Resolution of the Frauchiger-Renner paradox

[Demystifier]

You might not realize you are assuming it, but you are whenever you rely on evidence.

But I never absolutely rely on anything. I always doubt, I always admit that there is a possibility that I'm wrong. The only thing I don't doubt is: I think, therefore I am.

But I have also never seen in the literature any discussion of the implications of it being possible.

Yes you have. The FR paper is an example, as well as all the subsequent papers that analyze the FR paper.

"Measurement" is actually not really the relevant criterion here. The relevant criterion is decoherence. But the concepts of "measurement" and "observation" that we use in science implicitly rely on decoherence (at least if we assume that we and our measuring devices are ultimately quantum systems which must obey the laws of quantum mechanics), and on it being irreversible. The fact that this is not explicitly recognized does not mean it isn't true.

Are you claiming that decoherence is absolutely irreversible, that reversion is not just very difficult but fundamentally impossible? If you have a good argument for this claim, then you should publish it. It would be a very new and important result, contradicting everything we know about decoherence from the existing literature. In the meanwhile, I'll stick with the standard view that reversion of decoherence is just very difficult.

 

[PeterDonis]

Yes you have. The FR paper is an example

I meant they don't discuss the implications in terms of not being able to rely on evidence. I should have been more specific before.

Are you claiming that decoherence is absolutely irreversible

I am saying that extrapolating our current quantum mechanics unchanged to systems with $10^{30}$ or more degrees of freedom might be extrapolating it too far. The implications of being able to reverse decoherence, which is indeed a prediction of our current quantum mechanics, are part of the reason why I say that.

However, as far as this particular thread is concerned, I pointed out the implications in order to argue that the paper you referenced in the OP doesn't actually "resolve" anything about the Frauchiger-Renner scenario, because, as I said, if it is true that decoherence can be reversed, the rule prescribed in that paper cannot be followed--because that rule requires the friend to rely on evidence they are given about whether or not they will be "cat measured" in the future, and, as I have said, if decoherence can be reversed, nobody can rely on evidence. That, in itself, is not an argument that decoherence cannot be reversed. But it is an argument that the implications of the Frauchiger-Renner scenario can't be "resolved" by telling the "friend" to rely on evidence.

 

[Demystifier]

However, as far as this particular thread is concerned, I pointed out the implications in order to argue that the paper you referenced in the OP doesn't actually "resolve" anything about the Frauchiger-Renner scenario, because, as I said, if it is true that decoherence can be reversed, the rule prescribed in that paper cannot be followed--because that rule requires the friend to rely on evidence they are given about whether or not they will be "cat measured" in the future, and, as I have said, if decoherence can be reversed, nobody can rely on evidence. That, in itself, is not an argument that decoherence cannot be reversed. But it is an argument that the implications of the Frauchiger-Renner scenario can't be "resolved" by telling the "friend" to rely on evidence.

I think you see the problem because you think in black or white terms; either friend can rely on evidence or he cannot. By contrast, I think in shades of gray terms. I cannot absolutely rely on anything (except that I think and therefore exist), the best I can do is to rely with more or less confidence. The possibility of FR scenario adds one more reason to question the reliability of evidence, but there are many more. Perhaps the apparatus has a failure, perhaps my research assistant lies to me, perhaps I have schizophrenia, etc. Now you will say that I can use additional evidence to rule out all these other possibilities, but my point is that this additional evidence is also subject to doubt; maybe the apparatus for checking the apparatus also has a failure, etc. Or to be blatantly direct, yes, I think it's possible that someone cat measures my brain right now, but I estimate that probability for that is much smaller than the probability that I'm having a schizophrenic episode right now. And neither of the possibilities worries me much, because I estimate that probability for either of them is very small.

 

[PeterDonis]

I think you see the problem because you think in black or white terms; either friend can rely on evidence or he cannot.

No, that's not the issue. Once again, it's not a question of ordinary ways that people forget things or that evidence is not always reliable. Those have nothing to do with reversing decoherence. Reversing decoherence is a fundamentally different way for evidence to be unreliable. I've already explained why I think that and I don't think I can add anything further.

 

[Demystifier]

No, that's not the issue. Once again, it's not a question of ordinary ways that people forget things or that evidence is not always reliable. Those have nothing to do with reversing decoherence. Reversing decoherence is a fundamentally different way for evidence to be unreliable. I've already explained why I think that and I don't think I can add anything further.

At least we know where we disagree. But let us try to understand each other in a different way. Suppose that we model observers by using only classical physics. Would a possibility of reversal in the classical phase space be a fundamentally different way for evidence to be unreliable, compared to ordinary ways that people forget things or that evidence is not always reliable?

 

[PeterDonis]

Suppose that we model observers by using only classical physics. Would a possibility of reversal in the classical phase space be a fundamentally different way for evidence to be unreliable, compared to ordinary ways that people forget things or that evidence is not always reliable?

Since the effects of such a reversal, assuming classical physics (and therefore a fully deterministic time reversible model), would be to undo everything that happened during the period that got reversed, yes, I would say this is a fundamentally different way for evidence to be unreliable.

Note, however, that classical physics does not contain any operation that would actually do such a reversal. It contains pairs of solutions that are time reverses of each other, but does not contain any way of switching between them in mid-stream, so to speak. So there is no analogue in classical physics to the kinds of "reverse decoherence" unitary operations that the Frauchiger-Renner scenario uses.

 

[Demystifier]

Since the effects of such a reversal, assuming classical physics (and therefore a fully deterministic time reversible model), would be to undo everything that happened during the period that got reversed, yes, I would say this is a fundamentally different way for evidence to be unreliable.

Fine. And assuming classical physics, do you think that such reversal is possible in principle? (I'm not talking about reversal of the whole Universe, but about reversal of the friend and his classical laboratory.) And if it possible in principle, does it mean that we cannot do science? Or if you say that it is not possible in principle, can you explain why it is not possible?

 

[PeterDonis]

assuming classical physics, do you think that such reversal is possible in principle?

I thought I already answered that in post #36, second paragraph.