Harry Cliff: Particle Physics and the Large Hadron Collider | Lex Fridman Podcast #92

Lex Fridman 0:00 The following is a conversation with Harry Cliffe, a particle physicist at the University of Cambridge, working on the Large Hadron Collider beauty experiment that specializes in investigating the slight differences between matter and antimatter by studying a type of particle called the beauty quark, or b quark. In this way, he's part of the group of physicists who are searching for the evidence of new particles that can answer some of the biggest questions in modern physics. He's also an exceptional communicator of science, with some of the clearest and most captivating explanations of basic concepts in particle physics that I've ever heard. So when I visited London, I knew I had to talk to him. And we did this conversation at the Royal Institute Lecture Theatre, which has hosted lectures for over two centuries from some of the greatest scientists and science communicators in history, from Michael Faraday to Carl Sagan. This conversation was recorded before the outbreak of the pandemic. For everyone feeling the medical and psychological and financial burden of this crisis, I'm sending love your way. Stay strong. We're in this together. We'll beat this thing. This is the Artificial Intelligence Podcast. If you enjoy it, subscribe on YouTube, review it with five stars on Apple Podcasts, support it on Patreon, or simply connect with me on Twitter, @LexFridman, spelled F-R-I-D-M-A-N. As usual, I'll do a few minutes of ads now, and never any ads in the middle that can break the flow of the conversation. I hope that works for you and doesn't hurt the listening experience. Quick summary of the ads. Two sponsors, ExpressVPN and Cash App. Please consider supporting the podcast by getting ExpressVPN at expressvpn.com/lexpod and downloading Cash App and using code LEXPODCAST. This show is presented by Cash App, the number one finance app in the App Store. When you get it, use code LEXPODCAST. Cash App lets you send money to friends, buy Bitcoin, and invest in the stock market with as little as $1. Since Cash App does fractional share trading, let me mention that the order execution algorithm that works behind the scenes to create the abstraction of the fractional orders is an algorithmic marvel. So big props to the Cash App engineers for solving a hard problem that, in the end, provides an easy interface that takes a step up to the next layer of abstraction over the stock market, making trading more accessible for new investors and diversification much easier. So again, you get Cash App from the App Store or Google Play and use the code LEXPODCAST, you get $10 and Cash App will also donate $10 to FIRST, an organization that is helping advance robotics and STEM education for young people around the world. This show is sponsored by ExpressVPN. Get it at expressvpn.com/lexpod to get a discount and to support this podcast. I've been using ExpressVPN for many years. I love it. It's easy to use. Press the big power-on button and your privacy is protected. And, if you like, you can make it look like your location is anywhere else in the world. I might be in Boston now, but I can make it look like I'm in New York, London, Paris, or anywhere else. This has a large number of obvious benefits. Certainly, it allows you to access international versions of streaming websites, like the Japanese Netflix or the UK Hulu. ExpressVPN works on any device you can imagine. I use it on Linux, shout out to Ubuntu, Windows, Android. But it is available everywhere else too. Once again, get it at expressvpn.com/lexpod to get a discount and to support this podcast. And now, here's my conversation with Harry Cliffe.

Lex Fridman 3:52 Let's start with probably one of the coolest things that human beings have ever created, the Large Hadron Collider, LHC. What is it? How does it work?

Harry Cliff 4:02 Okay. So it's essentially this gigantic, 27 kilometer circumference particle accelerator. It's this big ring. It's buried a hun- about 100 meters underneath the surface in the countryside just outside Geneva in Switzerland. And really, what it's for, ultimately, is to try to understand what are the basic building blocks of the universe. So you can think of it, in a way, as, like, a gigantic microscope. And, and the analogy is actually fairly precise. So-

Lex Fridman 4:28 Gigantic microscope.

Harry Cliff 4:30 Microscope. Effectively. Uh, except it's a microscope that looks at the structure of the vacuum.

Lex Fridman 4:36 In order for this kind of thing to study particles, uh, which are these mi- microscopic entities, it has to be huge.

Harry Cliff 4:44 Yes.

Lex Fridman 4:44 So it's a gigantic microscope. So what do you mean by, "Studying vacuum?"

Harry Cliff 4:48 Okay. So, I mean, so particle physics as a, as a field, is kind of badly named, in a way. Because particles are not the fundamental ingredients of the universe. They're not fundamental at all. So the things that we believe are the real building blocks of the universe are objects, invisible, fluid-like objects called quantum fields. So these are fields like, uh, like the magnetic field around a magnet that exist everywhere in space. They're always there. In fact, actually, it's funny that we're in the Royal Institution 'cause this is where the idea of the field was effectively invented by Michael Faraday doing experiments with magnets and coils of wire. So he noticed that, you know, if he... Well, it's a very famous, uh, experiment that he did, where he got a magnet and put it, on top of it, a piece of paper, and then sprinkled iron filings. And he found the iron filings arranged themselves into these kind of loops, uh, of, of, uh, which was actually mapping out the invisible influence of this magnetic field, which is a thing, you know, we've all experienced. We've all felt, held a magnet and, or two poles of a magnet and pushed them together and felt this thing, this force, pushing back. So these are real, physical objects. And, uh, the way we think of particles in modern physics is that they are essentially little vibrations, little ripples in these otherwise invisible fields that are everywhere. They fill the whole universe.

Lex Fridman 6:03 You know, I, I don't, uh, I apologize perhaps for the ridiculous question. Are you comfortable with the idea of the fundamental nature of our reality being fields? 'Cause to me, particles, you know, uh, a bunch of different building blocks makes more sense sort of intellectually, sort of visually. Like it's-

Harry Cliff 6:23 Mm-hmm.

Lex Fridman 6:23 ... it seems to, uh, I, I seem to be able to visualize that kind of idea easier.

Harry Cliff 6:28 Yeah.

Lex Fridman 6:28 Are you comfortable, psychologically, with the idea that the basic building block is not a block, but a field?

Harry Cliff 6:35 I think it's, um, I think it's quite a magical idea. I find it quite appealing. And it, it's, well, it comes from a misunderstanding of what particles are. So like when you, when we do science at school and we draw a picture of an atom, you draw like, you know, a nucleus with some protons and neutrons, these little spheres in the middle, and then you have some electrons that are like little flies flying around the atom. And that is a completely misleading picture of what an atom is like. It's nothing like that. The electron is not like a little planet orbiting the atom. Um, it's this spread out, wibbly-wobbly, wave-like thing. And we know, we've known that since, you know, the, the early 20th century thanks to quantum mechanics. So when we, we, we carry on using this word particle because sometimes when we do experiments, particles do behave like they're little marbles or little bullets, you know? So in the LHC, when we collide particles together, you'll get, you know, you'll get like, uh, hundreds of particles all flying out through the detector, and they all take a, a trajectory, and you can see from the detector where they've gone, and they look like they're little bullets. So, they behave that way, um, you know, a lot of the time. But when you really study them carefully, you'll see that they are not little spheres. They are these ethereal disturbances in, in these underlying fields. So this is, this is really how we think nature is. Um, which is surprising, but also, I think kind of magic. So you know, we are, our bodies are basically made up of like little knots of energy in these invisible objects that are all around us.

Lex Fridman 8:01 (inhales deeply) And, uh, what, what is the story of the vacuum when it comes to LHC?

Harry Cliff 8:07 I-

Lex Fridman 8:07 So why, why did you mention the word vacuum?

Harry Cliff 8:10 Okay, so if we just, if we go back to like the physics we do know.

Lex Fridman 8:13 Yeah.

Harry Cliff 8:14 So atoms are made of electrons, which were discovered a hundred or so years ago, and then in the nucleus of the atom, you have two other types of particles. There's an up, something called an up quark, and a down quark. And those three particles make up every atom in the universe. So we think of these as ripples in fields. So there is something called the electron field, and every electron in the universe is a ripple moving about in this electron field. The electron field is all around us. We can't see it, but every electron in our body is a little ripple in this thing that's there all the time. And the quark field's the same. So there's an up quark field and an up quark is a little ripple in the up quark field, and the down quark is a little ripple in something else called the down quark field. So these fields are always there. Now, there are potentially, we, we know about a certain number of fields in what we call the standard model of particle physics, and the most recent one we discovered was the Higgs field. And the way we discovered the Higgs field was to make a little ripple in it. So what the LHC did, it fired two protons into each other very, very hard with enough energy that you could create a disturbance in this Higgs field. And that's what shows up as what we call the Higgs boson. So this particle that everyone was going on about eight or so years ago is proof really ... The particle in itself is, I mean, it's interesting, but the thing that's really interesting is the field, because it's the, the Higgs field that we believe is the reason that electrons and quarks have mass. And it's that invisible field that's always there that gives mass to the particles. The Higgs boson is just our way of checking it's there basically.

Lex Fridman 9:46 So the Large Hadron Collider, uh, in order to get that ripple in the Higgs field, you, it requires a huge amount of energy-

Harry Cliff 9:54 Yeah.

Lex Fridman 9:54 ... I suppose. And so that's why you need this huge ... That's why size matters here. So maybe there's a million questions here, but let's backtrack. Why does size matter in the context of a g- (laughs) of a particle collider? So why, um, does bigger allow you for higher energy collisions?

Harry Cliff 10:15 Right, so the reason ... Well, it, it's kind of simple really, which is that there are two types of particle accelerator that you can build. One is circular, which is like the LHC. The other is a great long line.

Lex Fridman 10:25 Mm-hmm.

Harry Cliff 10:25 So the advantage of, uh, a circular machine is that you can send particles around a ring, and you can give them a kick every time they go around. So imagine you have a ... There's actually a bit of the LHC that's about only 30 meters long, where you have a bunch of metal boxes, which have oscillating two million volt electric fields inside them-

Lex Fridman 10:42 Mm-hmm.

Harry Cliff 10:42 ... which are timed so that when a proton goes through one of these boxes, the field it sees as it approaches is attractive, and then as it leaves the box, it flips and becomes repulsive, and the, the proton gets attracted then kicked out the other side, so it gets a bit faster. So you send it, but then you send it back around again and-

Lex Fridman 10:57 That's incredible, like the timing of that, the synchronization. Wait, really?

