Biggest Mysteries in Physics: Antimatter, Dark Energy & ToE - Don Lincoln | Lex Fridman Podcast #497

In describing the search for a theory of everything in physics, you describe how the history of physics can be told effectively as a kind of history of unifications. There's this centuries-long quest to show that these distinct phenomena are actually linked by some unified underlying principles, even starting with Newton. You can think of the effort of physics as trying to unify the laws of nature. So I was wondering if we could talk through the history of unification through that lens of physics.

There are, of course, lots of different ways to do physics, but the way that particle physicists and cosmologists do it, I would say, is that they are trying to find the underlying principles that govern the laws of nature. If we go back, say, to the 1650s or so, you're the most brilliant person around, and you've noticed 2 things. One thing you've noticed is that when you trip, you fall. That is the nature of gravity that we all experience day to day.

But then there's astronomy, where you look out at the heavens and you see the stars march across the sky. You see the planets move through the stars. And there, that seems to have absolutely nothing to do with what happens when you drop your sandwich and the dog grabs it from you.

So the brilliant thing was when Newton looked at that and thought, "Maybe the moon is falling, but it's missing the Earth." So what we had in maybe 1650 was what we might call the laws of celestial gravity, the gravity that governs the heavens, and terrestrial gravity, the gravity that is here on Earth. Now, we don't think of it that way anymore. We think of it as just gravity.

And in fact, if you look in the books, Newton's theory is Newton's law of universal gravitation. The universal is there, and the reason is because he realized these 2 things that seemed to have nothing to do with one another were indeed one and the same. Newton is arguably one of the most brilliant humans of which I'm ever aware. At any rate, it is the first easily describable unification of physics that you can state in a way that makes sense to modern humans.

You can go back farther than that. But regarding unification, there are more examples. You go back to Democritus, who was wrong about very many things, but the idea that there was a smallest particulate form of matter was right. So it's kind of funny. You read the chemistry books, and they say that the idea of atoms goes back to Democritus.

His idea was that there was a smallest atom of oil, which was smooth, and it was smooth, of course, because oil is smooth. There was a smallest atom of vinegar because vinegar is tart and it pricks your tongue, so atoms were little sharp, pointy things. He was wrong about a lot, but he was right about the idea that there was a small particle, and we have a very different concept than he did.

If you go back to, say, 1830 or so, scientists were trying to understand electricity, for instance. There was a lot going on. People really understood things. At the time, you would have 2 phenomena that are familiar to us now. One is a magnet, which, at the time, mostly consisted of little pieces of iron that had been magnetized and could stick to steel.

Then you had electricity, which, at the time, meant generating little sparks that they could play with and have fun with, or, more broadly, a lightning bolt blazing across the sky. When you think about this, that lightning bolt and that little magnet seem to be unrelated. But over the 1800s, a number of scientists were exploring little aspects of it.

What happens when you run electricity through a wire? It seems to make a magnetic field. There was a whole bunch of experiments and there were a lot of names. But in about the 1860s, James Clerk Maxwell took all of those ideas that had been percolating around for the previous 50 years and wrote his laws of electromagnetism.

They're really fascinating. If you look at the laws of electromagnetism, they are differential equations or integral equations. Basically, what they say is that on one side, you have a bunch of terms that have electricity in them, and then, on the other side of the equals sign, you have a magnetism thing.

Forgetting all of the mathematical symbols, you have an electricity side equals a magnetism side. Electricity equals magnetism. That is a staggering concept: the fact that these 2 things, a lightning bolt and the magnet that holds your kids' art to the refrigerator, are one and the same.

This was another case where electricity and magnetism became unified into electromagnetism. So now we have 2 examples: gravity being unified—terrestrial and celestial gravity—and then electricity and magnetism. I'll tell you about some more in a moment, but one thing that's important is that the goal is, of course, to unify everything.

If I could do what I want to do, I would have some unified theory that would explain all the behavior of all energy, matter, space, and time, which is a grand goal.

And we should say that maybe one of the goals of science more broadly, outside of physics even, is to construct models that can generalize the world. If you look at Darwinian evolution, that was a very beautiful theory that captures another layer of reality—how this particular thing that we see here on Earth happens.

So when we talk about a theory of everything in physics, that's capturing a different layer of abstraction about the functioning of the universe.

Right. The whole Darwinian evolution, the fact that our genetics has significant overlap with the genetics of a banana, is pretty staggering. It's astonishing that that works. But for at least the class of scientists that I am part of, what we think of is, well, sure, biology is interesting and all, but when you get right down to it, whatever happens in biology is caused by the movement of molecules.

Then you say, "Well, that's great and all, but molecules do what they do because they're made of atoms." Then the next step is, "Atoms, that's great, but atoms work the way they do because of the nucleus and the electrons," and then the nucleus is protons and neutrons.

There are those of us, myself included, who want to dig down to the very, very bottom and find out what is the smallest building block of nature from which all of these other, far more complex, interesting, and abstract things are made. What is at the very, very bottom?

Also, that's great, but if you know what the smallest building blocks are, that doesn't tell you the story. It's like having a whole bunch of LEGOs but not knowing how to put them together. You also need to know how they interact, how they work, and so that's why we study forces.

There are the various subatomic forces with which we're familiar. For instance, electricity and magnetism are components of electromagnetism, which then governs the behavior of things like—

This is amazing.

Electromagnetism explains electricity and magnetism, but it explains how light works. It explains how much of chemistry works. So, electromagnetism—in 1860 or '70, the wonderful thing about that is that if you take Maxwell's equations and apply a little bit of calculus, it's very easy to see that the laws of electricity and the laws of magnetism combined together make what's called a wave equation.

That shows that these electric and magnetic fields oscillate. They vary, and if you have something that's varied, that's a wave, and the wave then moves. If you do the math, you find out that the speed at which these waves move is the speed of light. People said, "Wow, the speed of light comes out of those equations." That had to be, I think, very persuasive.

Electromagnetism also plays a really significant role in chemistry because, after all, atoms are held together by electromagnetic forces. There's more to how atoms work. There is all the quantum mechanics stuff. But if you did not have electromagnetism, or if electromagnetism was very different, then atoms would be very different.

So it plays a very big role in holding us together. It's a staggering advance in science to have a good theory of that. Being able to tame electromagnetism is why people can hear you when you do your podcast, through the miracles of the internet—or just electricity running the computers.

This is a case where, if I can get on a small soapbox, people back then said, "Why are you messing around with magnets and sparks?" And, "Who cares?" Well, that very fundamental digging into the laws of nature has spinoffs. One of the big spinoffs is our entire technological society.

Without being able to govern electricity, we'd still be farmers and shoemakers in cities. We certainly would not have everything that we do. So, off my soapbox.

But it’s really a lovely thing to show how this digging into deep, fundamental, not understood, mysterious things can, a hundred or two hundred years later, transform the world. The type of science I do now—people often ask, “What good is knowing about how the inside of atoms work, how the inside of quarks work?” And I don’t know the answer to that.

Just being a little more pragmatic, if I go back, say, a hundred years, to when people were trying to understand how the protons and neutrons inside atoms held together, how they split, how you could combine them, and so forth, this has led to nuclear power.

Now, whatever you think about nuclear power—and some people like it and some people don’t—but it is powerful. It will generate energy for humanity, and it may be that that is the path that we take as we move away from digging fossil fuels out of the ground. Humanity is going to need power no matter what.

Nobody is going to go back to the way things were in the 1700s. One enormous source of energy that is there for us to take, if we so choose, is the modification of the nucleus of atoms. It seemed to have absolutely nothing to do with anything, and yet it provides humanity with an opportunity, which of course requires that we think carefully about how we do that and if we want to, but it gives us something that we didn’t have before.

Yeah, it’s very clear that nuclear fusion and nuclear fission will unlock a huge amount of energy that’s required for a civilization to flourish, but that’s almost near-term. Longer term, you can think about things like the dark energy crisis—we’ll talk about that—and antimatter.

Maybe if you figure out some of the mysteries around antimatter, that, too, would lead to energy sources and ways to produce energy. That, too, might lead to counterintuitive propulsion systems for us humans to travel through the universe.

Right now, it seems far-fetched, too expensive, too complicated, too difficult, but breakthroughs in fundamental theoretical physics might lead us to unlock some incredible energy sources, incredible technologies that will allow humans to explore the universe.

And of course, we should also mention that, as always with technology, it’s a double-edged sword. It will most likely lead to the development of more dangerous weapons or other sources of harm. Then we, as a civilization, kind of have to walk that line and hope we figure out how to do more good than bad with the technologies we build.

Right. But we have to really remember: while people worry about nuclear weapons, which are admittedly very dangerous, and even nuclear power, which has waste that has to be dealt with, what science is doing is working out how to find power that nature has presented to us.

This is not new. Fire is like that, too. Fire can burn down your house or it can cook your steak. Power is like that, and that’s just something that we have to understand as humanity.

That’s why this needs to be a broad conversation by all of society. When we talk about science, it has to be a broad conversation by all of society, because what scientists can do is figure out how the world works. Society has to figure out how we wish to apply that or not apply that.

Also, solving the mysteries and the puzzles of the universe in itself is effing awesome.

It is. It is.

So that means the thing that makes us human, in part, is looking at a thing and saying, “How does this work?” Then together, a bunch of apes get together, poke the thing, shake the thing. And then over time, you have rockets going all into space. You build roads and bridges. You build the internet.

Anyway, we talked about Newton, we talked about Maxwell. That takes us into the 20th century in terms of unification. There’s a guy named Einstein, on whom you wrote a book, who made quite a lot of progress on the effort of unification.

Sure. So Einstein, he’s a pretty amazing guy. In 1905, he had his miracle year where he wrote multiple papers. The one that most people know about is special relativity, where he showed something that makes no sense to anybody who’s not really dug into it very hard, and that is that 2 people experience time differently.

Time is a fascinating thing. We don’t really understand what time is, which is weird. You’d think that that’d be something we’d understand very well, but we really don’t. We know a lot about it, but really understanding it, not so much.

But Newton thought that time was just universal for everyone, so my time, your time, some person’s time on Mars or on Alpha Centauri—everybody experienced time the same. What Einstein showed was that that wasn’t the case, that different people moving at different speeds with respect to one another experience time differently, which is an absolutely mind-blowing concept.

Now, most people think that Einstein then said, well, he invented spacetime, that space and time are the same thing, and he was behind that. But that actual insight came from one of his teachers, a guy by the name of Minkowski, who looked at Einstein’s equations.

Minkowski was a little bit more mathematically inclined than Einstein, and he saw that if you look at the equations, you have basically one person’s space and time equals some numbers times this person’s space and time. So that’s kind of a staggering thing. That is where Einstein and Minkowski really got to this unbelievable concept: space and time are actually pretty much the same thing.

That runs afoul of our understanding of how the world works because time just moves; it’s continuous. We know what it is at a visceral level and an experiential level. We might not understand it at a formal level, but we know what time is. It’s what makes today today, and not yesterday or tomorrow.

Space is a little different. You can walk somewhere, you can walk back, you can move around. You have more freedom to move in space than you have to move in time. You can always move forward in time. It’s just moving backward that turns out to be a little more difficult.

But yeah, Einstein’s understanding that this is the case caused everybody to think about the world very, very differently, and that was in 1908, when Minkowski really laid it out in a speech on spacetime.

And that also led to the work on special relativity, which led to the speed limit—the speed of light.

Well, I have to tell you, when I first encountered this, it was pretty freaking weird. It pegs the weird meter. But as you become more familiar with it, as you become more comfortable with the idea, the thing to remember is that the speed of light is the speed of light through spacetime.

Once you embrace that, it makes a whole ton of sense. It all of a sudden makes everything fall much more into place.

I think that there is an ultimate speed; that isn't that shocking. It simply says that it's a property of space, in the same way that space can transmit a certain-strength electric field. It can support certain things. Whatever space is—and we don't know what space is—but whatever it is, it has the capability of transmitting these things at that one speed through space or time, and everything else comes from our insisting that we keep space and time different. That's how I view it, and for me, once I accepted that, it all became very comfortable.

So the nature of my question here—which will apply over and over—is trying to empathize, trying to put ourselves in the shoes of the people before space and time were unified into spacetime, and really experience and think through how difficult a leap that is. We are now in the modern day, in the 21st century, and we're going to have to make leaps like that in our future. So what are the unifications we're not seeing in front of our eyes? For example, there are so many examples in your work, through your lectures, of Paul Dirac taking antimatter seriously—looking at what the math shows and saying, “I really think this thing exists.” I mean, it just sounds insane.

Huge.

The reason I sort of say that is we are now in the modern day, in the 21st century, and we're going to have to make leaps like that in our future. So what are the unifications we're not seeing in front of our eyes? For example, there are so many examples in your work, through your lectures, of Paul Dirac taking antimatter seriously—looking at what the math shows and saying, “I really think this thing exists.” I mean, it just sounds insane.

It does.

And so I think this is a good warm-up. The spacetime unification is a good warm-up as we march through the 20th century, because it gets, in my view at least, weirder and weirder, even with Einstein himself.

Well, let me give you an even more basic example: sodium and chlorine. Sodium is an explosive metal. You put it in water, and it doesn't quite explode, but it gets hot and pops around. Chlorine is a gas. It's going to kill you. So these 2 things are deadly. They're awful. And yet when you mix them, you put it on your food at night. It's salt, right?

This is a case where this whole—A, unification, and B, this deeper understanding, in this case of chemistry—of how 2 things that are dangerous can be brought together and turned into something not only innocuous but necessary for human life. So this is not unusual, what you're describing.

