Harry Cliff is a particle physicist at the University of Cambridge working on the Large Hadron Collider beauty experiment that specializes in searching for hints of new particles and forces by studying a type of particle called the "beauty quark", or "b quark". In this way, he is part of the group of physicists who are searching answers to some of the biggest questions in modern physics. He is also an exceptional communicator of science with some of the clearest and most captivating explanations of basic concepts in particle physics I've ever heard.

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EPISODE LINKS:
Harry's Website: https://www.harrycliff.co.uk/
Harry's Twitter: https://twitter.com/harryvcliff
Beyond the Higgs Lecture: https://www.youtube.com/watch?v=edvdzh9Pggg
Harry's stand-up: https://www.youtube.com/watch?v=dnediKM_Sts

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OUTLINE:
0:00 - Introduction
3:51 - LHC and particle physics
13:55 - History of particle physics
38:59 - Higgs particle
57:55 - Unknowns yet to be discovered
59:48 - Beauty quarks
1:07:38 - Matter and antimatter
1:10:22 - Human side of the Large Hadron Collider
1:17:27 - Future of large particle colliders
1:24:09 - Data science with particle physics
1:27:17 - Science communication
1:33:36 - Most beautiful idea in physics

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a conversation with Harry Cliff, 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 physicists 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 Theater, which has hosted lectures for over 2 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.

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And now, here's my conversation with Harry Cliff.

Let's start with probably 1 of the coolest things that human beings have ever created, the Large Hadron Collider, LHC.

Okay, so it's essentially this gigantic 27 kilometer circumference particle accelerator.

It's buried about a hundred 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 the analogy is actually fairly precise.

Effectively, except it's a microscope that looks at the structure of the vacuum.

In order for this kind of thing to study particles, which are the microscopic entities, it has to be huge, gigantic microscope.

Okay, so I mean, so particle physics as a field is kind of badly named in a way because particles are not the fundamental ingredients of the universe.

So the things that we believe are the real building blocks of the universe are invisible, fluid-like objects called quantum fields.

These are fields like the magnetic field around a magnet that exist everywhere in space.

In fact, actually, it's funny that we're in the wrong Institution because 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, it's a very famous experiment that he did where he got a magnet and put on top of it a piece of paper and then sprinkled iron filings.

He found the iron filings arranged themselves into these kind of loops, which was actually mapping out the invisible influence of this magnetic field.

We've all experienced, we've all held a magnet or 2 poles of a magnet and pushed them together and felt this thing, this force pushing back.

And 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.

You know, I don't, I apologize, perhaps for the ridiculous question.

Are you comfortable with the idea of the fundamental nature of our reality being fields?

Because to me, particles, you know, a bunch of different building blocks makes more sense sort of intellectually, sort of visually, like it seems to, I seem to be able to visualize that kind of idea easier.

Are you comfortable psychologically with the idea that the basic building block is not a block, but a field?

And it's, well, it comes from a misunderstanding of what particles are.

So like when we do science at school and we draw a picture of an atom.

You draw 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.

The electron is not like a little planet orbiting the atom.

It's this spread out, wibbly wobbly wave-like thing.

We've known that since the early 20th century, thanks to quantum mechanics.

We carry on using this word particle because sometimes when we do experiments, particles do behave like they're little marbles or little bullets.

In the LHC, when we collide particles together, you'll get hundreds of particles flying out through the detector.

They all take a trajectory, and you can see from the detector where they've gone, and they look like they're little bullets.

When you really study them carefully, you'll see that they are not little spheres.

They are these ethereal disturbances in these underlying fields.

This is really how we think nature is, which is surprising, but also I think kind of magic.

So we are, our bodies are basically made up of like little knots of energy in these invisible objects that are all around us.

And what is the story of the vacuum when it comes to LHC?

we go back to the physics we do know, 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 2 other types of particles.

There's an up, something called an up quark, and a down quark.

And those 3 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.

So 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.

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.

We know about a certain number of fields in what we call the standard model of particle physics.

The most recent 1 we discovered was the Higgs field.

The way we discovered the Higgs field was to make a little ripple in it.

What the LHC did, it fired 2 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 8 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 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.

And So the Large Hadron Collider, in order to get that ripple in the Higgs field, it requires a huge amount of energy, 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 particle collider?

So why does bigger allow you for higher energy collisions?

Right, So the reason, well, it's kind of simple really, which is that there are 2 types of particle accelerator that you can build.

1 is circular, which is like the LHC, the other is a great long line.

So the advantage of 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 2 million volt electric fields inside them, which are timed so that when a proton goes through 1 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 proton gets attracted and kicked out the other side so it gets a bit faster.

So you send it, and then you send it back around again.

like the timing of that, the synchronization, wait, really?

I think there's going to be a multiplicative effect on the questions I have.

Okay, let me just take that tension for a second.

The orchestration of that, is that fundamentally a hardware problem or a software problem?

I mean, I should first of all say I'm not an engineer.

So there are people much, much better at this stuff than I

But maybe, But from your sort of intuition, from the echoes of what you understand, what you heard of how it's designed, what's your sense?

Okay, I mean, okay, there's always challenges with everything.

But basically, you have these, the beams that go around the galaxy, the beams of particles are divided into little bunches.

So they're a bit like swarms of bees, if you like.

And there are around, I think it's something of the order

And 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 cars going past in a very fast motorway.

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 voltage in those boxes oscillates at hundreds of megahertz.

So the beams are like 40 megahertz, but it's oscillating much more quickly than the beam.

So I think, you know, it's difficult engineering, but in principle, it's not, you know, a really serious challenge.

There's probably engineers like screaming at you right now.

So in terms of coming back to this thing, why is it so big?

