressure of the chair, and it's stopping me from falling. I lie down in a bed and I feel heavy on the bed, and I think of that as gravity.

Einstein has a beautiful ability to remove all of these extraneous factors, including atoms. So let's imagine instead that you're in an elevator and you feel heavy on your feet because the floor of the elevator is resisting your fall. But I want to remove the elevator. What does the elevator have to do with the fundamental properties of gravity? So I cut the cable. Now I'm falling, but the elevator is falling at the same rate as me. So now I'm floating in the elevator.

If this happened to me, if I woke up in this state of falling or floating in the elevator, I might not know if I was in empty space just floating or if I was falling around the Earth. They would actually be equivalent situations. I would not be able to tell the difference. Actually, when I get rid of the elevator in this way, by cutting the cable, I'm experiencing weightlessness. And that weightlessness is the purest experience of gravity.

Speaker 1

And so this idea of falling is actually fundamental. It's how we talk about it all the time. The Earth is in free fall around the Sun. It's actually falling; it's not firing engines, right? It's just falling all the time, but it's cruising so fast.

So, actually, yeah. God, you said so many profound things. One of them is that one of the ways to experience spacetime is to be falling. To be falling is the purest experience of gravity. The experience of gravity, unfettered and uninterrupted by atoms, is weightlessness.

Speaker 1

Yeah. That observation—no, it has an unhappy ending. The elevator story, right? Because of atoms. Again, that's the fault of the atoms in your body interacting electromagnetically with the crust of the Earth or the bottom of the building, or whatever it is.

But this period of free fall—the first observation is that it's the purest experience of gravity. Now, I can convince you that things follow along curved paths because I could take a pen and, if I throw it, we both know it's going to follow an arc. It's going to follow an arc until atoms interfere again and it hits the ground.

But while it's in free fall, experiencing gravity at its purest, what the Einsteinian description would say is that it is following the natural curve in spacetime inscribed by the Earth. The Earth's mass and shape curve the paths in space, and then those curvatures tell you how to fall—the paths along which you should fall when you're falling freely.

And so the Earth has found itself on a free fall that happens to be a closed circle, but it's actually falling. The International Space Station uses this principle all the time. They get the space station up there, and then they turn off the engines. Can you imagine how expensive it would be if they had to fuel that thing at all times?

They turn off the engines. They're just falling.

But, yeah, to be able to visualize at the beginning of the 20th century that free falling in curved spacetime—boy, the human mind is capable of things. I mean, some of that is constructing thought experiments that collide with our understanding of reality.

Maybe in the collisions, in the contradictions, you try to think of extreme thought experiments that exacerbate that contradiction and see, like, okay, what actually is there? Is there another model that can incorporate this? But to be able to do that, I mean, it's kind of inspiring because there's probably another general relativity out there.

#468

Yeah, in all—not just in physics, but in all lines of work and in all scientific pursuits—there are certain theories where you're like, “Okay, I just explained a big elephant in the room here that everybody just kind of didn't even think about.”

There could be stuff like that for the origin of life on Earth. Everyone's like, “Yeah, okay,” in polite company. “Yeah, yeah, yeah, yeah. Somehow it started.”

Right. Nobody knows. I find it wild that that's so elusive.

#468

Yeah, it's strange. And the fact that it's so elusive—I think it's a general relativity thing. There's going to be some thing. It's going to involve aliens and wormholes and dimensions that we don't quite understand, or some field that's bigger than—it's possible, maybe not.

It's possible that it's a field that's different, that will feel fundamentally different from chemistry and biology. It'll maybe be through physics again. Maybe the key to the origin of life is in physics.

And the same thing there—it's like a weird neighbor is consciousness.

#468

Yeah. It's like, all right, a weird neighbor. It's like, okay, so we all know that life started on Earth somehow. Nobody knows how. We all know that we're conscious. We have a subjective experience of things. Nobody understands that. People have ideas and so on.

But it's such a dark sort of—we're entering a dark room where a bunch of people are whispering about, like, “Hey, what's in this room?” But nobody has a fucking clue.

So, and then somebody comes along with a general relativity kind of conception where it reconceives everything, and you're like, “Ah, it's a watershed moment.”

#468

Yeah. Yeah. It's there, and we're living in a time until that theory comes along. It'll be obvious in retrospect, but right now we're right.

Well, it was obvious to no one that spacetime was curved, but even Newton understood something wasn't right. So, he knew there was something missing. I think that's always fascinating, when we're in a situation where we're pressure-testing our own ideas.

He did something remarkable, Newton did, with his theory of gravity: just understanding that the same phenomenon was at work with the Earth around the Sun as with the apple falling from the tree. That's insane. That's a huge leap.

Understanding that mass—inertial mass, what makes something hard to push around—is the same thing that feels gravity, at least in the Newtonian picture, in that simple way. Unbelievable leap. Absolutely genius.

But he didn't like that the apple fell from the tree even though the Earth wasn't touching it.

#468

Yeah, the action-at-a-distance thing.

The action-at-a-distance thing. That is weird, too.

#468

Well, that is a really weird one. It's really weird. But see, Einstein solves that. Relativity solves that because it says the Earth created the curve in space. The apple wants to fall freely along it. The problem is the tree is in the way.

The tree is the problem. The tree is actually accelerating the apple. It's keeping it away from its natural state of weightlessness in a gravitational field. As soon as the tree lets go of it, the apple will simply fall along the curve that exists.

I would love it if somebody went back to Newton's time and told him all this. Probably some hippie would be like, “Gravity is just the curvature in spacetime, man.”

#468

I wonder if he would be able to—

I don't think there's—you know, every idea has its time. He might not even be able to load that in. I mean, sometimes even the greatest geniuses—you need to be standing on the shoulders of giants, and on the shoulders of those giants, and so on.

#468

I heard that Newton used that as an unkind remark to his competitor Hooke.

Oh no, people talked even back then. Trash-talking.

This is one of the hilarious things about humans in general, but scientists too—these huge minds. There are these moments in history where you'll see this in universities, but everywhere else too. You have gigantic minds, obviously, also coupled with the fact that everybody has an ego.

Sometimes it's just the same soap opera that played out among humans everywhere else. You're thinking about the biggest cosmological objects and forces and ideas, and you're still jealous: “Your office is bigger than my office.” Or maybe you got married to this person that I was always in love with—the betrayal of something. The one woman in the department.

#468

And it's just—I mean, that is also the fuel of innovation: that jealousy, that tension. You know the expression, I'm sure: the battles are so bitter in academia because the stakes are so low.

That's a beautiful way to phrase it. But also, we shouldn't forget—I love seeing that even in academia because it's humanity. The silliness is there.

#468

There is a degree to academia where the reason you're able to think about some of these grand ideas is because you still allow yourself to be childlike.

Oh yeah, there's a childlike nature to asking questions, but children can also be children-children. So you don't—I think when, in a corporate context, maybe the world forces you to behave and you're supposed to be a certain kind of way, there's something really beautiful to preserve and celebrate in academia. You're just allowed to be childlike in your curiosity and your exploration. You're exploring and asking the biggest questions.

The best scientists I know often ask the simplest questions. They're really—first of all, there's probably some confidence there, but also they're never going to lie to themselves that they understand something they don't understand.

So even this idea that Newton didn't understand the apple falling from the tree—had he lived another couple hundred years, he would have invented relativity, because he never would have lied to himself that he understood it. He would have kept asking this very simple question. I think there is this childlike beauty to that.

#468

Absolutely.

Lex Fridman

Yeah. Just some of the topics—I don't know why I'm stuck on those 2 topics, origin of life and consciousness—but some of the most brilliant people I know are stuck just like Newton and Einstein. They're stuck on that: “This doesn't make sense.”

I know a bunch of brilliant biologists, physicists, and chemists. They're thinking about the origin of life. They're like, “This doesn't—I know how evolution works. I know how the biological systems work, how genetic information propagates, but this part, the singularity at the beginning, doesn't make sense.”

#468

We don't understand. We can't create it in the lab. They're bothered by it every single day. That tension, that gap in knowledge, is the catalyst. That's the fuel, the catalyst for discovery.

The discovery is going to come because somebody couldn't sleep at night and couldn't rest. In that way, I think black holes are a kind of portal into some of the biggest mysteries of our universe. It's good terrain on which to explore these ideas.

Can you speak about some of the mysteries that black holes present us with?

#468

Yeah. I think it's important to separate the idea that there are these astrophysical states that become black holes from the idea of being synonymous with black holes. Black holes are this larger idea, and they might have been made primordially when the Big Bang happened. There's something flawless about black holes that makes them fundamental, unlike anything else.

They're flawless in the sense that you can completely understand a black hole by looking at just its charge—its electric charge—its mass, and its spin. Every black hole with that charge, mass, and spin is identical to every other black hole. You can't be like, “That one's mine. I recognize it. It has this little feature, and that's how I know it's mine.” They're featureless.

You try to put Mount Everest on a black hole, and it will shake it off in these gravitational waves. It will radiate away this imperfection until it settles down to be a perfect black hole again. There's something about them that's unlike anything else in the universe that's macroscopic. That's another reason why I don't like to call them objects in the traditional sense.

It's a little bit more like a fundamental particle. An electron is described by a certain short list of properties: charge, mass, spin, maybe some other quantum numbers. That's what it means to be an electron. There's no electron that's a little bit different. You can't recognize your electron. They're all identical in that sense.

In some very abstract way, black holes share something in common with microscopic fundamental particles. What they tell us about the fundamental laws of physics can be very profound. That's why even theoretical physicists and mathematical physicists—not just astronomers who use telescopes—rely on the black hole as terrain on which to perform their thought experiments. It's because there's something fundamental about them.

Yeah. General relativity meets quantum mechanics, which means singularity. Sadly, heartbreakingly so, it's out of reach for experiment at this moment, but it's within reach theoretically. It's in reach for thought experiments, which are quite beautiful. On that topic, I have to ask you about the paradox—the information paradox of black holes. What is it?

#468

This is what catapulted Hawking's fame when he was a young researcher. He was thinking about black holes and wanted to add just a little smidge of quantum mechanics. He wasn't going for full-blown quantum gravity, but was asking, “What if I allowed this nothing, this vacuum, this empty space around the event horizon—the star is gone, there's nothing there—to possess ordinary quantum properties, just a little tiny bit? Nothing dramatic. Don't go crazy.”

One of the properties of the vacuum that's intriguing is the idea that you can never say the vacuum is actually completely empty. We talked about Heisenberg, but the Heisenberg uncertainty principle really kicked off a lot of quantum-mechanical thinking. It says that you can never exactly know a particle's position simultaneously with its motion, with its momentum. You can know one or the other pretty precisely, but not both precisely.