When you think about it, forget about everything else, just the fact that we tell little kids that the world is made of atoms. Now, that's crazy. Most people have never seen atoms, and yet nobody really doubts it anymore. I think it's just a case of familiarity, and then the culture slowly accepts it, and it's real even without the evidence. In fact, one of the courses you described there, How We Know What We Know, I think that's a valid question: How do we know there are atoms? And, of course, there are ways we do.

And by the way, on that front, I would love to go through how we know the building blocks in the universe as we march toward quarks. That, in the course that you mentioned, is one of the most fascinating things about this philosophy of atoms being around for a very long time. Then you concretize it and actually can prove, or have strong observations that indicate, that there are atoms, and then there is a nucleus, there are electrons, there are photons, there are quarks, and it gets weirder and weirder. Now we're facing the mystery that there are building blocks even smaller than that.

But anyway, Einstein, it turns out, didn't just do special relativity. By the way, I really think he deserves 3 Nobel Prizes. He got it for the photoelectric effect. The fact that he didn't get it for general relativity is a crime against humanity. I don't understand. Obviously, he should have gotten it for general relativity and special relativity. I think special relativity is separate from general relativity insofar as Nobel Prizes go. So general relativity is another unification.

Yes, that's right. What Einstein realized was that if you were in a rocket ship, and it was very quiet and accelerating, it would feel like you were experiencing gravity. And so, as you say, it's one of his happiest moments when he realized that acceleration and gravity feel very much the same.

What I'm impressed by is that idea, which is already a pretty neat idea, somehow led him to take his spacetime idea, take this acceleration-gravity idea, and realize that he could describe gravity as the bending of spacetime. Spacetime being constant, like east, west, north, south, that's already hard enough. But now he's saying, “Well, take your map and crinkle it and bend it,” and so forth, and that's gravity. That is a staggering, mind-blowing idea.

I guess I wonder if you can comment on what you think is the idea-generation process that leads to that. Probably, in Einstein's case, it had to start with, “What if gravity is itself spacetime geometry?” You have to have a thought like that, right?

Yes, I think so. There's a lot about science. There's, of course, knowing what went before. There is knowing the mathematics that allows you to figure out the implications of your theory. There is the discipline to argue with yourself and other people, because most ideas are wrong. But then there's what you just described: that intuitive spark, and that is something that is very, very difficult to create.

There's a reason that we venerate these people. It is an unusual feature, and most people only have that aha moment once in their lifetime, if they have it at all. And then there's a tricky business, because I'm sure you do, and I get a lot of letters from creative thinkers who don't have all of the history and the mathematical discipline and the self-critique that's necessary. And so they come up with these ideas, and often it's easy to see where they just don't play out.

So in order to be that person who changes the way we see the world, ideas themselves are not enough. These creative ideas are not enough. You need those ideas with the discipline and the critique, and it's that amalgam of those things that makes you a genius whom history remembers.

But it's hard to know. In a field of people you might be tempted to call crazy, there could be geniuses there, and it's hard to know which is which. We should mention that Einstein himself couldn't see the genius in quantum mechanics initially—couldn't see the correctness, I should say. So he could see the insanity of gravity bending spacetime, but quantum mechanics was too weird for Einstein.

In all fairness, it's weird for me too. The thing is, even while that is true, Einstein maybe spent the last few years of his life trying to blend electricity, magnetism, and gravity into a single thing, and he was unsuccessful, but he still was a very, very valuable critic of quantum mechanics. It's not that he didn't understand it, because he did understand it. He thought about the implications and all this quantum-entanglement business—not all of it.

But he was responsible for saying, “Well, if you're right, then this.” And, of course, then people went out and found out that Einstein's implication of quantum mechanics was real, and so they could say, “See? Quantum mechanics is real.” He was thinking deeply about it, and he was doing exactly that thing I said. There's that spark idea, but there's that critique idea.

If you're able to critique an idea, you might kill it. It's always depressing when I have this brilliant idea and it gets killed, but it's better to be killed than to keep it around and waste time on it. So he was, in that case, not generating the aha. What he was saying is, “All right, let's take your aha. Let's see if it's right. What does it mean? It means this.” That allows people to go test it.

He was contributing very crucially to that other part of scientific advancement, which is not just the aha moment, but beating it to death, testing it, critiquing it, and making sure it's real. And it's only after all of that has been done that you can really assure yourself that you're right. That's why science is such a powerful tool. It is that combative, downright jerky critique that most people don't like. They don't like people saying, “Your ideas might be wrong.” But it is crucial. It is a crucial part of the scientific process.

Plus, there's that quote on the other side of it that I've heard you mention: “I believe your idea is crazy, but is it crazy enough?” Was that Niels Bohr?

I love that one. Yes, we all agree that your idea is crazy, but is it crazy enough?

And there is some degree of taking those leaps into the crazy, but it has to be backed with rigor.

Right.

And the unifications continue as we take steps toward the Standard Model, which is such an incredible part of physics in the 20th century. So can you describe that unification?

We're jumping forward now to the 1930s, or thereabouts. By that time, people had realized that there were 4 distinct forces that did not seem to be connected. 1 is gravity, 2 is electromagnetism, and those are things people are relatively familiar with. But there are 2 other forces that only have any real importance inside the nucleus of atoms, which is why most people have no experience with them.

One is the strong nuclear force, which holds the nuclei of atoms together, and the other one is what we call the weak nuclear force, which is responsible for some types of radioactivity. Since most people don't play around with nuclei, and most people don't play around with radioactivity, they don't know what that is. But by the 1930s, scientists had done enough experiments and enough theorizing to say that there were these 4 forces, and that was already a triumph.

In our goal for a theory of everything, we'd like to think that there is 1 force, which is what we're talking about: unification.

Maybe these 4 forces are just different ways of looking at a single underlying force. But in the ’30s, that’s where we were. There were the 4 forces. So we move ahead, and in the late ’50s and early ’60s, some people were thinking that maybe the weak nuclear force and electromagnetism actually were the same. So they were working on trying to bring together these 2 forces to show that they’re connected. And it came true. They were able to show that electricity and magnetism were actually 2 different facets of a single force that we now call the electroweak force.

Now, the story that you’re told in articles about this, about what people have called the Higgs boson or the God particle, is very, very simplified because in 1964, there were 3 groups with 6 individuals who came up with important papers talking about what’s called the Higgs field. And I’ll get to what that is in a minute, but the Higgs field is important. It wasn’t until 1967, 3 years later, that Steven Weinberg and some others actually unified electromagnetism and the weak force.

Sheldon Glashow, Abdus Salam, and Steven Weinberg successfully unified electromagnetism and the weak nuclear force, showing that at high energies—

Brian Greene

Right.

—these 2 forces were merged into a single electroweak force.

Right, and that was in 1967, all right? Everybody talks about this thing happening in 1964, but it really wasn’t. It happened over quite a few years, actually.

All right, so now let’s—what you said is true. Weinberg, Glashow, and Salam showed that electromagnetism and the weak force at high energies were the same. There was a problem, however, and the problem is that electromagnetism has an infinite range. We know that because we can see stars that are millions of light-years away. That shows you that the range of that force is essentially infinite.

The weak force, however, basically becomes nonexistent at distances much smaller than the size of a proton. So to say, “Oh, they’re the same,” and yet one can reach across the universe and one can’t reach out of an atom, well, that’s just dumb. The obvious thought here is, “Well, we just proved that that whole idea is stupid, so throw it away. Ridiculous.” And that is where these ideas from 1964 came in and saved the day.

So how can it be true that the electroweak force is real and electromagnetism and the weak force act so differently? The way that could happen is if these forces were transmitted by a particle moving from one subatomic particle to the other. In the case of electromagnetism, it’s the photon. In the case of the weak force, we now call them the W and Z particles.

The idea that Higgs and his colleagues came up with is this: “All right, the electroweak force is real. The way we make it so that there is now an electromagnetic force and a weak force is that the force-carrying particle of electromagnetism has no mass, while the force-carrying particle of the weak force has a mass.”

So what was done is that a field was postulated. There was this additional field that was kind of distinct from this electroweak field, and we call it the Higgs field. The Higgs field permeates all of space. Here’s the kicker: Some particles interact with the field, and some particles don’t interact with the field. The ones that interact with the field get mass, and the ones that don’t interact with the field don’t have mass.

That’s the idea: The Higgs field gives the weak-force particles mass. However, the photon laughs at the Higgs field, doesn’t see it, and has no mass.

And I should say here, going to Perplexity, the big-picture view is that the Higgs field is the quantum field that fills all of space and gives many elementary particles, just as you’re saying, their mass through their interaction with it. The Higgs boson is the particle associated with ripples or excitations of this field.

In modern particle physics, every type of particle corresponds to a field that exists everywhere. The Higgs field is one such scalar field, meaning at each point in space, it has a single numerical value rather than a direction. The Higgs field differs from most other fields because even in empty space—empty in quotes, by the way—its average value is not 0. This nonzero vacuum value is what enables it to endow particles with mass.

Right. So let’s talk about something a little more familiar just to try and hang some intuition on those words.

Mm-hmm. Awesome.

All right, so right in front of us, there is a gravitational field. You can’t see it, but right there. Right there. Check it out.

Yep.

If I were to take something, a pen or whatever, and put it there, it feels a force, and it falls. Very insightful, I know. So we have the gravity field, and we have the pen that has a mass, and the mass and the gravity field interact, and it drops.

Now, if we had another—

I have a—

Oh, yes—

—object for you.

All right, so, like—

For demonstration purposes. This is great.

Performance art. Here we go.

This is great.

This thing has mass, and we drop it. How remarkable. It falls. But when we step back and think about what really happens, it’s the mass of this thing and the interaction with this invisible field we see here. That’s what gives this weight.

Now, I have this particle here that you can’t see, but it’s there. It has no mass, and I leave it there. Well, since it has no mass, it doesn’t feel gravity. It’s still floating there. And that is really all the Higgs field is. Some particles have effectively what you could call the Higgs charge that interacts and sees the field, and other particles don’t. And that is really what you just read basically means.

It’s kind of neat because in ordinary life, there is a Higgs field right there, and the Higgs field is not 0, just like gravity is not 0, and things will get mass. But at super-high energies, the strength of the Higgs field goes to 0. So whether things have mass, or whether they have a Higgs charge or not, when the Higgs field is 0, they don’t interact. They have no mass.

So that’s kind of what Weinberg, Salam, and Glashow said: At very high energies, the Higgs field is 0. Since the Higgs field is 0, the weak-force particles don’t feel mass, and therefore they can travel at the speed of light, just like the photon does, and everything’s happy.

It is when the universe cooled down—after the Big Bang, it was very hot, at very high energy—that nothing had mass. The universe cooled, and at a certain temperature, what happened is the Higgs field turned on. At the moment it turned on, it gave mass to the weak-force particles but did not give mass to the photon.

That’s what we call electroweak symmetry breaking, so it’s a mouthful. But all it says is there was a moment in time early in the history of the universe, at 10⁻¹² seconds after the Big Bang, when the Higgs field turned on and particles got mass. So that’s the whole idea.

This is another really neat thing. The electroweak symmetry theory doesn’t need Higgs because that only really applies at very, very high energies. But in order to make it work at low energies, you need to fix the theory, and you need to fix the theory by effectively putting a Band-Aid on it. Higgs theory is just a Band-Aid on top of electroweak symmetry theory, and that is the Band-Aid that fixes it because it gives mass to particles at low energy.

But how does the Higgs—this Band-Aid, the field and the Higgs boson—come into play on the experimental front, on the evidence and discovery front? So what is this Higgs boson thing, then?

Okay, excellent. So we have never seen the Higgs field. The Higgs field is a hypothetical, theoretical thing. But that is true of most of our fields. We’ve never seen the electromagnetic field. We’ve never seen the gravity field. We’ve seen the effect of the field.

All of these theories are now what we call quantum field theories. The whole idea of quantum fields is that if you have a quantum field, that field can vibrate like a drumhead. It doesn’t vibrate exactly like a drumhead, but it vibrates locally. So you can have specific localized vibrations, and those specific localized vibrations are the particles.

In the electromagnetic field, the vibration is the photon. In the Higgs field, the vibration is the Higgs boson. So what we can do is not see the field, but we can actually excite the field, make it vibrate, and detect the vibrations.

The Higgs boson idea was predicted in 1964. It became useful in 1967, and then scientists started looking for it. So in the early 2000s, people were starting to think that we had built particle accelerators powerful enough to actually create these vibrations and detect them.

The accelerator that was working at the time was a large particle accelerator outside Chicago at Fermilab called the Tevatron. We were colliding protons and antiprotons at near the speed of light and at very high energy. That was the accelerator at which the top quark was discovered in 1995.

But we had upgraded our apparatus. We had 10 times the number of collisions per second. We had slightly more energy, and we were banging the protons and antiprotons together, hoping that we would actually find the Higgs boson.

Can you actually back up a little bit and look at the bigger picture? Fermilab has this legendary accelerator, and there’s also a personal story with you connected to it because there are a million questions I want to ask you, and we’ll ask you about some aspects of that.

This idea of an accelerator—the design and the physics of an accelerator—how is that productive for understanding and discovering different aspects of particle physics?

Well, I’m so glad you asked. This is fascinating. All right. Everybody has heard Einstein’s equation: E = mc².

Nobody knows what it means. Maybe they heard that energy equals mass and mass equals energy. But they've heard the equation, the most famous equation in all of science. Buried inside that equation is a thoroughly fascinating concept: energy and matter are equivalent, and you can, in fact, convert movement energy into mass.

This is something that we've known for a long time. This was predicted back in basically 1928, so a long time ago—almost 100 years ago, actually. It is not in the slightest bit controversial. We can do this all the time.

