0:00 - The following is a conversation with Don Lincoln, a particle physicist at Fermilab, who has spent decades working at the frontier of high energy physics. This was a mind-blowing and inspiring conversation. Don turned out to be one of my favorite people to talk to about physics. Truly a unique mind with that Richard Feynman ability of taking very complicated ideas and explaining them simply, without losing any of the essential, brilliant insights at the core of those ideas. This is a Lex Fridman podcast. To support it, please check out our sponsors in the description where you can also find ways to contact me, ask questions, give feedback, and so on. And now, dear friends, here's Don Lincoln.
0:49 In describing the search for a theory of everything in physics, you describe 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, that you can think of the effort of physics as one is trying to unify- ... the laws of nature. So I was wondering if we could talk through the history of unification- ... that lens of physics. - There are, of course, lots of different ways to do physics, but the way that I would say that particle physicists, cosmologists do
1:36 is they are trying to really find basically the underlying principles that govern the laws of nature. If we go back, say, to the, I don't know, 1650s or so, you're the most brilliant person around, and you've noticed two things. One 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 sort of 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-
2:21 ... the brilliant thing was when Newton looked at that and he thought about, maybe the moon is falling, but it's missing the Earth. So what we had is that in maybe 1650, you had 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. But at that time, that wasn't at all obvious. And in fact, if you look in the books, Newton's theory is Newton's law of universal gravity. The universal is there, and the reason is because he realized these two things that seemed to have nothing to do with one another were indeed one and the same. I mean, this is absolutely brilliant. I mean, Newton
3:07 is arguably one of the most brilliant humans of which I'm ever aware. But at any rate, it is the first sort of easily to describe unification of physics that you can state in a way that sort of makes sense to modern- ... Humans. I mean, you can go back farther than that, where people are talking about chemistry, the nature of atoms. You go back to Democritus, who was wrong about very many things, but the idea that there was a smallest particulate form of matter is right. So it's kind of funny. You talk-- You read the chemistry books, and they say that the idea of atoms goes back to Democritus, and, you know, he, his idea was that, like, there was a smallest atom of oil, which was smooth, and it was smooth, of course, because, well, oil is smooth. There was a smallest atom of vinegar because vinegar is tart and it pricks your tongue-
3:54 ... so therefore, atoms were little sharp, pointy things. Um, and so he was wrong about a lot, but he was right about the idea that there was a small particle, and we now know a very-- we have a very different concept than he did. So you can go back farther than that. But getting the unification, there are more examples. For instance, if you go back to, say, 1830 or so, scientists were trying to understand electricity, for instance. And there was a lot going on. People really understood things. But at the time, you would have two phenomena that are familiar to us now. One is a magnet, which, you know, at the time, mostly magnets were simply little pieces of iron that had been magnetized and they
4:42 could stick to steel. And then you had electricity, which was at the time they were generating little sparks that they could play and, and have fun with, or more broadly, a lightning bolt blazing across the sky. And so when you think about this, that lightning bolt and that little magnet seem to be really unrelated. Um, 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. You know, they, they-- There was a whole bunch of experiments and there were a lot of names. But in about the 1860s or so, James Clerk Maxwell took all of those ideas that had been percolating around for the previous 50 years and wrote his laws of
5:30 electromagnetism, and they're really fascinating. If you look at the laws of electromagnetism, they are-- they're differential equations or inter- integral equations. But basically, what they say is on one side, you have a bunch of terms that have electricity in them, and then you have equals on the other side, a magnetism thing. So forgetting all of the mathematical symbols, you have electricity side equals a magnetism side. Electricity equals magnetism. And that is a staggering concept, the fact that these two things, a lightning bolt and the magnet that holds your kids' art to the refrigerator, are one and the same. And this was another case where electricity and magnetism became unified into electromagnetism. So now we have two examples: one, gravity being unified,
6:18 terrestrial and celestial gravity, and then electricity and magnetism. So I'll tell you about some more in a moment, but one thing that's kind of important because the goal is, of course, to unify everything. That 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. So if you look at Darwinian evolution- ... that was a very beautiful theory that captures another layer of reality of, like, how this particular thing that we see here on Earth happens.
