Why Particles Might Not Exist | Sabine Hossenfelder, Hilary Lawson, and Tim Maudlin

The particle is just a vibration in the field. We don't have to postulate a metaphysical other thing. No, no serious physics. I don't think there's any serious physical theory that says that. People speak loosely that way, but it doesn't make any sense. So you can't think of particles as fluctuations of fields if the fields aren't in space-time. There's fields and particles as a separate sort of thing. Sure. There are always different ways to look at the same thing. So it doesn't really make sense to say the particle is the thing that is real if there's another description that's exactly as good but doesn't have the particle. You know, once we thought we knew that they were atoms, and then it became protons and neutrons, and now we're talking about quarks. And so the real question is basically what makes a particle a particle? Like, at what point do we give it a name and say, OK, this is a thing, as it were? And I think, Hilary, you refer to this as the sort of there's no end to these Chinese boxes. So what do you reckon? At what point do we say that something is a thing? Yeah. Well, I think, just to pick up on what Tim was saying there, I think the way that he described that is we've got these different accounts. You've got the particles. We've got fields. Then as an underlying thing, we could have spin networks, we could have strings, you know, whatever. And there's an idea that science usually operates with a rhetoric of, well, we're trying to find the right answer. And, of course, that's right. We're trying to find the best account. But the point I would want to make is you're not going to get to a final account. That's not what's going on. Of course, you can always try and improve a count, but usually the notion is that we're getting better, we're getting closer. And I don't think that's what's going on. It's you can have a certain way of holding the world, and you can apply that, and you can interpret it in that light, and you try and make it work. And you can then have another one, which achieves slightly different things, works in a slightly different way, enables you to intervene within that model. And as, I mean, Hawking came to the view in 2012 in his book that we operate within a model-theoretic realism, as he put it. And what he meant by that is that we operate within frameworks, theoretical models, and we can't get beyond those models. We can't somehow escape from the model to describe what's ultimately out there. So we should not imagine that at one day you or your children or your grandchildren are going to be told there's one version of what's going on out there, there's really fields or there's really information or there's really whatever it is. There will always be a variety of different models, and none of them reach through to say what the world is ultimately like. I feel like both of you should have a strong opinion about that. Do you want me to? Yeah, go on, talk to him. There are theories, right? There are proposals, there are hypotheses. You want them to be clearly articulated, sometimes they're not. Some articulated theories postulate their particles, some postulate their fields, some postulate their strings. Okay. Then the question is, what kind of evidence could we have that might make one of these plausible? Historically, there was a thing called the Parton model, which we now call the Quark model. Initially, when Murray Gell-Mann developed it, he's very clear. He said, this is just math. There's a mathematical way of finding all these particles. I can organize them mathematically by postulating, as it were, these purely mathematical quarks. Then he later was convinced, no, quarks actually exist. What convinced him? Deep inelastic scattering results from the Stanford Linear Accelerator Center. You just take electrons and whack protons really hard with them, and you notice that they're coming out as if they're three point-like scattering centers. So you have to look at any of these theories. What's the evidence? Sometimes the evidence becomes so overwhelming, we now know the moon is made of rock. We know it. Aristotle didn't know it. He thought it was made of ether. Why? We now have better evidence. So you have to ask, how good evidence can you have for these theories? And then you have to go theory by theory and see what does it predict, and so on. We have pretty good reason to believe that it's not Chinese boxes forever. We have really good reason to believe that electrons are not made of anything else. Why? Because of the electromagnetic form factor that goes into the scattering predictions. I mean, there's technical reasons for this. So this kind of very general high-level skepticism, I think, is inappropriate when you have definite theories and you're asking for definite reasons to believe them. Does that chime reasonably for you? Yeah. Broadly speaking, it's reasonable. Though one could say we don't yet have evidence that electrons are made of anything. I think it might be interesting to some people to know that it isn't all that easy to take the fundamental particles that we currently use in the Standard Model and just say they're made of something else. There's a fairly simple reason for this, which is if they're made of something else, then the constituent particles have to have smaller masses than the things that they combine to. But particles that have small masses would have been discovered long ago. Why? Because particle colliders produce all particles below the energy of the collision, generically. So you have to somehow get around this problem. And there are a few possibilities to get this done. One is