Harry Cliff 11:01 Yeah. Yeah, yeah, yeah.

Lex Fridman 11:02 That's ... I think there's going to be a mul- multiplicative effect on the questions I have.

Harry Cliff 11:06 (laughs)

Lex Fridman 11:07 Uh, is, uh ... Okay, let me just take that tangent for a second. Um, how, the orchestration of that, is that fundamentally a hardware problem or a software problem? Like what, how, how do you get that perfectly right?

Harry Cliff 11:20 I mean, I, so I should first of all say, I'm not an engineer, so the guys ... I did not build the LHC. So there are people much, much better at this stuff than I could probably-

Lex Fridman 11:26 For sure, but maybe (laughs) ...

Harry Cliff 11:30 (laughs)

Lex Fridman 11:30 But from, from your sort of, uh, in- intuition, uh, from the, the, uh, the echoes of what you understand-

Harry Cliff 11:37 Mm-hmm.

Lex Fridman 11:37 ... of what you heard of h- how it's designed, what, what's your sense? How, what's the, the engineering-

Harry Cliff 11:42 Well-

Lex Fridman 11:42 ... aspects of it?

Harry Cliff 11:43 ... the acce- the acceleration bit is not challenging, okay? I mean, okay, there is always challenges with everything, but, uh, basically, you have these, um, the beams that go around the LHC, the beams of particles are divided into little bunches. So they're called, they're, they're a bit like swarms of bees, if you like. Um, and there are around, I think it's something of the order 2,000...... bunches spaced around the ring. And they, if you're, if you're at a given point on the ring counting bunches, you get 40 million bunches passing you every second. So they come in like, you know, just like cars going past on a very fast motorway.

Lex Fridman 12:15 Yeah.

Harry Cliff 12:16 So, you need to have, if your electric field, that you're using to accelerate the particles, that needs to be timed so that as a bunch of protons arrives, it's got the right sign to attract them and then flips at the right moment. But I think the, the voltage in those boxes oscillates at hundreds of megahertz. So the beam's at like 40 megahertz, but it's oscillating much more quickly than the beam. So I'm, I think, you know, it's difficult engineering, but in principle it's not, you know, a really serious challenge. The bigger problem-

Lex Fridman 12:42 There's probably engineers like screaming at you right now (laughs) .

Harry Cliff 12:44 Probably.

Lex Fridman 12:44 Yeah (laughs) .

Harry Cliff 12:45 Well, and, but I mean, okay, so in terms of coming back to this thing, why is it so big?

Lex Fridman 12:48 Yeah.

Harry Cliff 12:48 Well, the reason is you wanna get the particles through that accelerating element over and over again.

Lex Fridman 12:53 Yeah.

Harry Cliff 12:54 So you wanna bring them back round, so that's why it's round. The question is, why couldn't you make it smaller? Well, the, the basic answer is that these particles are going unbelievably quickly. So they travel at 99.9999991% of the speed of light in the LHC. And if you think about, say, driving your car around a corner at a high speed, if you go fast, you need a very, you need a lot of friction in the tires to make sure you don't slide off the road. So the, the limiting factor is how powerful a magnet can you make, 'cause it's, what we do is magnets are used to bend the particles around the ring, and essentially the LHC when it was designed, was designed with the most powerful magnets that could conceivably be built at the time. And so that's your kind of limiting factor. So if you wanted to make the machine smaller, that means a tighter bend, you need to have a more powerful magnet. So it's this toss-up between how strong are your magnets versus how big a tunnel can you afford. The bigger the tunnel, the weaker the magnets can be. The smaller the tunnel, the stronger they've gotta be.

Lex Fridman 13:54 Okay, so

Lex Fridman 13:55 maybe c- can we backtrack to the-

Harry Cliff 13:56 Mm-hmm.

Lex Fridman 13:56 ... standard model and-

Harry Cliff 13:57 Mm-hmm.

Lex Fridman 13:58 ... say what kind of particles there are, period?

Harry Cliff 14:00 Mm-hmm.

Lex Fridman 14:00 And, uh, maybe the history of kind of assembling that, uh, the standard model of physics and then how that leads up to the hopes and dreams and the accomplishments of the Large Hadron Collider?

Harry Cliff 14:12 Yeah, sure. Okay. So-

Lex Fridman 14:14 (laughs)

Harry Cliff 14:14 ... sp- all of 20th century physics in like five minutes. Let's try.

Lex Fridman 14:16 Yeah, please.

Harry Cliff 14:17 So, okay. So, okay, the story really begins properly end of the 19th century. The basic view of matter is that matter is made of atoms, and that atoms are indestructible, immutable little spheres like the things we were talking about that don't really exist.

Lex Fridman 14:32 Yeah.

Harry Cliff 14:32 Um, and there's, you know, one atom for every chemical element, so there's an atom for hydrogen, for helium, for carbon, for iron, et cetera, and they're all different. Then in 1897, experiments done at the Cavendish Laboratory in Cambridge, which is where I, I'm still, uh, where I'm based, uh, showed that there are actually smaller particles inside the atom, which eventually became known as electrons. These are these negatively charged things that go round the outside. A few years later, Ernest Rutherford, very famous nuclear phys- one of the pioneers of nuclear physics, shows that the atom has a tiny nugget in the center, which we call the nucleus, which is a positively charged object. So then by like 1910, '11, we have this model of the atom that we learn in school, which is you've got a nucleus, electrons go around it. Fast forward, you know, a few years, the nucleus, people start doing experiments with radioactivity, where they use alpha particles that are spat out of radioactive elements-

Lex Fridman 15:22 Mm-hmm.

Harry Cliff 15:23 ... as, as bullets, and they fire them at other atoms. And by banging things into each other, they see that they can knock bits out of the nucleus. So these things come out called protons, first of all, which are positively charged particles about 2,000 times heavier than the electron. And then 10 years later, more or less, a neutral particle is discovered called the neutron. So those are the three basic building blocks of atoms. You have protons and neutrons in the nucleus that are stuck together by something called the strong force, the strong nuclear force, and you have electrons in orbit around that held in by the electromagnetic force, which is one of the, you know, the forces of nature. That's sort of where we get to by like 1932, more or less. Then what happens is physics is nice and neat. In 1932, everything looks great, we've got three particles, we know what the atoms are made of, that's fine. But then, uh, s- cloud chamber experiments, so these are devices that can be used to... The first device is capable of imaging subatomic particles so you can see their tracks, and they're used to study cosmic rays, particles that come from outer space and bang into the atmosphere. And in these, uh, experiments, people start to see a whole load of new particles. So they discover, for one thing, antimatter, which is a sort of a mirror image of the particles. So we discover that there's also, as well as a negatively charged electron, there's something called a positron, which is a positively charged version of the electron, and there's an antiproton, which is negatively charged. An- and then a whole load of other weird particles start to get discovered, and no one really knows what they are. This is known as the zoo of particles.

Lex Fridman 16:51 Are these discoveries on the first, uh, theoretical discoveries or are they discoveries in, in experiment? So like-

Harry Cliff 16:59 E- yeah.

Lex Fridman 16:59 ... wha- yeah, wha- what's the process of discovery for these early sets of-

Harry Cliff 17:02 It- it's-

Lex Fridman 17:03 ... of particles.

Harry Cliff 17:03 ... it's a mixture. I mean, the, the early stuff around the atom is really experimentally driven. It's not based on some theory. It, it's exploration in the lab using equipment. So it's really people just figuring out, getting hands-on with the phenomena, figuring out what these things are. And the theory comes a bit later. That there is, that's not always the case. So in the discovery of the antielectron, the positron, that was predicted from quantum mechanics and relativity by a very, uh, clever theoretical physicist called Paul Dirac-

Lex Fridman 17:30 Mm-hmm.

Harry Cliff 17:30 ... who was probably the second brightest, you know, physicist of the 20th century apart from Einstein, but isn't as w- anywhere near as well known. So he predicted the existence of the antielectron from basically a combination of the theories of quantum mechanics and relativity, and it was discovered about a year after he made the prediction.

Lex Fridman 17:46 What happens when an e- when an electron meets a positron?

Harry Cliff 17:49 They annihilate each other. So if you, when you bring a particle and its antiparticle together, they, they react, well, they react, they just wipe each other out and they turn, their mass is turned into energy, usually in the form of photons, so you get light produced.

Lex Fridman 18:03 So, um-When you have tha- that kinda situation, why, why does the universe exist at all if there's matter and antimatter?

Harry Cliff 18:10 Oh God, now we're getting into the really big questions. So... (laughs)

Lex Fridman 18:14 K- Maybe-

Harry Cliff 18:14 Depends if you... Do you wanna go there now? I mean-

Lex Fridman 18:15 Yeah. Let's, ma- maybe, may- may- maybe let's go there later. That's, uh-

Harry Cliff 18:19 'Cause tha- I mean that is a very big question, yeah.

Lex Fridman 18:20 Yeah. Let's, let's, uh-

Harry Cliff 18:22 (laughs)

Lex Fridman 18:22 Let's take it slow with the standard model. So, okay, so there's ma, uh, there's matter and antimatter in the '30s.

Harry Cliff 18:27 Mm-hmm.

Lex Fridman 18:28 So wha- what else?

Harry Cliff 18:29 So matter, antimatter, and then a load of new particles start turning up in these cosmic ray experiments, first of all. And they don't seem to be particles that make up atoms, they're something else. They all mostly interact with a strong nuclear force, so they're a bit like protons and neutrons. And by, uh, in the 1960s, in America particularly, but also in Europe and Russia, scientists started to build particle accelerators, so these are the forerunners of the LHC. So big, ring-shaped machines that were, you know, hundreds of meters long, which in those days was enormous. You never h- you know, most physics up until that point had been done in labs in universities, you know, with small bits of kit. So this is a big change. And when these accelerators are built, they start to find they can produce even more of these particles. So I don't know the exact numbers, but by around 1960, there are, of order, 100 of these things that have been discovered. And physicists are kind of tearing their hair out because physics is all about simplification, and suddenly what was simple has become messy and complicated, and everyone sort of wants to understand what's going on.