7:07 - Right. - So when we talk about 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, is astonishing- ... that that works. So that is amazing. But for at least the class of scientists that I am, what we think of is, well, sure, biology is interesting and all, but when you get right down to it, it's, it's caused-- whatever happens in biology is caused by the movement of molecules. And then you say, "Well, that's great and all, but molecules, they do what they do because they're made of atoms."
7:53 And then the next step is, well, you know, 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. And so 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 and interesting and abstract things are. But what is at the very, very bottom? And 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. So there are the various subatomic forces of which we're familiar and for instance,
8:41 electricity and magnetism are components of electromagnetism, which then governs the behavior of things like-- This is amazing. Electromagnetism explains, of course, electricity, magnetism, but it explains how light works. It explains how much of chemistry works. So electromagnetism, 1860 or '70 the wonderful thing about that is if you take Maxwell's equations and you 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, which 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
9:29 wave then moves. And if you do the math, you find out that the speed at which these waves move is the speed of light. And so people said, "Wow, the speed of light comes out of those equations." And that had to be, I think, very persuasive. And of course, 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. So it's a staggering advance in science to have a good behavior on that. And of course, being able to,
10:19 to tame electromagnetism is why people can hear you when you do your podcast because through the miracles of the internet just-- or just electricity running the computers. I mean, this is a case if I can get on a small soapbox, where people back then said, "Well, why are you messing around with magnets and sparks?" And who cares? Well, that very fundamental digging into the laws of nature has spinoffs, and it 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
11:06 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. And the type of science I do now, people often ask, "Well, 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 but just being a little more pragmatic, if I go back, say, a hundred years, where people were trying to understand how the protons and neutrons inside atoms held together, how they split, how they, how you could combine them and so forth, this has led to nuclear power. Now, whatever you think about nuclear power, and
11:52 some people like it and some people don't, but it is powerful. It will generate energy for humanity, and it may be 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. And one enormous source of energy that is there for us to take if we so choose is the modification of the nucleus of atoms. 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 of how we do that and if we want to, but it gives us something that we didn't have before.
12:38 - Yeah, it's very clear that nuclear fusion and nuclear fission will unlock huge amount of energy that's required for a civilization to flourish, but that's almost like near term. Longer term, you can think about things like we'll talk about dark energy crisis- antimatter. Maybe if you figure out some of the mysteries around antimatter, that too would lead to energy sources, how to produce energy. That too might lead to counterintuitive propulsion systems- ... for us humans to travel through the universe. Now, right now it seems far-fetched, too expensive, too complicated, too difficult, but
13:24 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. And then we as a civilization kinda 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,
14:12 what science is doing is working out, finding 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, and that's why this needs to be a-- you know, 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.
14:58 - So that means that's the thing that makes us human in part is looking at a thing and saying, "How does this work?" And then together a bunch of apes get together, like poke the thing, kinda shake the thing. And then over time you have rockets going all into space. You build roads and bridges. You build the internet. Anyway, so we talked about Newton, we talked about Maxwell. That takes us into 20th century in terms of unification. There's a guy named Einstein- ... on whom you wrote a book, who did- ... 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,
15:45 where he showed something that makes no sense to anybody who's not really dug into it very hard, and that is that two people experience time differently. Time, you know, 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
16:30 different speeds with respect to one another experience time differently, which is absolutely a mind-blowing concept. Now, most people think that Einstein then said, well, he invented spacetime, that, 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. And so that's kind of a staggering thing. So that is where Einstein and Minkowski really did this
17:20 unbelievable concept that 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 keeps, 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 backwards that turns out to be a little more difficult. But yeah, Einstein's understanding that that is the case, it caused everybody
18:06 to think about the world very, very differently, and that was in 1908 when Minkowski really laid it out in a strict spacetime. - And that also led to the work on special relativity, led to the speed limit, the speed of light. - Well, it was a premise. He had two premises. One was that the laws of nature are the same for everybody, so if you're moving at some speed, or if I'm moving at some speed, I can say I'm not moving and saying you're moving at some speed. That's not controversial. That is what we call Galilean relativity. It's from hundreds of years ago. But what Einstein said that was controversial was that everybody measures that the speed of light is the same. Irrespective of