The simplest thing is to take 2 particles that have no structure, the closest thing you can have to BBs that are just true mathematical BBs. If you smash those 2 things together, one is coming in with a huge amount of energy from one direction, and the other is coming in with a huge amount of energy from the other direction. The directions cancel, so the net momentum is zero. You have these 2 things coming in with exactly balanced energy, and if they collide, they could stop.

That energy has to go somewhere, and that energy can literally create mass, create particles. Now, there are special rules about what happens. If you have 2 things coming together and they create a particle, they have to create an antimatter particle to balance it. That's just the rules of the laws of nature.

Why is that the case? We have some ideas, but in many respects, the answer is that those are the laws of the universe, and those are the things that we try to understand. But this is absolutely true. What particle accelerators do, among other things, is simply transform energy into particles.

Basically, any particle that doesn't exist in nature, we can make in this way. You can make the antimatter electron by taking 2 particles and smashing them together. The energy sits there, and it will make an electron and an antimatter electron. It just does, and we know that. The antimatter electron was discovered in 1932. This is all pretty easy.

The antimatter proton was discovered in 1955 at the Berkeley Bevatron. This is just what you do. You can convert energy into a matter-antimatter particle. Now, the converse is also true, and that's something we might talk about. You can take matter and antimatter and bring them together, and it'll make energy. The process can go both ways: energy can make matter and antimatter, and matter and antimatter can make energy. This is just true. We do it all the time. There's no question that this is the case.

We should also mention that this is the reason why Fermilab had a nice stash of antimatter particles. As a side effect, you can also collect antimatter in this kind of way. You can produce antimatter, but it's extremely costly—

Oh, very, very costly. At the Fermilab machine, we would have to smash 100,000 protons into something to make 1 antimatter proton. It took some work.

Is there some extremely precise recipe for being able to produce particular kinds of particles? When you smash 2 things together, is there a way to accurately control which kind of particles you're trying to produce?

If you want to make antimatter electrons, it's just easier with electrons because, to the best of our knowledge, electrons have nothing inside them, so they're simple. They have a certain mass, and that's that. If you smash particles together with the right energy, you can make them very, very easily.

It's like an old-style radio back in the day where you had to dial it in. You could get right on the station, and you could hear the signal. If you were off a little, it didn't work.

The problem for things like protons and so forth is that they're not point-like particles. They're kind of like garbage cans full of stuff, and so it's very difficult to make antimatter protons.

You can get more of them by increasing the energy at which you collide 2 particles together. If you're below a certain energy and you collide 2 protons together at low energy, you just don't have enough energy to make an antiproton, and so it doesn't happen. You get to a certain energy, and you can just barely make them. The more energy you collide them together with, the more you make. That's just sort of how it works. More is better.

Then, with CERN, if you compare maybe CERN and Fermilab—I'm going to Perplexity here—CERN's accelerator, the Large Hadron Collider, or LHC, is the world's highest-energy proton collider, while Fermilab's current and planned accelerators focus on intense proton beams for neutrino physics rather than pushing the absolute energy frontier.

Correct.

The absolute energy frontier means the highest possible energy smashing of protons together.

Correct. We were talking about accumulating antimatter, and there, that is typically making antiprotons, as opposed to making all particles in general. So let's focus on the antiproton side to begin with.

Fermilab doesn't make antiprotons anymore. We stopped making them in 2011, because we shut our big accelerator down to concentrate on a different facet of particle physics. However, at the time, we would smash protons with an energy of 120 GeV, and in that, we would make antiprotons. That's a ton of energy.

It's true that the CERN accelerator, the big accelerator, is now much higher energy than the Fermilab accelerator was. No problem. But that's not how they make antiprotons.

All of these big-beam laboratories have more than 1 accelerator. At Fermilab, there were 5 distinct accelerators, and it was basically like shifting an old standard car, because you couldn't just go from zero to super speed in 1 accelerator. You had to go from 1 to another, getting higher and higher.

At CERN, they use basically their second gear in their very big accelerator complex to make antimatter protons, and their accelerator is only 26 GeV compared to the 120 GeV at Fermilab. Fermilab's not operating, but when it was operating, it operated at an energy about 4 times higher than what CERN is doing now.

Why is that? It's because what CERN needs to do is not make as many antiprotons as Fermilab did. They are doing a very different current experimental program. They're doing a fascinating experimental program, including trying to figure out whether antimatter falls up or down, which is kind of neat. We sort of know the answer to that. That's separate.

Okay, so that's the antiproton thing, and if we get back to antimatter, we can talk about that, because that is way cool.

Yeah, super cool. But now, the other side of your thing about making high-energy unknown particles: bigger is better.

It is true that the LHC is a very high-energy machine. It is about 7 times more powerful in terms of energy per collision. It is also about 100 times more collisions per second than the Fermilab machine. So it is true that the LHC can make bigger, heavier particles than the old Fermilab Tevatron ever could. And that is true.

If you want to look at high-energy stuff, you go to CERN now, which is why many of my colleagues, including myself, once we had measured all of the frontier measurements we thought we could make with the Fermilab accelerator, saw this bigger, more powerful machine with 7 times the energy and 100 times more collisions per second. We said, "Heck yeah, let's go work on that."

To give you a sense of scale, the top quark, which is the heaviest particle ever discovered, was discovered at Fermilab in 1995. There were 2 discovery papers, and in the one on which I was a co-author, we had worked for a good chunk of between 6 months and a year collecting collisions. There were a lot of collisions, and our paper had 38 top quark candidates. 38.

We knew that half of them were crap, because when you make a detector like that, there's what you call background. You have the background and the good stuff, and we knew it was about 50/50. So we had maybe 19 top quarks after working for between 6 months and a year collecting data.

But now, at the LHC, we make a top quark every second. That's what higher energy and more collisions per second will do for you. That extra energy means you're above threshold, so you make a ton of them. When you compare the 1995 Fermilab accelerator to the current CERN accelerator, it's probably 1,000 times as many collisions per second.

It went from painstaking, pulling teeth, to, yeah, now top quarks are a background. We try to get rid of them. There's just too many of them. They get in the way of searching for the stuff we really want to search for. They are so 30 years ago.

By the way, is there something to be said about the signal processing here—how you remove the noise, how you remove the background, how you determine which particle is which? There's probably some incredible nuance there, even outside the scope of this conversation.

Let me just throw some numbers out. At the CERN accelerator, when it's operating, the collisions occur at a prodigious rate. We get about a billion, with a B, collisions per second.

Wow.

Yeah, that's what I said.

Wow.

Now it turns out some of them are happening at the same time, so there are about 40 million moments in time per second where you would take a shot. Inside that moment, there might be 20 collisions. That's why we get to the billion.

Yeah, but can you individually pinpoint the collisions?

Sort of, to a degree. The beams are…

When people think of beams, they think of laser beams, but that’s not really what particle beams look like. Particle beams look like little tiny sticks of spaghetti, except they’re much thinner. They’re not as fat as a stick of spaghetti, and at the LHC—different accelerators are different—they’re about this long.

You have one of them going one way, full of protons, and another one going the other way, full of protons, and they pass through each other. As they pass through each other, you should think of this as a swarm of bees.

Yep.

This is like a swarm of bees, and mostly the bees pass through each other and don’t do anything. But every so often, some of the bees hit nose-on, and there are stripes and wings and everything everywhere.

That’s awesome. That’s awesome.

As they collide through each other, one collision’s here, and one’s here, and one’s here. You can’t tell too much side to side because the beams are really small. They’re sort of the thickness of a human hair, but you can see along the direction. This is about the right size.

We have detectors around them, and we can actually see, “Oh, particles came from here, and particles came from here.” That’s amazing. All right, so at any one crossing, there are maybe 20 collisions.

Most collisions are absolutely boring. They’re boring because they exemplify physics that we know a great deal about already. We’ve tested it for decades. We know all about it. We don’t care. I mean, it’s kind of blasé that we can say, “Oh, yeah, yeah, we’re making a billion subatomic particles every second,” but who cares?

That’s just the way of frontier scientists. What you need is to pick out the cool ones, the weird ones, the ones that nobody’s seen before. So what happens is these beams collide, and we surround the collision point with an enormous detector.

There are 2 absolutely ginormous detectors at the LHC, one of them called CMS, which is the one I’m on, and the other one is called ATLAS—which is the other one and we don’t speak of. No, they’re both really amazing.

Okay. It’s good to know that there’s friendly competition even inside CERN.

Yes, yes.

That’s awesome.

The fact is, they are both amazing, absolutely amazing detectors.

Right. But CMS is just a little cooler than ATLAS.

Oh, yeah, yeah. In particle physics, we really absolutely want our competitors to do extremely well, just not quite as well as we do.

Got it.

Yeah, yeah.

All right.

So—

So, these 2 giant detectors.

Right. One of them, our detector, the CMS detector, is the small one. It is 70 feet long, 50 feet high, and 50 feet wide. It’s 5 stories tall, and it weighs 14,000 tons.

Small one, yep.

Small. The ATLAS experiment is 150 feet long and 80 feet across. It weighs only 7,000 tons. Just a piece of cake.

You could take the ATLAS detector, and you could put 4 of them on a soccer or football field, and it would fill the field up with just enough room on the sidelines for the cheerleaders, the water boy, the coaches, and stuff. That’s how big they are.

Nice.

These are absolutely ginormous detectors. Basically, they’re cameras, and they can take pictures 40 million times per second.

All the data comes streaming off that detector, and we can’t record it all. It would just fill up all of our tapes, and they’d be full of all these boring things we don’t care about.

So, as the beams pass through one another, we teach our detectors to say, “We only want the ones where there are certain configurations that might be interesting,” like there’s a gob of energy in the detector, or there’s a gob of energy on one side and nothing on the other side, or there are 4 gobs of energy, or whatever. These are called triggers.

We have fast electronics that take the 40 million possible pictures per second and say, “You know, about 100,000 of those are really cool. We should think about them.” Then it passes not all of the 40 million, but those 100,000, to the next level, which consists of commercial processors that have basically our final analysis code, but optimized to run very, very quickly.

They do a really quick-and-dirty analysis to further refine what’s good and what’s not. That computer farm then accepts about 1,000 collisions per second, and we record those for further analysis.

That’s what’s really happening. Of the 40 million possible collisions per second, the fast electronics and then the computers pick the 1,000. Then we pass those through analysis software and hand them to the graduate students, and they pick through them, looking for and finding the handful that are the next Nobel Prize.

That’s how that works, and that is truly astonishing. Hats off to the accelerator builders, the detector builders, the people who make the software work, and the people who make not gigabytes, not terabytes, but petabytes of data flow around the world seamlessly. It’s really amazing. I’m very grateful.

So, take me to July 4, 2012, the discovery of the Higgs boson.

This is really fun because the people searching for the Higgs were a community, and the entire community knew that the LHC was coming online. Even though many of us had been working on the Tevatron, a lot of us were transitioning to the LHC.

We were in the very funny business of wearing our Fermilab detector-people hats, trying desperately to find the Higgs boson at Fermilab—

Mm-hmm.

—while simultaneously wearing our CERN hats, knowing that the LHC was going to be able to find it if it existed. We were a little neurotic. We wanted our old stuff to work, and there were an awful lot of people on both experiments.

Did you have a sense that one of the 2 places would be able to find the Higgs first? First, did you think the Higgs boson existed? Second, did you think that these accelerators had a chance to find it?

I was cognizant of the fact that the Higgs boson might not exist, but there was a lot of evidence pointing in the direction that it might. I knew that both experiments—the Tevatron and the LHC—would either find or rule out the Higgs if it existed.

Rule out?

That’s a possibility. Maybe the Higgs theory was wrong, right? Until you know it’s there, it might be wrong. It’s like dark matter. People talk about dark matter; it might not be real. I think it probably is, but it might not be.

So, you knew at these energy levels you should be able to find the Higgs boson?

Yes. That’s the nice thing about this kind of physics, because there was a theory, and that theory made predictions.

There were parameters in the theory that we didn’t know. If the mass was this, we’d get this thing. If the mass was that, we’d get that thing. But we could do the calculation for every conceivable Higgs mass, and then we could search.

Let’s say the Higgs mass is 100 in some units. Did we see it there? No. Then it’s not 100. Let’s look at 103. Is it there? No. So we could do that.

Both accelerators could either find it or definitively rule out the predictions of simple Higgs theory, 100% guaranteed. However, the LHC had 10 times the collisions per second and 3.5 times the energy.

Remember when I said, with the top quarks, it was like 6 months for 19 versus 1 a second? There’s no question. The writing was on the wall. The LHC was going to have an easier time of it if it was real.

However, I’m a Fermilab scientist, and we wanted Fermilab to win. So we were busting our butts, and we had done what I said. We had ruled out certain mass ranges. We knew it wasn’t there.

We finally said that if a Higgs boson existed, its mass was somewhere between, if I recall, 120 and 145. We’d ruled out all the other stuff. Wearing our CERN hats, we said, “Okay, we’re going to find that.” But we were really, really trying to do it.

If we’d had another 2 or maybe 3 years of running the Tevatron, Fermilab would have discovered or ruled out—or, in this case, it turned out, discovered—the Higgs boson, because it’s a real thing. We would have found it without question.

Unfortunately—or fortunately—in 2008, the LHC had turned on. It broke, and they had to fix it. It turned on again in 2010. It ran poorly in 2011. In 2012, they pushed up their sleeves and said, “Let’s do this,” and it turned on.

Fermilab knew that if it didn’t have it then, it was too late. Anyway, 2012 rolls around, and about 2 days before the announcement at CERN—which was July 4, so 2 days before that—Fermilab made a measurement and said, “We can rule out certain regions, but there are certain regions we can’t rule out.”

