Steven Strogatz asks why physicists keep testing a theory that no experiment has ever contradicted. Halliwell objects to colleagues in quantum foundations who told him the work is pointless because quantum physics is simply true, calling that a kind of fundamentalism. He then walks through what Bell and Leggett-Garg tests rule out and through Bohmian mechanics, a picture in which the world is definite but non-local, before landing here.
Sure. I mean, what has been done is interferometry experiments for pretty big systems. I think Vladko Vedrell from Oxford is an expert on this thing. He talked about shredding as a virus as the sort of scale we might be getting up to. Those experiments simply confirm there's an interference pattern when you do a double-slit experiment. Leggett-Gog tests, they do more than that. They not only confirm quantum, but they rule out alternative classical explanations, which is a stronger thing to do. This is sort of interesting. You could still get an interference pattern, but there's a classical model for it, actually, if the interference isn't too big. The thing is, interference is created by the fact that the wave goes through two holes and what come out is two waves. They're not probabilities. They're waves that can be positive and negative. And they can constructively interfere, so they get bigger. And that's like a classical effect, actually, because if you add two probabilities, they get bigger, but they can destructively interfere, so they cancel each other out. So you can get two waves coming in, and you get nothing, which can't happen with probabilities. Now, if the destructive interference isn't too big, you can still model them as like classical probabilistic flows. And that would satisfy the Leggett-Gog inequalities, which actually means that there is a classical model of interference patterns in a certain limited regime. So for one of these big interferometries experiments with big particles to really say this is definitely quantum and not pseudo-classical, you'd need to violate a Leggett-Gog inequality, basically. That can all be done in principle. I mean, even just with the data from existing experiments, basically.
Let me start backing away from some of the more theoretical points and get into some more personal things. One of them is just a matter of opinion, really, but I bet you have a strong opinion, which is so far, no experiment has contradicted quantum mechanics or quantum field theory. I'm not sure how far we should push this. So why do we keep testing it? What are we hoping to learn by pushing these quantum tests to larger and larger systems?
Yes, that is a very good point. It is very, very striking indeed that there isn't, in terms of experimental predictions that have been measured to many, many, many dismal places. There are no contradictions in the formalism. To the point, actually, that I know experts in quantum foundations who even said to me, why do you work on Leggett-Gog inequalities, which confirm quantum behavior, when in their Words, we know quantum physics is true anyway. I have a problem with that. It's this bordering on a kind of fundamentalism about accepting something as truth. That could be a wider discussion, but whatever theories are, they must be exposed to experimental test and confirmation. I mean, really, absolutely. I mean, it would be a wonderful thing if we started seeing differences. But it is still a really striking thing that there have been no differences with theory ever observed. I think behind this, it is about what is the true interpretation of quantum physics behind all these investigations, Bell inequalities, Legend Gog inequalities. What these things do, they eliminate certain very natural worldviews. So, Bell inequalities show that quantum physics and experiment are consistent with the worldview of local realism. So, the idea is that a particle has definite properties independent of what distant particles are doing. Similarly, Leggett Gog is testing macroscopic realism. A particle has definite properties irrespective of measurements in the past or measurements in the future for that matter. So, we know what reality is not. But there are other worldviews, if you like, other interpretations. One of the most famous complements to these ideas is Bohmian mechanics, if you're familiar with that one, or De Broglie Bohm theory, which basically does take standard quantum mechanics, which is based on a wave function, but says the particle has a wave function and it also has a position trajectory. Then, you don't need to worry about is the particle here or there. The particle definitely has a trajectory, it's definitely somewhere. The wave function has a back seat as a kind of they call it a guidance field, like a fluid that sort of flows along and tells the particle what to do. But the particles always has a definite position, and that's very appealing. It was invented actually shortly after quantum physics was invented. And a lot of people like it because you know where you are, basically, you've got a definite path through, but it has a feature which is that it's non-local. Because there's a wave function and a particle that where the particle goes, the wave function can be spread out all over the place. So, the guidance field for the particle can depend on very distant observers. It basically says it's an entanglement situation with distant particles, that one particle really can depend on what the other one is doing or what you measure on the other particle. But that might be it. I mean, you can have a world in which everything is definite, but it's non-local. And that might also be true, actually. So, that's why I don't believe in truth. I believe in multiple perspectives.
Do you want to expand on that idea? In some of my preparation for our conversation, I was told you have an interest and have practiced things like yoga, meditation. Is that related to the sorts of perspectives you're describing here?
It's not directly related, but in terms of the sort of attitude and perspective I take towards physics and scientific ideas, it sort of helps me in that sense. So, I think in the early days, when I wanted to figure out, you know, how does the universe actually start? We really want to know. Now, I'm a lot happier to sit with the mystery. When I've given popular talks, I have the sense that the general public interested in physics feel discomfort at the lack of understanding, that they want the physicist to deliver something mythologically meaningful to them in terms of the Big Bang, and they're dissatisfied. So, I mean, I found a sort of other perspective and a more, let's say, spiritual side, where I'm just happier to sit with the mystery of multiple perspectives. There was an American cosmologist who's no more of a sort of mythologist and writer called Brian Swim, S-W-I-M-M-E, who you may have come across. And he's basically starts with the idea that all societies have creation myths of one form or another. And modern society doesn't necessarily have that. But he said, Well, look, let's take the Big Bang cosmology as a modern myth for our time. We don't have to take it as true, but we can take it as a meaningful mythological tale from which we can draw a sense of guidance or purpose or perspective and so on. And I felt in popular talks I've given, I felt that actually filled a gap in things where people have the Big Bang, but it's presented to them as almost as truth. But it doesn't really fulfill people to a degree that they actually want.
It's interesting to me, the analogy between the perspective you've described with holding different possibilities in mind at the same time and being comfortable, I want to say, sort of as a superposition, that you seem to resist collapse in your own thinking.