18:52 how we're moving with respect to each other, you'll measure the speed of light to be a number, I'll measure the speed of light to a number, and that's very, very different from what Newton would've said or Galileo or any of the old guys. And it was taking those two things together that caused all of the weirdnesses of special relativity. Now, you could then very easily say, well, that second premise that everybody measures the speed of light to the same is just dumb, and that, you know, you could test that. So that's where testing relativity comes in. And Einstein's equations, which include those two assumptions, it predicts the behavior of everything perfectly well. Now we've actually measured, done experiments where we can say that the speed of light is the same for
19:41 everybody. That's not how that's been in the beginning. It was really that assumption leads to predictions. The predictions are true, so the assumption is true. Now, there is a-- for those people, for your viewers who wanna say, "Well, how do you measure that the speed of light is the same for everyone?" The particle physicists do this, and the way you do this is the following: There are some subatomic particles that when they decay, they emit light. That's their decay product. And so you collide two things together so you know when the particle was created, then you have surrounded your collision point by a detector, and you measure how long it takes
20:26 for light to get to your detector, and by God, it's the speed of light, which it should be. However, sometimes in these collisions, some of these subatomic particles you make are coming out at very high speed. They might be coming out at ninety-five or ninety-seven or very large fraction of the speed of light, and then they decay into photons. And so you measure how long it takes for the photon to get to your detector, and it says it's-- light travels at the speed of light. Now, if it were that if Einstein's conjecture was incorrect, you'd have a particle coming out at near the speed of light, it would be decaying into a particle traveling at the speed of light, then that particle should have traveled at, say, two times the speed of light or something like that, so it
21:12 should have taken half as much time to get to the detector, but it doesn't. So this is a hard, serious measurement that shows that something, you know, we can measure the speed at which light comes out of this stationary created particle, and it's the speed of light. Then we can measure what the speed is of it coming out of something that's moving, and it's still the speed of light. So that is an actual measurement, but that is not something that was possible in Einstein's day- ... but it is now. - Just to take a small tangent, how weird is it in the full ranking of weirdness that is physics, how weird is it that there's that speed limit of the speed of light? - Well, I have to tell you, when I first encountered this, it's pretty freaking weird. It peaks the weird meter. But as you become more familiar with
22:03 it, as you become more comfortable with the idea, the thing to remember is the speed of light, it's the speed of light through spacetime. Once you embrace that, that 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 isn't that shocking. It just simply says that it's a property of space in the same way that there is, you know, space can transmit a certain strength electric field. It can transmit-- 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
22:50 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 at least for me, that once I accepted that, it all became very comfortable. - So the nature of my question actually here- ... that will apply over and over is trying to empathize, trying to put ourselves in the shoes of the people before space and time are unified- ... into space time and really experience and think through how difficult of a leap is that. - Huge. - The reason I sort of say that is we are now in the modern day in the twenty-first century, and of course, we're gonna have to make leaps like that in our future.
23:36 So what are the unifications we're not seeing in front of our eyes? So for example, there's so many examples through 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 space-time unification- ... is a good warm-up as we march through the twentieth century 'cause 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 chloride. Sodium is an explosive metal. You put it in water and it's kind of
24:23 neat. You put it in water, and it just, it doesn't quite explode, but it gets hot, and it pops around. Chlorine, it's a gas. It's going to kill you. So these two things are deadly. They're awful. And yet when you mix them, you put it on your food at night. It's salt, right? And so this is a case where, where this whole, A, unification and B, this deeper understanding, in this case of chemistry, of how—Two things that are dangerous can be brought together and turned into something not only innocuous, but necessary for human life. And so this is not unusual, what you're describing. I mean, when you think about it, forget about everything else, just the fact that, you know, we tell little kids, little kids, that the world is made of atoms. Now,
25:12 that's crazy. Most people have never seen atoms, and yet nobody really doubts it anymore. And I think it's just a case of familiarity, and then the culture slowly accepts it, and it's then, 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, 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 towards quarks. That, in the course that you mentioned, is one of the most fascinating things of this philosophy of atoms being around for a very long time. Then you concretize and you actually can prove or have
25:57 strong observations that indicate that there is atoms, and then there is a nucleus, there is electrons, there is photons, there is quarks and I mean, it gets weirder and weirder, and now we're facing the mystery is there's building blocks even smaller than that. But anyway, Einstein, turns out, didn't just do special relativity. By the way, I really think he deserves three Nobel Prizes. He got it for photoelectric effect. The fact that he didn't get it for general relativity is a crime against humanity. I don't understand. Obviously should have gotten it for general relativity and, and special relativity. I mean, I think special relativity is separate for general relativity in as far 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