Lex Fridman 19:31 As a quick kind of aside, and probably a really dumb question-

Harry Cliff 19:34 Mm-hmm.

Lex Fridman 19:34 ... but, um, how is it possible to take something like a, like a photon or an electron and be able to control it enough, like, to be able to, uh, do a controlled experiment where you collide it against something else?

Harry Cliff 19:51 Yeah.

Lex Fridman 19:52 I- i- uh, is that, is that... That seems like an exceptionally difficult engineering challenge, 'cause you mentioned vacuum too, so you, you basically wanna remove every other distraction and really focus on this collision.

Harry Cliff 20:05 Mm-hmm.

Lex Fridman 20:05 How difficult of an engineering challenge is that? Just to get a sense.

Harry Cliff 20:08 I mean, it is very hard. Um, I, I mean, in the early days, particularly when the first accelerators are being built in like 1932, um, L- Ernest Lawrence builds the first what we call a cyclotron, which is like a little accelerator this big or so. There's another one-

Lex Fridman 20:22 Literally that big?

Harry Cliff 20:22 This tiny little thing, yeah.

Lex Fridman 20:24 Yeah.

Harry Cliff 20:24 I mean, so most of the first accelerators were, um, what we call fixed target experiments. So you had a ring. You accelerate particles around the ring and then you fire them out the side into some target. So it's ei- that makes the kind of... The colliding bit is relatively straightforward, 'cause you just fire it at whatever it is you wanna fire it at. The hard bit is the steering the beams with the magnetic fields, getting, you know, strong enough electric fields to accelerate them, all that kinda stuff. The first colliders, where you have two beams colliding head-on, that comes later. And I, I don't think it's done until maybe the 1980s. Uh, I'm, I'm not enti- I'm not entirely sure, but it takes, it's a much harder problem.

Lex Fridman 21:05 That's crazy, 'cause you have to like perfectly-

Harry Cliff 21:08 Yeah.

Lex Fridman 21:08 ... get them to hit each other. I mean, we're talking about, I mean, what scale? Like s- what's the s-, the, the s- the... I mean, the temporal thing is, is, is a giant mess.

Harry Cliff 21:18 Mm-hmm.

Lex Fridman 21:18 But the, the... Spatially, like the size-

Harry Cliff 21:22 Mm-hmm.

Lex Fridman 21:22 ... is tiny.

Harry Cliff 21:23 Well, to give you a sense, so the LHC beams, the cross-sectional diameter is, I think, around a dozen or so microns. So, you know, ten, ten millionths of a, of a meter, so they-

Lex Fridman 21:37 And, and a beam, sorry, um, just to clarify, a beam contains how many... Is it the bunches that you mentioned?

Harry Cliff 21:43 Yeah. So-

Lex Fridman 21:43 Is it multiple part or is it just one part of it?

Harry Cliff 21:45 Oh, no, no. The bunches contain, say, 100 billion protons each. So a bunch is, it's, it's not really bunch-shaped. It, they're actually quite long. They're like 30 centimeters long, but thinner than a human hair. So they're like very, very narrow, long sort of objects. Those are the things. So wha- what happens in the LHC is you steer the beams so that they cross in the middle of the, uh, detector. So they basically have these swarms of protons that are flying through each other.

Lex Fridman 22:09 Mm-hmm.

Harry Cliff 22:10 And most of the ha- you have, so you have 100 billion coming one way, 100 billion another way, maybe f- 10 of them will hit each other.

Lex Fridman 22:17 Oh, okay. So this, okay, that makes a lot more sense.

Harry Cliff 22:19 So it's

Narrator 22:19 (laughs)

Lex Fridman 22:19 That's nice. So th- you're trying to use sort of, it's like probabilistically. You're not...

Harry Cliff 22:24 You can't make a single particle collide with a singular particle.

Lex Fridman 22:26 Yeah. So okay, yeah-

Harry Cliff 22:27 That's not an efficient way to do it. You'd be waiting a very long time to get anything. Yeah.

Lex Fridman 22:31 So you, you're basically... Uh, right. So you're relying on probability to be that some fraction of them are gonna collide-

Harry Cliff 22:38 Yeah.

Lex Fridman 22:38 ... and then you know which... 'Cause it's, it's a, it's a swarm of the same kind of particle.

Harry Cliff 22:44 So it doesn't matter which ones-

Lex Fridman 22:45 Okay.

Harry Cliff 22:45 ... hit each other exactly.

Lex Fridman 22:46 Yeah.

Harry Cliff 22:46 I mean, that, that's not to say it's not hard. You've got to... One of the challenges to make the collisions work is you have to squash these beams to very, very... They're basically, the narrower they are, the better, 'cause the higher the chances of them colliding. If you think about two flocks of birds flying through each other, if the birds are all far apart in the flocks, there's not much chance that they'll collide. If they're all flying densely together then they, they're much more likely to collide with each other. So that's the sort of problem, and it's tuning those magnetic fields, getting the magnetic fields powerful enough that you squash the beams and focus them so that you get enough collisions.

Lex Fridman 23:16 That's super cool. Do you know how much software is involved here? I mean, sort of I come-

Harry Cliff 23:21 Mm-hmm.

Lex Fridman 23:21 ... from the software world and it's fascinating. Uh, this, this seems like it's, the software is buggy and messy and-

Harry Cliff 23:28 (laughs)

Lex Fridman 23:29 ... sec- so like you, you almost don't want to rely on software too much. Like if you do, it has to be like low level, like Fortran style programming.

Harry Cliff 23:36 Mm-hmm.

Lex Fridman 23:36 Do you know how much software is in the Large Hadron Collider?

Harry Cliff 23:39 I mean, uh, uh, it depends at which level. A lot. I mean, the whole thing is obviously computer controlled. So I mean, I, I don't know a huge amount about how the software for the actual accelerator works, um, but, you know, I've been in the control center. So at CERN there's this big control room which is like a bit like a NASA mission control with big banks of, you know, desks where the engineers sit and they monitor the LHC, 'cause you obviously can't be in the tunnel when it's running, so everything's remote. Um, I mean, one sort of anecdote about the sort of software side, in 2008 when the LHC first switched on, they had this big launch event and then, you know, big press conference, party-... to inaugurate the machine, and about 10 days after that, they were doing some tests and the, there's dramatic event happened where a huge explosion basically took place in the tunnel that destroyed or damaged, badly damaged about, about half a kilometer of the machine. But the story's view, the engineers were in the control room that day. They, they, one guy told me this story about, you know, basically, these, all these screens they have in the control room started going red. So all these al- alarms like, you know, kind of in software going off, and then they assumed, oh, there's something wrong with the software 'cause there's no way-

Lex Fridman 24:45 Yeah.

Harry Cliff 24:45 ... something this catastrophic could've, could've happened.

Lex Fridman 24:47 Yeah.

Harry Cliff 24:48 Um, but I mean, when I worked on, when I, when I was a PhD student, one of my jobs was to help to maintain the software that's used to control the detector that we work on. And that was, it, it's relatively robust, not such, you don't want it to be too fancy, you don't want it to sort of fall over too easily. The more clever stuff comes when you're talking about analyzing the data, and that's where the sort of, you know-

Lex Fridman 25:10 Are we jumping around too much? Did we finish with the standard model?

Harry Cliff 25:13 We didn't, no.

Lex Fridman 25:13 We didn't. So we, uh, have we even started talking about quarks?

Harry Cliff 25:17 We haven't talked about them yet, no.

Lex Fridman 25:18 Okay.

Harry Cliff 25:18 We got, we got to the messy zoo of particles.

Lex Fridman 25:20 (laughs) Let me, uh, let, let's-

Harry Cliff 25:21 (laughs)

Lex Fridman 25:21 ... go back there if it's okay.

Harry Cliff 25:23 Okay, that's fine. Yeah, yeah.

Lex Fridman 25:23 Can you take us through th- the rest of the history of physics in the 20th century?

Harry Cliff 25:27 Okay, sure. Okay, so circa 1960, you have this cis- you have these 100 or so particles. It's a bit like the periodic table all over again, so you've got like, uh, like having 100 elements, sort of a bit like that. And people try, start to try to impose some order. So, uh, Murray Gell-Mann, uh, he's a theoretical physicist, American from New York, he realizes that there are these symmetries in these particles that if you arrange them in certain ways, they re- they relate to each other, and he uses these symmetry principles to predict the existence of particles that haven't been discovered, which are then discovered in accelerators. So this starts to suggest there's not just random collections of crap, there's like, you know, actually some order to this un- underlying it. A little bit later in 1960, again, it's right around the 1960s, um, he proposes along with another physicist called George Zweig that these symmetries arise because, just like the patterns in the periodic table arise because atoms are made of electrons and protons, that these patterns are due to the fact that these particles are made of smaller things, and they are called quarks. So these are the particles. They're predicted from theory. For a long time, no one really believes they're real. A lot of people think that they're a kind of theoretical convenience that happen to fit the data, but there's no evidence, no one's ever seen a quark in any experiment. And lots of experiments are done to try to find quarks, to try to knock a quark out of a... so the idea if protons and neutrons, say, are made of quarks, you should be able to knock a quark out and see the quark. That never happens, and we still have never actually managed to do that. S-

Lex Fridman 26:58 Wait, really?

