> your claim that the QM description is not correct
That's not my claim. My claim is that the change in the mathematical description of Bob's particles (from mixed states to pure states) when Alice measures her particles does not correspond to any actual physical change in Bob's particles.
We agree about the math, and we agree about the outcomes of all experiments performed by Alice and Bob.
There is one thing we disagree about, and that is the outcome of the thought experiment I described above (which may in fact be realizable in practice). I predict that you will observe "retrocausality" in phase 4, i.e. when the experiment halts, Bob's choices will correspond with Alice's measurements with (essentially) 100% certainty. (Of course, this isn't really retrocausality, but it appears that way.) AFAICT, on your view of the world, you would predict (essentially) a 0% chance of Bob's choices corresponding with Alice's measurements.
Just out of curiosity, are you a physicist? Because you obviously know what you're talking about.
BTW, just in case it wasn't obvious, I left out a detail in the description of the thought experiment: In phase 3, if the experiment does not halt at step 4 it loops back around to step 1, just as it does in phase 4.
>> your claim that the QM description is not correct
> That's not my claim. My claim is that the change in the mathematical description of Bob's particles (from mixed states to pure states) when Alice measures her particles does not correspond to any actual physical change in Bob's particles.
Well, according to quantum mechanics there is a physical change when Alice measures her particles because:
before Alice's measurement => the pair "A/B" is entangled
after Alice's measurement => the pair "A/B" is not entangled
According to you alternative description, is the pair A/B entangled or not after Alice's measurement? Or is entanglement not an actual physical property of the pair?
I am (or used to be, for a brief time many years ago) a physicist. We had the opportunity to discuss in person last week. I enjoyed that (and this) discussion, thanks. By the way, I said I would send you a comment from David Mermin about the recent paper from Frauchiger and Renner. Actually I was thinking about this article from Jeffrey Bub: https://arxiv.org/abs/1804.03267
> Well, according to quantum mechanics there is a physical change when Alice measures her particles because:
> before Alice's measurement => the pair "A/B" is entangled
> after Alice's measurement => the pair "A/B" is not entangled
No. That's the whole point. The presence of absence of entanglement is a function of the perspective you choose to take when modeling the situation, not a function of the actual physical situation.
Thanks for the pointer to the Bub paper, that's a great reference.
Maybe we should move this discussion to email? It's getting pretty deeply nested and I don't think anyone else is paying attention.
That's not my claim. My claim is that the change in the mathematical description of Bob's particles (from mixed states to pure states) when Alice measures her particles does not correspond to any actual physical change in Bob's particles.
We agree about the math, and we agree about the outcomes of all experiments performed by Alice and Bob.
There is one thing we disagree about, and that is the outcome of the thought experiment I described above (which may in fact be realizable in practice). I predict that you will observe "retrocausality" in phase 4, i.e. when the experiment halts, Bob's choices will correspond with Alice's measurements with (essentially) 100% certainty. (Of course, this isn't really retrocausality, but it appears that way.) AFAICT, on your view of the world, you would predict (essentially) a 0% chance of Bob's choices corresponding with Alice's measurements.
Just out of curiosity, are you a physicist? Because you obviously know what you're talking about.
BTW, just in case it wasn't obvious, I left out a detail in the description of the thought experiment: In phase 3, if the experiment does not halt at step 4 it loops back around to step 1, just as it does in phase 4.