26:47 and the rocket ship was a very quiet rocket ship and it was 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 space-time idea, take this acceleration gravity idea, and realize that he could describe gravity as the bending of space-time. Space-time being constant like east, west, north, south, that's already hard enough. But now he's saying, "Well, you know, take your, your map and crinkle it and bend it," and so forth, and that's
27:35 gravity. That is a staggering, mind-blowing idea. - I guess, I wonder if you can comment on what do you think is the idea generation process that leads to that. So probably an Einstein case has to start with what if gravity is itself space-time geometry? You, 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,
28:21 and that is something that is very, very difficult to, to create. There's a reason that we venerate these people, is because 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, from creative thinkers who don't have all of the history and the mathematical discipline and the self, you know, 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
29:09 we see the world, ideas themselves are not enough. These creative ideas, that's not enough. You need it with the discipline and the critique, and it's that amalgam of those things that you know, make you a genius that 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 space-time, but quantum mechanics
29:56 was too weird for Einstein. - In all fairness, it's weird for me too. But... ... but the thing is, even while that is true, and Einstein maybe spent the last few years of his life trying to blend electricity and magnetism and gravity in a, 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, you know, 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, that Einstein's implication of quantum mechanics was real, and so they could say, "See?
30:42 Quantum mechanics is real." So, you know, 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. And if you're able to critique an idea, you might kill it. And that is—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. And 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 it's right. What does it mean? It means this." That allows people to go test it, and so he was contributing very
31:28 crucially to that other part of scientific advancement, which is not just the aha moment, but the beat it to death, test it- ... critique it, and make sure it's real. And it's only after all of that has been done that you really assure you're right. And that's why science is such a powerful tool. It is that, that combative, just downright kinda jerky critique that most people don't like. They don't like people saying, "Uh, your ideas, you know, might be wrong." But that is-- 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, which is you know, "I believe your idea is crazy, but is it crazy enough?"
32:15 Was that Niels Bohr? - I love that one. Yes, yes, we all agree that your idea is crazy, but is it crazy enough? - And there is some degree of taking those leaps of crazy, but it has to be backed with rigor. - Right. - And the unifications continue that as we take steps towards the Standard Model- ... which is such an incredible part of physics in the 20th century. So can you describe that unification? - So, you know, we're sort of jumping forward here now to the 1930s or thereabout. And at, by that time, people had realized that there are four distinct forces that do not seem to be connected. One is gravity, two is electromagnetism, and those are things people are relatively familiar with.
33:01 But there are two 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 nucleus of the atoms together, and the other one is what we call the weak nuclear force, which is responsible for some types of radioactivity. And 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 '30s, scientists had done enough experiments, done enough theorizing to say that there were these four forces, and that was already a triumph. I mean, we, in our goal for a theory of everything, we'd like to think that there is one force, which is what we're talking
33:48 about, the unification. Maybe these four forces are just different ways of looking at a single underlying force. But in the '30s, that's where we were. There were the four 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 two forces to show that they're connected. And it came true. They were able to show that electricity and magnetism were actually two different facets of a single force that we now call the electroweak force.
34:37 Now, the story that you're told in articles about this, about what people have called the Higgs boson or the God particle, the story is very, very simplified because in 1964, there were three groups with six individuals who came up with important papers talking about what's called the Higgs field. And I'll get back, get to what that is in a minute, but the Higgs field is important. But it wasn't until 1967, so three 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
35:25 force that showing that at high energies- - Right - ... these two forces were merged into a single electroweak force. - Right, and that was in '67, all right? Everybody talks about this thing happening in '64, but it really wasn't. It happened over quite a few years actually. But all right, so now let's-- what you said is true. So 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, And we know that because we can see stars that are millions of light-years away. I mean, that shows you that the range of that force is essentially infinite.
36:15 The weak force, however basically becomes nonexistent on distances much smaller than the size of a proton. So that, you know, 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. I mean, 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?