The uncertainty isn't a lack of ability that we will technologically overcome. It's foundational. In some sense, when it's in a precise location, it is fundamentally no longer in a precise motion. That uncertainty principle means I can't precisely say a particle is exactly here, but it also means I can't say it's not.

So it led to this idea: What do I mean by a vacuum? I can't know with 100% precision. In fact, it's not really meaningful to say that there are zero particles here. What you can say, however, is that maybe particles froth around in this seething quantum sea of the vacuum.

Maybe 2 particles come into existence, and they're entangled in such a way that they cancel out each other's properties. They have the properties of the vacuum; they don't destroy the properties of the vacuum because they cancel out each other's spin, maybe each other's charge, maybe things like that. They froth around. They come, they go, they come, they go, and that's what we really think is the best that empty space can do in a quantum-mechanical universe.

Now, if you add an event horizon—which, as we said, is really fundamentally what a black hole is, the most important feature of a black hole—if the particles are created slightly on either side of that event horizon, now you have a real problem. The pair has been separated by this event horizon. They can both fall in. That's okay.

But if one falls in and the other doesn't, it's stuck. It can't go back into the vacuum because now it has a charge, or it has a spin, or it has something. It's no longer a property of the vacuum it came from. It needs its pair to disappear. Now it's stuck. It exists. It's like you've made it real.

In a sense, the black hole steals one of these virtual particles and forces the other to live. If it does, it'll escape, radiate out to infinity, and look to an observer far away like the black hole actually radiated a particle. The particle did not emanate from inside. It came from the vacuum. It stole it from empty space, from the nothingness that is the black hole.

The reason why this is very tricky is because of this separation on either side of the event horizon. The particle it absorbs has to do with the switching of space and time that we talked about. From the outside, you might say, “Oh, it had negative momentum. It was falling in.” From the inside, you say, “Well, this is actually motion in time. This is energy.”

It has negative energy, and it absorbs negative energy. Its mass goes down. The black hole gets a little lighter, and as it continues to do this, the black hole really begins to evaporate. It does more than just radiate; it evaporates away.

It's intriguing because Hawking said, “Look, this is going to look thermal,” meaning featureless. “It's going to have no information in it. It's going to be the most informationless possibility you could possibly come up with when you're radiating particles. It's just going to look like a thermal distribution of particles, like a hot body.”

The temperature is only going to tell you about the mass, which you could tell from outside the black hole anyway. You know the mass of the black hole from the outside, so it's not telling you anything about the black hole. It has no information about the black hole. Now you have a real problem.

When he first said it, a lot of people described it, but not everyone understood how really naughty he was being. He did. Some people who loved quantum mechanics were really annoyed. People like Lenny Susskind and Gerard ’t Hooft, a Nobel Prize winner, were mad because it suggested something was fundamentally wrong with quantum mechanics if it was right.

The reason it says there's something fundamentally wrong with quantum mechanics is that quantum mechanics does not allow this. It does not allow quantum information to simply evaporate away, poof out of the universe, and cease to exist. It's a violation of something called unitarity.

Really, the idea is that the loss of quantum information is intolerable. Quantum mechanics was built to preserve information. It's one of the sacred principles, as sacred as conservation of energy. In this example, it's more sacred, because you can violate conservation of energy with Heisenberg's uncertainty principle a little tiny bit.

It was so sacred that it created what became known as the black hole wars. People were saying, “Look, general relativity is wrong. Something's wrong with our thinking about the event horizon. Or quantum mechanics isn't what we think it is.” The two were not getting along anymore.

Just to tell you how dramatic it is, the temperature goes down with the mass of the black hole. The heavier a black hole is, the cooler it is, so we don't see black holes evaporate. They're way too big. But as they get smaller and smaller, they get hotter and hotter.

As the black hole nears the end of this cycle of evaporating away—it takes a very long time, much longer than the age of the universe—it will be as though the curtain, the event horizon, is yanked up. It'll literally explode away. The event horizon, in principle, would be yanked up. Everything's gone.

All that information that went into the black hole, all that sacred quantum stuff, is gone. Poof. It's not in the radiation, because the radiation has no information. It was an incredibly productive debate, because in it are the signs of what will make gravity and quantum mechanics play nicely together—a quantum theory of gravity.

Whatever these clues are, they're hard to assemble. If you want a quantum-gravity theory, it has to correctly predict the temperature of a black hole and the entropy of a black hole. It has to have all of these correct features. The black hole is the place where we can test quantum gravity, but it still has not been resolved.

It has not been fully resolved. I looked up all the different ideas for the resolution. There’s information loss, which is what you referred to. It’s perhaps the simplest yet most radical resolution: information is truly lost. This would mean quantum mechanics as we currently understand it—specifically, unitarity—is incomplete or incorrect under these extreme gravitational conditions.

I’m unhappy with that. I would not be happy with information loss. I love that it’s telling us there’s this crisis, because I do think it’s giving us the clues, and we have to take them seriously. For you, the gut is like, unitarity is going to be preserved?

Speaker 1

Preserved. So quantum mechanics has to come to the rescue, as Lenny Susskind in his book The Black Hole War says. The subtitle is My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics. I love something to that effect.

So then, from string theory, one of the resolutions is called fuzzballs. I love physicists so much. Originating from string theory, this proposal suggests that black holes aren’t singularities surrounded by empty space and an event horizon. Instead, they are horizonless, complex, tangled objects, also known as fuzzballs, made of strings and branes roughly the size of the would-be event horizon. There’s no single point of infinite density and no true horizon to cross.

In some sense, it says there’s no interior to the black hole. Nothing ever crosses. So, I gave you this very nice story that there’s no drama. Sometimes that’s how it’s described at the event horizon: you fall through and there’s nothing there. This other idea says, well, hold on a second.

If it’s really strings, as I get close, with this magnifying quality and the slowing of time near the event horizon, it is as though I put a magnifying glass on things. Now the strings aren’t so microscopic. They kind of smear around, and then they get caught like a tangle around the event horizon, and they actually never fall through. I don’t think that either, but it was interesting. So it’s just adding a very large number of extra complex degrees of freedom.

Speaker 1

Yeah, there are no teeny-tiny marbles to fall through, but it’s similar to what we already have with quantum mechanics. It’s just really saying the interior is not there ever. Nothing falls in. So the information gets out because it never went in in the first place.

Oh, interesting. So there is a strong statement there. A strong statement there. Okay. Soft hair challenges the classical no-hair theorem by suggesting that black holes do possess subtle quantum “hair.” This isn’t classical hair, like charge, but very low-energy quantum excitations—soft gravitons or photons at the event horizon—that can store information about what fell in.

Speaker 1

Worth trying, but I also don’t think that’s the case. The no-hair theorems are formal proofs that the black hole is this featureless, perfect fundamental particle that we talked about. All you can ever tell about the black hole is its electrical charge, its mass, and its spin, and it cannot possess other features. It has no hair, as one way of describing it, and those are proven mathematical proofs in the context of general relativity.

The idea is, therefore, I can know nothing about what goes into the black hole, so the information is lost. But if it could have hair, I could say, “That’s my black hole,” because it would have features that I could distinguish, and it could encode the information that went in in this way. The event horizon isn’t so serious. There isn’t such a stark demarcation between events inside and outside, where I can’t know what happened inside or outside. I don’t think that’s the resolution either, but it was worth a try.

Okay, the pros and cons of that one. The pros: it works within the framework of quantum field theory in curved spacetime, potentially requiring less radical modifications than fuzzballs or information loss. Recent work by Hawking, Perry, and Strominger strongly revitalized this idea. The con is that the precise mechanism by which information is encoded and transferred to the radiation is still debated and technically challenging to work out fully. Indeed, it needs to store a vast amount of information.

Okay, another one. This is a weird one. ER equals EPR. This is probably it, though. Oh boy. So, ER equals EPR—Einstein–Rosen bridge equals Einstein–Podolsky–Rosen—posits a deep connection between quantum entanglement and spacetime geometry. Specifically, an Einstein–Rosen bridge, commonly known as a wormhole. It suggests that entangled particles are connected by tiny, non-traversable wormholes.

Speaker 1

Okay, I can say that this is not a situation where we can follow the chalk. We can’t start at the beginning and calculate to the end. So it’s still a conjecture. I think it’s very profound, though. I kind of imagine Juan Maldacena, who’s part of this with Lenny Susskind. They were kind of like, “Huh, it’s like ER equals EPR.” They couldn’t even formulate it properly. It was an intuition that they had kind of landed on, and now they’re trying to formalize it.

But to take a step back, one way of thinking about ER equals EPR is that you have to talk about holography first. Holography—Juan Maldacena really formalized it, and Lenny Susskind suggested it. The idea of a black hole hologram is that all of the information in the black hole, whatever it is—whatever entropy as a measure of information, whatever the entropy of the black hole is, which is telling you how much information is hidden in there, how much information you don’t have direct access to in some sense—is completely encoded in the area of the black hole.

Meaning, as the area grows, the entropy grows. It does not grow as the volume. This actually turns out to be really, really important. If I tried to pack a lot of information into a volume—more information than I could pack, let’s say, on the surface of a black hole—I would simply make a black hole, and I would find out, “Oh, I can’t have more information than I can fit on the surface.”

So, Lenny coined this a hologram. People who take it very seriously say, well, again, maybe the interior of the black hole just doesn’t exist. It’s a holographic projection of this 2-dimensional surface. In fact, maybe I should take it all the way and say, so are we. The whole universe is a holographic projection of a lower-dimensional surface, right?

People have struggled—nobody’s really landed it—to find a universe version of it. Maybe there’s a boundary to the universe where all the information is encoded, and this entire 3-dimensional reality that’s so compelling and so convincing is actually just a holographic projection.

Juan Maldacena did something absolutely brilliant. It’s the most highly cited paper in the history of physics. It was published in the late 1990s. It has a very opaque title that would not lead you to believe it’s as revelatory as it is.

He was able to show that a universe like in a box with gravity in it—it’s not the same universe we observe; it doesn’t matter, it’s just a hypothetical called anti-de Sitter space—is a universe in a box. It has gravity, it has black holes, it has everything gravity can do in it. On its boundary is a theory with no gravity, a universe that can be described with no gravity at all. So, no black holes and no information loss problem. And they’re equivalent.

The interior universe in a box is a holographic projection of this quantum mechanics on the boundary: pure quantum mechanics, purely unitary, with no loss of information. None of this stuff could possibly be true. There can’t be loss of information if this dictionary really works, if the interior is a hologram—a projection of the boundary. I know that’s a lot.

Yeah. So there’s some mathematics there. There’s physics, and then there’s trying to discern what that actually means practically for us.

Speaker 1

What it would mean for us is that information can’t be lost, even if we don’t know how to show it in the description in which there are black holes. It means it can’t possibly be lost because it’s equivalent to this description with no gravity in it at all: no event horizons, no black holes, just quantum mechanics.