What we know—and this is important—is that if the Higgs boson exists, it must be in this region that we’re not yet capable of ruling out. That’s where we were 2 days before the LHC said, “We got it.” That was July 4, 2012.

So, detecting the Higgs boson confirmed the existence of the Higgs field, the mechanism through which fundamental particles like electrons and quarks acquire mass in the Standard Model.

Correct. Although, let’s be very specific about what we did then. We found a particle consistent with the existence of the Higgs boson.

There were alternative theories at the time that predicted not 1, but multiple Higgs bosons. There’s a theory called supersymmetry, which said that there were not 1, but 5 Higgs bosons.

The standard original 1964 Higgs theory says there was one. All we really knew at the time was that we had found a particle. We did not necessarily confirm that Higgs was right. We found data that said it looked like Higgs was right, but until we ran for longer, we were unable to rule out other alternative theories. So that's the deal.

Now, in the fullness of time, after all, 14 years later, we have been able to basically rule out some of those other things, and by now we have validated things. We found the mass of the particle. We know the spin of the Higgs boson. It has a spin of zero.

We have discovered that the Higgs boson decays. It preferentially decays into the heaviest particles it can through energy conservation. It can't decay into top quarks; it's too light to decay into top quarks, but it can decay into bottom quarks. It can decay into W and Z particles, and it can decay in a weird way into photons. We have looked for all of the hypothesized decays of the original Higgs theory, and we have validated that it decays in those ways at the rates that theory predicted.

So now, in the fullness of time, I'm pretty comfortable saying Peter Higgs, Robert Brout, François Englert, and his colleagues were right back in the ’60s. But we weren't sure on July 4th. All we knew was that we had found a particle consistent with the theory. The thing is, with these discoveries, they're often just barely discoveries. It takes a while to go and do the more complex, detailed measurements, and that's what we've done.

At the time, I remember it being referred to as the “God particle.” You also had a minor in theology, so throwing that all together, calling it the God particle is speaking to the importance, the potential importance, of discovering this particle. Do you think that is, in some degree, justified? If we look at the big impact of it on the history of physics, how important was it to find and show that the Higgs field is real?

Well, I don't think it is as important as, for instance, some of Einstein's work. I mean, it was an important prediction. The prediction of quarks was very important and interesting, and validating this—the Higgs was kind of like validating that quarks existed. It's an important stepping stone, and I do not wish to denigrate it in any way, but there are discoveries that changed the way we thought about the world, like Einstein did. It wasn't that sort of thing.

There is a funny story. The reason they call it the God particle is this book by Leon Lederman, and if you read his book, he says, “Well, you know, we call it the God particle, but we should call it the goddamn particle because it's been causing us so much trouble trying to find it.” Leon ran Fermilab, and he wrote a foreword for one of my books. I talked to him, and he was a really funny guy.

The real truth was that the book was called The God Particle because his publisher thought it would sell more copies. That got into the mindset of the reporters and so forth, and we called it the God particle. Leon never really thought of it as having anything to do with religion. He was an incredible jokester: the goddamn particle.

It is a really important part of our model of the universe.

It is.

There's this field that gives mass to some particles and not others.

Right. It's a huge thing, but it was part of the Standard Model. The Standard Model had known forces, known particles; it had all that. The Higgs boson—the one thing that is true is that it was the last unvalidated piece of the Standard Model.

The Standard Model does not answer all questions, which is why we have unanswered questions in physics, but it was a punctuation point, the end of about 50 years of discovery and searching, where we finally were able to say the Standard Model, while incomplete, is mostly right as far as it goes.

We did a whirlwind tour of the history of physics and took a little tangent on this incredible discovery of the Higgs boson. But we didn't go all the way yet. There's this dream of the grand unified theory, the GUT, that's a step toward the TOE, the theory of everything. Can we talk about the GUT first? What's entailed in the GUT?

The GUT is short for grand unified theory. We talked about how there were four known subatomic forces: the electromagnetic force, gravity, the strong force, and the weak force. Electroweak symmetry unification merged the weak force and electromagnetism into the electroweak force. What GUT hopes to do is merge the electroweak force and the strong force into one grand unified force.

That leaves gravity outside because gravity is seemingly fundamentally significantly different. Subsequently, it is hoped that, at a higher energy, we will be able to blend the theory of everything together with all of the known subatomic forces—the strong, weak, and electromagnetic forces—and then gravity. As you say, GUT is sort of a way station along the way.

That's the goal, and at this point, I would have to say that I do not see fast progress in the immediate future. I think we're a ways away from that at this point.

You mean on the gravity front?

Maybe we'll come up with something really cool. We certainly had some ideas back in the early ’80s that we tested, and they didn't pan out.

Speaking of which, string theory is the thing you're referring to. String theory posits that particles are tiny vibrating strings, and by tiny, we mean extremely tiny, at the scale of a Planck length. Then there are other leading candidates, like loop quantum gravity. Maybe there are some alternate theories in the works. Can you elaborate on that a little bit more? Do you think a theory of everything exists?

I hold personally that there are rules that govern matter and energy, space and time, and they probably are rules that I don't know. There are probably phenomena I'm not aware of. But I do believe that there is a rule that governs reality. In that sense, once we understand the fundamental rules that govern reality, that would be a theory of everything.

There are things that are unknowable, like, for instance, inside black holes. We don't know what's inside there, but that doesn't mean that there's not something inside there. There's a distinction between what we can know and truth.

I do believe that there are rules, and I do believe that with sufficient time, technology, and effort, we will be able to figure this all out. Now, this isn't a thing in my lifetime. It's not a thing in my grandchildren's lifetime or even their grandchildren's lifetime.

Whoa, whoa, whoa. That's a pretty strong statement, right? That's a pretty strong statement, saying we're 50 to 100 years out from finding a theory of everything.

It took 200 years to go from unifying gravity to unifying electromagnetism. It took 100 years to go from unifying electromagnetism to unifying the electroweak force. Now, you could say, well, gee, that went from 200 to 100, so it's getting faster. But it's also getting harder because the unification scale is of order 10¹⁵, which we can do the math. That's a quadrillion times higher than the highest-energy accelerator we can build today.

We are reaching diminishing returns. We get something like a factor of 7 increase in particle accelerator energy every 20 years, and so we have to get to a quadrillion times. If you really did believe in a factor of 7 every 20 years, then we're talking like 500 years. But this is like Moore's law—it doesn't continue forever. We're not going to get another factor of 7 every 20 years. So yes, I think it's a very long time. That's my prediction. Some people are far more optimistic, and we can talk about that.

We should also—I should mention that I guess your intuition behind that is not just the part where you come up with a theory that's beautiful and seems to be internally consistent, but you have to have a theory that's making falsifiable, testable predictions. You have to have a feasible engineering construction, a methodology for creating an experiment that tests that prediction.

I think a lot of your “this is 50, 100, 200 years from now” intuition is maybe about the second part of that, which is that you need to have an experiment.

Yes. But let's say—I mean, you alluded to superstring theory. I haven't answered that question. I'll table that for a moment. Superstring theory is a fascinating idea. I don't believe it, but I love it. I hope it's true.

There's a real aphorism, and it says you should absolutely never believe what you think. So even if you think superstring theory is true, you shouldn't believe it because it hasn't been tested.

Now, let's say superstring theory is correct—hypothetically, it's 100% correct. I don't know that it's correct, so I don't care. It could be correct, but until it's validated, it's just a wild-ass guess, you know? We have to have a way of validating it.

Yes, the empirical side of it is important. You could wake up tomorrow and have a theory that is the perfect theory, but if I can't prove it, I don't care.

If we were to think—this is going back to The Great Courses on the evidence for modern physics—we're talking about energy levels and tiny particles to the degree where the kind of prediction we would be making is not accelerator-type predictions. So it's probably going to be impossible to build an accelerator that detects something like a string.

So you have to make predictions about macroscale behaviors.

That’s another alternative.

It’s a different kind of prediction. Do we even have intuitions about what kind of predictions they would be? One line of intuition, of course, has to do with black holes or the singularity. The physics of black holes combines certain elements of general relativity and quantum mechanics, so you could see some kind of predictions you could make.

But you can’t really mess with a black hole. It’s not like you can create a black hole in the lab.

The energies that we’re talking about, the sizes that we’re talking about, are inside a black hole, which you can intrinsically never see. You can only see the outside of a black hole, not the inside of a black hole.

What you said was incredibly important and incredibly correct and probably won’t happen, but that’s still good, okay? So we have 2 choices. When you talk about superstrings, either superstrings are correct and they’re making predictions up at the Planck energy scale, at which point we have to somehow build facilities that can generate Planck energies. That’s possibility 1.

Possibility 2 is that this theory, which is currently only applicable at Planck energy scales, is solved by someone figuring out a way to take those equations and solve them in a way that, say, predicts the mass of the electron.

Mm-hmm. Right?

That is a tricky business. I am not a string theorist, so I can’t tell you that’s likely, but I can tell you that they’ve been working on it since the ’80s and they haven’t gotten very far.

Furthermore, I think it’s fair to characterize string theory as still a vague idea, and that’s unfair, but let me tell you why I say that. What they have are approximate solutions to approximate equations, and that is already saying that we’re a ways away from really getting a handle on that.

So yes, there could be some bright young lad or lass out there—someone listening to this podcast right now—who figures out a way to take superstring theory and solve it in tractable ways that make predictions from the scale at which it currently applies down to a measurable scale today. If that happens, then I might retract my question or my concept. There’s a reason why I think that probably isn’t true. That’s probably not valid.

I love this. All right, so let’s back up. I wrote this book for Oxford, Einstein’s Unfinished Dream, and Einstein’s unfinished dream was to come up with a theory of everything. It was unfinished because, well, it’s unfinished.

The second part, the tagline of that book, is Practical Progress Toward a Theory of Everything, with the emphasis on practical. When you read books about theories of everything, when you see podcasts, when you listen to YouTube videos or whatever, they are often written by theorists.

Theorists are big-idea people. They’re very, very smart, but there’s a pragmatism that is often missing, in the sense that they say, “Well, superstrings, look, have these little vibrating things, and wouldn’t it be cool?” But you gotta get to the question: Do you know it? So let’s pretend superstring theory, or something like it, is correct.

The energy scale at which that should occur is of order 10^15 times higher: 10^19 GeV. We can currently do things at 10^4 GeV, give or take. So that is 10^15. That’s a quadrillion times higher energy.

What we are doing now is looking at the world with our very best measurements, and we are trying to project out a quadrillion times higher and figure out a theory that explains everything.

Suppose that you were some Joe Australopithecus 2 million years ago or something, in Africa, wandering around somewhere in Kenya. You’re about a meter in size, so you can walk a meter. The meter scale is like your scale. You can walk 10 meters in every direction—that’s 30 feet, no problem.

You can walk 100 meters, 300 feet. You can walk 1,000 meters; that’s half a mile. You can walk 10,000 meters; that’s 60 miles. 100,000 meters is 10^5, and that’s unlikely.

But the distance that we need to go from what we can see to the Planck scale—it’s not 10^5; it’s 10^15. In my analogy, think about this guy who’s walking around Africa. If he walks 100 feet or something, it looks a lot like what it is now. He can make a prediction about what he sees, and when he goes to that new place, it’s probably going to be okay.

But if he starts walking 500 miles east, walking around the center of Africa, he has no concept of, for instance, the Indian Ocean. He would never predict sperm whales or kraken. He would never predict what the bottom of the ocean is like, or what it’s like going north. He’s in Africa.

He would never, ever have a clue about the Alps or Antarctica. Going even smaller distances, going a mile up, things wouldn’t be very different. But if he goes 10 miles up, he wouldn’t breathe and he’d freeze. If he goes 100 miles up, he would die. If he goes 2 miles down, he would roast.

The point is that we are like that Australopithecus. We have a realm that we can study, and we can even predict with some validity what would happen if we go some distance away. But the farther away we go, the less and less our local prediction really represents the reality of those more distant scales.

Basically, his theory about the world would be totally bogus. So even if he had the best theory, his theory would not have anticipated the Alps. It would not have anticipated penguins. Flamingos—not there. That is just the case.

That’s what we’re doing now. We have reason to understand what we know, and we can predict a factor of 10 or 100. But I think it is the absolute pinnacle of arrogance to think that what we can do, given the understanding that we have from what we’ve measured now, is predict it out a quadrillion times higher than we can see now.

My opinion—and this is partly because I’m an experimentalist—is that the correct way to make progress, practical progress, toward a theory of everything is to look around at the things that we don’t have answers to right now.

For instance, are there some things smaller than quarks? I don’t know. Is dark matter real? I don’t know. If it’s real, what is it? I don’t know. Is dark energy real? Yes, probably, but I don’t know. What is the nature of space and time? I don’t know.

These are questions we can explore, and I would expect—and this is my prediction—that we’re going to figure things out at a factor of 10 or 100 times better than we can do now. We might be able to do that in my kids’ lifetime or something like that.

But in order for us to predict a quadrillion times higher, I’m pretty sure superstring theory is wrong. Not because people aren’t smart, but because something new is going to happen.

If you were talking about chemistry, you would have never predicted nuclear physics, and that’s a small increase in energy. The idea that there’s something in the nucleus of atoms that causes the sun to burn—there’s a reason why people didn’t believe it. They calculated how old the sun should be, and it should only be 10 million years old because otherwise it would burn out. Well, that’s clearly wrong, and it’s wrong because of nuclear physics.