37:02 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 call them now the W and Z particles. So the idea is that Higgs and his colleagues came up with is saying, "All right, electroweak force is real. The way we make it so that there is now an electromagnetic force and a weak force is the force-carrying particle of electromagnetism has no mass. The force-carrying particle of the weak force has
37:52 a mass." And so what was done is a field was postulated that there was this additional field that was kind of distinct from this electroweak field, and we call it the Higgs field. And the Higgs field permeates all of space. And 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. And so that's the idea, is that the Higgs field gives the weak force particles mass. However, the photon laughs at the Higgs field, doesn't see it, and it has no mass.
38:40 - And I should say here, going to Perplexity, the big-picture view, 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, empty space, its average value is not zero. This non-zero vacuum value is what enables
39:29 it to endow particles with mass. - Right. So let's talk about something a little more familiar just to try and hang some, some intuition on those words. - Mm-hmm. Awesome. - All right, so right in front of us, there is a gravitational field. Now- ... 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- - Demonstration purposes. This is great. - Performance art. Here we go. - This is great. - This thing has mass, and we drop it.
40:16 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 and that is really what you read just basically means. Now, it's kinda neat because in the
41:02 ordinary day, there is a Higgs field right there, and the Higgs field is not zero, just like gravity is not zero, and things will get mass. But at super high energies, the Higgs field, the strength of the Higgs field goes to zero. So whether things have mass or whether they have a Higgs charge or not, they have no or they have the Higgs charge, Higgs field zero, they don't interact. It has no mass. So that's kind of what, uh, Weinberg and Salam and Glashow said, is at very high energies, the Higgs field is zero. Since the Higgs field is zero, 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
41:49 was very hot, very high energy. Nothing had mass. The universe cooled, and at a certain temperature, what happened is the Higgs field turned on. And at the moment it turned on, it gave mass to the weak force particles, did not give mass to the photon. So 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 to the minus 12 seconds after the Big Bang, the Higgs field turned on and particles got mass. So that's the whole idea. So this is another really neat thing. So the electroweak symmetry theory doesn't need Higgs because that only really applies at very, very high energies.
42:36 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 the theory. 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 then the Higgs— this Band-Aid, the field and the Higgs boson come into play on the experimental front, on the evidence- - Okay. - ... discovery front? - Right. - So what is this Higgs boson thing then? - Okay, excellent. So we have never seen the Higgs field. 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
43:23 gravity field. We've seen the effect of the field. And so all of these theories are now what we call quantum field theories, and that-- the whole idea of quantum fields, if you have a quantum field, but that field can vibrate like a drumhead. And so it doesn't vibrate just 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. And so what we can do is not see the field, but we can actually excite the field, make it vibrate, and detect the vibrations. So the Higgs
44:11 boson idea was predicted in sixty-four. It became useful in sixty-seven, 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 to be able to create these vibrations and detect them. So the accelerator that was working at the time was a large particle accelerator outside Chicago at Fermilab called the Tevatron. And we were colliding protons and antimatter protons at near the speed of light at very high energy, and that was the accelerator at which the top quark was discovered in ninety-five. But we had upgraded our apparatus. We had ten
44:58 times the number of collisions per second. We had slightly more energy, and we were banging the protons and the antimatter protons together, hoping that we would actually find the Higgs boson. - Can you actually back up a little bit and look at the bigger picture? So Fermilab has this legendary accelerator that there's also a personal story with you connected to it because, I mean, there's a million questions I wanna ask you, and we'll ask you about some aspects of that. So 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 I mean, this is fascinating. All right. Everybody has heard Einstein's equation E equals MC squared. Nobody
45:46 knows what it means. Maybe they heard that energy equals mass and mass equals energy. I don't know. You know. But they've heard the equation, the most famous equation in all of science. But buried inside that equation is a really thoroughly fascinating concept, that energy and matter are equivalent, and you can, in fact, convert movement energy into mass. And so this is something that we've known for a long time. This was predicted back in basically 1928, so a long time ago, actually almost a hundred years ago. And it is not in the slightest bit controversial. We can do this all the time. So the simplest thing is to take two