Harry Cliff 26:59 No. So the way it, the, the way that it's done in the end is, uh, this machine that's built in California at Sta- uh, the Stanford, uh, Lab, Stanford Linear Accelerator, um, which is essentially a gigantic, three-kilometer-long electron gun. It fires electrons, almost the speed of light, at protons. And when you do these experiments, what you find is, at very high energy, the electrons bounce off small, hard objects inside the proton, so it's a bit like taking an X-ray of the proton. You're firing these very light, uh, high-energy particles and they're pinging off little things inside the proton that are like ball bearings, if you like. So you actually, that way, they resolve that there are three things inside the proton which are quarks, the quarks that Gell-Mann and Zweig had predicted. So that's really the evidence that convinces people that these things are real. The fact that we've never seen one in an experiment directly, they're always stuck inside, um, other particles. And the reason for that is, uh, essentially to do with the strong force. The strong force is a force that holds quarks together and it's so strong that it's impossible to actually liberate a quark. So if you try and pull a quark out of a proton, what actually ends up happening is that the, uh, you kind of create this, th- this springlike bond in the strong force. Imagine two quarks that are held together by a very powerful spring. You pull and pull and pull, more and more energy gets stored in that bond, like stretching a spring, and eventually the tension gets so great the spring snaps and the energy in that bond gets turned into two new quarks that go on the broken ends. So you started with two quarks, you end up with four quarks.

Lex Fridman 28:34 (laughs)

Harry Cliff 28:34 So you never actually get to take a quark out, you just end up making loads more quarks in the process.

Lex Fridman 28:40 So how do we... again, forgive the dumb question, how do we know quarks are real then?

Harry Cliff 28:44 Well, A, from these experiments where we can scatter... you fire electrons into the protons-

Lex Fridman 28:49 Mm-hmm.

Harry Cliff 28:49 ... they can burrow into the proton and knock off, and they can bounce off these quarks. So you can see from the angles the electrons come out at-

Lex Fridman 28:58 I, I see. You can infer.

Harry Cliff 28:59 You can infer that these things are there.

Lex Fridman 29:01 Yeah.

Harry Cliff 29:01 Um, the quark model can also be used... it, it has a lot of successes, so you can use it to predict the existence of new particles that hadn't been seen. So a- and it basically, there's lots of data basically showing from, you know, when we fire protons at each other at the LHC, a lot of quarks get knocked all over the place, and every time they try and escape from, say, one of their protons, they make a whole jet of quarks that go flying off-

Lex Fridman 29:25 Yeah.

Harry Cliff 29:25 ... as, uh, bound up in other sorts of particles-

Lex Fridman 29:28 Yeah.

Harry Cliff 29:28 ... made of quarks. So the, all the sort of, the theoretical predictions from the basic theory of the strong force and the quarks all agrees with what we are see in experiments. We've just never seen a, an actual quark on its own because unfortunately it's impossible to get them out on their own.

Lex Fridman 29:41 So quarks, these crazy smaller things that are hard to imagine are real. So what else? What else is part of the story here?

Harry Cliff 29:50 So the other thing that's going on at the, at the time, around the '60s, is an attempt to...... uh, understand the forces that make these particles interact with each other. So you have the electromagnetic force, which is the force that was, sort of, discovered, to some extent, in this room, or at least in this building. Um, so the first, what we call quantum field theory of the electromagnetic force, is developed in the 1940s and '50s by, uh, Feynman, Richard Feynman, amongst other people, uh, Julian Schwinger, Tomonaga, um, who come up with the first, what we call, a quantum field theory of the electromagnetic force. And this is where this description of which I gave you at the beginning that particles are ripples in fields, well, in this theory, the photon, the particle of light, is described as a ripple in this quantum field called the electromagnetic field. Um, and the attempt then is made to try or can we come up with a quantum field theory of the other forces, of the strong force, and the weak, the oth- the, the, the third force which we haven't discussed, which is the weak force, which is, uh, a w- a nuclear force, we don't really experience it in our everyday lives, but it's responsible for radioactive decay. It's the force that allows, you know, when a radioactive at- atom to turn into a different element, for example.

Lex Fridman 31:01 And it's, I don't know if you have explicitly mentioned, but so there's technically four forces.

Harry Cliff 31:06 Yes.

Lex Fridman 31:06 Th- th- I guess, three of them would be in, in, uh, in the standard model, like the weak, the, the strong, and the ele- electromagnetic, and then there's gravity.

Harry Cliff 31:14 And there's gravity, which we, we don't worry about that 'cause it's too hard. (laughs)

Lex Fridman 31:17 Yes. (laughs) Who cares?

Harry Cliff 31:17 Well, no. Maybe, maybe bring that up at the end, but yeah.

Lex Fridman 31:19 Yeah. Yeah.

Harry Cliff 31:19 Gravity, so far, we don't have a quantum theory of, and if you can solve that problem, you'll win a Nobel Prize well pretty soon.

Lex Fridman 31:25 Well, we're gonna have to bring up the graviton at some point, I'm gonna ask you. But, uh-

Harry Cliff 31:28 Yeah.

Lex Fridman 31:29 (laughs) ... let's, let's, let's leave that to the side for now. So those three, okay, Feynman, elec- uh, electromagnetic force, the, the quantum field.

Harry Cliff 31:38 Yeah. So-

Lex Fridman 31:39 And, uh, where does the weak force come in?

Harry Cliff 31:42 So, so yeah, we... well, first of all, I mean, the strong force is a bit easier. So the strong force is a little bit like the electromagnetic force, it's a force that binds things together. So that's the force that holds quarks together inside that proton, for example. So a quantum field theory of that force is discovered in the, I think, it's in the '60s, um, and it predicts the existence of new force particles called gluons. So gluons are a bit like the photon. The photon is the particle of electromagnetism, gluons are the, the particles of the strong force. Uh, and so there is, there's, just like there's an electromagnetic field, there's something called a gluon field, which is also all around us. Uh-

Lex Fridman 32:19 So these par- there's some of these particles, I guess, are the force carriers or whatever.

Harry Cliff 32:23 Yeah.

Lex Fridman 32:23 They carry the force?

Harry Cliff 32:24 Well, it depends how you wanna think about it. I mean, really the field, the strong force field, the gluon field is the thing that binds the quarks together. Um, the gluons are the little ripples in that field. So that like-

Lex Fridman 32:36 I see.

Harry Cliff 32:36 ... in the same way that the photon is a ripple in the, in the electromagnetic field. But the thing that really does the binding is the field. I mean, you may have heard people talk about things like virtu- have you heard the phrase virtual particle? Um-

Lex Fridman 32:49 (laughs)

Harry Cliff 32:49 ... so some- sometimes in some des- if you hear people describing how forces are exchanged between particles-

Lex Fridman 32:54 Yeah.

Harry Cliff 32:54 ... they quite often talk about the idea that, you know, if you have an electron and another electron say, and they're repelling each other through the electromagnetic, electromagnetic force-

Lex Fridman 33:02 Mm-hmm.

Harry Cliff 33:03 ... you can think of that as if they're exchanging photons, so they're kind of firing photons backwards and forwards between each other, and that causes them to repel.

Lex Fridman 33:11 Uh, that photon is then a virtual particle?

Harry Cliff 33:13 Yes. That's what we call a virtual particle. In other words, it's not a real thing, it doesn't actually exist. (laughs) So it's an artifact of the way theorists do calculations. So when they do calculations in quantum field theory, rather than there's no, no one's discovered a way of just treating the whole field, you have to break the field down into simpler things. So you can basically treat the field as if it's made up of lots of these virtual photons. But you, there's no experiment that you can do that can detect these particles being exchanged. What's really happening in reality is that the electromagnetic field is warped by the charge of the electron, and that causes the force. But the way we do calculations involves-

Lex Fridman 33:48 It's just a-

Harry Cliff 33:48 ... particles. So it's a bit confusing. Um-

Lex Fridman 33:50 Yeah. Yeah.

Harry Cliff 33:50 ... but it's, it's really a mathematical technique, it's not something that corresponds to reality.

Lex Fridman 33:55 I mean, that's part, I guess, of the Feynman diagrams.

Harry Cliff 33:58 Yes.

Lex Fridman 33:58 Is, is this these virtual part- okay.

Harry Cliff 34:00 That's right. Yeah.

Lex Fridman 34:01 Uh, some of these have mass, some of them don't.

Harry Cliff 34:03 Mm-hmm.

Lex Fridman 34:04 Is that, is that wha- what, what does that even mean not to have mass? And maybe you can say wh- which, which one of them is, have mass and which don't?

Harry Cliff 34:12 Okay. So, um-

Lex Fridman 34:14 And why is mass important or relevant in this k- uh, in this, in this field-

Harry Cliff 34:19 Mm-hmm.

Lex Fridman 34:20 ... view of the universe?

Harry Cliff 34:22 Well, there a- there are actually only two particles in the standard model that don't have mass, which are the photon-

Lex Fridman 34:26 Yeah.

Harry Cliff 34:27 ... and the gluons. So they are massless particles. But the electron, the quarks, um, and there are a bunch of other particles I haven't discussed. There's something called a muon and a tau, which are basically heavy versions of the electron that are unstable. You can make them in accelerators, but they, they don't form atoms or anything. They don't exist for long enough. But all the matter particles, there are 12 of them, six quarks, and, uh, six what we call leptons, which includes the electron and its two heavy versions and three neutrinos. All of them have mass and so do, this is the critical bit, so the weak force which is the third of these f- quantum forces, which was the, one of the hardest to understand, um, the, the force particles of that force have very large masses. Um, and there are three of them, they're called the W plus, the W minus, and the Z boson. And they have masses of between 80 and 90 times that of the, the protons. They're very heavy.

Lex Fridman 35:25 Proton, wow.

Harry Cliff 35:26 They're very heavy things.

Lex Fridman 35:26 So they're what? The heaviest, I guess, particles?

Harry Cliff 35:29 They're not the heaviest. The heaviest particle is the top quark, which has a mass of about 175-ish protons. So that's really massive, and we don't know why it's r- so massive. But so coming back to the weak force, so the, the, the problem in the '60s and '70s was that the reason that the electromagnetic force is a force that we can experience in our everyday lives. So if you have a magnet and a piece of metal, you can hold it, you know, a meter apart if it's powerful enough, and you'll feel a force. Whereas the weak force only is becomes apparent when you basically have two particles touching at the scale of, um, a nucleus. So we get to very short distances before this force becomes-... manifest.

Lex Fridman 36:09 Mm-hmm.