to say, well, they interact very rarely or very weakly. Then you can do it. And the other thing to say is kind of similar as it is with the strong nuclear force, which is that they are bound together very strongly, and it takes a lot of energy to pull them apart. So this is why we don't have quarks floating around individually, but they're bound together in neutrons and protons. So that said, I think I kind of agree and disagree with both of you. So I think it's right, of course. You know, we call a thing a particle if we have evidence for it, and we have mathematics to explain this evidence. And then we can define something that we say, well, this is a particle. So this is my instrumentalist view on things. But that doesn't necessarily mean that this is the only way we can talk about it. There are actually quite interesting examples in the foundations of physics where you can have two completely different mathematical formulations that give you the same predictions for observables. That's called a duality. Maybe the best-known duality is the area of CFT duality that I don't want to talk about in great detail because then I'll still be talking tomorrow hours. But broadly speaking, it means that we can try to explain what we see in a particle collision as a sort of gravity theory in a space with one dimension more. So it's a completely different thing. And the important thing about this is that the stuff that is a particle, for us in the laboratory, is no longer a particle in this other description, which is mathematically exactly as valid. So I think that that is kind of a physics example that supports your point that there are always different ways to look at the same thing. So it doesn't really make sense to say the particle is the thing that is real if there's another description that's exactly as good but doesn't have the particle. And just to pick up on the point you're making, Smina, which is really exactly along the lines that I was arguing, which is that many of you might imagine that if you think of an electron, you probably have a picture of those drawings. You have a sort of atom in the middle, and the electrons are going around the outside like a ball. But, of course, there are plenty of circumstances where scientists are not thinking of it as a particle going round like that. But it's a probability space in which there's no particle to be found anywhere at all in that probability space as such. It is found in a particular place as a result of you observing it. Now, furthermore, what is this particle if you imagine it, if you think of the electron as a particle? Well, in field theory, we see that particle as a vibration in a field. That's what it is in terms of Einsteinian physics. We don't really think of it in terms of particles anymore. We think of it in terms of fields. But what is that vibration in the force field, the electromagnetic force field? The point that I'm making, allied really with the things that Sabine is saying, is that we can hold these different models for understanding what's going on, and some of them are very, very powerful, and we can try and improve them in the light of our experiments. But we shouldn't imagine that somehow we can get to the bottom of that. It's not like a deep mystery that there's a weird thing that it's both a wave and a particle, for example. It's just they're different ways of holding that stuff. And I think you can explore both of them, and we shouldn't necessarily imagine that we can have a single answer, because there isn't a single answer. The world isn't made of bits that might have answers like that. Interesting. Okay. So, yeah, let me intervene here. So there's a claim made that the AD... Again, forgive me for a minute. ADS-CFT is supposed to be an example of, as it were, equivalent, mathematically equivalent, empirically equivalent, I don't know, equivalent in some ways, formulations of a theory that give you entirely different pictures of what the world is made of. Yes? ADS-CFT is a conjecture. It's not proved. Nobody's proven it. I don't know if you can prove it. Nobody has proven it. There are these tiny little mathematical suggestions if you take different theories out to various infinite limits you get similar... Okay, but it's not proven. It's just not true that you can say, oh, here are two equivalent ways of describing the same thing. That is a conjecture. Even if it were true, we have that all the time in physics, and we don't think they have to be equal. So Lorentz had an understanding of spacetime where there's absolute simultaneity. Einstein got rid of it, but you can prove in their applications they make the same predictions. Nobody thinks they're the same theory. There's different theories. So this idea that, oh, of course, if you can describe it this way, then you can describe it that way. We actually do not have lots of examples of that. And then you have to think hard, if you have alternative theories, what the evidence is that makes one more plausible than another. It's not a matter of proof, but it's a matter of plausibility. Okay. So the picture that I'm getting right now, I don't know about you guys, is that it seems to me that this idea of particles is something that can be useful, and if there's evidence for it, then we can hold it to be true. So evidence seems to be important, but also there's this idea that it has a function for how we think about things and how we can proceed through the sort of scientific endeavours and the phenomena that we're trying to explain. But at the start, we also mentioned that we were going to talk about fields. So can we, Tim, can I just ask you to sort of give us an explanation of what's the difference, basically, between