That is why I feel fairly confident to say that while someone could think superstring theory might be right, and maybe it’s right, I hope it’s right. It would be awesome if it’s right. But what are the odds when you’re making something with that tiny lever arm, predicting it out a quadrillion away, and saying, “Oh, yeah, we got it right”? What are the odds? My answer is, you gotta be kidding me.

Now, I could be wrong, and I admit that I could be wrong, but that’s why I think the real issue is not the brilliance of humanity. It’s the stuff we haven’t found.

We don’t know. The simple one—and I’m saying it’s simple, and it’s not—is: What is dark matter? We don’t have a bleeping clue. Not a clue. We know a lot of what it isn’t, but we don’t know what it is.

And so, talk about superstrings. All right, well, maybe dark matter fits in superstrings, or maybe dark matter is governed by a physics that is completely diametrically opposed to the superstring concept.

Allow me a bit of a thought experiment here, a brief thought. My intuition says that when you propose a theory of everything, the kind of prediction you want to make involves a leap of conceptual understanding that Einstein made.

For example, you want to come up with something like spacetime and then gravity—

Brian Greene

Sure.

—bends spacetime. So it’s not merely that you have this beautiful mathematical framework, but that framework allows you to rethink how you see reality enough to make a prediction that’s about the macroworld.

To come up with something like spacetime, there’s one idea, for instance, that space and time aren’t real. They emerge from entropy. That’s a new way of thinking, and maybe there’s some validity to it. I want people to think about it.

But in the end, it’s just an idea, and that’s the real key thing. As you say, it has to tie to the macroworld. You have to validate it. If you don’t validate it, it’s a crazy idea.

Theorists are incredibly creative, smart, wonderfully interesting people, but I don’t care. I want a measurement that validates the idea because there are so many—if you read the journals, there are so many theoretical papers with all these nifty ideas that die.

One that I liked recently, and that might still be true, was that dark matter—our simple model of dark matter is that there’s a subatomic particle out there that’s heavy, and it’s floating around, and it’s causing gravity.

But someone said, “Well, you know, maybe there’s complex dark matter,” which means there’s a whole dark sector, so there are dark atoms, and they interact with one another. That is a nifty idea, and I love it. That was all the rage for a while, and we looked at it, and it may still be true, but the simple ideas have been mostly invalidated because we’ve tested them and they don’t work.

Same thing: there was a talk about large extra dimensions. The reason that gravity is so much weaker than the other forces was, well, maybe gravity can sneak into more dimensions than the other forces.

It leaks into those dimensions.

Brian Greene

That was a cool idea. But that’s the point: you have these lovely, cool, interesting ideas that constantly die. I would love for a new nifty idea to be the idea, but I don’t know how to pick it out of the hurricane of wrong ideas.

I mean, that’s the real beauty of science. It really is. The theories kind of get some of the glory sometimes, but the real beauty emerges from the experiment and the demonstration that the theory is correct.

There are 2 directions. You’re talking top-down: someone comes up with this big idea that’s testable. But you also have the other way that science advances, and it’s not with a theory that is then tested. It’s with, “Huh, that’s weird.”

For instance, either in the 1930s with Fritz Zwicky or in the 1970s with Vera Rubin, she did a simple thing. She said, “How fast are galaxies rotating?” Because it’s an easy thing to calculate. You could literally calculate that with high school physics. You get an answer, and then you measure it, and it’s wrong.

That’s the “Wow, huh, I don’t know what that is.” And that led to the hypothesis of dark matter. Now, dark matter is not a theory of everything, but it’s a clue. It’s a powerful clue. We should pull, tug at that thread. Maybe our entire theoretical edifice unravels, or maybe it doesn’t. Maybe it’s just a snag, and we can fix what we have now. I’m not sure.

That’s another option: to simply look at many measurements that are very precise and find ones where the outcome and the prediction with established theory disagree. That is a clue.

Before we leave the topic, we’ve got to talk about string theory. In your view, is it basically dead? As I understand, one of the primary flaws of string theory, outside of the testable experiments that we’re talking about, is because it relies on these unobserved extra dimensions. There was a hope that it uniquely could explain our universe, but it turns out this, quote, “landscape”—there’s an enormous so-called landscape of possibilities it leads to. It renders the theory basically unpredictive, because it can describe all kinds of universes, and therefore you can just select, tune it to describe ours.

I agree to a degree. But I bring it back to my prior objection. It is absolutely true that superstring theory, in its current manifestation, aside from the extra dimensions, which are some level of small potatoes, allows for an extremely large number of possible universes. But if we were able to take those predictions and somehow connect them to a physical measurement, then what we would do is lop off those alternatives. We’d throw them away, saying, “Well, you know, those are like an equation, X plus 5. I can put in any number I want in there; it doesn’t matter. But if X plus 5 equals 9, then I’ve ruled out a whole bunch of numbers except 4.”

This is a case where string theory does allow for many predictions, but if we could rule them out by connecting them to a measurement, then it would no longer do that. We would modify string theory, and we would retain the vibrating string concept, which I really, really like. I mean, I really like it. But until we can validate this, we can’t.

So now you ask, is it dead or not? In my opinion, it is very difficult to kill such a theory—really, truly kill it—because killing it means making a prediction and seeing that it fails. But what can happen, and what is happening, is that people have been working on it since the ’70s. We’re talking about 50 years of people working on it, and it has not solved the problem.

I think what’s happening is people are looking at that and saying, “Do I want to spend my life working in this direction, with the very likely possibility that 30 years from now we’ll be not much farther along than we are now?” It’s a lot like back in the 1940s, when people started thinking about the meaning of quantum mechanics. I wanted to do that when I was a kid in the ’70s, but then when I went to grad school, I realized that very smart people—people smarter than me—had been working on that for most of their lives and made no definitive progress.

You have to decide, as a scientist who wants to answer questions, do I really want to take on a question that is so hard that it will not be answered in my lifetime? I think that’s what’s happening with a lot of superstring theory. People are saying, “It’s really neat. It might be right, but I don’t want to devote my life to something where I might not see progress in my lifetime.”

What do you think about the alternate theories? Do you think there’s anything interesting in those?

You know, many of those theories are espoused by passionate people. They have fans. People love them, but they don’t do what science needs to do, which is make predictions.

Now, loop quantum gravity is a little different. That one is better developed, and that one is not a theory of everything, so we should make that clear. Loop quantum gravity is not a theory of everything. It is simply a theory of quantum gravity, period. It does not aspire to include all of the known forces.

It simply tries to take gravity, which currently intrinsically treats space as smooth and continuous. For those of your viewers who are mathematically inclined, in Einstein’s theory of general relativity, gravity is infinitely divisible. There is no smallest bit, and so essentially the laws of calculus apply.

However, it is possible that at a small enough scale, space is no longer divisible, in the same way that you can take a cup of water out of a swimming pool, and then a quarter cup, and so forth. But eventually, once you’ve taken out a single molecule of water, you can no longer take out a smaller thing. Loop quantum gravity attempts to quantize gravity. That’s what it does.

This is unlike string theory, which attempts to bring gravity in with the other forces. In fact, the fundamental reason why string theory became so interesting to the theoretical community is that string theory was not being developed as a theory of everything. It was being developed as a theory of the strong force, and it was in competition with QCD, which is the currently accepted theory of the strong force.

The 2 groups—the string theory groups and the QCD groups—competed for a while, and string theory basically failed the race. People paid attention to quantum chromodynamics, QCD. But then somebody noticed in string theory that one of the things it predicted was a massless spin-2 particle.

You can prove that any massless spin-2 particle is the graviton. If you see a theory that has a massless spin-2 particle, you now have a candidate for dragging gravity in. Then, oh my gosh, people got terribly excited, because this theory, which was working in the direction of the other quantum forces, brought in gravity. Now it was a candidate theory for everything.

But that’s not what loop quantum gravity is. Loop quantum gravity is simply trying to understand the nature of space itself, which is already a fantastic thing. I talk to Rovelli every so often. I write about his theory and point out some of the issues with the theory, but I’m usually about 2 months behind as he and his colleagues are developing it and so forth.

One of the things is that originally, loop quantum gravity predicted that the speed of light would not be universal. The speed of light would depend on the frequency of the light. High frequency would travel at 1 speed, and low frequency would travel at a different speed. It had to do with the wavelength of light basically interacting with the structure of space.

That was an issue with loop quantum gravity. If you look at gamma-ray bursts, which are super-explosions of astronomical events that are a billion light-years away or more, they spit out light in all wavelengths. If that loop quantum gravity prediction were correct, when you saw 1 of these gamma-ray bursts, you would see 1 wavelength of light appearing on Earth at a different time than another wavelength because of the different speeds.

That wasn’t the case. They appear at the same time. I went on to say, “Well, this pretty much killed loop quantum gravity,” only to get a prickly note from Dr. Rovelli saying, “You know, we’ve disproved that. We’ve changed the theory. That’s no longer true, and now that prediction, that old prediction of loop quantum gravity, is no longer valid.”

That observation of the uniformity of the speed of light no longer kills the new loop quantum gravity. It would’ve killed the old one, but it didn’t kill the new one.

By the way, that example of different speeds of light based on wavelength—that’s a beautiful thing that a theory can predict. That’s a testable thing.

It is.

Right? Those kinds of things—and if it in fact did explain a phenomenon of that sort, that’s a good sign for the theory, right? If it correctly predicted it.

There was another brilliant observation recently. I love it: the observation of gravitational waves. It was from 2 neutron stars orbiting and coalescing. They made gravitational waves, fantastic, but they also, because they were not black holes—they were neutron stars—they hit and exploded, giving off a tremendous bright flash of light.

And so astronomers saw the flash. Gravitational-wave astronomers saw the ripples of space-time. It was 140 million light-years away, which means light would have traveled 140 million years to get here, and the 2 incidents—light and gravity—both arrived within 1.7 seconds of one another. That tells you that gravity travels at the speed of light. That was a brilliant, fantastic measurement.

Now, we thought gravity traveled at the speed of light, but now we have a measurement. We proved it, and damn it, I am impressed.

Our universe is so fascinating. Speaking of which, since we brought up antimatter, we'll have to talk about it. You've talked about it in several of your lectures from different angles, including the dark energy crisis and empty space and the vacuum, and so on. So let's look at the empty-space angle. It turns out that empty space is not empty.

It's true, which is kind of bizarre.

Can you speak about what we know about what makes up empty space?

That's a hard question because we don't know what space is. But let's start out with something simple. We'll assume that space is not quantized, okay? Now, it probably is; I don't know, but we have to start somewhere. So let's start with the space of calculus—the space that you can divide forever.

The modern version of quantum mechanics is called quantum field theory, and it postulates that, A, space exists. Then it postulates that within space there exist fields for every known subatomic particle. So there is a photon field, an electron field, an up-quark field, a down-quark field—there are all the fields. Those fields can vibrate, and when they do, those vibrations are the subatomic particles. An electron field vibrating in a characteristic way is an electron.

Now, it's also possible for the electron field to vibrate not in the characteristic way, but in a way that's still vibrating, though it's not an exact electron. This is what we call virtual particles. There are lots of ways to talk about virtual particles, and the way I'm talking about them now is the most correct and sophisticated way that we can talk about them. I will talk about them briefly in a simpler way to help, but right now, the important thing is that there are these fields.

Specific vibrations are the known particles. Vibrations that are a little different are these virtual particles. They're particles that don't truly exist. So that is what we think space is: all of these fields. They're all vibrating a little. If you insert the right amount of energy, you can get a field to vibrate in the characteristic way and make that subatomic particle. But even when you don't, the fields are there and they are vibrating. Those vibrations are what we call virtual particles.

Now, your viewers may have heard of virtual particles in other ways, in which case it says that space is just empty, and what happens is that matter and antimatter particles briefly appear for a very short period of time before they coalesce and disappear, reemerging into the field. These are both correct. What quantum field theory says is that these ripples are appearing and disappearing, or these particles are appearing and disappearing.

That just sounds nuts. You look at empty space, and you're not seeing anything happening. But they're happening fast enough that they can't be seen, yet they do have consequences. There are 2 experimental measurements that I can think of that validate that this thing that sounds crazy is really happening, and 1 is called the Casimir effect.

In the Casimir effect, you take 2 metal plates, parallel plates, and put them near one another—very, very close. If these virtual particles exist, then between the plates, particles are appearing and disappearing, and outside the plates, particles are appearing and disappearing. However, because these plates are close to one another, this puts a constraint on the wavelengths of the particles that can occur between the 2 plates, because the particles cannot extend outside the plates.

Short-wavelength particles can exist between the plates, but the longer-wavelength ones cannot. However, outside the plates, there is no constraint, so short-wavelength and long-wavelength particles can exist there. The net effect is that there are more virtual particles outside and fewer particles inside, and therefore you have a net pressure that would push those 2 plates together. That is a prediction we've been talking about, and guess what? It happens. Those plates push together. So that is a validation for the existence of these particles in empty space.

Now, there is another measurement, and this changes the magnetic properties of particles like the electron, the muon, and so forth. This was discovered in 1948. If you take old-school, standard quantum mechanics, you know the spin of an electron and its charge, and you can calculate its magnetic moment. It comes out to a number. If you do the measurement, what you find is that the measurement disagrees with the 1930s quantum-mechanical prediction by 0.1 percent, and that was measured in 1948.

People went, “Huh.” This happened at the Shelter Island Conference in New York. On the way home, someone who saw this measurement thought about it, and they invented what we now call quantum electrodynamics. Old quantum mechanics quantizes matter. The second quantization quantizes both matter and the fields—in this case, it quantizes the electric fields.

In this quantized field, it predicts that surrounding a bare, say, electron, which is spinning and has a charge, there is this bath of virtual particles appearing and disappearing all around it. The ensemble of all those particles appearing and disappearing will alter the magnetic properties that you can measure for the subatomic particle, and it changes them by 0.1 percent.