Harry Cliff 36:09 It's not, doesn't, we don't get weak forces going on in this room. They, we don't notice them. Um, and the reason for that is that the particle, well, (laughs) the, the field that transmits the weak force, the particle that's associated with that field has a very large mass, which means that the field dies off very quickly.

Lex Fridman 36:26 Mm-hmm.

Harry Cliff 36:26 So as you... Whereas an electric charge, if you were to look at the shape of the electromagnetic field, it would fall off with this, you have this thing called the inverse square law, which is the idea that the force halves every time you double the distance. Uh, no sorry, it doesn't halve, it quarters every time you s- every time you double the distance between, say, the two particles. Whereas the weak force kind of, you move a little bit away from the nucleus and it just disappears. The reason for that is because these, these fields, the particles that go with them have a very large mass. Um, but the problem that was, that theorists faced in the '60s was that if you tried to introduce massive force fields, the theory gave you nonsensical answers, so you'd end up with infinite results-

Lex Fridman 37:08 Mm-hmm.

Harry Cliff 37:08 ... for a lot of the calculations you tried to do. So the basically, it turned, it seemed that quantum field theory was incompatible with having massive particles. Not just the force particles actually, but even the electron was a problem. So this is where the Higgs that we sort of alluded to comes in, and the solution was to say, "Okay, well, actually all the particles in the standard model are mass, they have no mass." So the quarks, the electron, they don't have a mass. Neither do these weak particles, they don't have mass either. It, what happens is, they actually acquire mass through another process. They get it from somewhere else. They don't actually have it intrinsically.

Lex Fridman 37:43 Mm-hmm.

Harry Cliff 37:43 So this idea that was introduced by, well, Peter Higgs is the most famous, but actually there are about six people that came up with the idea more or less at the same time, um, is that you introduce a new quantum field, which is another one of these invisible things that's everywhere-

Lex Fridman 37:57 Mm-hmm.

Harry Cliff 37:57 ... and it's through the interaction with this field that particles get mass. So you can think of, say, an electron in the Higgs field, it kind of, the Higgs field kind of bunches around the electron. It's sort of tr- drawn towards the electron, and that energy that's stored in that field around the electron is what we see as the mass of the electron. But if you could somehow turn off the Higgs field, then all the particles in nature would become massless and fly around at the, at the speed of light. So this, this idea of the Higgs field allowed, uh, other people, other theorists, to come up with a, well, it was another, a u- basically a unified theory of the electromagnetic force and the weak force. So once you bring in the Higgs field, you can combine two of the forces into one. So it turns out the electromagnetic force and the weak force are just two aspects of the same fundamental force. And at the LHC, we go to high enough energies that you see these two forces unifying, um, effectively.

Lex Fridman 38:59 So tha- so first of all, it started, uh, as a theoretical notion. Like this is some-

Harry Cliff 39:05 Yeah.

Lex Fridman 39:05 ... and then, I mean, wasn't the Higgs called the God particle at some point?

Harry Cliff 39:10 It was, by a guy trying to sell popular science books, yeah. Yeah.

Lex Fridman 39:13 Yeah, but I mean-

Harry Cliff 39:15 (laughs)

Lex Fridman 39:15 I mean, I remember 'cause, uh, when I was hearing it, I thought it would, um, I mean, that would solve a lot of the, unify a lot of our ideas of physics-

Harry Cliff 39:24 Mm-hmm.

Lex Fridman 39:24 ... is was was my notion. But, um, maybe you can speak to that. Was-

Harry Cliff 39:29 Yeah.

Lex Fridman 39:29 Is it as big of a leap, is it as, is it a God particle or is it a Jesus particle?

Harry Cliff 39:34 (laughs)

Lex Fridman 39:35 Which, uh, which, you know, what's the big contribution of Higgs in terms of this unification power?

Harry Cliff 39:40 Yeah. I mean, to understand that, I, it maybe helps to know the history a little bit. So when the, what we call electroweak theory was put together, which is where you unify electromagnetism with the weak force, and the Higgs is involved in all of that. So that theory, which was written in the mid '70s, predicted the existence of four new particles, the W+ boson, the W- boson, the Z boson, and the Higgs boson. So there were these four particles that came with the theory, that were predicted by the theory. In 1983, '84, the Ws and the Z particles were discovered at an accelerator at CERN called the Super Proton Synchrotron, which was a seven kilometer particle collider. So three of the bits of this theory had already been found, so people were pretty confident from the '80s that the Higgs must exist, because it was a part of this family of particles that this theoretical structure only works if the Higgs is there. So what then happens, so this question about why is the LHC the size it is-

Lex Fridman 40:39 Yes.

Harry Cliff 40:39 ... well actually, the tunnel that the LHC is in was not built for the LHC. It was built from a, for a previous accelerator called the Large Electron Positron Collider. So that f- that was b- began operation in the late '80s, early '90s. Um, they basically, they, that's when they dug the 27 kilometer tunnel and they put this accelerator into it, collider, that fires electrons and anti-electrons at each other, electrons and positrons. So the purpose of that machine was, well, it was actually to look for the Higgs. That was one of the things it was trying to do, but didn't, man, it didn't have enough energy to do it in the end. But the main thing it achieved was it studied the W and the Z particles at very high precision. So it made loads of these things. Previously, you could only make a few of them at the previous accelerator. So you can, you could study these really, really precisely, and by studying their properties, you could really test this electroweak theory that had been invented in the '70s a- and really make sure that it worked. So actually by 1999, when this machine turned off, people knew... Well, okay, you never know until you, until you find the thing, but people were really confident this electroweak theory was right, and that the Higgs almost, the Higgs or something very like the Higgs had to exist, 'cause otherwise the whole thing doesn't work. It'd be really weird if you could discover and these particles, they all behave exactly as your theory tells you they should, but somehow this key piece of the picture-

Lex Fridman 41:59 Is missing.

Harry Cliff 42:00 ... is not there. So in a way, it depends how you look at it. The discovery of the Higgs on its own, um, is, is obviously a huge achievement in many, both experimentally and theoretically.... on the other hand, it's this, it's like having a jigsaw puzzle where every piece has been filled in. You have this beautiful image, there's one gap and you-

Lex Fridman 42:19 Yeah.

Harry Cliff 42:19 ... kind of know that that piece-

Lex Fridman 42:21 (laughs)

Harry Cliff 42:21 ... must be there somewhere, right?

Lex Fridman 42:22 Yeah.

Harry Cliff 42:23 So if-

Lex Fridman 42:24 (laughs)

Harry Cliff 42:25 So the discovery in itself, although it's important, is not so interesting in that sense.

Lex Fridman 42:30 It's like a confirmation of the obvious-

Harry Cliff 42:32 Confirmation. Yeah.

Lex Fridman 42:32 ... at th- at th- at that point.

Harry Cliff 42:33 Yeah. But what makes it interesting is not that it just completes the standard model, which is a theory that we've known abo- had the basic layout of s- for 40 years or more now. Um, it's that the Higgs actually is a, is a unique particle. It's very different to any of the other particles in the standard model, and it's a, a theoretically very troublesome particle. There are a lot of nasty things to do with the Higgs, but also opportunities, uh, so that we basically, we don't really understand how such an object can exist in the form that it does. So there are lots of reasons for thinking that the Higgs must come with a bunch of other particles, or that it's perhaps made of other things, so it's not a fundamental particle, that it's made of smaller things. I can talk about that, if you like, a bit.

Lex Fridman 43:19 Well, that's, that's still a notion. So-

Harry Cliff 43:21 Yeah.

Lex Fridman 43:21 ... so, so the Higgs might not be a fundamental particle?

Harry Cliff 43:24 Yeah.

Lex Fridman 43:25 There, there might be some, it might... Oh, man.

Harry Cliff 43:27 So that, that is an idea. It's not, you know, uh, it's not been demonstrated to be true. But, I mean, this, this, all of th- these ideas basically come from the fact that, um, it's a, this is, this is a problem that motivated a lot of development in physics in the last 30 years or so. And there's this basic fact that the, the Higgs field, which is this field that's everywhere in the universe, this is the thing that gives mass to the particles. And the Higgs field is different from ev- all the other fields, in that, let's say you take the electromagnetic field, which is, you know, if we actually were to measure the electromagnetic field in this room, we would measure all kinds of stuff going on, 'cause there's light, there's gonna be microwaves and radio waves and stuff. But let's say we could go to a really, really remote part of empty space and shield it and put a big box around it and then measure the electromagnetic field in that box. The field would be almost zero, apart from some little quantum fluctuations. But basically, it goes to naught. The Higgs field has a value everywhere, so it's a bit like the whole, it's li- uh, yeah, the entire of space has got this energy stored in the Higgs field, which is not zero, it's, it's finite, it's got some, it's a bit like having the, the temperature of space raised to-

Lex Fridman 44:30 Mm-hmm.

Harry Cliff 44:30 ... you know, some background temperature. Um, and it's that energy that gives mass to the particles. So the reason that electrons and quarks have mass is through the interaction with this energy that's stored in the Higgs field.

Lex Fridman 44:43 Mm-hmm.

Harry Cliff 44:44 Now, it turns out that the precise value this energy has, has to be very carefully tuned if you want a universe where interesting stuff can happen. So if you push the Higgs field down, it has a tendency to collapse to, well, there's a ten- if you do your sort of naive calculations, there are basically two possible likely configurations for the Higgs field, which is either it's zero everywhere, in which case you have a universe which is just particles with no mass that can't form atoms and just fly about at the speed of light-

Lex Fridman 45:18 Mm-hmm.

Harry Cliff 45:18 ... or it explodes to an enormous value, what we call the Planck scale, which is the scale of quantum gravity, and, uh, at that point, if the Higgs field was that strong, even an electron would become so massive that it would collapse into a black hole, and then you have a universe made of black holes and nothing, like us. So it seems that the, the strength of the Higgs field is, to it could, to achieve the value that we see requires what we call fine-tuning of the laws of physics. You have to fiddle around with the other fields in the standard model and their properties, to just get it to this right sort of Goldilocks value that allows atoms to exist. This is deeply fishy. People really dislike this.