46:34 particles that have no structure, so, you know, the closest thing you can have to BBs that are just true mathematical BBs. If you smash those two things together, it's coming in with a huge amount of energy from one direction, a huge amount of energy from the other direction. The directions cancel, so the net momentum, the net energy of this has no motion. So you have these two things coming in with a, you know, exactly balanced energy, and if they collide, they could stop. Well, 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 two things coming together and it creates a particle, it has to create an antimatter
47:19 particle to balance it. That's just kind of the rules of the laws of nature. Why is that the case? Well, we have some ideas, but in many respects, the answer is because those are the laws of the universe, and that's the things that we try to understand. But this is absolutely true. So what, what particle accelerators do, among other things, is simply transform energy into particles. And so basically, any particle that doesn't exist in nature, we can make in this way. You can make the antimatter electron by taking two particles, smashing them together. The energy sits there, and it will make an electron and an antimatter electron, and it just does. And we know
48:05 that. The antimatter electron was discovered in 1932. This is all pretty easy. The antimatter proton was discovered in 1955 at the Berkeley Bevatron. And so this is just what you do. You can convert energy into a matter-antimatter particle. Now, the converse goes true, and that's something we might talk about. You can take matter and antimatter and bring it together, and it'll make energy. It's the the process can go both ways. Energy can make matter and antimatter. Matter and antimatter can make energy, and this is just true. We do it all the time. There's no question that this is the case. - And we should also mention that this is the reason why Fermilab had a nice stash of antimatter particles. So
48:52 as a side effect, you can also collect antimatter in this kind of way. You can produce antimatter, but it's at an extremely costly- - Oh, very, very costly. In order at, at the Fermilab machine, we would have to smash 100,000 protons into something to make one antimatter proton. So I mean, it took some work. - Is there some extremely precise recipe of being able to produce particular kinds of particles and all this kind of stuff? When you smash two things together, together, is there like how can you control accurately which kind of particles you're trying to produce? - If you wanna make antimatter electrons, you smash together energy at a certain-- It's just
49:39 easier with electrons because the electrons, to the best of our knowledge, have nothing inside them, so they're simple. They have a certain mass, and that's that. So if you smash particles together with the right energy, you can make them very, very easily because you can-- It's like a 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, the signal, and if you were off a little, it didn't work. The problem for things like protons and so forth is 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. Now, you can get more of them by increasing the energy at which you collide two particles together. If you're at below a certain energy,
50:25 the-- and say you collide two protons together at kind of 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, the more you make. So that is just sort of how it works. More is better. - And then with CERN, if you compare maybe CERN and Fermilab, going to Perplexity here, CERN's accelerator, the Large Hadron Collider, LHC, is the world's highest energy proton collider, while Fermilab's current and planned accelerators focus on intense proton beams for neutrino physics- - Correct - ... rather than pushing the absolute energy frontier. Absolute energy frontier meaning highest possible energy smashing of protons together.
51:16 - Correct. So one, we were talking about, like, accumulating antimatter. All right? And so 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. All right? So Fermilab doesn't make antiprotons anymore. We stopped making them in 2011, and it's 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. So that's a ton of energy. It's true that the CERN accelerator, the big accelerator,
52:04 is now much higher energy than the Fermilab accelerator was. No problem. But that's not how they make antiprotons. The-- A-all of these big beam big laboratories, it's not one accelerator. At Fermilab, there were five distinct accelerators, and it was basically like shifting an old standard car, 'cause you couldn't just go zero to super speed in one accelerator. You had to go from one to another, getting higher and higher. Well, at CERN, they use one of the, 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
52:51 an energy about four times higher than what CERN is doing now. And why is that? Well, it's because what CERN needs to do is to 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 does antigravity fall up or down, which is kinda neat. And we sort of know the answer to that. That's separate. But anyways, so getting back to the antiproton business- ... while Fermilab doesn't do it now, it was top dog. It's not anymore. The only really big antiproton accelerator creator is a small accelerator at CERN. Okay, so that's the antiproton thing, and if we get back to antimatter, we can talk about that- ... 'cause that is way cool.