So it really strongly suggested that quantum mechanics was going to win in this battle, but it didn’t show exactly how it was going to win. So then comes ER equals EPR. A visual way to imagine what this means: ER has to do with little wormholes, and EPR—Einstein–Podolsky–Rosen—has to do with quantum entanglement.

The idea was, well, maybe the stuff that’s interior to the black hole is quantum-entangled, like EPR, with the Hawking radiation outside the black hole that’s escaping. That quantum entanglement is what allows you to extract the information, because it’s not actually physically moving from the interior to the exterior. It’s just subtle quantum entanglement.

In fact, I can kind of think of the entire black hole: if I look at it, it looks like a solid shadow cast on the sky, some region of spacetime. If I look at it very closely, I will see, “Oh no, it’s actually sewn from these quantum wormholes,” like it’s embroidered. When I get up close, it’s almost as though the event horizon isn’t the fundamental feature of spacetime. The fundamental feature is the quantum entanglement embroidering the event horizon. The embroidering is just tiny wormholes.

So the quantum entanglement is when 2 particles are connected at arbitrary distances, and they’re connected by a wormhole. In this case, they would be connected by a wormhole.

Mhm. So the reason why that’s helpful is that it helps you connect the interior to the exterior without trying to pass through the horizon. The con of this theory is that it’s highly conceptual and abstract. The exact mechanism for information retrieval via these non-traversable wormholes is not fully understood. It’s primarily explored in theoretical toy models, like anti-de Sitter spacetime, rather than realistic black holes.

Whoa, Gemini going hard.

#468

True. We do what we can in baby steps. Another idea to resolve the information paradox is the firewall proposal by Almheiri, Marolf, Polchinski, and Sully—AMPS. This is a more drastic scenario arising from analyzing the entanglement requirements of Hawking radiation to preserve unitarity and avoid information loss.

They argued that the entanglement structure requires the event horizon not to be smooth—not to be the smooth, unremarkable place predicted by general relativity and the equivalence principle. Instead, it must be a highly energetic region, a so-called firewall that incinerates anything attempting to cross it.

Okay. That's a nice solution: just destroy everything that crosses it. Do you find this at all a convincing resolution to the information paradox?

#468

I would say the firewall papers were fascinating, very provocative, and very important in making progress. I don't even think the authors of those papers thought firewalls were real. I think they were saying, “Look, we've been brushing too much under the rug.”

If you look at the evaporation process, it's even worse than what you thought previously. It's so bad that I can't get away with some of these prior solutions that I thought I could get away with. There was a kind of duality idea, or a complementarity idea, that one person thinks they fell in and one person thinks they never fell in, and that's okay—no big deal.

They exposed flaws in these kinds of approaches, and it actually reinvigorated the campaign to find a solution. It stopped it from stalling. I don't think anyone really believes that at the event horizon you'll find a firewall. But it did lead to things like entangled wormholes embroidering a black hole, which was born out of an attempt to address the concerns that AMPS raised. So it did lead to progress.

So for you, the resolution would be—?

#468

I'm going back to the vacuum.

You're going back to the empty space, the beautiful event horizon.

#468

Yeah. I'll give up locality, meaning that I will allow things to be connected nonlocally by a wormhole.

So that is the weirdest thing you're willing to allow for: an arbitrary-distance connection of particles through a wormhole?

#468

But quantum mechanics must be preserved. I'll entertain pretty weird things, but I think that's the one that sounds promising.

The implications are so dramatic, because this is why you start to hear things like, “Wait a minute. If the event horizon only exists when it's sewn out of these quantum threads, does that mean that gravity is fundamentally quantum mechanics?”

#468

Not that gravity and quantum mechanics get along and I have a quantum gravity theory and now know how to quantize gravity. Actually, something much more dramatic: gravity is just kind of emerging from this quantum description. Gravity isn't fundamental.

What is the only thing that we have when we go rock bottom, when we go deeper and deeper, smaller and smaller? It's quantum mechanics. All of this spacetime looks nice and smooth and continuous, but if I look at the quantum realm, I'll see everything sewn together out of quantum threads and that spacetime is not a smooth continuum all the way down.

People already thought that, but they thought it came in chunks of spacetime instead. Maybe it's just quantum mechanics all the way down.

Quantum threads. These entangled particles are connected by wormholes. So that's how you would even visualize a black hole in that way?

#468

From our perspective, in terms of detecting things, the light going in is all still the same. But when you zoom in a lot—when you zoom in to the quantum-mechanical scale at which you're seeing the Hawking radiation—you would notice that there's some entanglement between the radiation that I could not explain before and the interior of the black hole.

It's no longer a perfectly thermal spectrum with no features that only depends on the mass. It actually has a way of having an imprint of the information interior to the black hole in the particles that escape. Now, in principle, I could sit there for a very long time—it might take longer than the age of the universe—and collect all the Hawking radiation and see that it actually had details in it that would explain to me what was interior to the black hole. So the information is no longer lost.

So, yeah, information is not being destroyed. In theory, you should be able to get information.

#468

I can't do that any more than I can recover the words on that piece of paper once it's been burned. But that's a practical limitation, not a fundamental one. When I burn a piece of paper, technically the information is all there somewhere. It's in the smoke, in the currents, in the molecules, and in the ink molecules.

But in principle, if I had the age of the universe, I should be able to reconstruct the piece of paper and all the words on it.

Do you think a theory of everything that unifies general relativity and quantum mechanics is possible? So, we're skirting around it?

#468

Yeah, we're skirting around it. I think this is the way to find that out. It's going to be in the terrain of black holes that we figure out if that's possible.

I think that this is suggesting that there might not be a theory of quantum gravity, that gravity will emerge at a macroscopic level out of quantum phenomena. Now, we don't know how to do that yet, but these are all hints of emergence.

A lot of the mathematics of anything that emerges from a complex system is very difficult. The transition is very difficult, right? So if that's the case, there might not be a simple, clean equation that connects everything.

There are examples of emerging phenomena that are very simple and clean. I can take electromagnetic scattering, a law of physics where particles scatter electromagnetically, and have a lot of them. I have a lot of them in this room, and they come to some average. Well, I call that temperature, right?

That one number—the fact that there's 1 number describing all of these gazillions of particles—is an emergent quantity. There's no particle that carries around this fundamental property called temperature. It emerges from the collective behavior of tons and tons of particles.

In some sense, temperature is not a fundamental quantity. It's not a fundamental law of nature. It's just what happens from the collective behavior. That's what we'd be saying: this emerges from the collective behavior of lots and lots and lots of quantum interactions.

When do you think we would have some breakthroughs on the path toward a theory of everything—showing that it's possible or impossible, all that kind of stuff? If you look at the 21st century, say you move 100 years into the future and look back, when do you think the breakthroughs will come?

Let me give you some hard problems. I guess my question is, how hard is this problem? What does your gut say? Finding the origin of life, figuring out consciousness, solving some of the major diseases—then there's the theory of everything, understanding this, resolving the information paradox.

These are the puzzles that are before us as a human civilization. In physics, this feels like really one of the big ones. Of course, there could be other breakthroughs in physics that don't solve this.

#468

Yeah, we could discover dark matter and dark energy. We could discover extra spatial dimensions. We could discover that those 3 things are linked, that there's a dark sector to the universe that's hiding in these extra dimensions. That's something that I love to work on. I think it's really fascinating.

All of those would also be clues about this question, but they wouldn't solve this problem. I think it's impossible to predict. There has been real progress, and the progress, as we've said, comes from the childlike curiosity of saying, “Well, I don't actually understand this. I'm going to keep leaning on it because I don't understand it.” Then suddenly you realize nobody really understood it.

So I don't know. Do I think it's a harder problem than the problem of the origin of life? I think it's technically a harder problem. But I don't know. Maybe the breakthrough will come.

When you mention discovering extra dimensions, what do you mean? What could that possibly mean?

#468

We know that there are 3 spatial dimensions. We like to talk about time as a dimension. We can argue about whether that's the right thing to do, but we don't know why there are only 3.

It very well could be that there are extra spatial dimensions, that there's a little origami of these tightly rolled-up dimensions. Not all the models require that they're small, but most do. String theory requires extra dimensions to make sense.

Even if you feel very hostile toward string theory, there are lots of reasons to consider the viability of extra dimensions. We think that they can trap little quantum energies in such a way that might align with dark energy. The numerology is not perfect. It's a little bit subtle. It's hard to stabilize them.

It's possible that there are these kinds of quantum excitations that look a lot like dark matter. It's an interesting idea that, in the Big Bang, the universe was born with lots of these dimensions. They were all wrapped up in the early universe. What we're really trying to understand is why 3 got so big and why the others stayed so small.

Is it possible to have some kind of natural selection of dimensions?

#468

There is, actually, and people have worked on that.

Is there a reason why it's easier to unravel 3?

#468

Some people think about strings and branes wrapping up in the extra dimensions, causing a kind of constriction but preferentially loosening up in 3.

#468

Sometimes we look at exactly models like that, which have to do with origami being resistant to change in a certain way that only allows 3 to unravel and keeps the others really taut. But then there are other ideas that we're actually living on a 3-dimensional membrane that moves through these higher dimensions. The reason we don't notice them isn't because they're small. Maybe they're not small at all, but it's because we're stuck to this membrane, so we're unaware of these extra directions.

Is it possible that there are other intelligent alien civilizations out there that are operating on a different membrane? Is this a bit of an out-there question? But I ask it more seriously: Is it possible, do you think, from a physics perspective, to exist on a slice of what the universe is capable of?

#468

I think it is certainly mathematically possible, on paper, to imagine a higher-dimensional universe with more than 1 membrane. And if things are mathematically possible, I often wonder if nature will try them out.

Yeah.

#468

Which is how people get into the strange territory of talking about a multiverse. One of the aspirations was that, in the same way that we identified the law of the electroweak theory of matter—a single description that exactly landed on the description that matched observations—the same thing would happen for a kind of theory that also incorporated gravity. There would be this one beautiful law, but instead they got a proliferation, all of which did okay or did equally badly.

They suddenly had trouble not only finding a single one, but finding one would just beg a new question: Why that one? And if nature can do something, won't she do anything she can try? So maybe we really are just 1 example in an infinite sea of possible universes with slightly different laws of physics.

So, if I can do some of these things on paper, like imagine a higher-dimensional space in which I'm confined to a brane, and there's another brane, or maybe a whole array of them, maybe nature's tried that out somewhere. Maybe that's been tried out here. And then, yes, is it possible that there's life and civilizations on those other branes?

Lex Fridman

Yeah, but we can't communicate with them. They'd be in a shadow space. Can you seriously say we can't communicate with them?