Lex Fridman 45:57 Well, yeah, I guess, well, so what would be th- so two e- two explanations. One, there's a god that designed this perfectly-

Harry Cliff 46:03 Mm-hmm.

Lex Fridman 46:03 ... and two is, there's an infinite number of alternate universes, and we're just happen to be in the one in which life is possible?

Harry Cliff 46:11 Yeah. Yeah.

Lex Fridman 46:11 Complexity? So when you say, um, I mean, life, any kind of complexity that's not either complete chaos or black holes.

Harry Cliff 46:20 Yeah. Yeah.

Lex Fridman 46:21 I mean, how does that make you feel? What do you make of that? That's such a fascinating notion, that this perfectly tuned field that's the same everywhere-

Harry Cliff 46:30 Yeah.

Lex Fridman 46:30 ... is there. What do you make of that? Yeah. What do you make of that?

Harry Cliff 46:34 I mean, y- yeah, so you laid out two of the possible explanations. I-

Lex Fridman 46:36 Really?

Harry Cliff 46:37 Somewhat.

Lex Fridman 46:37 Is this a s- (laughs)

Harry Cliff 46:37 Yeah, I mean, well, someone, you know, some cosmic creator went-

Lex Fridman 46:41 Yeah.

Harry Cliff 46:41 ... "Yeah, let's fix that to be at the right level." That's one possibility, I guess. It's not a scientifically testable one, but, you know, theoretically, I guess it's possible.

Lex Fridman 46:48 Sorry to interrupt-

Harry Cliff 46:49 Yeah.

Lex Fridman 46:49 ... but there could also be, not a designer, but couldn't there be just, uh, I guess I'm not sure what that would be, but, uh, some kind of force that, um, that, uh, some kind of mechanism by which this, this, um, this kind of field is enforced in order to create complexity? Basic- basically forces that, um, pull the universe towards an interesting complexity.

Harry Cliff 47:19 I mean, yeah. I mean, there are people who have those ideas.

Lex Fridman 47:22 B- as-

Harry Cliff 47:22 I don't really subscribe to them. I mean-

Lex Fridman 47:23 As I'm saying it, it sounds really stupid, so I'm gonna-

Harry Cliff 47:25 No, I mean, I, I, there are definitely people that make those kind of arguments. Um, you know, there's ideas that, uh, I think it's Lee Smolin's idea, or one o- uh, I think, that, you know, universes are born inside black holes, and so universes, they're basically this idea of, like, Darwinian evolution of the universe, where universes give birth to other universes, and the universes where black holes can form are more likely to give birth to more universes, so you end up with universes which have similar laws. I mean, I don't know, whatever. But-

Lex Fridman 47:52 Well, I talked to, uh, I talked to Lee, uh, recently on this, on this podcast-

Harry Cliff 47:57 Yeah.

Lex Fridman 47:57 ... and he's, uh, he's a reminder to me that, uh, the physics community has, like, so many interesting characters in it (laughs) .

Harry Cliff 48:05 Yeah.

Lex Fridman 48:05 It's fascinating.

Harry Cliff 48:06 Yeah.

Lex Fridman 48:07 Anyway, sorry, so ...

Harry Cliff 48:08 I mean, as an experimentalist, I tend to sort of think, uh, these are interesting ideas, but they're not really testable, so I tend not to think about them very much.

Lex Fridman 48:14 Okay.

Harry Cliff 48:14 So, I mean, going back to the science of this-... there wasn't, there, there is an explanation. There is a possible solution to this problem of the Higgs, which doesn't involve multiverses or creators fiddling about with the laws of physics. I- if, the, the most popular solution was something called supersymmetry, which is a theory, uh, which is invol- involves a new type of symmetry of the universe. In f- in fact, it's one of the last types of symmetries that is possible to have that we haven't already seen in nature, which is a symmetry between force particles and matter particles. So what we call fermions, which are the force, the matter particles, and bosons, which are the force particles. And if you have supersymmetry, then there is a super partner for every particle in the standard model. And the, without going into the details, the effect of this basically is that you have a whole bunch of other fields, and these fields, uh, cancel out the effect of the standard model fields, and they stabilize the Higgs field at a nice sensible value. So in supersymmetry, you naturally, without any c- tinkering about with the constants of nature or anything, you get a Higgs field with a nice value, uh, which is the one we see. So this is one of the rea- and, and supersymmetry's also got lots of other things going for it. It predicts the existence of a dark matter particle, which would be great. It, you know, it potentially in- suggests that the, the strong force and the, the electroweak force unify at high energy. So lots of reasons people thought this was a productive idea. And when the LHC was, just before it was turned on, there was a lot of hype, I guess, a lot of, uh, an expectation that we would discover these super partners because... And particularly the main reason was that if, if supersymmetry stabilizes the Higgs field at this nice Goldilocks value, these super particles should have a mass around the energy that we're probing at the LHC, around the energy of the Higgs. So it was kind of thought you discover the Higgs, you probably discover super partners as well.

Lex Fridman 50:03 So once you start creating ripples in this Higgs field, you should be able to see these kinds of, uh-

Harry Cliff 50:08 Yeah.

Lex Fridman 50:08 ... you should be s- yeah, the ma-

Harry Cliff 50:10 So these super fields would be there.

Lex Fridman 50:11 Yeah.

Harry Cliff 50:11 When I, when at the very beginning I said, "We're probing the vacuum," what I mean is really that, you know, okay, let's say these super fields exist. The vacuum contains super fields, they're there, these supersymmetric fields. If we hit them hard enough, we can make them vibrate. We see super particles come flying out. That's the sort of, that's the idea.

Lex Fridman 50:26 That was the hope, okay, so-

Harry Cliff 50:26 That's the whole point.

Lex Fridman 50:29 Uh, but we-

Harry Cliff 50:29 But w-

Lex Fridman 50:29 ... we haven't.

Harry Cliff 50:30 But we haven't. So, so far at least, I mean we've had now a decade of data-taking at the LHC. Um, no signs of super partners have, supersymmetric particles have been found. In fact, no signs of any physics, any new particles beyond the stand- the model have been found. So supersymmetry is not the only thing that can do this. There are other theories that involve additional dimensions of space or potentially involve the Higgs boson being made of smaller things, being made of other particles.

Lex Fridman 50:58 Yeah, that's an interesting... you know, I haven't heard that before. That's really, that's an interesting point. Could you maybe linger on that? Like what, uh, what could be, wh- what could the Higgs particle be made of?

Harry Cliff 51:08 Well, so the, the oldest, I think the original ideas about this was these theories called technicolor, which were basically like an analogy with the strong force. So the idea was the Higgs boson was a bound state of two very strongly interacting particles that were a bit like quarks. So like quarks, but I guess higher energy things, with a super strong force. So not the strong force, but a new force that was very strong. And the Higgs was a bound state of, of these, these objects. And the Higgs would in principle, if that was right, would be the first in a series of technicolor particles. Technicolor, I think, uh, not being a theorist, but I th- it's not, it's basically not done very well for s- particularly since the LHC found the Higgs that kind of, it rules out, you know, a lot of these technicolor theories. But there are other things that are a bit like technicolor. So there's a theory called, um, partial compositeness, which is an idea that some of my colleagues at Cambridge have worked on, which is a, a similar sort of idea, that the Higgs is a, a bound state of some strongly interacting particles, and that the standard model particles themselves, the more exotic ones like the top quark, uh, are also sort of mixtures of these composite particles. So it's a kind of, uh, an extension to the standard model, which explains this problem with the Higgs boson's Goldilocks value, but also, um, helps us understand we have, we're in this situation now, again, a bit like the periodic table where we have six quarks, six leptons in this kind of, you can arrange them in this nice table and they, you can see these columns where the patterns repeat and you go, "Mm, okay. Maybe there's something deeper going on here," as, you know. And, and so this would potentially be something, this partial compositeness theory could explain, uh, sort of enlarge this picture that allows us to see the whole symmetrical pattern and understand what the ingredients. Why do we have with... So one of the big questions in particle physics is, why are there three copies of the matter particles?

Lex Fridman 53:05 Mm-hmm.

Harry Cliff 53:06 So in what we call the first generation, which is what we're made of, there's the electron, uh, the electron neutrino, the up quark and the down quark. They're the most common matter particles in the universe. But then there are copies of these four particles in the second and the third generations. So things like muons and top quarks and other stuff. We don't know why. We see these patterns, we have no idea where it comes from. So that's another big question, you know. Can we find out the deeper order that explains this particular ta- periodic table of particles that we see?

Lex Fridman 53:36 Uh, is it possible that the, uh, the deeper order includes like almost a single entity? So like s- something that I guess like string theory dreams about?

Harry Cliff 53:45 Mm-hmm.

Lex Fridman 53:46 Is this, is this e- is this pa- is this essentially the dream, is to discover something s- simple, beautiful, and unifying?

Harry Cliff 53:54 Yeah. I mean, that is the dream. And it, well... and, and I think for some people, for a lot of people, it still is the dream. So there's a great book by, uh, Steven Weinberg, who is one of the theoretical physicists who was instrumental in building the standard model. So he came up with some others with the electroweak theory, the theory that unified electromagnetism and, and the weak force. And he wrote this book, I think it was towards the end of the '80s, early '90s called Dreams of a Final Theory, which is a very lovely, quite short book about this idea of a final unifying theory that brings everything together. And I think you get a sense reading his book written at the end of the '80s, early '90s, that there was this feeling that such a theory was-... coming. Um, and that was the time when string theory had been, was, was very exciting. So string theory, there's been this thing called the superstring revolution and theoretical physics getting very excited. They discovered these theoretical objects, these little vibrating loops of string that, in principle, not only was a quantum theory of gravity, but could explain all the particles in the standard model and bring it all together, and you, as you say, you have one object, the string, and you can pluck it, and the way it vibrates gives you these different notes, each of which is a different particle. So it's a very lovely idea, um, but the problem is that, well there's, there's a few... People discovered that mathematics is very difficult, so people have spent three decades or more trying to understand string theory, and I think, you know, if you spoke to most string theorists, they would probably freely admit that no one really knows what string theory is yet.