53:36 - Yeah, super cool. - But now the other side of your thing about making high-energy unknown particles, bigger is better. And it is true that the LHC is a very high-energy machine. It is about seven 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 that the old Fermilab, the Tevatron, ever could. And that is true. So if you wanna look at high-energy stuff, yeah, you go to CERN now, which is why many of my colleagues, including myself, once we had measured all of the sort of frontier measurements we thought we
54:25 could make with the Fermilab accelerator, we saw this bigger, more powerful machine with seven times the energy and 100 times more collisions per second. We said, "Heck yeah, let's go work on that." And to give you a sense of scale, the top quark, which is the heaviest particle ever discovered, discovered at Fermilab in 1995, there were two discovery papers, and the one on which I was a co-author, we had worked for a good chunk of between six months and a year of collecting collisions, and there were a lot of collisions, and our paper had 38 top quark candidates. 38. And we knew that half of them were crap because when you make a detector like that, there's what you call background, so you have the background and the good stuff, and we know it was about 50/50.
55:13 So we had maybe 19 top quarks after working for between six months and a year of collecting data. But now, at the LHC, we make a top quark every second. And that's what higher energy and more collisions per second will do for you. That extra energy, you're above threshold, you make a ton of them, and when you compare the 1995 Fermilab accelerator to the current CERN accelerator, it's probably 1,000 times of collisions per second. So 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 wanna search. They are, like, so 30 years ago.
55:59 - By the way, is there something to be said about the kind of sort of signal processing here, how you remove the noise, how you remove the background, how you determine which particle is which? There's probably some, like, incredible nuance there, even outside of the scope of this conversation. - So let me just throw some numbers out, all right? So 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. Now, each one of those - Wow. - Yeah, yeah, that's what I said. - Wow. - Now, it turns out some of them are happening at the same time, so there's about, of order 40 million moments in time per second where you would take a shot, and inside that moment, there might be 20 collisions.
56:46 So that's why we get to the billion. - Yeah, but can you individually pinpoint the collisions? - Sort of, to a degree. So the beams are, you know... When people think of beams, they think of, like, 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 they're at the LHC, different accelerators are different, they're about this long. And so 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. And as they-- ...pass through each other, you should think of this as like a swarm of bees. - Yep. - And 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
57:35 stripes and wings and everything everywhere. - That's awesome. That's awesome. - And so as they collide through each other, you know, one collision's here, and one's here, and one's here. And 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. And they're, you know, this is about the right size. And so we have detectors around them, and we can actually see, oh, particles came from here, and particles came from here. And that's amazing. All right. So at any one crossing, there's maybe 20 collisions. Now, most collisions are absolutely boring. They're boring because they are-- they exemplify physics that we know a great deal about already. We've tested it for
58:22 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 because, you know? But that's just the way of, you know, frontier scientists. So what you need is you need to pick out the cool ones, the weird ones, the ones that nobody's seen before. And so what happens is, these things, these beams collide, and we surround the collision point with an enormous detector. There are two 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.
59:08 - Okay. It's good to know that there's friendly competition even inside CERN. - Yes, yes. - That's awesome. - I mean, the fact is they are both amazing, absolutely amazing detectors. - Right. But CMS is just a little cooler than ATLAS. - Oh, yeah, yeah. I mean, you know, 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 two giant detectors. - Right. So, but one of them, our detector, the CMS detector, it's the small one. It is 70 feet long, 50 feet high, 50 feet wide. It's five stories tall. It weighs 14,000 tons. - Mm-hmm. Small one, yep. - Small. ATLAS experiment is 150 feet long, 80 feet across. It weighs only 7,000 tons. Just piece of cake. You could take the ATLAS detector, and you could put four of them on a soccer or
59:56 a football field, and it would fill the field up with just enough room on the sidelines for the cheerleaders and the water boy and the coaches and stuff. That's how big they are. - Nice. - So these are absolutely ginormous detectors, and basically they're cameras, and what they can do is they can take pictures 40 million times per second. Now, 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 it would be full of all these boring things we don't care about. So what we do is as the beams pass through one another, we teach our detectors to say, "We only want one where there are certain configurations that might be interesting," like there's gobs of energy in the detector, or there's a gob of energy on one side and nothing on the other side, or there's four gobs of energy or whatever. These are called triggers.
