- I mean, wasn't the Higgs called the God particle at some point? - It was by a guy trying to sell popular science books, yeah. - Yeah, but I mean, I remember 'cause when I was hearing it, I thought it would, I mean, that would solve a lot of, that unify a lot of our ideas of physics, was my notion.

But maybe you can speak to that. Is it as big of a leap? Is it a God particle, or is it a Jesus particle? (laughing) Which, you know, what's the big contribution of Higgs in terms of this unification power? - Yeah, I mean, to understand that, it maybe helps to know the history a little bit.

So when the, what we call electro weak 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 plus boson, the W minus 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 W's 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? Well, actually the tunnel that the LHC is in was not built for the LHC.

It was built for a previous accelerator called the large electron positron collider. So that began operation in the late '80s, early '90s. They basically, that's when they dug the 27 kilometer tunnel. They put this accelerator into it, the 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 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 can only make a few of them at the previous accelerator. So 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 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 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 because 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 is not there. So in a way, it depends how you look at it. The discovery of the Higgs on its own is obviously a huge achievement in many, both experimentally and theoretically.

On the other hand, 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 kind of know that that piece must be there somewhere, right? So the discovery in itself, although it's important, is not so interesting. - It's like a confirmation of the obvious at that point.

- But what makes it interesting is not that it just completes the standard model, which is a theory that we've known, had the basic layout of for 40 years or more now. It's that the Higgs actually is a unique particle. It's very different to any of the other particles in the standard model.

And it's a theoretically very troublesome particle. There are a lot of nasty things to do with the Higgs, but also opportunities. So 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. - That's still a notion. So the Higgs might not be a fundamental particle, that there might be some, oh man. So that is an idea. It's not been demonstrated to be true.

But I mean, all of these ideas basically come from the fact that this is a problem that motivated a lot of development in physics in the last 30 years or so. And it's this basic fact that 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 all the other fields in that, let's say you take the electromagnetic field, which is, 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 like the entire space has got this energy stored in the Higgs field, which is not zero. It's finite. It's a bit like having the temperature of space raised to some background temperature. 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. 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 tendency, 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, or it explodes to an enormous value, what we call the Planck scale, which is the scale of quantum gravity.

And 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 strength of the Higgs field is 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. - Well, yeah, I guess, so what would be, so two explanations.

One, there's a God that designed this perfectly, and two is there's an infinite number of alternate universes, and we just happen to be in the one in which life is possible. - Yeah, yeah. - Complexity. So when you say, I mean, life, any kind of complexity, that's not either complete chaos or black holes.

- Yeah, yeah. - 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 is there. What do you make of that? Yeah, what do you make of that? - I mean, yeah, so you laid out two of the possible explanations.

- Really? - Somewhat. Yeah, I mean, well, someone, some cosmic creator went, 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 theoretically, I guess, it's possible. - Sorry to interrupt, but there could also be, not a designer, but couldn't there be just, I guess, I'm not sure what that would be, but some kind of force that, some kind of mechanism by which this kind of field is enforced in order to create complexity.

Basically, forces that pull the universe towards an interesting complexity. - I mean, yeah, I mean, there are people that have those ideas. I don't really subscribe to them. - As I'm saying, it sounds really stupid. - No, I mean, there are definitely people that make those kind of arguments.

There's ideas that, I think it's Lee Smolin's idea, one, I think, that universes are born inside black holes. And so universes, they basically have like Darwinian evolution of the universe, where universes give birth to other universes. And if 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. - Well, I talked to Lee recently on this podcast, and he's a reminder to me that the physics community has so many interesting characters in it. It's fascinating. Anyway, sorry, so. - I mean, as an experimentalist, I tend to sort of think, these are interesting ideas, but they're not really testable, so I tend not to think about them very much.

So, I mean, going back to the science of this, 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. If the most popular solution was something called supersymmetry, which is a theory which involves a new type of symmetry of the universe.

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 matter particles, and bosons, which are force particles. And if you have supersymmetry, then there is a super partner for every particle in the standard model.