Lex Fridman 55:24 Mm-hmm.

Harry Cliff 55:25 I mean, there's been a lot of work, but it's not really understood. And, um, the other problem is that string theory mostly makes predictions about physics that occurs at energies far beyond what we will ever be able to probe in the laboratory.

Lex Fridman 55:40 Ever?

Harry Cliff 55:40 Yeah. Probably ever. But-

Lex Fridman 55:42 By the way, so sorry, take a million tangents-

Harry Cliff 55:44 Yeah.

Lex Fridman 55:44 ... but, uh, is there room for complete innovation of how to build a particle collider that could give us an order of magnitude increase in-

Harry Cliff 55:53 Mm-hmm.

Lex Fridman 55:53 ... in the kind of energies? Or do we need to keep just increasing (laughs) the size of thing?

Harry Cliff 55:58 I mean, maybe, yeah. I mean, there, there are ideas, but to give you a sense of the gulf that has to be bridged, so the LHC, uh, collides particles at an energy of, uh, what we call 14 tera-electron volts, so that's basically the equivalent of you accelerated a proton through 14 trillion volts. That gets us to the energies where the Higgs and these weak particles live. They're very massive. The, the scale where strings become manifest is something called the Planck scale, which I think is of the order of 10 to the, uh, hang on, get this right, it's 10 to the 18 giga-electron volts, so about 10 to the 15, um, tera-electron volts. So you're talking, you know, trillions of times more energy, more that-

Lex Fridman 56:45 So 10, uh, yeah, 10 to the 15th or 10 to the 14th larger, oh boy, I don't even-

Harry Cliff 56:49 I may have got the number slightly wrong, but it's, it's of that order. It's a very big number. Um, so, you know, we're not talking just an order of magnitude increase in energy, we're talking 14 orders of magnitude energy increase. So to give you a sense of what that would look like, were you to build a particle accelerator with today's technology...

Lex Fridman 57:04 Bigger or smaller than, than our solar system? (laughs)

Harry Cliff 57:07 As s- the size of the galaxy.

Lex Fridman 57:09 The galaxy?

Harry Cliff 57:10 So you'd need to put-

Lex Fridman 57:10 Yes.

Harry Cliff 57:10 ... a particle accelerator that circled the Milky Way, to, to get to the energies where you would see strings, if they exist. So that is a fundamental problem, which is that most of the predictions of the unified, these unified theories, quantum theories of gravity, only make statements that are testable at energies that we will not be able to probe. Um, l- and barring some unbelievable, you know, completely unexpected (laughs) technological or scientific breakthrough, which is almost impossible to imagine. You never, never say never, but it seems very unlikely.

Lex Fridman 57:42 Yeah. I can just see the news story, "Elon Musk decides to build-"

Harry Cliff 57:47 (laughs)

Lex Fridman 57:47 "... a particle collider the size of our galaxy."

Harry Cliff 57:50 It would have to be, we'd have to get together with all our galactic neighbors to, to pay for it, I think.

Lex Fridman 57:55 What is the exciting possibilities of the Large Hadron Collider? What is there to be discovered in this, in this order of magnitude, of scale?

Harry Cliff 58:04 Mm-hmm.

Lex Fridman 58:04 Is there other bigger efforts on the horizon, like, in this space, what are, what are the open problems, the exciting possibilities? You mentioned supersymmetry.

Harry Cliff 58:14 Yeah. So, well there, there are lots of new ideas. Well, there are lots of problems that we're facing. So there's a problem with the Higgs field, which supersymmetry was, was supposed to solve. Um, there's the fact that 95% of the universe, we know from cosmology and astrophysics, is invisible, that it's made of dark matter and dark energy, which are really just words for things that we don't know what they are.

Lex Fridman 58:35 Uh-huh.

Harry Cliff 58:35 It's what Donald Rumsfeld called the known unknown.

Lex Fridman 58:37 (laughs)

Harry Cliff 58:38 So we, we know we don't know what they are.

Lex Fridman 58:39 Well, that's, it's better than an unk- unknown unknown.

Harry Cliff 58:42 Yeah, well, there may be some unknown unknowns, but by-

Lex Fridman 58:44 Within that...

Harry Cliff 58:44 ... definition, we don't know what those are. (laughs) So, yeah.

Lex Fridman 58:47 But, but, uh, the, the hope is the, uh, uh, a particle, um, accelerator could help us make sense of dark energy, dark matter? There's still, there's, uh, some hope for that?

Harry Cliff 58:57 There's hope for that, yeah. So one of the hopes is the LHC could produce a dark matter particle in its collisions. And, you know, it may be that, uh, the LHC will still discover new particles, that it might still... Supersymmetry could still be there, we just, it's just maybe more difficult to, to find than we thought originally, and, and, you know, dark matter particles might be being produced, but we're just not looking in the right part of the data for them. That, that's possible. It might be that we need more data, that these processes are very rare and we need to collect lots and lots of data before we see them. But I think m- a lot of people would say now that, um, the chances of the LHC directly discovering new particles in the near future is quite slim. It may be that we need a decade more data before we can see something. Or we may not see anything.

Lex Fridman 59:43 Mm-hmm.

Harry Cliff 59:44 That's the, that's where we are. So, I mean, the, the, the physics, the experiments that

Harry Cliff 59:48 I work on, so I work on a detector called LHCb, which is one of these four big detectors that are spaced around the ring, and we do slightly different stuff to, uh, the big guys. There's two big experiments called ATLAS and CMS, uh, 3,000 physicists and scientists and computer scientists on them each. They're the ones that discovered the Higgs and they look for supersymmetry and dark matter and so on. What we look at are standard model particles called b quarks, which depending on your, your preference is either bottom or beauty. We tend to say beauty-

Lex Fridman 1:00:17 Beauty, yeah.

Harry Cliff 1:00:17 ... because it sounds sexier.

Lex Fridman 1:00:18 For sure.

Harry Cliff 1:00:18 Yeah.

Lex Fridman 1:00:19 For sure.

Harry Cliff 1:00:20 Um, but these particles, um, are interesting because they, uh, we can make lots of them, we make billions, or billy- hundreds of billions of these things. You can therefore measure their properties very precisely, so you can make these really lovely precision measurements. And-What we are doing really is a sort of complementary thing to the other big experiments, which is they, if you think, the sort of analogy they'll often use is if you imagine you're looking in, you're in a jungle and you're looking for, um, an elephant, say.

Lex Fridman 1:00:48 Mm-hmm.

Harry Cliff 1:00:49 And you are a hunter, and you're kind of like... Let's say there's the elephant's very rare. You don't know where in the jungle. The jungle's big. So there's two ways you'd go about this. Either you can go wandering around the jungle and try and find the elephant. The problem is if the elephant, if there's only one elephant and the jungle's big, the chances of running into it are very small. Or you could look on the ground and see if you see footprints left by the elephant. And if the elephant's moving around, you've got a chance that, you've got a better chance maybe of seeing the elephant's footprints. If you see the footprints, you go, "Okay, there's an elephant. I maybe don't know what kind of elephant it is, but I got a sense there's something out there." So that's sort of what we do. We are the footprint people. We-

Lex Fridman 1:01:23 (laughs)

Harry Cliff 1:01:23 ... we are, we're looking for the footprints, the impressions that quantum fields that we haven't managed to directly create the particle of, the effects these quantum fields have on the ordinary standard model fields that we already know about. So these-

Lex Fridman 1:01:37 I see.

Harry Cliff 1:01:37 ... these b particles, the way they behave can be influenced by the presence of, say, superfields or dark matter fields or whatever you like. Um, and they're, the way they decay and behave can be altered slightly from what our theory tells us they ought to behave.

Lex Fridman 1:01:52 Got you. And it's easier to collect huge amounts of data on, on b-

Harry Cliff 1:01:55 Yeah.

Lex Fridman 1:01:55 ... on b quarks.

Harry Cliff 1:01:56 We get, you know, billions and billions of these things. You can make very precise measurements. And the, the only place really at the LHC, or in really in high energy physics at the moment, where there's fairly compelling evidence that there might be something beyond the standard model is in these b, these beauty quarks decays.

Lex Fridman 1:02:15 Just to clarify, um, which, oh, is the difference between the different, the four experiments, for example, that you mentioned, is it the kind of particles that are being collided? Is it the energies that were which they're collided? What's the, what's the fundamental difference between-

Harry Cliff 1:02:29 No, it's that the-

Lex Fridman 1:02:29 ... the different experiments?

Harry Cliff 1:02:30 The collisions are the same. Um, what's different is the design of the detectors. So ATLAS and CMS are called, they're called, what are called general purpose detectors, and they are basically barrel-shaped machines, and the collisions happen in the middle of the barrel, and the barrel captures all the particles that go flying out in every direction. So in a sphere, effectively, they come flying out, and it can record all of those particles. And-

Lex Fridman 1:02:52 What's the... Sorry to be interrupting, but what's the, what's the mechanism of the recording?

Harry Cliff 1:02:57 Oh. So these detectors, I don't know if you've seen pictures of them. They're huge, like ATLAS is, uh, 25 meters high and 45 meters long. They're vast machines. Um, instruments, I guess you should call them really. Uh, they are, they're kind of like onions. So they have layers, concentric layers of detective, detectors, different sorts of detectors. So close into the beam pipe, you have what are called, usually made of silicon. They're tracking detectors, so they're little, made of strips of silicon or pixels of silicon, and when a particle goes through the silicon, it gives a little electrical signal, and you get these dots, you know, electrical dots through your detector, which allows you to reconstruct the trajectory of the particle. So that's the middle. And then the outsides of these detectors, you have things called calorimeters, which measure the energies of the particles, and on the very edge, you have, um, things called muon chambers, which basically me- These muon particles, which are the heavy version of the electron, they are, they're like high velocity bullets, and they can get right to the edge of the detector. So if you see something at the edge, that's a muon. So that's broadly how they work.