1:00:45 And so these-- what we have is fast electronics that take the 40 million possible pictures per second, and it says, "You know, about 100,000 of those are really cool. We should think about them." And then it passes not all of the 40 million, but those 100,000 to the next level, which are commercial processors that have, Basically our final analysis code, but optimized to run very, very quickly. And what they do is they do a really quick and dirty analysis to say, to further refine what's good and what's not. And that, that computer farm then accepts about 1,000 collisions per second, and we record
1:01:32 those for further analysis. So that's what's really happening. Of the 50 million possible collisions per second, the fast electronics and then the computers pick the thousand. And then we pass those through analysis software and hand them to the graduate students, and they pick through them, looking and finding the handful that are the next Nobel Prize. So that's how that works, and that, that is truly astonishing. I'm-- hats off to the accelerator builders, the detector builders, the people who make the software work, the people who make the 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 4th, 2012, the discovery of Higgs boson.
1:02:19 - So this is really fun because the people searching for the Higgs, it's a community, and the entire community knew that the LHC was coming online. So even though many of us had been working on the Fermilab accelerators, a lot of us were transitioning to the CERN accelerator. So 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 CERN was going to be able to find it if existed. And so, you know, we were a little neurotic, kind of wanted, you know, our, our old stuff to work
1:03:05 and, and there was an awful lot of people on both experiments. Did you have a sense that one of the two places would be able to find the Higgs? First, did you think the Higgs boson existed, and second, did you think that these accelerators have the chance to find them? - So 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 be. I knew that both experiments, the Fermilab accelerator and the CERN accelerator, would either find or rule out the Higgs if it existed. - Rule out? - Well, that's a possibility. I mean- ... maybe the Higgs theory was wrong. Right? Until you, till you know it's there, it might be wrong. It's like dark matter. People talk about dark matter, it might not be real.
1:03:52 I mean, I-- it probably is, but it might not be. - So you knew at these energy levels, you would be-- you should be able to find- - Yes - ... the Higgs boson. - Yes. So that's the nice thing about this kind of physics, because there was a theory, that theory made predictions. Now, there were parameters in the theory we didn't know. If the mass was this, we'd get this thing. If the mass was this, we'd get this thing. But we could do the the calculation for every conceivable Higgs mass, and so then we could search. Look, well, let's say the Higgs mass is 100 in some units. Did we see it there? No. Then it's not 100. Well, 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
1:04:39 predictions of simple Higgs theory, 100% guaranteed. However, the CERN accelerator had 10 times the collisions per second and three and a half times the energy. So remember when I said it-- with the top quarks, it was, like, six months for 19 versus one a second? There's no question. The writing is on the wall. The Hi- the, the LHC was gonna have an easier time of it- ... if it was real. However, so but, you know, I'm a Fermilab scientist, and we wanted Fermilab, you know, come on, we want Fermilab- ... to win. - Of course. - Not, you know. So we were busting our butt, and we had done what I said. We had ruled out this region. We had certain mass ranges.
1:05:24 We knew it wasn't there. And we finally said, if there was a Higgs boson, if it existed, which we didn't know at the time, its mass was somewhere between, if I recall, between, like, 120 and 145. All right? We'd ruled out all the other stuff. And so wearing our CERN hat, we said, "Okay, we're gonna find that." But we were really, really, really trying to do it. Now, if we had had another two years or maybe three years of running the Fermi accelerator, Fermilab would have discovered or ruled out, or in this case, it turned out, discovered the Higgs boson, 'cause it's a real thing. We would have found it without a question. But we didn't have enough data in July of 2012. We needed a couple
1:06:12 more years. Unfortunately, in 2008, or fortunately, the LHC had turned on. It broke. They had to fix 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. And so, you know, there was this-- the Fermilab knew if it didn't have it now, it wasn't-- it was too late. Anyways, come 2012 rolls around, and like two days before, The announcement at CERN was July 4th, so two days before that, Fermilab made a measurement and said, "We can rule out certain regions, but certain regions we can't rule out. But what
1:06:59 we know, and this is important, if the Higgs boson exists, it must be in this region for which we are not capable yet of ruling out." So that's where we were two days before the LHC said, "We got it." That was July 4th, 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 of 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 one,
1:07:47 but multiple Higgs bosons. So there's a theory called supersymmetry, which said that there was not one, but five Higgs bosons. The standard original 1964 Higgs theory says there was one. And so all we really knew at the time was we found a theory. We did not necessarily confirm that Higgs was
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