We have measured this, and we have not measured this imprecisely. We have measured the magnetic properties of both the electron and the muon to 12—count them—12 significant figures. The theory and the data agree number for number for 10 places. Once you get out to the very end, where both the theory and the data have some imprecision, they then disagree. Maybe there's some interesting stuff going on there. But 10 figures—it's just staggering.

So virtual particles refer to matter and antimatter particles coming to life.

Correct.

Can we just talk about the antimatter part of that? Starting with Paul Dirac, this is 1 of the most legendary examples of mathematics leading to physics: the math suggesting that something like antimatter should exist, and Paul Dirac taking it seriously and eventually showing that it does exist. What evidence do we have for antimatter?

Antimatter was predicted in 1928. Paul Dirac was trying to merge quantum mechanics and relativity because the original Schrödinger equation was not relativistic. In doing so, the equations were complex, but in the end it came down to something like E² = 1. You take the square root of both sides, and you get E = +1 or −1. The +1 was the electron; the −1 was something. He didn't know what it was.

For a while, people thought maybe it might be the proton, but that didn't seem to work out. So he insisted that his equations were right and that there was an antimatter—he didn't call it antimatter—but a positively charged sibling of the electron, what we now call the positron, the antimatter electron.

It was predicted. It was discovered in 1932 by Carl Anderson and his student Seth Neddermeyer. They saw an antimatter electron, and that was pretty cool. So right there, they knew it was real. Antimatter was predicted, it was observed, and that's that.

In 1955, the antiproton was created, and that required a large particle accelerator with high enough energy to make it. That was done at Berkeley. A year later, the antineutron was discovered. At this point—and now jumping ahead to today—we can use energy from smashing particles together to make antiprotons and positrons. We have gone so far as to make antimatter helium nuclei. We have made 2 antiprotons and 2 antineutrons, combined them together to make an antimatter helium nucleus. This has been done and observed, no question.

At CERN, they have gone so far as to make antimatter hydrogen, or antihydrogen. They take a beam from 1 of their lower-energy accelerators and make antiprotons. They collect them, slow them down, and cool them to almost absolute zero. They take Sodium-22, which makes positrons. They slow them down, bring them together, and coalesce them, making literal antimatter hydrogen atoms with an antiproton surrounded by a positron.

They have done incredible measurements. They have agitated the atoms and caused them to emit light. They have looked at the light that comes out of antimatter atoms. The question is: Does the light coming out of antimatter hydrogen atoms have exactly the same spectral characteristics as ordinary hydrogen, which we predict it does? The answer is that it does.

The tests have been staggering. We now know a great deal about antimatter hydrogen. Recently—around 2023, I believe it was—an experiment called ALPHA at CERN made antimatter hydrogen, put it in a bottle, released it, and watched which way it would go. Did it fall up, or did it fall down?

Because, while it makes sense, maybe, to think that antimatter falls up, in the same way that, with Coulomb's law, electric charges might attract or repel.

However, there were ample theoretical reasons to believe that antimatter would also fall down. So they did this fantastic measurement. First, they put in hydrogen, and they calculated that if they did this, something like 80% of the hydrogen atoms would fall through the bottom of the bottle, and 20% would go through the top, just because gravity is very weak and the atoms will escape wherever they can. But there will be a bias pulling hydrogen atoms down.

They did exactly the same thing, and what did they find? They found that antimatter falls down. Now, they do not have a good enough measurement at this time to say that the gravity antimatter experiences is 100% that of matter. What they have measured is that antimatter fell down with 75% of the strength of regular matter, but there were big uncertainties.

There was plus or minus 0.13 due to the experiment, which was good but imperfect, and plus or minus 0.16 due to their theoretical model. So it's like 0.75 plus or minus something like 0.29. That means there's a good chance it's between 0.5 and 1, which means it's consistent with 1. So they are improving their measurements.

Well, if I can, I would love to take a bit of a tangent on that topic because I went down a rabbit hole watching some of your videos on antimatter. Fermilab was the hub for the production of antimatter for quite a while.

I saw that NASA said that the global estimate for the current rate of production of antimatter is 1 nanogram per year. Can you speak to how hard it was to make antimatter? You also mentioned in a video that if matter and antimatter meet, they produce a lot of energy. I think 20 grams of antimatter is equivalent to a 1-megaton nuclear warhead in terms of explosive energy. So, all of those questions together: how hard is it to produce antimatter?

It's freaking hard. So here's the deal. Until 2011, Fermilab was the most powerful antiproton production facility on the planet. Every 2.3 seconds, we would smash 10^13 protons into a target, and we would get out 10^8 antiprotons.

Basically, in order to get a single antiproton, we needed to smash 100,000 protons into material. Every 2.3 seconds, we would get on the order of 10^8 antiprotons. We would collect them over the course of 12 hours or so, and in the end, after collecting them and cooling them down and so forth, we would have on the order of 10^12 antiprotons every 12 to 24 hours.

Ten to the 12th sounds like a lot. It really does. That is a trillion. But you need to remember that a gram of antimatter is 10^23 antiprotons. That means that over the course of a day, we were able to create something like 1 hundred-billionth of a gram. If we did that for a year, then that would be about a nanogram, give or take. That's a reasonable estimate.

A nanogram—1 billionth of a gram—means that at that rate, with that facility, it would take 1 billion years, running with very little downtime, to make a single gram of antimatter. If you combine 1 gram of antimatter and 1 gram of matter together, the energy release is equivalent to the combined Hiroshima and Nagasaki explosions. If you wanted a megaton, you would need about 25 times more, so you would have to run for 25 billion years to get a megaton of explosive power.

Let me lay it all out, because I think it's pretty interesting. This is a NASA estimate of how much it costs to produce antimatter. Looking at all the costs of the accelerator, everything combined together, to do enough for a 1-megaton antimatter bomb, if such a thing were even possible—on the order of 25 grams, like we mentioned—would cost about $1.5 quadrillion based on the NASA estimate.

By the way, NASA wasn't talking about a bomb. That's just me adding that. NASA was talking about the estimate of $62 to $63 trillion per gram of antimatter, actually, which is what they're referring to. Compared to that, I was looking at estimates for how much it takes to produce a 1-megaton nuclear warhead. Everything combined is about $10 million to $50 million in the United States. So you're talking about a difference, in terms of a weapon with equal power, of $50 million versus $1.5 quadrillion.

To me, what's interesting is that weapons are just one indication of this. One other possibility, and NASA also writes about this, is the use of antimatter in propulsion systems, just like you can use nuclear fission and maybe even nuclear fusion down the line. In propulsion systems, I saw that 1 gram can help get us to the Alpha Centauri star system if we can get to 0.2 times the speed of light in 20 years. That would mean it would take us 20 years to get to Alpha Centauri.

Is any of this a possible future—the use of antimatter for the generation of energy? We should mention that it's extremely compact. It has the obvious downside that it's extremely costly to produce, and we don't know how to do that at that kind of scale. The upside is that it's compact.

It's very powerful. The short answer is that it is not a physics problem; it's an engineering problem. I have people for that.

Well, yeah.

But no, the truth is that antimatter, if you are able to assemble it and store it, would be able to heat up matter and shoot it out the back of a rocket. It would do what rockets do, make us go quickly, and that would be fine.

And we should mention that the thing you just mentioned is correct. One of the hugest challenges is containment—

Oh, 100%, yeah.

—because antimatter, when it comes in contact with matter—

Is a problem.

Right. So if you were unable to contain your trip to Alpha Centauri for even a millionth of a second, boom, that would not be good. It reminds me of Star Trek, where Scotty says, “Captain, look, we're losing containment. It's gonna blow.” And that's exactly what would happen.

The short answer is yes: antimatter, in principle, we could make and use as a source of energy. But there are probably far less expensive sources of energy. It depends on what you need to do.

The Voyager probes are still chugging along with plutonium. They're running out of energy at this point, but we could presumably do a somewhat better job if we needed to. I like the idea of antimatter, but the reality is the danger—not the obvious danger of weapons, but the danger if you wanted to be in a ship run by antimatter. If it ever got loose, you would never know it. That would be that.

The reason I find this kind of inspiring is that antimatter is in a space of physics that has a lot of mysteries. There's a lot of exploration to be done. This connection to energy means that if we have a bunch of breakthroughs on the antimatter side, that might lead to a better propulsion system and better energy-generation systems.

In principle.

There's some combination of engineering here, but there's also some combination of understanding the fundamental physics.

We know how to do this. We take energy and make antimatter. You have to contain it, store it, and do all the hard things. But I would be shocked if there was some new addition to the theory that made antimatter production easier.

Interesting. So we know how to produce antimatter with accelerators. You're saying there aren't breakthroughs in physics that could lead to different mechanisms for the generation of antimatter?

You have to concentrate energy. That's it. If there's another way to concentrate energy, that would work too.

And our best knowledge of how to concentrate energy is the accelerator.

Remember, we're talking about concentrating it into volumes the size of a proton. If you concentrate it to the size of your thumb, then it's really the density that matters—the local density. When you smash 2 protons together, all of that is occurring in a tiny, tiny volume, so it's the local density of energy that matters.

If you had a lot of energy in a thimble or something, it's probably not dense enough. It really has to be in close proximity for that to happen, and then when it does, it's okay. If there's another way, we know how to make that density with accelerators. If someone has a bright idea on how to make highly dense energy, then, yeah, making antimatter is a piece of cake. But that's the crux: concentrated energy.

Yeah, and how to do so in a cost-efficient manner, not for trillions of dollars.

Well, yeah.

So one of the big mysteries with antimatter is the bigger “why.” Where is the antimatter that should be there if the whole idea is that any time you generate matter, you generate the same amount of antimatter? Yet when we look out into the observable universe, it seems like there's not antimatter there, for the most part.

Correct.

So what do we understand about this mystery? What are the possible explanations as to why?

There's this thing called baryogenesis. As you say, reiterating a little bit of what you just said, these are both Einstein things. Einstein says that when you take energy, you make matter and antimatter in equal quantities, and Einstein says that after the Big Bang, there was a lot of energy in the universe, which should have made matter and antimatter. We only see matter. Where did the antimatter go?

The answer is we don't know. However, there are some ideas, and there's a lot of thinking on it. In fact, Fermilab is doing an experiment right now with neutrinos, trying to better understand what it was that made matter and antimatter not be the same.

Now, we do have a measurement of how much different it should be, and it’s kind of neat. We can do this by counting the number of protons in the universe, just looking at galaxies and so forth. Then we can look at the cosmic microwave background, which is sort of the aftermath of the Big Bang, and count the number of photons from the cosmic microwave background.

With a little bit of math, we can say that somehow, in the early universe, something made a very, very tiny asymmetry, so that for every billion—billion with a B—antimatter particles that existed in the universe, there were a billion and one matter particles. The billions canceled, annihilated, destroyed each other, and that extra one that’s left over is us.

And so what physics mechanism made that ever-so-slight asymmetry is not understood. There are some thoughts. One thought is that it’s just how it was: when the universe was formed, there was an asymmetry. It was not made by matter and antimatter.

Another possibility is that there are various theories, all under the word baryogenesis—“baryo” coming from the word baryon, which basically means protons, and “genesis” meaning the creation of something. We say that simply because the protons are the heaviest particles. Baryogenesis is just the creation of matter.

There are a number of theories in quantum mechanics that say that matter and antimatter can oscillate back and forth into one another, and there is a slight asymmetry in how that happens. We know that this is true to a degree. We measured it in the 1960s with a different form of matter—not protons, but a type of ephemeral matter that only exists in particle accelerators.

We know that there is a slight difference between matter and antimatter, but it’s not enough. It doesn’t explain that. We’re not sure.

So at Fermilab, we have this idea, which kind of turns things on their head. It’s not baryogenesis; it’s leptogenesis. Leptons are the electrons, and because Fermilab is currently the world’s most powerful neutrino accelerator, and neutrinos are leptons, there is this idea.

Now, leptogenesis is incredibly complicated, but the idea is that it’s possible. We know that neutrinos actually change their identity. There are 3 different types of neutrinos, like cats, jaguars, and tigers. If you have a beam of just cats and go along a little while, you find cats and jaguars and then tigers, and then they’ll be back to all cats again.

This oscillation is called neutrino oscillation. We’ve known this has been true since 1998, and what we’re studying is a beam of neutrinos and another beam of antineutrinos. We’re going to study the oscillation behavior of the 2 of them.

It is possible—it is unlikely, but it is possible—that the 2 of them will oscillate at slightly different rates. If the neutrinos oscillate at slightly different rates, then that, along with several other highly improbable things, could tie together and might explain why there is more matter in the universe.

If I were going to bet the farm, I’d bet that they oscillate at the same rate. But I don’t know, and you don’t know until you do the measurements, so that’s what we’re doing. There are some other experiments trying to measure it right now, so there’s a big race between the Fermilab group and another group in Japan to see who gets there first and makes this measurement. We will find out.

If it turns out, though, that there is a difference in this oscillation rate between matter and antimatter, it will be a huge clue in this very, very difficult puzzle. I wish I could tell you I knew what the answer is, but literally nobody knows. That’s the thing about being a research scientist like me: if you’re not confused, you’re not doing your job.

So there is this desperate—or not desperate, exciting—search for this tiny asymmetry.

Yes.

It’s so crazy to think that everything we see around us is a result of this tiny asymmetry, that there was this gigantic annihilation of matter and antimatter in the early universe, and this is just some little accident. But, yeah.

Yeah, that’s crazy.

It’s a happy accident.

That is totally crazy.