And without going into the details, the effect of this basically is that you have a whole bunch of other fields, and these fields 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 tinkering about with the constants of nature or anything, you get a Higgs field with a nice value, which is the one we see.

So this is one of the reasons, 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 potentially suggests that the strong force and 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 an expectation that we would discover these super partners, because, and particularly the main reason was that 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. - So once you start creating ripples in this Higgs field, you should be able to see these kinds of, you should be, yeah. - So these super fields would be there. 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. - That's the hope, okay. - That's the whole point. - But we haven't. - But we haven't.

So, so far at least, I mean, we've had now a decade of data taking at the LHC. No signs of super partners have, supersymmetric particles have been found. In fact, no signs of any physics, any new particles beyond the standard 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. - 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, what could be, what could the Higgs particle be made of?

- Well, so 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 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, not being a theorist, but it's not, it's basically not done very well, 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 partial compositeness, which is an idea that some of my colleagues at Cambridge have worked on, which is a similar sort of idea that the Higgs is a bound state of some strongly interacting particles, and that the standard model particles themselves, the more exotic ones like the top quark, are also sort of mixtures of these composite particles.

So it's a kind of an extension to the standard model, which explains this problem with the Higgs bosons, Goldilocks value, but also helps us understand, we're in a situation now, again, a bit like the periodic table, where we have six quarks, six leptons, in this kind of, you can arrange in this nice table, and you can see these columns where the patterns repeat, and you go, "Hmm, okay, "maybe there's something deeper going on here." And so this would potentially be something, this partial compositeness theory, could explain, sort of enlarge this picture that allows us to see the whole symmetrical pattern, and understand what the ingredients, why do we have, so one of the big questions in particle physics is, why are there three copies of the matter particles?

So in what we call the first generation, which is what we're made of, there's the electron, 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, can we find out the deeper order that explains this particular periodic table of particles that we see? - Is it possible that the deeper order includes almost a single entity?

So like something that I guess like string theory dreams about, is this essentially the dream? Is to discover something simple, beautiful and unifying? - Yeah, I mean, that is the dream. And I think for some people, for a lot of people, it still is the dream. So there's a great book by 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 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. And that was the time when string theory had been, was very exciting. So string theory, there's been this thing called the super string 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 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. But the problem is that, well, there's a few people discover 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. I mean, there's been a lot of work, but it's not really understood. And the other problem is that string theory mostly makes predictions about physics that occurs energies far beyond what we will ever be able to probe in the laboratory.

Yeah, probably ever. - By the way, so sorry, take a million tangents, but is there room for complete innovation of how to build a particle collider that could give us an order of magnitude increase in the kind of energies, or do we need to keep just increasing the size of things?

- I mean, maybe, yeah. I mean, there are ideas, to give you a sense of the gulf that has to be bridged. So the LHC collides particles at an energy of what we call 14 tera electron volts. So that's basically the equivalent of you've 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 scale where strings become manifest is something called the Planck scale, which I think is of the order 10 to the, hang on, get this right, it's 10 to the 18 giga electron volts.

So about 10 to the 15 tera electron volts. So you're talking, you know, trillions of times more energy, more than- - Yeah, 10 to the 15, or 10 to the 14th larger. - I may be wrong, but it's of that order. It's a very big number. 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- - Bigger or smaller than our solar system? - As the size of the galaxy. - The galaxy. - So you'd need to put a particle accelerator that circled the Milky Way 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 these unified theories, quantum theories of gravity, only make statements that are testable at energies that we will not be able to probe. And barring some unbelievable, you know, completely unexpected technological or scientific breakthrough, which is almost impossible to imagine.

You never say never, but it seems very unlikely. - Yeah, I can just see the news story. Elon Musk decides to build a particle collider the size of our- - It would have to be, we'd have to get together with all our galactic neighbors to pay for it, I think.

It'd be like CERN on mega steroids. (static) (silence) (silence) (silence) (silence) (silence) (silence) (silence) you