#468

No, that's fair. I'm limited in my communication because I'm glued to the brane. But some things can move through the bulk. Gravity, for instance—a gravitational wave. So I could design a gravitational communication system and send gravitational waves through the bulk, like SETI is doing with light into space. I could send signals into the bulk.

Nice.

#468

Telling them where we are and what we do, and of course singing songs. Sending gravitational waves is very expensive. We don't know how to send them; they're very expensive and very hard to localize. They tend to be long-wavelength and very hard to produce. A lot of energy is moving around.

Lex Fridman

A lot of energy. So, is it possible that the membranes are “hairy” in other ways?

#468

It is possible that there are other things that live in the bulk. I mean, last night I was calculating away, looking at something that lives in the bulk.

Okay, this is fascinating. Can we take a little bit more seriously the whole question of when I look out there at the stars?

#468

Mm-hm.

Lex Fridman

From a basic intuition, I cannot possibly imagine that there aren't just alien civilizations everywhere.

#468

Yeah.

Lex Fridman

Life is so damn good. Like you said, nature tries stuff out.

#468

Yeah.

Lex Fridman

Nature's an experimenter. And I just can't, from basic observation—you said somewhere that you like extremophiles. Life just figures it out; it just finds a way to survive.

#468

Yeah.

Lex Fridman

Now, there could be something magical about the origin of life, the first spark, but I can't even see that it's not happening over and over and over. I bet, actually, once the story is fully told and figured out, life originated on Earth almost right away—and did that billions of times in multiple places, just over and over and over and over.

That seems to be the thing that, whatever is the life force behind this whole thing, seems to create life, seems to be a creator of different sorts.

#468

Yeah.

Lex Fridman

From the very original primordial soup of things, it just creates stuff. So I just can't imagine—but we don't see the aliens.

#468

Right.

Lex Fridman

Yeah. We don't even have to go to something as crazy as extra dimensions and brane worlds and all of that. What's happening right now, in the past 30 years in astronomy, looking at real objects, is that the number of planets—exoplanets outside our solar system—has absolutely proliferated.

#468

There are probably more planets in the Milky Way galaxy than there are stars. And now we have a real quandary.

No, I don't think it's a quandary. I think it's really exciting.

#468

What you just said, I totally agree with. It becomes impossible to imagine that life was not sparked somewhere else in our Milky Way galaxy, and maybe even in our local neighborhood of the Milky Way galaxy, maybe within a few hundred light-years of our solar system. So my gut says that some crazy amount of solar systems have had life—bacterial life—somewhere at some point in their history, some bacterial-type of life. Maybe it's totally different kinds of life.

Then I'm faced with a question: Why have we not clearly seen alien civilizations? And the answer—I just don't find any great filter answer convincing. There's just no way I can imagine an advanced alien civilization not avoiding its own destruction. I can see a lot of them getting into trouble. I could see how we humans are really 50/50 here.

Well, isn't that kind of appalling? I mean, just take that statement. We've only been around for a couple hundred thousand years at most. That is not very long, and we're at 50/50. I mean, that's unbelievable. It's indisputable that we have created the means, at least potentially, for our own destruction.

Will we learn from our mistakes? Will we alter course and save ourselves? One hopes so, right? But even the concept that it's conceivable whales have not invented a way to kill themselves, to wipe out all whales on Earth and life on Earth—that's one way to see it.

But I actually see it as a feature, not a bug, when you look at the entirety of the universe, because it does seem that the mechanism of evolution constantly creates things that operate on the verge of destruction. It seems like the predator-and-prey dynamic is really effective at creating and accelerating evolution and development.

It seems like our being able to destroy ourselves is a really powerful way to give us a chance to really get our shit together and to flourish, to develop, to innovate, to go out amongst the stars—or 50/50 destroy ourselves. Which, I think, as a human, is a horrible thing. But if there are a lot of other alien civilizations, that's a pretty cool thing.

You want to give everybody nuclear weapons: half of them will figure it out, half of them won't. And the ones that figure it out will figure out some incredible technologies about how to expand, how to develop, and all that kind of stuff, right?

#468

You could use a kind of evolutionary Darwinian natural selection on that, where survival isn't just in a harsh, naturally induced climate change, but is because of a nuclear holocaust. And then something will be created that is now impervious to that, that now knows how to survive.

Yep. Exactly. So why haven't we seen them?

#468

Right. Well, because that's a pretty big bar. If you look, just for a comparison, at dinosaurs—250 million years—I mean, maybe not very bright. They didn't invent fire, didn't write sonnets, and didn't contemplate the origin of the universe, but they lived in a benign situation without confronting their own demise at their own hands. So it's just a sheer numbers game. That's a long time, 250 million years.

I do think, though, that life can flourish without wanting to manipulate its environment. And we do see many examples of species on Earth that are very long-lived, very, very long-lived, and have very different states of consciousness. The jellyfish does not even have a localized brain. I don't think they have a heart or blood. I mean, they're really different from us.

Okay.

#468

And that's what I think we have to start thinking about when we think about aliens. Those species have lived for a very, very long time. They even show some evidence of immortality. You can wound one badly, and there are certain jellyfish that will go back into a kind of previous state and start over.

So I think we're very attached to imagining creatures like us that manipulate technology. And I think we have to be way more imaginative if we're going to really take seriously life in the universe.

Lex Fridman

Yeah. They might not prioritize conquest and expansion.

#468

Mm-hm.

Lex Fridman

They might not be violent. They might not be violent like us humans. They might be solitary. They might not be social. They might not move in groups.

They might not want to leave records. They might, again, not have a localized brain or have a completely different kind of nervous system.

#468

I think all we can say about life is that it has something to do with moving electrons around. Neurologically, we move electrons through our nervous system. Our brain has electrical configurations. We metabolize food, and that has to do with getting energy—electrical energy, in some sense—out of what we're eating.

We have organisms on Earth that can eat rocks—minerals. It's quite amazing. I mean, talk about extremophiles. They can metabolize things that I would have thought were impossible to metabolize.

#468

And so, again, I think we have to open our minds to how strange that could be and how different from us. We are the only example, even here on Earth, that manipulates its environment in that extreme way.

Can you think of life as—as you said, electrons—is there some degree of information processing required? So, it does something interesting, in quotes, with information?

#468

I think there are arguments like that. How entropy is changing from the beginning of the universe to today, how life lowers entropy by organizing things, but it costs more as a whole system. So, the whole entropy of the whole system goes up.

Of course, I organized things today and reduced the entropy of certain things in order to get up and get here. Even having this conversation involves organizing thoughts out of a cloud of information, but it comes at the cost of the entire system increasing entropy. So, I do think there's probably a very interesting way to talk about life in this way.

Yeah, yeah. It creates local pockets of low entropy, and the kind of mechanism, the kind of object, the kind of life form that could do that probably can take arbitrary forms. If you reduce it all to information, you can start to think about physics in the realm of physics, with the multiverse and all this kind of stuff. You could start to think, “How do I detect those pockets of low entropy?”

#468

Mhm. Yeah. People have tried to make arguments like that: Can I look for entropic arguments that might suggest we've done this before? The Big Bang has happened before.

So, is it possible that there's some kind of physics explanation for why we haven't seen the aliens? Like we said, branes—I don't think branes are going to explain why we don't see them in the Milky Way. I think that is just a problem we're stuck with.

#468

Whether or not there are extra dimensions, or whether or not there's life in another brane, I think we know that even just in our galaxy—which is a very small part of the universe, with 300 billion stars, something like that—there's a whole variety of possibilities to be explored by nature in the same way that we're describing.

I think you're absolutely right: when life first sparked here on Earth, it was voracious. It took a really long time, though, to get to multicellularity. I think that's interesting. That's weird. It took a really, really long time to become multicellular, but it did not take long just to start.

What do you think is the hardest thing on the chain of leaps that got to humans?

#468

I would say multicellularity, which is strictly an energy problem. I think, again, it's just: Can electrons flow the right way? Is it energetically favorable for multicellularity to exist? Because if it's energetically expensive, it's not going to succeed, and if it's energetically favorable, it's going to take off. It's really just that.

That's why I also think that going from inanimate to animate is probably gray. The transition is gray. At what point do we call something fully alive? Famously, it's hard to make a nice list of bullet points that need to be met in order to declare something alive.

Is a virus alive? I mean, I don't know. Was a prion alive? They seem to do some things, but they kind of rely on stealing other DNA and replicating. I don't know. I guess they're not alive.

The point is that, at the end of the day, I really think it's just—if you asked if it's just physics—I think it's just these rules of energetics. The gray area between the nonliving and the living is way simpler just on Earth. You said it's already complicated on Earth, but it's probably even more complicated elsewhere, where the chemistry could be anything.

Carbon is really cool and really useful because it binds a lot. It's nice. It finds a lot of ways to combine with other things, and that's complexity. Complexity is the kind of thing you need for life. You can't have a very simple linear chain and expect to get life. But I don't know—maybe sulfur would do.

Okay, okay. As we get progressively toward crazier and crazier ideas, we talked about these microscopic wormholes, which—my mind is still blown away by that. But if we talk a little bit more seriously about wormholes in general, also called Einstein-Rosen bridges, to what degree do you think they're actually possible as something to study, creeping toward the possibility, maybe centuries from now, of engineering ways of using them—of creating wormholes and using them for transportation of humanlike organisms?

Speaker 1

I think wormholes are a perfectly valid construction to consider. They're just a curve in spacetime. Topologically—which has to do with the connectedness of space—it's a little tricky, because we know that Einstein's description is completely in terms of local curves and distortions, expansion, and contraction. But it doesn't say anything about the global connectedness of space, because he knew that it could be globally connected on the largest scales.

This kind of origami that we're talking about—you could travel in a straight line through the universe, leave our galaxy behind, watch the Virgo Cluster drift behind us, and eventually find ourselves coming back to the Virgo Cluster, then the Milky Way, and eventually the Earth. We could find ourselves on a connected, compact spacetime.

Topologically, there's something we know for sure—something beyond Einstein's theory that has to explain that to us. Wormholes are a little funky because they're topological. They create these handles and holes in these connected spaces. It's like Swiss cheese or something.

I could have 2 flat sheets connected by a wormhole, but then wrap them around on the largest scale. There's nothing wrong with it, as far as I can see. There's nothing that violates the laws of physics about a wormhole.

We can reverse-engineer it. We were saying, “If I know how matter and energy are distributed, I can predict how spacetime is curved.” I can reverse-engineer that and say, “I want to build a curved spacetime like a wormhole. What matter and energy do I need to do that?” It's a simple process, and it's the kind of thing Kip Thorne worked on. He's a very imaginative, creative person.

The problem was that he said, “Oh, here's the bummer: The matter and energy you need doesn't seem to be like anything we've ever seen before. It has to have negative energy.” That's not great.