a particle and a field? And so if particles are a little bit, some particles have evidence, others don't, does that mean that they are instead fields or that we should be thinking about fields? Okay, so let's start with some definitions. A particle is always somewhere. A field is always everywhere, right, just classically. Electromagnetic field classically is postulated at every point in space and time it has a value. A particle is always somewhere, and typically what people mean by a particle is more than that, that its trajectory through space-time is continuous. You don't think of particles as discontinuously jumping around, okay? All right, so particles are not fields, fields are not particles. Electromagnetic fields we're familiar with from classical physics, they were postulated, very useful. Particles have been postulated, very useful. The quote from Bell that I read is from John Bell, is that we have phenomena, like little dots appearing on screens, and certainly one handy thing for explaining that phenomenon is to think there was a little invisible thing that pretty much hit the screen there and caused this flash or this emulsion or whatever. That's not the only possible explanation, but it's a pretty reasonable one. So yeah, particles and fields are different, and you should be clear about what you're postulating. So are you saying, so there are two things going on here. When we try and picture the electron, you're saying there's the particle, which is the electron, and there's the field, which is the electromagnetic field. Well, classically there is, and quantum mechanically you can have particles and then what is represented by the wave function, which is a different non-classical kind of postulated entity. But don't, I mean, in the context of field theory, don't we just think of the particle as just a vibration in the field? We don't have to postulate a metaphysical other thing. No, no serious physics. I don't think there's any serious physical theory that says that. People speak loosely that way, but it doesn't make any sense. I mean, there are vibrations in electromagnetic fields because they're local things. You can ask of an electromagnetic field, a classical electromagnetic field, what is it here, what is it here? Is it fluctuating here, is it fluctuating there? In quantum theory, the field-like thing is the wave function. It's not even definable in space-time. So you can't think of particles as fluctuations of fields if the fields aren't in space-time. Okay, so within Einstein, you know, there's fields and particles as a separate sort of thing. Sure. I mean, that's a classical theory. I think we're confusing two different fields. Like, there's the classical field, the electromagnetic field, and then there's the quantum field. There's the vacuum of the quantum field, and the particles are excitations over the quantum field of the vacuum. So this is why physicists talk about this, as excitations over something. And this is very similar to what I was talking about earlier with the quasi-particles, where you can think of the material as kind of the background, and over that background, you have excitations, which are kind of sound waves, for example. It's a simple example. So you have a bunch of an excitation that travels locally. It's in some place, okay? So you can see it going along. And in a quantum field theory case, the background is just the vacuum, and then you have these excitations over the vacuum, which is kind of... This is why I say it's kind of suggestive with the quasi-particles, that maybe they're also made of something else, but this is where this idea comes from. So just so I'm very clear on this, you're sort of suggesting that there's, in the field view of the universe, there's a field, and then what we can think of as particles, let's say, or where we might think of defining something as being a particle, is where something's happening to the field, that there's some sort of movement in the field. So it's a bit like the ocean, basically. So the peak of the wave is kind of where we might look with a different lens. Amazing. Love this. Not the way it is. I'm sorry. You're giving a very classical picture of a field. Yes, a classical field is local. Electromagnetic field, it's local. You can take a look. You say, what is it here? What is it here? It's a local thing. Quantum mechanics doesn't... I mean, the quantum field, the wave function, just isn't defined on physical space. It's not like that. Does that mean that you can't measure it? Is that what you're saying? Measure it. Well, measure it is a matter of finding some interactions that you think give you information about the physical values of various variables. Which you can't do. Of course, you can. You can. I mean, you can check all kinds of things in the lab. I'm not... I mean, the point is that classical fields are local objects. They're what John Bell called local beables. You can just pick out a little region of space-time and ask, what is it there? What is it there? You can specify it here independently of anywhere else. The quantum wave function just mathematically isn't like that. You can't understand it that way. You can't. Mathematically, you can't understand it that way. Okay, it's a new thing. But that's... The quantum wave function is a very different thing than a classical field. Okay, so we have two different kinds of... I mean, Sabina, you kind of said that as well. And Hilary, I feel like this talks to your point. To continue watching this video, click the link in the top left or in the description below. With a free trial, you can enjoy the full talk and thousands more. Thank you for being part of the conversation.