Well the reason is you want to get the particles through that accelerating element over and over again,

The question is why couldn't you make it smaller?

Well the basic answer is that these particles are going unbelievably quickly.

And if you think about, say, driving your car around a corner at high speed, If you go fast, you need a lot of friction in the tires to make sure you don't slide off the road.

So the limiting factor is how powerful a magnet can you make.

Because 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 machines 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 got to be.

LRH Okay, so maybe can we backtrack to the Standard Model and say what kind of particles there are, period, and maybe the history of assembling the standard model of physics and then how that leads up to the hopes and dreams and the accomplishments of the Large Hadron Collider.

Okay, so all of 20th century physics in like 5 minutes.

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 the atoms are indestructible, immutable little spheres, like the things we were talking about that don't really exist.

And there's 1 atom for every chemical element.

There's an atom for hydrogen, for helium, for carbon, for iron, etc.

Then in 1897, experiments done at the Cavendish Laboratory in Cambridge, which is where I'm based, showed that there are actually smaller particles inside the atom, which eventually became known as electrons.

These are these negatively charged things that go around the outside.

A few years later, Ernest Rutherford, a very famous nuclear physicist, 1 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 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 a few years, the nucleus, people start doing experiments with radioactivity where they use alpha particles that are spat out of radioactive elements 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 2000 times heavier than the electron.

Then 10 years later, more or less, a neutral particle is discovered called the neutron.

Those are the 3 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 1 of the forces of nature.

That's sort of where we get to by late 1932, more or less.

Then what happens is physics is nice and neat.

You've got 3 particles and all the atoms are made of, that's fine.

But then cloud chamber experiments, these are devices that can be used to- the first device is capable of imaging subatomic particles, so you can see their tracks.

They're used to study cosmic rays, particles that come from outer space and bang into the atmosphere.

In these experiments, people start to see a whole load of new particles.

So they discover, for 1 thing, antimatter, which is a sort of 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.

And then a whole load of other weird particles start to get discovered and no 1 really knows what they are.

Are these discoveries, are not the first theoretical discoveries or are they discoveries in an experiment?

So like, What's the process of discovery for these early sets of-

I mean, the early stuff around the atom is really experimentally driven.

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.

So in the discovery of the anti-electron, the positron, that was predicted from quantum mechanics and relativity by a very clever theoretical physicist called Paul Dirac, who was probably the second brightest physicist of the 20th century apart from Einstein, but isn't anywhere near as well known.

So he predicted the existence of the anti-electron from basically a combination of the theories of quantum mechanics and relativity, and it was discovered about a year after he made the prediction.

What happens when an electron meets a positron?

So if you, when you bring a particle and its antiparticle together, they react, well, they don't 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.

So when you have that kind of situation, why does the universe exist at all if there's matter and antimatter?

Oh God, now we're getting into the really big questions.

Yeah, let's take it slow with the standard model.

So, okay, so there's matter and antimatter in the 30s.

So matter, antimatter, and Then a load of new particles start turning up in these cosmic ray experiments, first of all.

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.

In the 1960s, in America particularly, but also in Europe and Russia, scientists started to build particle accelerators.

These are the forerunners of the LHC, big ring-shaped machines that were hundreds of meters long, which in those days was enormous.

Most physics up until that point had been done in labs, in universities, with small bits of kit.

When these accelerators are built, they start to find they can produce even more of these particles.

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.

As a quick kind of aside, and probably a really dumb question, but how is it possible to take something like a photon or electron and be able to control it enough, like to be able to do a controlled experiment where you collide it against something else.

Is that, that seems like an exceptionally difficult engineering challenge, because you mentioned vacuum too, so You basically want to remove every other distraction and really focus on this collision.

How difficult of an engineering challenge is that, just to get a sense?

I mean, in the early days, particularly when the first accelerators are being built, in like

Ernest Lawrence builds the first, what we call a cyclotron, which is like a little accelerator, this big or so.

There's another one- Luke Literally that big?

I mean, so most of the first accelerators were 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.

The colliding bit is relatively straightforward because you just fire it at whatever it is you want to fire it at.

The hard bit is steering the beams with the magnetic fields, getting strong enough electric fields to accelerate them, all that kind of stuff.

The first colliders where you have 2 beams colliding head on, that comes later.

And I don't think it's done until maybe the 1980s.

I'm not entirely sure, but it's a much harder problem.

That's crazy because you have to like perfectly get them to hit each other.

I mean, we're talking about, I mean, what scale, like what's the, I mean, the temporal thing is a giant mess, but the spatially, like the size, it's tiny.

Well, to give you a sense, so the LHC beams, the cross-sectional diameter is, I think, around a dozen or so microns.

And a beam, sorry, just to clarify, a beam contains how many, is it the bunches that you mentioned, is it

Oh no, no, the bunches contain, say, 100 billion protons each.

So a bunch is, it's not really bunch shaped, they're actually quite long.

They're like 30 centimeters long, but thinner than a human hair.

So like very, very narrow, long sort of objects.

So what happens in the LHC is you steer the beams so that they cross in the middle of the detector.

So they basically have these swarms of protons that are flying through each other.

And most of that, you have 100 billion coming 1 way, 100 billion another way, maybe 10 of them will hit each other.

Oh, okay, so this, okay, that makes a lot more sense.

That's nice, so you're trying to use sort of, it's like probabilistically, you're not.

You can't make a single particle collide with a single

You'd be waiting a very long time to get anything, yeah.

So you're basically, right, so you're relying on probability to be that some fraction of them are gonna collide, and then you know which, because it's a swarm of the same kind of particle.

So it doesn't matter which ones hit each other exactly.

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