There are some conjectures that we shouldn't allow things that have that kind of property, that have negative energy. Only things that have positive energy are going to be stable and long-lived. But we actually know of quantum examples of negative energy, so it's not that crazy.

There's something called the Casimir effect. You have 2 metal plates and put them really close together. You can see this kind of quantum fluctuation between the plates. It can have a negative energy and actually cause the plates to attract or repel, depending on how they're configured.

You could imagine doing something like that, like having wormholes propped up by these kinds of quantum energies. People have thought of imaginative configurations to try to keep them propped up. Are we at the point of me saying, “Oh, this is an engineering problem”? I'm not saying that quite yet, but it's certainly plausible.

Yeah. So, you have to get a lot of this kind of weird matter. You need a lot of this weird matter to send a person through, right? That's going to be really telling. I'm not saying it's simply an engineering problem, but it's all within the realm of plausible physics.

I think that's super interesting. I think it's obviously intricately, deeply connected to black holes. Is it fair to think of wormholes as just 2 black holes that are connected somehow? Have people looked at that?

Speaker 1

They tend to be nontraversable wormholes. They're not trying to prop them open. But yeah, some of this ER = EPR—quantum entanglement—they're trying to connect black holes. It's really cool.

It's not quite, again, following the math. By that I mean we can't exactly start at a concrete place and calculate all the way to the end yet.

So, if I may read off some of the ideas that kept running through his head about how to artificially construct wormholes, the first method involves quantum mechanics and the concept of quantum foam. This is the thing we've been talking about.

Now, to create a wormhole, these tiny wormholes would need to be enlarged and stabilized to be useful for travel. But the exact method of doing this remains entirely theoretical. No? You think so? So, these tiny wormholes that are basically for the quantum entanglement of the particles—somehow enlarged.

Man, playing with the topology of the Swiss cheese would be so interesting, even just to get a hint.

Speaker 1

Mhm.

Lex Fridman

That would be in the top 3, if not 1—maybe even number 1—question for me to ask if I got a chance to ask an omniscient being a question that I could get an answer to. Maybe with some visualization.

Speaker 1

Mhm.

Lex Fridman

The shape, the topology of the universe.

Speaker 1

Yeah.

Lex Fridman

But I need some details. Unfortunately, I'll get an answer that I can't possibly comprehend.

Speaker 1

Right. It's a hyperbolic manifold that's identified across...

Exactly. You need to be able to ask a follow-up question.

Speaker 1

Exactly.

Lex Fridman

Yeah, that would be so interesting. Anyway, the classical-quantum strategy: The second approach combines classical physics with quantum effects. This method would require an advanced civilization to manipulate quantum gravity effects in ways we don't yet understand.

Speaker 1

There's a lot we don't understand, and then there are exotic-matter requirements. But I can tell you I'm pretty sure all of them have in common the feature that they're saying, “Here's what I want my wormhole to look like” first. So it's like saying, “I want to build a building” first. They construct an architecture of the spacetime that they're after, and then they reverse the Einstein equations to say, “What must matter and energy—what are the conditions that I impose on matter and energy—to build this architecture?”

That's unfortunately a very early step of figuring it out, but it's important because it's how they realized, “Oh, wow, they have to have these negative energies. They have to violate certain energy conditions that we often assume are true.” Then you either say, “Oh, well, then all bets are off. They'll never exist,” or you look a little harder and say, “Well, I can violate that energy condition without it being that big a deal.” Again, quantum mechanics often does violate those energy conditions.

Do you think studying black holes and some of the topics we've been talking about will allow us to travel faster than the speed of light, travel close to the speed of light, or make some kind of really innovative breakthroughs in the propulsion technology we use for traveling in space?

Speaker 1

Sometimes I assign, in an advanced general relativity class, the assignment of inventing a warp drive, and it's kind of similar. The idea is: here's a place you want to get to, and can you contract the spacetime between you with something antithetical to dark energy—the opposite—and skip across, and then push it back out again? That's all: can you do that in the context of general relativity?

I can't find the energy that has these properties, but I also can't find dark energy. We've already been confronted with something that, when we look at the spacetime—the spacetime is expanding ever faster—we say, “What could possibly do that?” We don't know what it is, but I can tell you about its pressure. I can tell you certain features about it, and I just call it dark energy, but I actually have no idea.

That name is just a proxy for what this is. It should be called invisible because it's not actually dark. It's in this room. It's not hard to see through. It's not dark. It's literally invisible. Maybe that was a misnomer. The point being, I still don't fundamentally know what it is. That's not so terrible. That's the state of the world that we're actually in.

So maybe warp drive is just kind of a version of that. I don't know what form of matter can do that yet, but at least I can identify the features that are needed.

So figuring out what dark energy is might land some clues.

Speaker 1

Yeah, it actually might. It is positive energy and a negative pressure, which is a rubber-band quality. We think of pressure as pushing things outward, and dark energy has a very strange sort of quality: as things move outward, you feel more energy as opposed to less energy. The energy doesn't get lower; it gets more.

It doesn't have the right features for the wormhole, but those are some pretty surprising features. We can again conjecture, “Hey, the quantum energy of the vacuum kind of behaves that way.” That would be a great resolution to the dark energy problem. It's just the energy of empty space, and it's the quantum energy of empty space.

That's an excellent answer.

Speaker 1

The problem is, by all our methods and all the understanding we have, that energy is either really, really huge—way bigger than what we see today—or it's like zero. So that's a numbers problem. We can't naturally fine-tune the energy of empty space to give us this really weird value, so that we just happen to be seeing it today.

Again, we can think of a kind of dark energy that exists. The question becomes, why is it such a weird value? It's not that this is inconceivable, because we can conceive of it.

Yeah. But if it's a weird value, that means there is a phenomenon we don't understand.

Speaker 1

Yes, there's absolutely a phenomenon. Nobody's going to say they're happy with that. We're all going to say there's something we don't understand, which is why we look to the extra dimensions. Then you can say, “Oh, maybe it has to do with the size of the extra dimensions or the way that they're wrapped up.” Maybe it's foisted on us because of the topology, the connectedness of the higher-dimensional space. These are all things that we're exploring. Nobody's landed on one that's so compelling that your friends like it as much as you do.

What do you think would lead to the breakthroughs on dark matter and dark energy?

Speaker 1

I think dark matter might be less peculiar than dark energy. My hope is that they're tied together. That would be very gratifying. These aren't just separate problems coming from different sectors, but they're actually connected—that the reason dark matter is where it is, in terms of how much it's contributing to the universe, is connected with why dark energy is showing up right now.

I would love that. That would be a solution like no other, right? If it revealed something about extra dimensions, that would be a happy day.

Correct me if I'm wrong: dark matter could be localized in space.

Janna Levin

Yeah, dark matter is localized in space. So it clumps. It doesn't clump a lot, but it's around the galaxy. It's in a halo around the galaxy.

Lex Fridman

Oh, it's really compelling.

Janna Levin

You see these images of clusters that pass through each other, and you can see where the light is, where the luminous matter is distributed. Then, by looking at the gravitational lensing, which shows you where the actual mass is distributed—the light bends around the most massive parts in a particular way—you can reconstruct where the mass is gravitationally, quite separate from looking at the luminous matter, which is not dark.

They are separate because, as they pass through each other, the interacting stuff, the luminous stuff, collides and gets stuck. You can see it colliding and lighting up, while the dark stuff—which, by definition, is dark because it doesn't interact—passes right through each other. This is so compelling.

Dark energy is harder to get a hold of. Dark energy is much harder to get a hold of. The Higgs field could have also explained dark energy.

If you've heard of the God particle—I don't know if you know—the Nobel laureate Leon Lederman co-authored a book, and he wanted to call it “The Goddamn Particle” because he couldn't find it. His publisher convinced him to call it “The God Particle.” He said they managed to offend 2 groups: those that believed in God and those that didn't.

Lex Fridman

That's a good line, too. Oh, boy.

Janna Levin

He was very funny. He was very witty. The Higgs turned out to be Higgs's great discovery. It was unbelievable. They built this massive collider at CERN in Switzerland, and there it was, kind of where you expected it to be. Unbelievable.

The reason I say it could be dark energy is because the Higgs particle, like a particle of light, also has a field like an electromagnetic field. Light can have this field that's distributed through all space—an electric and magnetic field—and you shake it around and it creates little particles.

The Higgs field is actually more important than the Higgs particle, the complement to the Higgs particle, because that's what you and I connect with to get mass in our atoms. The idea is that our atoms are interacting with this gooey field that's everywhere.

But we don't actually interact with a lot of quanta lying around; there aren't a lot of Higgs particles lying around because they decay. It's the field that's really important, and that field could act like a dark energy. It's just not in the right place, meaning the energy is too high to explain this tiny, tiny value today.

Again, we're back to this mismatch. It's not that we can't conceive of forms of dark energy; it's that we can't make one where we're finding it.

Lex Fridman

I wonder if you can comment on something that I've heard recently. There are some people outside of physics who say that dark matter and dark energy are just something physicists made up to put a label on the fact that they don't understand a very large fraction of the universe and how it operates. Is there some truth to that? What's your response to that?

Janna Levin

There's some truth to it, but it's really missing a huge point, which is that if we did not understand the universe as incredibly precisely as we do, it's stunning that there's modern precision cosmology. It's absolutely incredible.

When COBE, which was an experiment that measured the light left over from the Big Bang, first revealed its observations in the 80s, there was applause. People were cheering. It was unbelievable. We had predicted and measured the light left over from the Big Bang.

Because of all the precision that's happened since then, we're able to confront the fact that there are things we don't know. We're able to confront, “Wow, this is really everything everybody has ever seen and ever will see, as far as we understand, makes up less than 5% of what's out there.”

But to dismiss that as some kind of, “Oh, they just don’t know,” is actually quite the opposite. It is a stunning achievement to be able to stare that down and to have that so precise and so compelling that we’re able to know that there’s dark energy and dark matter. I don’t think those are disputed anymore, and they were up until recently. They were still disputed.

I think we’re still at such early stages where we’re not really even at a good explanation.

Lex Fridman

Right. You’ve mentioned a few.

Janna Levin

Well, I can think of examples of dark matter that exist that we really know for sure are real versions of dark matter, like neutrinos. Right now, they’re radiating through us. That’s very well confirmed. They’re technically dark. They don’t interact with light, and so we can’t see them.

Right now, they’re raining through us. If we could see the dark matter in this room—and we absolutely know it’s coming from the Sun—it would be wild. It would be a rainstorm, but they’re just invisible to us. Mostly, they pass through our bodies. Mostly, they pass through the Earth. Occasionally, they get caught in some fancy detector experiment that somebody built specifically to catch solar neutrinos.