This is one of the areas of physics where there’s a lot of mystery. Okay, so can we pull at that thread a little further? Let’s talk about our intuition of what dark energy is as it connects to empty space and everything we’ve been talking about. What’s the cleanest definition of dark energy?

Dark energy is either energy of space or energy in space. The most common statement is the energy of space, and it is essentially a repulsive form of gravity. We believe this is real, and the reason we believe this is real is from observation.

This is one of those things we talked about a while ago, where I said that you can think about theoretical stuff and try to come up with a measurement, or you can make measurements and see where they disagree with predictions and let that lead you in a direction.

Back in the late 1990s, some astronomers were looking at the expansion rate of the universe. The Big Bang occurred, the universe is expanding, and the universe is full of matter. Matter attracts, so the gravity due to the matter of the universe should slow the expansion of the universe. The only question was, how much?

There were 3 possibilities. The first was that there was so much gravitational force that the expansion of the universe would slow, stop, and be pulled back together in a Big Crunch. The second was that the universe would continue expanding, slowing down, but never really stopping. The third possibility was the exact critical case, where expansion would slow forever and approach 0 only at infinity, never quite stopping or reversing.

Those were the possibilities: door number 1, 2, or 3. So they did the measurement, and what did they find? It was door number 4. The universe was not only expanding, but the expansion was speeding up. The only way that could happen, given that gravity slows it down, is if there was a repulsive force. The name we give to that repulsive force is dark energy.

This is something that Einstein postulated early on in his development of general relativity. At the time, he knew that his theory predicted that the universe would collapse, but he believed the universe was eternal and unchanging. He needed something to counterbalance that collapse, so he invented dark energy. He didn’t call it that; he called it the cosmological constant.

A few years later, Edwin Hubble discovered that the universe was indeed expanding. Since the universe was no longer static, Einstein said, “No need for the cosmological constant,” and took it back out. He thought it was a dumb idea that he had put in and was embarrassed.

However, in 1998, it became clear that his original idea—that there should be some sort of repulsive form of gravity—was real, and it was put back into the theory. That’s what it is. We are pretty confident at this point that the expansion of the universe is speeding up, and the thing driving it is dark energy.

Now, what is dark energy? I don’t know. As I said, the most common thought is that it is the energy of space itself, but it is at least conceivable that there is a field in space, where space exists, and that field is pushing space apart. That’s another possibility that I’m not sure we have the instrumentation to distinguish, but that’s not what people normally think. People think it is literally a property of space.

But there is what you call the worst prediction in physics, which is a nice—

Oh, yeah. That’s another one.

—a nice little insight about the complicated nature of dark energy. The observations, as you describe, say that empty space has a tiny energy density that accelerates the expansion of the universe. But quantum field theory’s prediction for what vacuum energy should be when coupled with gravity is much larger.

This is what makes for the quote—you have a video on this—“the worst prediction in physics.” Can you explain this crisis?

Well, there’s a measurement, and you can measure how fast the universe is expanding. From that, you get a measurement of dark energy.

However, if you then say, “Suppose the dark energy is due to fields in space,” that’s quantum field theory. I know a lot about quantum field theory. We can take quantum field theory and calculate what the density of energy is due to quantum field theory.

Basically, what you do is take, within a volume, all of the wavelengths: the longer wavelengths, the shorter wavelengths, the shorter and shorter wavelengths. You can add them all up. Each wavelength adds a certain amount of energy, and if you add that all up, you get a number.

That number is the rather embarrassing 10^120 times—that’s a 1 with 120 zeros after it—bigger than the measurement of dark energy. You go, “Yuck, that is not fun at all.”

That is because the equation comes to the highest energy, or the smallest wavelength particle that you can imagine, to the 4th power. Anything to the 4th power is a big deal, so that’s where you get that awful number.

Now, if it turns out that there is some new physics just about at the energy scale we can measure using our biggest particle accelerators—remember I told you that the maximum energy scale, the Planck scale, is 10^15 times bigger than what we can measure now—let’s say that we don’t have to calculate up to the Planck scale because something happens, something changes, at the energy that we know right now.

That means we don’t have to integrate to the Planck scale. We integrate to 10^15 less than the Planck scale, and this thing is to the 4th power, so 10^15 to the 4th power is 10^60.

So now, even if we say, “Don—he’s brilliant; he’s going to find something at the LHC tomorrow that’s going to solve all these problems,” then we’ve solved it—it’s much better. It’s only different by 10^60, which is still pretty big. So the short answer is, there is very clearly something going on, something very badly wrong in quantum field theory. We have to have—maybe there’s another field that balances out the energy, that cancels it down.

Even that isn’t so outrageous. You could imagine another field. We have matter and antimatter; they balance pretty well. Maybe there’s something going on that could cancel that out. That would be perfect.

But canceling something to zero is easy because +1 and −1 make 0; +2 and −2 make 0. But we still have dark energy. Dark energy is a little bit, so if it cancels, it doesn’t cancel exactly, because there’s a little bit of dark energy left over. That is its own curiosity. Perfect cancellation is pretty easy. Theorists do that 8 times before breakfast. Imperfect cancellation is much harder.

Just to elaborate that a little bit, what do you think “solving” dark energy would look like?

What you would do is hypothesize that there existed some other field that had the reverse effect of existing quantum fields.

But not to zero.

But not to zero.

How would we then demonstrate the existence of that field?

That would depend on the prediction.

How do you even come up with a new field?

Like all theorists do. Let’s add something to my equation and see what happens. And that’s okay. I’m being glib about that, but that is precisely what you do.

You say, “What changes? We have this thing that works quite beautifully except it fails here. What is the addition that we need to make that changes very little in the realm that we’ve measured and yet fixes this hard thing?” And so you literally just go, “Okay, what do I need? Plus X or something.” As long as it makes no changes where it would hurt our measurements and fixes the big thing, then that is at least a candidate theory.

Now, that doesn’t mean it’s right, but it at least gives you an understanding of what the right answer should look like. And so that’s the first step: what should the real answer look like, or what is a possible real answer? Once you know that, other people can look and say, “Let me think about a theory that has the required properties to do what we need it to do.”

It’s a multistep process, but the first step is: how do we tame this problem without coming up with really terrible predictions that we’ve already ruled out? That is literally a sensible, viable theoretical thing, because you have to explore cool ideas.

One of the reasons dark energy is super interesting is that it gives us a mechanism by which we can talk about the deep future of the universe. We have observations about the expansion of the universe, but it’s also giving us the mechanism for that, right?

David Tong

Sure.

So any weirdness—any good model we have that captures some of the weirdness of dark energy—might give us insights into how this thing ends, about the deep future of the universe, right?

Absolutely. As it stands right now, if dark energy is real—and who knows?—if it’s real exactly as we’ve measured it, then as the universe gets bigger and bigger, dark energy becomes a bigger and bigger component of the energy balance of the universe. It takes over, and it drives the continued accelerated expansion of the universe.

If dark energy gets lower for some reason that we don’t understand—maybe it changes over time and gets smaller—that could change things. If it gets bigger, it could change things.

That is one of the big open questions: whether it’s constant over time or not.

Right. There has been a recent measurement that suggests that dark energy is getting smaller. However, that is a new measurement, not confirmed. Nobody should believe it, but it’s a hint that maybe it’s changing, which is kind of cool in itself, because the current belief until recently was that dark energy is constant.

Now, I want to be super careful because it’s misleading. People say dark energy is constant. Dark energy is a density. Think about that. You have a certain density; let’s start with that. Then the universe expands. Energy is volume times density. If the universe gets bigger and the density is constant, that means dark energy is increasing.

It’s not just increasing as a fraction and overwhelming ordinary matter. Ordinary matter, as the universe expands, has a decreasing density because it is constant and the volume gets bigger, so the density drops. Dark energy, until recently, was thought to have a constant density.

So that’s what’s implied when you say “constant”: constant density, which means it’s actually increasing because space is increasing. The size of space—

David Tong

Right.

—is increasing. Interesting.

And so that’s a weirdness, and that then ties into the nature of space. Why does that tie into the nature of space? Well, because if dark energy is a field in space, if you increase the volume, you would think the energy density would drop.

But if space is increasing and space is quantized—and I don’t know if it is—then maybe what’s happening is space isn’t stretching, but little space particles are appearing as space expands. There are bubbles of space appearing, and each bubble contains a certain amount of dark energy. Therefore, that would give you a sense that dark energy is a property of space rather than a field in space. But that’s all very hand-wavy, guesswork-y stuff.

So if you had to bet all your money, is dark energy a real physical—what does that even mean?—thing that exists, or is this just a renaming of the cosmological constant?

Unfortunately, I think it’s both. It is describing a reality, but it’s also maybe telling us something about space.

Literally a property of space.

Yeah. That’s kind of what it looks like. Given that it seems to be constant density, that seems to me—now, this is not something anybody should believe. Please, nobody believe this—but it seems to me that this is leaning toward the idea that, A, it’s a property of space; B, space is quantized; C, as space is expanding, little quanta of space are appearing; and D, each one of those quanta has a certain amount of energy associated with it. That would kind of explain the constant density.

But a lot of what you just said is probably experimentally testable. You can probably experimentally construct the bubbles of—

David Tong

Well, finding the bubbles of space—but those quanta conceivably are Planck-sized bubbles.

Yeah, the quanta.

David Tong

Well, they’d be quanta of space. The idea is, you look at a sand dune and it looks smooth and continuous, but you can see individual grains of sand, right? And so what this is saying is, as this dune expands, new grains of sand are appearing, and each one of them is a quantum of space.

So what kind of experiments can we do in the coming decades or centuries to understand dark energy better?

People have been talking about quantum entanglement of gravity. In standard quantum mechanics, a particle can be in 2 places at the same time. Now you have 2 particles. This particle can be in 2 places at the same time, and this particle can be in 2 places at the same time, and you put them near one another.

If they’re close to each other, there’s a certain gravitational force; if they’re far apart, there’s another. If one is close and one is far, you have yet another. You can calculate the effects of gravity having to do with quantum-entangled particles being in 2 places.

People are talking about doing this and trying to see if, in making such a measurement, they might be able to definitively determine whether gravity is a quantum phenomenon or a continuous phenomenon. That is potentially a measurement that could be done soon-ish, because the technologies inherent in all of this recent work on quantum mechanics are allowing people to make instrumentation that might be precise enough to do this measurement.

Now, this will not tell us what quantum gravity is. It will not tell us anything, but it will tell us that gravity is quantized. And just knowing that—for one thing, it shuts out a whole realm of continuous gravity, and the theoretical community will then turn its attention, forget this stuff, and think over here.

That doesn’t tell you that space is quantized, but it tells you that gravity is quantized, if it bears out. So if gravity is quantized, then people will start thinking more about space being quantized.

I have to ask, because you mentioned dark matter is perhaps even more mysterious than dark energy.

Okay.

Can you build up the intuition for why it’s more mysterious? What is dark matter?

Oh gosh, what is dark matter? A: I don’t know. B: It’s terribly fascinating.

The first thing, and the most important thing because I’m an experimentalist, by God, is: why do we believe there’s dark matter? The reason is that astronomical measurements do not agree with predictions by Newtonian or relativity theory. Galaxies spin too fast, clusters of galaxies move too quickly, and the distortion of very distant galaxies due to the gravitational field of nearer galaxies disagrees with the prediction from what we see from the observed matter.

So there are 3 very distinct reasons why we are predicting that something is wrong in our understanding of either the laws of physics or the matter budget of the universe. The easiest one to talk about is spinning galaxies. What I'm saying is not unique to spinning galaxies, just easiest to talk about. Galaxies are observed to spin more quickly than they should if we add up the gravity we see.

By all rights, galaxies spinning that fast should blow themselves apart, and they don't. So what can be the answer? You have the force required for a star to orbit and move in a circle, and you have the force due to gravity. They're connected by an equal sign, and the prediction is wrong. So either the force due to gravity is wrong, the force needed to move in a circle is wrong, or the equal sign is wrong. This is really simple: one of those things is wrong.

One possibility is simply that Newton's law of gravity—mass times mass over R² times a constant—is just wrong. Another possibility is that Newton's F = ma, which we are taught in introductory physics, is wrong. Both of those are eminently possible. Over here, maybe we don't understand gravity, or maybe there's more mass than we can see. It's nice that you can look at this really simply and come up with a list, a cookbook of things we can test. And so we've done that. We've gone and said, “What are the possibilities?”

The most obvious possibility is that there is more mass than we can see. There are black holes, hydrogen gas that we can't see, whatever—there's something out there. So that was the first thing. You go and look, and there's no hydrogen gas because we can see that with radio waves. That's not it. In the ’90s, we went looking for black holes, rogue planets, things like that. Those exist, but not enough of them. That's not it.

Now we're left with there's some sort of matter that we can't see, or we don't understand gravity, or we don't understand inertia. If you asked me this 25 years ago, I would've said the most likely answer is that we don't understand inertia or gravity. If you asked me 25 years ago, that's what I would've said, no problem. However, there have been a couple of observations that have caused me to change my thinking, and I think that dark matter is more likely.

One of them is called the Bullet Cluster. In the Bullet Cluster, there are 2 large clusters of galaxies. In these large clusters of galaxies, any galaxy consists of a couple of components: there are the galaxies themselves, there is the hydrogen gas that surrounds the galaxies, and maybe there is dark matter. If dark matter is real or dark matter is not real, you will get different answers if those 2 clusters pass through one another.

The galaxies themselves should pass through one another, basically not interacting, but the big thing is the gas clouds. If there are big clouds of gas, as the galaxies pass through one another, the clouds should interact, and the gas clouds should stop in the middle and be really, really hot. If there were no dark matter, you would see a cluster of galaxies, a cluster of galaxies, and a big gas cloud in the middle. Because the big gas cloud in the middle is much more massive than the galaxies themselves, you would expect to see distortions that we call dark matter distortions in the middle.