Lex Fridman 1:03:54 And all of that is being recorded?

Harry Cliff 1:03:55 That's all being fed out to, you know, computers that-

Lex Fridman 1:03:58 The data must be awesome. Okay. Uh-

Harry Cliff 1:04:00 But L- so LHCb is different. So we, because we're looking for these b quarks-

Lex Fridman 1:04:04 Yes.

Harry Cliff 1:04:04 ... b quarks tend to be produced, uh, along the beam line. So in a collision, the b quark tend to fly sort of close to the beam pipe.

Lex Fridman 1:04:12 Mm-hmm.

Harry Cliff 1:04:12 So we built a detector that's sort of pyramid, cone-shaped basically-

Lex Fridman 1:04:16 Oh.

Harry Cliff 1:04:16 ... that just looks in one direction. So we ignore... If you have your collision, stuff goes everywhere. We ignore all the stuff over here and going off sideways. We're just looking in this little region, uh, close to the beam pipe where most of these b quarks are made. So-

Lex Fridman 1:04:29 Is there a different aspect of the sensors involved in the collection of the b quark trajectories?

Harry Cliff 1:04:37 Yeah. There are some differences. So one of the differences is that one of the ways you know you've seen a b quark is that b quarks are actually quite long lived by particle standards. So they live for 1.5 trillionths of a second, which is if you're, if you're a fundamental particle, is a very long time.

Lex Fridman 1:04:51 Mm.

Harry Cliff 1:04:52 (laughs) 'Cause, you know, the Higgs boson I think lives for about, uh, a trillionth of a trillionth of a second, or maybe even less than that. So these are quite long-lived things, and they will actually fly a little distance before they decay. So they will fly, you know, a few centimeters maybe, if you're lucky, then they'll decay into other stuff. So what we need to do, in the middle of the detector, you wanna be able to see... You have your place where the protons crash into each other, and that produces loads of particles that come flying out. So you have loads of lines, loads of tracks that point back to that proton collision, and then you're looking for a couple of other tracks, maybe two or three, that point back to a different place that's maybe a few centimeters away from the proton collision, and that's the sign that little b particle has flown a few centimeters and decayed somewhere else. So we need to be able to very accurately resolve the proton collision from the b particle decay. So we are, the middle of our detector is very sensitive, and it gets very close to the collision. So you have this really beautiful, delicate silicon detector that sits, I think it's seven mil, millimeters, from the beam, and the LHC beam has as much energy as a jumbo jet at takeoff. So it's enough to melt a ton of copper.

Lex Fridman 1:05:57 Nice.

Harry Cliff 1:05:57 So you have this furiously powerful thing sitting next to this tiny, delicate, you know, sen- silicon sensor. Um, so the in- so those aspects of our detector that are specialized to des- to disc- to measure these particular b quarks that we're interested in.

Lex Fridman 1:06:10 And is there... I mean, I remember seeing somewhere that there's, uh, some mention of matter and antimatter connected to the b, the, these beautiful quarks.

Harry Cliff 1:06:18 Mm.

Lex Fridman 1:06:18 Is that... Um, what, what's the connection, uh, wha- uh, yeah, what, what's the connection there?

Harry Cliff 1:06:26 Yeah. So there is a connection, which is that, um-... when you produce these B particles, the B, these particles can... 'cause you don't see the B quark, you see the thing the B quark is inside. So they're bound up inside what we call beauty particles, where the B quark is joined together with another quark or two, maybe two other quarks, depending on what it is. There are a particular set of, uh, these B particles that exhibit this property called oscillation. So if you make a, for the sake of argument, a matter version of one of these B particles, as it travels, because of the magic of quantum mechanics, it oscillates backwards and forwards between its matter and antimatter versions. So it does this weird flipping about, backwards and forwards. And what we can use this for is a laboratory for testing the symmetry between matter and antimatter. So if the, if the symmetry between matter and antimatter is precise, it's exact, then we should see these B particles decaying as often as matter as they do as antimatter, 'cause this oscillation should be even. It should spend mo- as much time in each state. But what we actually see is that one of the states, it spends more time in. It's more likely to decay in one state than the other. So this gives us a way of testing this fundamental symmetry between matter and antimatter.

Lex Fridman 1:07:39 So what can you... sort of returning to the, the question we were... before about this fundamental symmetry, it seems like if there's perfect symmetry between, uh, matter and an- antimatter, if we have the equal amount of each in our universe-

Harry Cliff 1:07:54 Mm-hmm.

Lex Fridman 1:07:54 ... it would just destroy itself.

Harry Cliff 1:07:56 Mm-hmm.

Lex Fridman 1:07:57 And just like you mentioned, we seem to live in a very unlikely universe where it, it doesn't destroy itself.

Harry Cliff 1:08:03 Yeah.

Lex Fridman 1:08:03 So, uh, do you have some intuition about, about why that is?

Harry Cliff 1:08:07 I mean, (laughs) well, I, I, I'm not a theor- I don't have any particular ideas myself. I mean, I, I sort of do ex- measurements to try and test these things. But the... I mean, so in terms of the basic problem is that, in the Big Bang, if you use the standard model to figure out what ought to have happened, you should've got equal amounts of matter and antimatter made. 'Cause whenever you make a particle, in our coll- collisions, for example, when we collide stuff together, you make a particle, you make an antiparticle. They always come together. They always annihilate together. So there's no way of making more matter than antimatter that we've discovered so far. So that means, in the Big Bang, you get equal amounts of matter and antimatter. As the universe expands and cools down during the Big Bang, not very long after the Big Bang, I think a few seconds after the Big Bang, you have this event called the Great Annihilation, which is where all the particles and antiparticles smack into each other, annihilate, turn into light mostly, and you end up with a universe later on. If that was what happened, then the universe we live in today would be black and empty, apart from some photons. That would be it. So there's stuff in the u- there's, there is stuff in the universe. It appears to be just made of matter, so there's this big mystery as to where the... how did this happen? And there are various ideas, um, which all involve sort of physics going on in the first trillionth of a second or so of the Big Bang. So it, it could be that... one possibility is that the Higgs field is somehow implicated in this, that there was this event that took place in the early universe where the Higgs field basically switched on. It, it acquired its modern value.

Lex Fridman 1:09:31 Mm-hmm.

Harry Cliff 1:09:31 And when that happened, um, this caused all the particles to acquire mass, and, and the universe basically went through a phase transition where you had a hot plasma of massless particles, and then in that plasma, it's almost like a, a gas turning into droplets of water. You get kind of these little bubbles forming in the universe where the Higgs field has acquired its modern value. The particles have got mass. And this phase transition, in some models, can cause more matter than antimatter to be produced, depending on how matter bounces off these bubbles in the early universe. So that's one idea. There's other ideas to do with neutrinos, that there are exotic types of neutrinos that can decay, uh, in a biased way to just matter and not to antimatter. So, uh, and people are trying to test these ideas. That's what we're trying to do at LHCb. It's, there's neutrino experiments planned that are trying to do these sorts of things as well. So yeah, there are ideas, but at the moment, no clear evidence for which of these ideas might

Harry Cliff 1:10:22 be right.

Lex Fridman 1:10:23 S- so we're talking about some incredible ideas. By the way, never heard anyone be so eloquent about describing, uh, even just the standard model. So, uh, I'm in awe just listening.

Harry Cliff 1:10:34 (laughs) Oh, thank you.

Lex Fridman 1:10:35 And ju- and just in- and just ha- having fun enjoying it. So the, yes, the theoretical, the, the particle physics is fascinating here. To me, one of the most fascinating things about the Large Hadron Collider is the human side of it-

Harry Cliff 1:10:48 Mm-hmm.

Lex Fridman 1:10:48 ... that a bunch of sort of brilliant people that probably have egos got together and were collaborate together, and countries, I guess, collaborate together, you know, for the funds and e- uh, ev-

Harry Cliff 1:11:00 Mm-hmm.

Lex Fridman 1:11:01 ... it's just collaboration everywhere. Could you maybe... uh, I, I don't know what the right question here to ask, but almost, what's your intuition about how it was possible to make this happen, and what are the lessons we should learn for the future of human civilization in terms of our scientific progress? 'Cause it seems like this is a great, great illustration of us working together to do something big.

Harry Cliff 1:11:24 Yeah. I think it's possibly the best example, maybe a- a... I can think of, of international collaboration that isn't for some unpleasant purpose, basically (laughs) . You know, I, it's, I mean, so I, I, when I started out in the field in 2008, I, as a new PhD student, the LHC was basically finished. So I didn't have to go around asking for money for it or trying to make the case. So I have huge admirat- admiration for the people who managed that, 'cause this was a project that was first imagined in the 1970s, in the late '70s was when the first conversations about the LHC were, were mooted. And it took two and a half decades of campaigning and fundraising and persuasion until they started breaking ground and building the thing in the early noughties, in 2000. So when I think the reason, just from a sort of, from the point of view of the sort of science, the scientists there, I think the reason it works, ultimately, is that everywhere, everyone there is there for the same reason, which is, well, in principle at least, they're there because they're interested in the world. They want to find out, you know, what are the basic ingredients of our universe? What are the laws of nature? And so everyone is pulling in the same direction. Now of course, everyone has their own...... things they're interested in, everyone has their own careers to consider, and, you know, I wouldn't pretend that there isn't also a lot of competition. So there's this funny thing in these experiments where your collaborators, your 800 collaborators in LHCb, but you're also competitors, because you're academics in your various universities, and you wanna be the one that gets the paper out on the most exciting, you know, new measurements. So there's this funny thing where you're kind of trying to stake out your territory while also collaborating and having to work together to make the experiments work. And it does work amazingly, um, well, actually, considering all of that. And I think there was actually, I think McKinsey or one of these big management consultancy firms went into CERN, uh, maybe a decade or so ago, to try to understand how these organizations functioned.

Lex Fridman 1:13:15 Did they figure it out? (laughs)

Harry Cliff 1:13:16 I don't think they could.