So dark matter is known to exist. It’s just, again, there’s not enough of it. It’s not the right mass to be the dark matter that makes up this missing component.

Lex Fridman

I wanted to say that I’ve been recently fascinated by the flat-Earth people because there’s been a split in the community.

Janna Levin

First of all, the community is a fascinating study of human psychology. They did this experiment—I forgot who funded it—but they sent physicists and flat-Earthers to Antarctica.

Lex Fridman

Mhm.

Janna Levin

And this split happened because half of them got converted into round-Earthers.

Lex Fridman

Really?

Janna Levin

Yeah.

Lex Fridman

Wow. Well, good for them.

Janna Levin

But then the other half just said that it was all a psy-op.

Lex Fridman

Really? That’s fascinating. Did somebody film that? That’d be a great documentary.

Janna Levin

Yeah, they did. They made a whole thing. This was just at the end of last year.

Lex Fridman

I think that’s such a clean study of conspiracy theories because so many conspiracy theories have some inkling of truth in them. There are some elements about the way governments operate or human psychology; it’s too messy. Flat Earth, to me, is just clean. It’s like the Flying Spaghetti Monster or something. It’s just a cleanly wrong thing.

So it’s a nice way to understand the psychology of how a large number of people can believe a thing. And why do they want to believe a thing?

Janna Levin

What’s very interesting is trying to use rational arguments. That makes it even more confounding to me. I would understand more somebody who just said, “Look, I have faith and I believe these things, and it’s not about reason and it’s not about logic.” Okay. I don’t relate to it, but okay.

But to say, “I’m going to use reason and logic to prove to you this completely orthogonal conclusion”—I find that really interesting. So there’s some kind of romance about reason and logic.

Lex Fridman

Yeah, but also there’s a questioning of institutions that’s really interesting and important to understand.

Janna Levin

Well, I actually appreciate the skeptics’ stance. I think scientists also have to be skeptics. We have to be childlike, naive, and, in some sense, really open to anything, right? Otherwise, you’re not going to be flexible. You’re not going to be at the forefront. But also to be skeptical.

Lex Fridman

I guess that’s exactly what I’m saying is more confusing, because to invoke skepticism and then to want to use rational argument, what is the other component that’s going into this? Because, as you said, this is something that’s easily verified. We have people in space. So you have to believe a lot more machinery that’s a lot more difficult to justify or explain as a wild conspiracy.

So there’s something about the conspiracy that stirs a positive emotion.

I think one of the most incredible things that humans have ever accomplished is LIGO. We have to talk about gravitational waves. The very fact that we’re able to detect gravitational waves from the early universe is fucking wild. It’s crazy.

Janna Levin

Mhm.

Lex Fridman

Can you explain what gravitational waves are? We should mention you wrote a book about the whole journey of detecting gravitational waves and LIGO. Black Hole Blues is the book. Can you talk about gravitational waves and how the hell we’re able to actually do it?

Janna Levin

Let’s just start with the idea of gravitational waves. I have to move around a lot of mass to make anything interesting happen in gravity. If you think about it, gravity is incredibly weak. Right now, the whole Earth is pulling on me, and I can still get out of this chair and walk around. That’s insane. The whole Earth—gravity is weak, right?

So to get something going on in gravity, I need big objects and things like black holes. If black holes curve space and time around them in the way that we’ve been describing, things follow along the curves in space. If the black holes move around, the curves have to follow them, right? But they can’t travel faster than the speed of light, either.

So what happens is, as black holes move around—maybe I’ve got 2 black holes in orbit around each other. That can happen. It takes a while. A wave is created in the actual shape of space, and that wave follows the black holes as the black holes are undulating.

Eventually, those 2 black holes will merge, and, as we were talking about, it doesn’t take an infinite time even though there’s time dilation, because they’re both so big. They’re really deforming spacetime a lot. I don’t have a little tiny marble falling across an event horizon; I have 2 event horizons.

In the simulations, you can see them bobble, and they merge together and make 1 bigger black hole. Then it radiates in gravitational waves. It radiates away all those imperfections, and it settles down to 1 quiescent, perfectly silent black hole that’s spinning. Beautiful stuff.

It emits E = mc² energy. So the mass of the final black hole will be less than the sum of the 2 stellar black holes. That energy is radiated away in this ringing of spacetime.

It’s really important to emphasize that it’s not light. None of this has to do literally with light that we can detect with normal things that detect light. X-rays are a form of light. Gamma rays are a form of light. Infrared, optical—all of this whole electromagnetic spectrum—none of it is emitted as light. It’s completely dark. It’s only emitted in the rippling of the shape of space.

A lot of times, it’s likened more closely to sound. Technically, we’ve kind of argued—I haven’t done an actual calculation—but if you’re near enough to 2 colliding black holes, they actually ring spacetime in the human auditory range. The frequency is actually in the human auditory range, so the shape of space could squeeze and stretch your eardrum even in a vacuum. You could hear—literally hear—these waves ringing.

The idea is that they’re closer to something that you would want to map as a sound than to something like a picture.

Lex Fridman

Sorry. So what do you think it would feel like to ride the gravitational wave? To exist, as you mentioned here, would literally bob around, right? If you were orbiting these black holes—2 black holes—you’d be on a kind of complicated orbit, but your orbit would get tossed about. How would the experience be, because you’re inside spacetime?

#468

Yes, I see. The black hole is experienced within spacetime as a squeezing and stretching. So you would feel it as a sort of squeezing and stretching, and you would also find your location change. Where you would fall would be redirected.

It’s literally like a squeezing and stretching. That’s the way to think about it. It’s very detailed, the sort of nature of this.

For many years, people thought, “Well, these gravitational waves kind of have to exist,” for these intuitive reasons I’ve described. Spacetime’s curved. I move the curve. The wave has to propagate through that curved spacetime. But people didn’t know if they really carried energy. The arguments went on back and forth, and papers were written for decades.

I like this sound analogy because I liken the black holes to mallets on a drum. The drum is spacetime. As they move, they bang on the drum of spacetime and it rings.

Mhm.

#468

Remarkably, things don’t interfere with those gravitational waves very much. So they can travel for 2 billion light-years in distance, 2 billion years in time, and get to us kind of as they were when they were emitted—quieter, more diffuse. Maybe they’ve stretched out a little bit from the expansion of the universe, but they’re pretty preserved.

The idea of LIGO, this instrument, is to build a gigantic musical instrument. It’s kind of like building an electric guitar, where the electric guitar is recording the shape of the string and plays it back to you through an amplifier. LIGO is trying to record the shape of the ringing drum, and they literally listen to it in the control room. It just sort of hums and wobbles, and they’re trying to play this recording of the drum back to you, as opposed to taking a snapshot.

Lex Fridman

But to construct this guitar—this gigantic instrument—it has to be very large and extremely precise. It’s unbelievable. I can’t believe they succeeded. Honestly, I can’t believe they succeeded. It was so insane. It was such a crazy thing to even attempt. It took them 50 years.

#468

Really?

It’s people who started in their 30s and 40s who were in their 80s when it succeeded. Imagine that tenacity, that unbelievable commitment.

The sensitivity that we’re talking about is that we have this musical instrument that spans 4 kilometers in a kind of L-shape, with these tunnels where they created some of the largest vacuums on Earth, because they pulled a vacuum in these tunnels to build this instrument.

Speaker 1

They're measuring—they're trying to record the wobbling of spacetime as it passes this sort of undulation. That amounts to less than 1/10,000th the variation in a proton over the 4 kilometers. It's an insane achievement. Great engineering. I don't know how they did it. I follow them around just for fun. I'm very theoretical; I don't build things. I'm always super impressed that people can translate something on the page and it looks like wires. I don't know how. I'm always surprised at what it looks like.

I walked the tunnels with Ray Weiss, who won the Nobel Prize along with Kip Thorne and Barry Barish, one of the project managers. I walked the tunnels with Ray, and it was a delight. Ray is one of the most delightful people. Kip is one of the most wonderful people I've ever known.

Ray said to me, “You know, the reason why it was called Black Hole Blues and Other Songs from Outer Space is because about a month before they succeeded, he said to me, ‘If we don't detect black holes, this whole thing's a failure. We've led this country down this wrong path.’” He really felt this tremendous responsibility for the project to succeed, and it weighed on him. It was quite tremendous—the integrity, the scientific integrity.

The first instrument he built was outside MIT, on a tabletop, and his colleagues said, “You're not going to get tenure. You're never going to succeed.” They just kept going. People like that. Huge teams, huge collaborations—it's how the world moves forward, because it's an example. There's a growing cynicism about bureaucracies, about whether a large number of people, especially connected to government, can be productive. Bureaucracies slow everything down. So it's nice to see an incredibly unlikely, exceptionally difficult engineering project like this succeed.

Oh, yeah. I understand why there's this weight on his shoulders, and I'm grateful that there are great leaders who push it forward like that.

Speaker 1

Yeah, it really is. You see so many moments when they could have stumbled. They built a first-generation machine just after 2000, and it wasn't a surprise to them, but it detected nothing. Crickets. Crickets. They just had the wherewithal to keep going.

Then, with the second generation, they're about to turn the machine on, quote-unquote. It's a little bit of a simplification, but they're about to do their first science run, and they decide to postpone because they feel they're not ready yet.

It's September 14, 2015, and the experimentalists are out there in the middle of the night. They're working all night long, and they're banging on the thing—literally driving trucks and slamming on the brakes to see the noise that it creates. They're really messing with the machine, interfering with it just to calibrate how much noise it can tolerate.

I guess they get tired. There's an instrument in Louisiana and one in Washington State. They put their tools down and go home. The instrument is locked, mercifully. Within the span of an hour of them driving back to their humble abodes in these remote regions where they built the instruments, a gravitational wave washes over them.

I think it hits Louisiana first. It travels across the United States and rings the instrument in Washington State. It began over 1.5 billion years ago, before multicellular organisms had emerged on Earth. Just imagine this from a distant view: this collision course. It's the centenary—it's the year Einstein published general relativity. Just think about where that signal was when Einstein, in 1915, wrote down the general theory of relativity. It was on its way here. It was almost here.

What do you think is cooler: Einstein's general relativity or LIGO?

Speaker 1

I can't disparage my friends, but of course relativity is just so all-encompassing.

No, but see, hold on a second. It's an all-encompassing, super-powerful leap of a theory. Yeah, and they built it. They built it. I don't know, man—the greatest engineering on Earth. Humans getting together and building the thing is really ultimately what impacts the world, right?

Speaker 1

I mean, as I said, my admiration for Ray and Kip and the entire team is enormous. Just imagining Ray had been out there on-site, and he had just left to go back home, then wakes up in the middle of the night and sees it—you can imagine. There's a signal, something in the log. He's like, “What the hell is that?”