If, however, dark matter is real, the galaxies pass through one another, the cloud stops, and dark matter doesn't interact with the cloud, so it passes through. In that case, you would expect to see the distortions where the galaxies are. And that's what we see. The Bullet Cluster is strong evidence in my mind that dark matter is a real thing.

There is another example, which is much more recent. The Bullet Cluster was a while ago. It's called the Dragonfly galaxies. There's DF2 and DF4. These are galaxies that rotate exactly according to Newton's laws. The fact that they rotate exactly according to Newton's laws says that whatever's causing galaxies to rotate too fast is not a property of matter.

If you had a galaxy where there was no dark matter because, for whatever reason, it got stripped off or something, this is one of those lovely ironies: the existence of a galaxy with no dark matter is very strong evidence that dark matter is real because you can take the dark matter out. DF2 and DF4 also suggest to me that dark matter is real. So now, while it remains possible that we need to modify the laws of inertia or the laws of gravity, those are still possible. In my opinion—and now this is Don's opinion, but it's probably the opinion of most of the scientific community—dark matter is likely a real thing.

Now, that's great. I've taken you all the way to dark matter. So now you're going to ask me, you're going to say, “Don, what is dark matter?” I'm going to go, “I don't know.” But I know what it isn't. I know that it is not black holes. I know that it is not rogue planets. I know that we've done the measurements. We've looked across nearly every mass range for compact objects and ruled them out. So if dark matter is real, it can't be made of those.

So then you're left with the idea that dark matter is a particle, and that's what we've thought. The name for the dark matter particle that we've called for a long time is a WIMP, for a weakly interacting massive particle, and we have spent the last, God, 30 years looking for them in various ways. There are 3 ways that we might see dark matter.

The direct way says that dark matter exists literally everywhere—in this room, in our laboratory—and the dark matter is passing through the Earth like a wind. We put up detectors trying to see it. We have done that, and we've seen nothing.

So we should say we have done that for neutrinos.

We've done that for many different types of dark matter. We simply put detectors in labs deep underground, and we can see neutrinos in them, it's true, but dark matter, especially heavy dark matter like these WIMPs, has a different signature, and we've seen no evidence of dark matter interaction in these detectors.

Got it.

So neutrinos are also weakly interacting and also have mass, but—

They are—but not enough, so WIMPs are—

Heavy on the M.

Right. Neutrinos are indeed WIMPs of a sort. Now, we have to be careful what we mean by WIMPs. They are weakly interacting massive particles, but we can calculate, and there's just not enough mass in them. It's not it.

Got it. Got it.

So we need another form, and we have seen zero evidence of this wind of dark matter through the Earth. Another possibility is to look where you think dark matter might be concentrated, at the center of galaxies. If dark matter exists and there's antimatter dark matter, maybe they annihilate and make photons. So we look for gamma rays and various other signatures of annihilating dark matter, and there are always constant announcements: “Oh, we saw it. Oh, we didn't.” You know, it's—

The problem is that way of looking for dark matter is hard because there are other ways of making, for instance, gamma rays, like neutron stars and stuff. You really need to understand the details of galaxies really, really well to believe that.

Then the final option is what I do, where we smash particles together at high energy. We try to make dark matter particles. If you make dark matter particles, because they don't interact except via gravity, they escape from your detector. What you're seeing, what you hope to see, is an event where you collide particles, a dark matter particle escapes, and you don't see it, but you see the recoil on the other side because momentum is conserved. So you see a blob of energy on this side, nothing on the other side. Maybe that's dark matter, and that also happens with neutrinos.

You need to understand everything about neutrinos and calculate how many of those you see, then hope you see more, and then that might be dark matter. Again, that hasn't worked. So we've ruled out some dark matter particles, but the range of possible masses, if dark matter is of a particulate form, ranges from something like the mass of an asteroid to far lighter than an electron and everywhere in between. We have looked. We've ruled out some little spots in that phase space, but that's a big range.

Is it really possible to miss a particle the size of an asteroid?

The astronomical searches were not sensitive to that level of dark matter, but then you would expect that there would be some of those in the solar system. If they're what we think, like asteroids or something, then we'd heat them up, and we'd eventually see them. But if they're really, truly dark matter and don't interact with matter, they wouldn't absorb energy from the sun, so they'd be really dark. I don't know. Maybe they're out there.

The only way we searched for them was a thing called microlensing. If you have a distant star and a massive object passes between that star and your eye, that star will momentarily brighten. So you just look for what they call microlensing events, and you count them, and you see some. We did see some. Black holes pass in front of stars, and we've seen them, but we just haven't seen enough.

For very low-mass particles like asteroids, they just wouldn't make enough of a brightening effect to see. There's a minimum sensitivity to brightening, and that's about a third the mass of our moon. That's about the sensitivity that we had. Nobody, I think, really thought that these low-mass guys were likely. What they thought was more likely was that they were just unseen black holes, which I thought—

I think is completely reasonable. And then, when that got ruled out, I thought, “Okay, modified gravity or inertia.” Well, now the Bullet Cluster and Dragonfly seem to have ruled that out. So I'm stuck in my head with dark matter seems to be real, and we don't know what it is.

And it makes up a giant percentage of matter in the universe.

It is—

What is it?

Five times more prevalent than ordinary matter.

Wow. This is incredible.

It is incredible.

This is so fascinating.

And that's why it's cool. So if someone out there is a young person who wants to get into this, understanding dark matter is a big deal. I mean, it's five times more prevalent. The problem is, as I told you, if the mass is ranging from an asteroid to far lighter than an electron, if you get on an experiment that looks at one little range of mass, maybe you weren't the lucky guy who measured the right place.

And that's one of the reasons why, as fascinating as I think it is, I'm not doing dark matter experiments, because if you make an experiment that searches one mass range, it'll be blind to another mass range. So what you need is many groups doing all sorts of radically different experiments, exploring all sorts of parameter space. And with all that said, until you see it, there still is the possibility that maybe we don't understand gravity or inertia right. You can't rule that out.

If there is dark matter out there, you're hoping it's actually somehow detectable.

I don't know what it is. I think it's cool. It's very, very fascinating. That is one thing I really do hope will be understood in my lifetime, because I'd like to know the answer to that.

And that's the thing that you could legitimately see a discovery of.

You gotta get lucky, though. I mean, you gotta look in the right place, whatever it is.

Just imagine.

Or you have to come up with that really cool theoretical idea that everybody's overlooked, which is another possibility. And there are people who are really, really religiously hating dark matter, largely because we've looked so hard for so many years, and the experiments in today's world are a million times more sensitive than when I was a starting student—and they still haven't seen anything.

And that's why people really hate dark matter. Some of them do because they think we should have seen it by now, but I don't know.

I'm a sucker for direct observation. Indirect is obviously also really great, but direct—just imagine pointing your telescope in a certain direction and, because of some artifact of cosmology, being able to directly detect a giant amount of a thing that you could say is dark matter.

Yeah, you would see it orbit; things would orbit it.

Yeah.

Or it would eclipse things in front of it—

Yeah, like in an obvious way. Because some of the stuff you mentioned with DF-2 and DF-4, those are brilliant indirect deductions that there should be something like dark matter. But some obvious blocking, occluding, this kind of thing.

We did that in the ’90s with experiments called MACHO, OGLE, and some others. They looked for a black hole that you just can't see—a black hole you can't see. It's perfect. It's a perfect candidate for dark matter. And if there's enough of them out there—now remember, there's five times the number of stars—which means there's a whole lot of freaking black holes out there. We should have seen them, and we didn't.

What a grand mystery. We've covered so many of them. I could talk to you for a thousand more hours, Don. Let me ask you a little bit more on the personal side. You have a really inspiring life story. Your folks didn't go to college. Can you just tell me about your childhood and where you found the love for physics and science, and maybe how you found your journey to become a physicist, given the context of where you came from?

I grew up a poor kid in the boondocks. I had great parents, but they weren't ones who could guide me terribly academically; they were very, very nurturing. My mom would laugh that she could stop helping me with math after about sixth or seventh grade. But they were supportive.

There were a couple—three things, I think, that folded into it. One is that I was a voracious reader as a kid. I loved science fiction. I would read a book a day. It drove my mother nuts because she would try to be nice—she'd buy me a book, and I'd say, “Thank you,” and the next day it'd be done. It just drove her completely nuts.

Science fiction is good for fostering imagination, and so that's precisely what it did. In addition, and this is where the more serious science came along, there were lovely science communicators who were popular in the 1970s: Isaac Asimov, Carl Sagan, and a guy by the name of George Gamow. They wrote books about science aimed at a layperson. As a kid, I surely couldn't read a textbook and understand it, but I could read and get a hint of what science was.

And on top of that, as most people who became scientists are, I was irrepressibly curious about everything. I had a quasi-philosophical mind. I was interested in questions that have in the past been theological, then philosophical, and now are more scientific: How did the universe come into existence? Why is the universe the way it is? Why are the laws of the universe what we see them to be? Was it created? How will it be destroyed?

These are big questions that have bothered humanity for thousands of years. You said I had philosophy and religion minors in college, and I did, because I was curious about that. I was hoping that learning that history might help me understand these questions. And it was in college where I came to realize that the answers I was searching for were not to be found in those directions, but I still learned about how those questions have been asked in the past.

And so I became a scientist, and the only question was whether I was going to be a cosmologist, an astrophysicist, or a particle physicist. When I had to make that decision, it was the mid-’80s, and at the time there were a lot fewer cosmology measurements. There was an awful lot of thinking about the universe and not enough measuring. Whereas with particle physics, by God, you could do experiments.

What attracted me was the ability to actually get an answer and not just mull over what an answer might be. And so I became a particle physicist. It was difficult without having family mentors or anything like that, but I managed.

And that actually is why I'm here and why I have spent a fair bit of my time writing books and so forth, because I figure that there has to be some other kid out there in Iowa, Kansas, Montana, somewhere out in some little town without a lot of access to the kinds of things that people who have highly educated parents have access to. I'm hoping that some of them will have read some of the things I've written and will find their own path forward, because I found it very rewarding over the years.

I've been doing this long enough that I'm sure this is true. I've had kids come up to me at the lab and say, “Hey, I'm a summer intern because I saw your video or read your book,” or whatever. So I know that at least I've made a small impact. I always would like to do more, and I appreciate the opportunity that your audience affords me, because I think it's important to talk about these things.

These are really cool, fascinating questions. They are unanswered, and they are just waiting for youngsters to come and spend some time thinking about them, because one of your viewers might be one of the people who answer these questions that have stymied very smart people for decades.

And we should also say that you're a legit scientist. We'll mention Sean Carroll, who's a legit scientist, a legit physicist, but is also a good science communicator. Anyway, I did want to mention—I don't know if this is true, but I kind of heard you talk about this—that when you first showed up to Fermilab, you were working crazy hours, working extremely hard, from 8 AM to midnight.

I did.

First of all, I love that. Can you speak to what drove you and maybe the value of hard work in those contexts, in your early career, when you discover a thing you're passionate about?

Obviously, being smart matters. If you're Einstein, maybe you can slack, I guess, although even he didn't do that. But I'm not Einstein. The fact is, when I was young and unencumbered—no family, no kids—I couldn't imagine anything I wanted to do more.

Some people want to go out to the club. They want to play soccer or something. But I wanted to make measurements, and I wanted to understand and learn, and that was fantastic. So as a graduate student—and this isn't for everybody—I worked outrageously.

From Monday through Saturday, I would be at the lab voluntarily because I wanted to be there from 8 AM to midnight. On Sunday, I would work from 8 AM until about 5, and that's because from 5 to midnight I had to wash clothes, buy groceries, and do things like that.

I loved it. And I still love it. I can't do that anymore, but that's simply because I have other obligations. Had I been rich, I would've done the same thing. It's something I truly, truly loved.

There is absolutely nothing more fascinating to me than having a hard problem and figuring it out. And that work ethic—well, there are a couple of things that separate smart people from no-kidding scientists, because all scientists are smart.

But the thing that separates them, that many scientists have, is a drive and real grit. For me and for so many scientists that I know, trying to measure something and having it not work just ticks me off, and I am not going to let the universe in my lab or whatever beat me. Some people, if the thing breaks, say, “Oh, man, that didn’t work.” A lot of people say, “Well, I’m going to go home. I’m fed up.” No, it would just make me mad, and I’d put more effort into it.

I was crazy. I worked long hours. But I think the people who are really good at this will do maybe not that much. Some people have to have a better life than that. But for a lot of people, it’s just that you can’t imagine not knowing the answer.

When you see that as an older guy—not maybe to that degree, but when you see that kind of drive, that intensity of trying to get the answers—you know that person’s a winner. So if some student out there, if it doesn’t bring you joy, as uh, what's her name? The Japanese girl says, "If it doesn't bring you joy, then it might not be for you." Then you could be a person who reads about it and is involved. But if you want to be a real scientist, it has to be just part of who you are.

And by the way, it is a hard life, but it is also a very fulfilling one. So working hard toward the thing you love is a really fulfilling way to be.

I think that’s true for an artist or something—anybody, a musician. A musician, they just keep practicing because it is who they are.

Mm-hmm. Well, I’m glad there are people like you at a place I admire, like Fermilab, one of the many places in the United States and the world that is carrying the beacon of great science and great engineering forward. Don, thank you so much for everything you do, for all the teaching you do online, for all the incredible physics work that you do at Fermilab, and thank you so much for talking today.

Thank you for having me.