Transcript discussion Deutsch

July 20 15.00
Apart from universes
Speaker: David Deutsch
Floor speakers (in order of appearance):
Vaidman
Wallace
Saunders
Ladyman
Hemmo
Barbour

69

[questions only]

Vaidman
1. I’m certainly very much with you that Everett’s is by far the best interpretation, that it’s correct and it’s a miracle why it’s not understandable until now. I’m trying to think why and I think you may be partly responsible. The first reason is the biggest; it’s many-world and you said they’re really separate and it looks like your multiverse, which had before been given different names, you consider theories of the multiverse. And I think Everett means, and especially Schrodinger, it’s a single physical universe. I don’t think multiverse, it’s not a theory of multiverse, the important thing is no collapse, that’s the main point. And the other thing, and you made a big contribution which put it forward, but you also didn’t ask things which can be achieved which might be not, and I think Everett was the first to say that you can get probability out of many-worlds, if he didn’t say so I think now it would be accepted because this was not proven at that time, it’s not proven until today. And when ………show that this doesn’t work they kill the whole theory. But the main thing is there’s no collapse and that does not contradict our experience. This is Everett’s theory. If you would not add some other things which are maybe not achievable then it would be accepted.

Deutsch
2. Okay, two things there. First of all, I think what you say about universe/multiverse, I don’t care about the terminology, I’m quite happy to call the multiverse the universe and the universe a branch, in fact I did that in one paper. I don’t care. Certainly the point of Everett’s theory is that we don’t need collapse and that quantum theory can be regarded as a complete theory of the universe. As for probability, I agree with you that the objection to Everett that it didn’t describe probability properly was always a mistake. What was happening there was that there is a problem at the foundations of probability theory which was obscured by silly interpretations. But in Everett’s interpretation you could actually see that problem. It wasn’t a problem with Everett’s interpretation, it’s something that you could see through Everett’s interpretation and then, we believe, solve. If we hadn’t solved it it would be no argument against Everett.

[new question]

Wallace
3. It’s a sort of friendly amendment I suppose. You had your particle, a single particle in space, this classical multiverse approximation breaks down. It seems to me that it’s a retrograde move to describe that thing as multiversal in the sense of describing it as living in different universes at different points. Those sorts of position-basis eigenstates are there and…….by all means, but calling them universes doesn’t seem to me to be necessarily that helpful. There not autonomous, they’re not that sort of thing, they’re just….

Deutsch
4. Yes, I entirely agree with that and if only we had a proper mathematical description of the multiverse then I’d be very happy to say that what’s actually happening with a single particle is so-and-so and you can see than you can’t then make the universe approximation and have that be accurate. Yeah, I agree.

Saunders
5. When you say we don’t have a proper understanding of the multiverse what sort of understanding do you want? Is it mathematical, is it….

Deutsch
6. There’s a mathematical physics issue and also a philosophical issue. The mathematical physics issue is that we don’t know what mathematical object the equations of quantum theory are supposed to apply to. We know how to get answers out of them and we know how to describe something about this object in emergent approximations but we don’t have an exact theory of it. We don’t have a theory of it that’s in principle exact. For instance, in that work I did on time travel with – going back in time and coming back in a different universe, I had a diagram there where there’s a spacetime and the loop in spacetime gets unfolded into a – I don’t know if any of you are familiar with that work, but anyway the point is that that manifold which is a sort of hybrid conception of a relativity thing with a quantum thing – there’s no mathematical object that I can say that’s an approximation to. There ought to be. So that’s the mathematical physics part of it, I think we need that in order to make progress in certain directions. There’s also the philosophical thing that we don’t have a vocabulary and a language. We have a vocabulary to talk about situations where and observable is sharp, and we have vocabulary to talk about parallel universes but most of the multiverse isn’t like that and we don’t have a good way of talking about what that is like, even in the case of a single particle. So, those are the two things that are lacking.

[new question]

Ladyman
7. I thought you overstated the argument against anti-realism and also you ran together two distinct things. An anti-realist about science doesn’t have to deny that there’s an objective reality, they can just deny that we’re finding out about it, so you shouldn’t run those things together because obviously one’s much less plausible than the other. I’m not personally an anti-realist but it is an intelligible view to take about science as is exemplified by the attitude a lot of people take towards Newtonian mechanics; you can think it’s a wonderful theory, use it every day, predict lots of stuff with it, don’t believe that there really are Newtonian action-at-a-distance forces, that’s not a crazy view to have of the world so someone might think about quantum mechanics just like that: it’s a good theory to use but I don’t see why I should believe it to be telling me the truth about unobservables.

Deutsch
8. First of all, I’m glad you’re not an anti-realist because it’s very hard to argue with someone who denies that they exist. About whether I was too hard on anti-realism, all I said about it was – you say it could be that there is a reality but our science doesn’t have access to it – all I said about it was that if you take that view then science isn’t about anything, apart from your own mind, and I think that would be true of the person that you outlined as well. Now, as for explanation, the trouble is, if you regard a theory as being purely a set of observable predictions then you must, logically, be thinking of observations as unanalysable primitive things and that is incompatible with having a universal scientific theory. Also there’s the fact that explanation – there are infinitely many interpretations of those things which make different predictions for the future, then you’ll run into the problem of induction and so on. I’d have to refer you to ‘The Fabric of Reality’ for why science has to be explanatory.

[new question]

Hemmo
9. We had a discussion this morning about….probability

Deutsch
10. Sorry, I wasn’t there for that

Hemmo
11. and many people …………………….derivation?..of the Born rule is not based solely on rationality…..[faint voice obscured by noises]….and second it might not even be possible to justify that in the context of many-world theory.

Deutsch
12. At the moment all I can say - I think they are justified but all I can say about that at the moment is the same as my reply to Lev’s question: if it turns out that there is a hidden assumption there, and we have to admit that there have been …[crosstalk]…if this really is an assumption over and above rationality then it’s no argument against Everett’s theory, it’s simply says that there’s a remaining puzzle about what probability really is. And, as I say, we have to admit that all previous attempts for whatever it is, three hundred years, to solve this problem of deriving a “tends-to” from a “does” have contained hidden assumptions and it could be that we’re wrong as well. We’re not.

[new question]

Barbour
13. David, you’ve completely persuaded me that the central task is to find the mulitverse, what it is like, would you agree with me in Popperian lines that ultimately someone’s got to conjecture what it’s like and then we’ve got to take…

Deutsch
14. Absolutely, I agreed entirely with what you said about that yesterday. Yes, they’ve got to conjecture an explanation, that is, an assertion about what the reality’s like, observed and unobserved, and then that has to be tested.

Barbour
15. As a follow-up to that: there is an argument for saying that science actually progresses in small steps so I wouldn’t be quite so quick in dismissing the ghost within the thing because that’s really actually - the classical ghost within the quantum - it’s a bit disliking not making a giant leap rather than a series of small leaps and to some extent I have made a conjecture about what the multiverse is like, I think it’s probably, almost certainly, too naïve and will be wrong, but that is essentially what Brice deWitt conjectured forty years ago, so I wouldn’t totally dismiss that as a possibility

Deutsch
16. I said it was worth trying heuristically, and it has been tried but it seems to be running out of steam. There seems to be an obvious explanation for why it’s running out of steam.