Speaking of the human story, you also wrote the book A Madman Dreams of Turing Machines. It connects 2 geniuses of the 20th century, Alan Turing and Gödel. What specific threads connect these 2 minds?

Speaker 1

Yeah, I was really mesmerized by these 2 characters. People know Alan Turing for having conceived of the computer, for being the person to really imagine it. But his work began with thinking about Gödel's work. That's where it began, and it began with this phenomenon of undecidable propositions, or unprovable propositions.

There was something huge that happened in mathematics: people imagined that any problem in math could technically be proven to be true. It doesn't mean human beings are going to prove every fact about everything in mathematics, but it should be provable, right? It seemed like a not-that-wild supposition, and everyone believed this—all the great mathematicians. Hilbert's call was to prove that.

Gödel was a very strange character, very unusual. He was a Platonist. He literally believed that mathematical objects had an existential reality. He wasn't so sure about this reality—this reality he struggled with. He was distrustful of physical reality, but he absolutely took very seriously a Platonic reality. In his own way of thinking, he proved that there were facts, even among the numbers, that could never be proven to be true.

You have to think about how wild that is: even a fact about numbers that seems very simple could be true and unprovable, could never exist as a theorem, for instance, in mathematics—unreachable. This incompleteness result was very disturbing. Essentially, it's equivalent to saying there's no theory of everything for mathematics. It was very disturbing to people, but it was very profound.

Alan Turing got involved in this because he was thinking about uncomputable numbers.

What's an uncomputable number?

#468

A number like 0.175... It just goes on forever with no pattern, and I can't even figure out how to generate it. There's no rule for making that number. He was able to prove that there were such things as these uncomputable, effectively unknowable numbers. That might not sound like a big deal, but it actually was really quite profound.

He was relating to Gödel intellectually, in the space of ideas. But he goes in a very different path, almost philosophically the opposite direction. He starts to think about machines. He starts to think about mechanizing thought. He starts to think: What is a proof? How does a mathematician reason? What does it mean to reason at all? What does it mean to think?

He begins to imagine inventing a machine that will execute certain orders, mechanize thought in a specific way. Maybe I can imagine a machine that does this kind of thinking. He can prove that even a machine could not compute these uncomputable numbers.

Where he ends up is the idea of a universal machine that computes—essentially, one that can take different software and execute different jobs. We don't have a different computer to connect to the internet than we do to write papers. It's 1 machine, 1 piece of hardware, but it can do all of these different tasks. So he really does invent the computer, essentially.

Famously, he uses that thinking in a very primitive form in the war effort, where he's recruited to help break the German Enigma code, which was heavily encrypted and largely believed to be uncrackable. People believe that Turing and his very small group actually turned the tide of the war in favor of the Allies, precisely by using a combination of this thinking, sheer ingenuity, and some luck.

The other profound revelation that Turing has is that maybe we're just machines—just biological machines. This feels very different from Gödel, who doesn't really believe in reality and thinks numbers are Platonic realities. Turing is thinking that we're actually machines and that we could be replicated.

So, of course, Turing's influence is still widely felt on many levels.

#468

On many levels, yeah—in complexity theory, theoretical computer science, and mathematics, but also in philosophy with his famous Turing test paper.

What is the connection that I guess Gödel never really made between mathematics and humanity, that Turing did? I think another connection to those 2 people is that they're both, in their own way, tormented humans. What aspect of that contributed to who they are and what ideas they developed?

#468

I think they were very tormented. I don't want to promote the trite trope of the mad genius: if you're brilliant, you are insane. I don't think that. I don't think if you're insane, you're brilliant.

#468

But I do think if somebody who's very brilliant also chooses not to go for regular gratification in life, they don't go for money. They don't necessarily value creature comforts. They're not leveraging for fame. They're really after something different.

I think that can lead to a kind of runaway instability, actually. Sometimes they're already outside of social norms. They're already outside of normal connections with people. They've already made that break, and I think that makes them more vulnerable.

So Gödel did have a wife and a strong relationship, as far as I understand, and was a successful mathematician. He ended up at the Institute for Advanced Study, where he walked with Einstein to the institute every day. They talked, and he proved certain really unusual things in relativity.

You made reference to these rotating galaxies. We were talking, and actually Gödel had a model of a rotating universe that you could travel backward in time. It was mathematically correct. He showed Einstein that within relativity, you could time travel.

Just an unbelievably influential and brilliant man. But he was probably a paranoid schizophrenic. He did have breaks with reality. He was, I think, quite distrustful and feared the government. He feared his food was being poisoned and ultimately, literally, starved himself to death.

It's such an extreme outcome for such a brilliant mind. I think it's important not to glorify or romanticize madness or suffering, but to me, you can flip that around and just be inspired by the peculiar maladies of a human mind, how they can be leveraged and channeled creatively.

#468

Oh, yeah.

I think a lot of us—obviously, probably every human—have those peculiar qualities. I talk to people sometimes about my own psychology. I'm extremely self-critical, and I'm drawn to the beauty in people, but because I make myself vulnerable to the world, I can really be hurt by people.

You can lay that out: that's this particular human. There are a bunch of people who will say, “Well, you know, many of those things you don't want to do. Maybe don't be so self-critical. Maybe don't be so open to the world. Maybe have a little bit more reason in how you interact with the outside world.”

It's like, yeah, maybe. Or maybe be that, and be that fully, and channel that into a productive life. We're all going to die, and in the time we have on this Earth, make the best of the particular weirdness that you have. Maybe you'll create something special in this world, and in the end it might destroy you. I think a lot of these stories are like that.

It's not like saying, “Because in order to achieve anything great, you have to suffer.” No. If you're already suffering, if you're already weird, if you already don't quite fit in your particular environment, your particular part of society, use that somehow. Use the tension of that, the friction of that, to create something.

I mean, that's what I think of Gödel, who suffered a lot from even stupid stuff like stomach issues and migraines. Migraines are psychosomatic or psychophysical, but all of those things can somehow be channeled into a productive life. It should be inspiring. A lot of us suffer in different ways.

#468

Yeah. I'm a big believer in the tragic flaw. Actually, I think the Greeks really had that right. You're describing it: what makes us great is ultimately our downfall. Maybe that's just inevitable. The choice could be not to be great.

I guess that's sort of what I mean by they had already broken from a traditional path, because they decided to pursue something so elusive, and that would isolate them to some extent inevitably. It could fail, and its rewards were hard to predict.

I do think that all the character traits that went into their accomplishments were the same traits that went into their demise. I think you're right. You could say, “Well, you know, Lex, maybe you should not be so empathetic. Hold yourself, cut yourself off a little, protect yourself.” But isn't that exactly what you're bringing—one of the elements that makes something extraordinary in a space that lots of people try to break through?

Yeah. We should mention that for every Gödel and Turing, there are millions of people who have tried and who have destroyed themselves without reason.

#468

I would find it impossible not to pursue a discovery that I could imagine my way through. If I can really see how to get there, I cannot imagine abandoning it for some other reason—fear that it would be misused, which is a real fear. It's a real concern.

I don't think in my work, since I'm doing extra dimensions in the early universe or black holes, I feel pretty safe. But who knows, right? Bohr couldn't think of a way to use quantum mechanics to kill people. I cannot imagine pulling back and saying, “Nope, I'm not going to finish this.”

I'll give you a counterexample of an exceptionally brilliant person: Terence Tao. Brilliant mathematician. Of all the brilliant people I've ever met in the world, he's better than anybody else at working on a hard problem and then realizing when it's, for now, a little too hard.

#468

Oh, that I can do.

And stepping away. He's like, “Okay, this is now a weekend problem.” He has seen too much. Everybody's different, but Grigori Perelman or Andrew Wiles give themselves completely, for many years, over to a problem.

#468

Yes. And for every one of them, they might not have cracked it.

So you choose your life story.

#468

I totally agree. I'm not going to say sometimes I take too long to come to that conclusion, but I will proudly say, as most theoretical physicists should, that I kill most of my ideas myself.

Okay, so you walk away.

#468

I am absolutely able to say, “I'm not going to deny that sometimes I maybe take a while to come to that conclusion, longer than I should, but I absolutely will. I will drop it.” Any self-respecting physicist should be able to do that.

The problem is with somebody like Andrew Wiles, whom you were describing. To prove Fermat's Last Theorem, it took him 7 years. Was that the number?

#468

Something like that. He went up into his mother's attic or something and did not emerge for 7 years. He was on the right track. He wasn't wrong.

But that could have been interminable. He still might not have gotten there in the end. That's the really difficult space to be in: you're not wrong, you're onto something, but you're just asymptotically approaching that solution and you're never actually going to land it.

#468

That happens, and it would break me—straight-up break me.

He had a proof.

#468

Yes. He announced it, and somebody found a mistake in it.

That would just break me.

#468

Because now everybody gets excited, right? And now you realize that it's a failure, and you have to go back. It took a year for people to check it. It's not the kind of thing you look over in an afternoon. To have the will, the confidence, and the patience to go back and rigorously work through it is unbelievable. It's a great story.

But then there's another great story: Grigori Perelman, who spent 7 years—he turned down the Fields Medal. He did it all alone. After he turned down the Fields Medal and the Millennium Prize for proving the Poincaré conjecture, he just walked away.

#468

Yeah. Now that's a very different psychology. That's wired differently. He doesn't care about money, fame, or anything else.

In fact, where is he now? St. Petersburg, Russia. I'm trying to get a conversation with him. It turns out when you walk away and you're a recluse and you enjoy that, you also don't want to take some weird dude in a tie. So it turns out I'm trying. I'm trying.

#468

Well, if you look at someone like Turing, his eccentricities were completely different, right? It's not as though there's some mold, and I really don't like it when it's portrayed that way. These are really individuals who were still lost in their own minds, but in very different ways.

Turing was openly gay during this time. He was working during World War II, so we understand the era, and it was illegal in Britain at the time. He kind of refused to conceal himself. There was a time when the attitude was, “Well, we're just going to ignore it.”

But he had been robbed by somebody he had picked up somewhere. I think it was in Manchester. It was such a small thing. I don't know what they took. It was nothing. But he couldn't tolerate it. He went to the police and told them, and then he was arrested. He was the criminal because it involved this homosexual act.

Now here you have somebody who made a major contribution to the Allies winning the war. It's just unbelievable, not to mention the genius, the mathematical genius. He saved the lives of the people who were doing this to him, and they essentially chemically castrated him as punishment. That was his sentence.

He became very depressed and suicidal. The story is that he was obsessed with Snow White, which had recently been released. He used to chant one of the little—I don't know if you would call them poem-songs—“Dip the apple in the brew, let the sleeping death seep through.” It was a chant from Snow White.

The belief is that he dipped an apple in cyanide and bit from the poison apple. I don't know if this is apocryphal, but people think that the Apple logo on the Macintosh, with the bite out of it, is a reference to Turing.

#468

Now some people deny this. That's nice. But some people say he did that so his mother could believe that maybe it was an accident. But yeah, quite a terrible end.

Yeah, but two of the greatest humans ever.

#468

I think the reason why I tie them together, not just because ultimately their work is so connected, but because there's this sort of impossibility of understanding them. There's this sort of impossibility of proving something about their lives, that even if you try to write factual biography, there's something that eludes you. And I felt like that's kind of fundamental to the mathematics: the incompleteness, the undecidable, the uncomputable.

Yeah. So structurally, it was about what we can kind of know and what we can believe to be true but can't ever really know. Limitations of formal systems, limitations—

#468

Exactly. Biography, limitations of fiction and nonfiction.

Limitations. There are so many layers to you. One of which is this romantic notion of just understanding humans, exploring humans, and there's exploring science, exploring the very rigorous, detailed physics and cosmology of things. So there's art, the kind of artistry. I saw that you're the chief science officer of Pioneer Works, which is mostly an artist-type situation. It's a place in Brooklyn. Can you explain to me what that is and what role art plays in your life?

#468

Yeah, I can start with Pioneer Works. Pioneer Works, in some sense, was inevitable. It felt like I was marching there for many years, and it came together again like at this collision. It was founded by this artist, Dustin Yellin. A very utopian idea: he bought this building, this old ironworks factory called Pioneer Iron Works in Brooklyn. It was in complete disrepair, but it was a beautiful old building from the late 1800s.

He wanted to make this kind of collage. Dustin's definitely a collage artist. He works in glass, making very big pieces that are very imaginative and wild and narrative, and are about nature and consciousness. I think he wanted to do that with people. He wanted a place, a collage, a living example of artists and scientists. It was founded by Dustin, and Gabriel Florenz was the founding artistic director.

It was started just before Hurricane Sandy. I don't know if people feel as strongly about Hurricane Sandy as New Yorkers do, but it was a real moment around 2012 or 2013. It sort of paused the project, and you can even see the waterline on the brick showing where Sandy was. I came in and collided with these two shortly after that, and it really was like a collision.

I'm science, you know; they're art. Gabe makes everything, builds everything with his bare hands. Dustin's a dreamer. They love science. They really wanted science, but science is hard to access. I've always loved the translation of science in literature and art. I love fiction writers, really literary fiction writers who dabble in thinking about science.

I very firmly believe science is part of culture. I know it to be true. I don't think of myself as doing outreach or education. I don't like those labels. I'm doing culture. An artist in their studio is working out problems, understanding materials, building a body of work. Nobody says to them when they exhibit, “Why are you doing outreach?” or “Are you doing education?” It's the logical extension.

So I feel that if you've had the privilege of knowing some of these people, of seeing a little bit from the summit, if you've had a little glimpse yourself, then you bring it back to the world. So we exploded. Pioneer Works became science and art. It's not artists who all do science or scientists who do art. It's real hardcore scientists talking about science, and a lot of live events.

We have a magazine called Broadcast, where we feature all of the disciplines rubbing together, artists working on all kinds of things. When I first started doing events there, my first guest was, like you, someone I was talking to. I know how to talk to people because I know these guys, and I've been on the interviewee side so much. I knew exactly how to do those conversations; it was fully formed for me.

Yeah, you're extremely good at that, too.

#468

Yeah, thank you. I appreciate that. You learn how to do it, too, though. I don't think the first one I did was perfect. I think I've learned, right? You acquire it; you get better. It's really interesting.

I love to study. I think you do, too. I really look into the material, and I love science. I really do. I want to talk to a CRISPR biologist because I don't understand it and I want to understand it.

And I saw there's a bunch of cool events and a very, very fascinating variety of humans.

#468

Yes, we have a really fascinating variety of humans. That's a good way of putting it.

Yeah. So it made me put in my mental map that it's a cool place to go and visit when in New York.

#468

Yes. You have to come see us. I think you would love it.

Also, I should mention fashion. I've seen you do a bunch of talks, and there's a lot of fashion.

#468

Yeah, there's an appreciation of fashion going on. You're giving me an opportunity to give a shout-out to Andrea Lara, who's a designer who makes these amazing jumpsuits that I often wear at a lot of my events. She has a jumpsuit design line called Risen Division, and she just makes these incredible—

They're fantastic. We also design patches for all of our events. So there are these string theory patches and consciousness patches—

We should show this as overlays. Hopefully there'll be nice pictures floating about everywhere.

#468

So, I think all of this is just—I just like to experiment with life. I think making the magazine was a big, wild experiment.

You said “with life.” Nice.

#468

This kind of idea that we were just describing is—I find it hard to stop the momentum if I think I can make something. I have to try to make it. To me, this is the closest I come to experimentation and collaboration, because even though I collaborate theoretically, I have great collaborators: Brian Greene, Massimo Porrati, Dan Kabat. These are my really close collaborators.

A lot of theoretical physics is alone, and you're in your mind a lot. This is something that really was built—this triad of Dustin, Gabe, and I, and all our amazing people who work there, and our amazing board. We really are doing it together. You take one element out and it starts to change shape, and that's a very interesting experience, I think. Making things is an interesting experience.

Since you mentioned literature, are there books that had an impact on your life, whether it's literature, fiction, or nonfiction?

#468

I love fiction, which I think people expect me to read a lot of science fiction or nonfiction. I mostly read fiction. I had a syllabus of great fiction writers that had science in it, and I love that syllabus.

Can you ever make that public, or no?

#468

Yeah, I suppose I could, but I can tell you some of them as they come to mind. Kazuo Ishiguro, who won the Nobel Prize and wrote The Remains of the Day, probably most famously. His book Never Let Me Go—it's unbelievable, totally devastating, stunning.

I see.

#468

I really love literature. When people can do that with these very abstract themes, it's sort of my favorite space for literature. Martin Amis wrote a book that runs backwards, Time's Arrow. I love some of his other books even more, but Time's Arrow is pretty clever.

So, you like it when these nontraditional mechanisms are applied to tell a story that's fundamentally human, that there's some—

#468

Yes.

Lex Fridman

—some beauty of language. I really appreciate that.

#468

Even Orwell is amazing. Hitchens's writing on Orwell is amazing. There were some plays on the syllabus. I have to think of what else was in there.

But there was one book that I think was kind of surprising, that I think is an absolute masterpiece, which is The Road. You might say, “In what sense is The Road science?” Well, first of all, Cormac McCarthy absolutely loves scientists and science. You can feel this very subtle influence in that book. It's remarkable: precise, stunning, ethereal, all of these things at once.

And there's no who, what, where, when, or how. You might guess it's a nuclear event that kicks off the book. A lot of people know The Road, I think, from the movie, but really, the book is magnificent. It's very, very abstract, but there's a sense to me in which science is structuring the world, and still, fundamentally, that book is about the human story, the human connection.

Boy, yeah. So the science plays a role in creating the world, and within it there's still really—it's a different way to explore human dynamics in a way that's maybe landed some clarity and depth that maybe a more direct telling of the story would not.

#468

Yeah. Even surreal worlds—to me, I don't know why, but I return to Orwell's Animal Farm a lot. It's another art form to be able to tell a simple story with some surreal elements.

Mhm.

#468

Well, just simple language.

Lex Fridman

Mhm.

#468

Animal Farm is incredible. In fact, I've kind of played with, “Some animals are more equal than others.” In good old Turing's work, there were some infinities that are bigger than others.

Certain books just kind of inject themselves into our culture in a way that just reverberates. I don't know—it creates culture, not just influences. It's quite incredible how writing and literature can do that.

Lex Fridman

Yeah. If you could have one definitive answer to one single question—this is the thing I mentioned to you.

#468

So hard.

Yeah. Well, there's an oracle, and you get to talk to that oracle. You can ask multiple questions, but it has to be on that topic. So, just to clarify—

What mystery of the universe would you want that oracle to help you with?

#468

You know, it’s funny. I should say the obvious thing, but I feel like it would almost be greedy. I think I have a complicated response to this. The obvious thing for me to say would be, “I want to understand quantum gravity,” or whether gravity is emergent. It’s not even something I work on day-to-day. I mostly just look with interest at what others are doing, and if I think I can jump in, I would, but I’m not jumping into the fray.

Obviously, that’s the big one, and there is a sort of sense that with that will come the answers to all these other things. My complicated relationship is that part of the scientific disposition is not having stuff you don’t know the answer to. We’re not going to have all the answers. I hope, because then, sort of, then what, right? It’s sort of dystopian.

I totally agree with you. I like the mysteries we have.

#468

Yeah. I had this assumption that there will always be mysteries, so you want to keep solving them. They will lead to more. In the same way that relativity led to black holes, black holes led to the information loss paradox, the Big Bang, what happened before, or the multiverse. It’s because we learned so much that we were able to escalate to the next level of abstraction.

Yeah, by the way, we should mention that if you’re talking historically, and even if you ask the obvious question about quantum gravity, I almost guarantee, with 100% probability, that even if all your questions are answered, it’s impossible to get to the end of your questions, because the oracle will say, “No, you can’t unify.” But then you say, “Well, wait,” and then you say, “Emergent,” and then the oracle will say, “Well, everything you think is fundamental is not; it’s emergent.” It’s like, okay, well, we need more questions, right? It’s been 100 years or more since relativity, and we’re still picking it apart.

#468

No, and there may be new ones.

Mm-hm. You write that eventually all our history in this universe will be erased. How does that make you feel?

#468

Yeah, that’s a tough thought. But again, I think there’s a way in which we can come to terms with that. That’s kind of poetic. You build something in the sand, and then you erase it. So, I think it’s just a reminder that we have to be concerned about our immediate experience too, right? How we are to those around us, how they are to us, what we leave behind in the near term, what we leave behind in the long term, have we contributed, and did we contribute overall net positive?

Eventually, I think it’s hard to imagine, but yes, all of these Nobel Prizes, all of these mathematical proofs, all of these conversations, all these ideas, all the influence we have on each other—even the AI—eventually will expire.

Well, at the very least, we can focus on drawing something beautiful in the sand.

#468

Yeah. Before it’s washed away.

Well, this was an incredible conversation. I’m truly grateful for the work you do and for your work.

#468

Thanks so much for having me.

Thank you for talking today.

#468

Yeah, lots of fun.

Thanks for listening to this conversation with Channel 11. To support this podcast, please check out our sponsors in the description. And now, let me leave you with some words from Albert Einstein on the topic of relativity.

“When you're courting a nice girl, an hour seems like a second. When you sit on a red hot cinder, a second seems like an hour. That's relativity.”

Thank you for listening and hope to see you next time.

Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions | Lex Fridman Podcast #468 | BidClub