On 19 Aug 2019, at 04:02, Bruce Kellett < [email protected]> wrote: On Sat, Aug 17, 2019 at 11:00 PM Bruno Marchal <[email protected]> wrote:
Brent, Bruce,
On 16 Aug 2019, at 22:26, 'Brent Meeker' via Everything List < [email protected]> wrote:
I think you can interpret it as decoherence spreads at
light speed from Alice's measurement event and decoheres
Bob's system when it comes within the future light cone of
Alice's measurement....and vice versa, which is why it
needs to assume MWI to maintain symmetry between Alice and
Bob.
That is my understanding. That explains entirely, it seems
to me, the violation of Bell’s inequality in a local, but
multi-versal type of reality. When Alice and Bob separates,
they simply never meet again, but both can meet their
correlated counterparts. Each Alice and each Bob can meet only
their correlates, that they enforce by decoherence, at a speed
lower than light.
But as Bruce says, it's a kind of magic as stated. To not be magic
there must be some physical interactions communicating Alice's
result to Bob's system: Photons would the obvious candidate, but how
exactly do they interact with Bob and his system to make them
orthogonal to one of Alice's results and not the other?
The state is ud - du, for d = down and u = up, with the usual sqrt(2) = 1.
The idea is that when Alice see her photon being u, she knows that whatever she will be interact with will be consistent wit her photon being u and with bob photon being d, including the bob she could ever manifest herself relatively.
That is exactly the magic that needs to be explained.
I don’t see this. ud - du predicts this, by the quantum formalism.
Yes. And that is the standard non-local argument. Don't forget that, as Maudlin points out, the quantum wave function is, itself, a non-local object.
I can hardly imagine a notion more local than a wave.
But, yes, if a wave describe an amplitude of probability concerning a single particle, then, if that wave collapse, it can only do this in a highly non-local way. That was the reason why Einstein criticise Bohr’s QM, notably in 1927 at the Solvay Congress in Brussels, and which lead to EPR.
I think Maudlin said that the quantum wave is a non-local object in the context of the “one-world” assumption. In my edition of his book on non-separability he explains this is no more true in the non-collapse theory (but you said he changes his mind on this?).
And reciprocally with Bob. It is just that when Alice see u, it means that her accessible histories will all be consistent with u for her photon and d for bob’s photon. If Bob sees u to, that will be the same: he knows that he will meet Alice having seen d. Both possibilities will exist. The Alices seeing u will access a world with Bob seeing d. The Alice seeing d, will access a world with Bob seeing u. The same for Bob. Both Alices and Bobs observations will spread toward each other at the speed of light, or slower, and no physical influence exist at all. They both only localise themselves in the multiverse, at different possible cosmic branches.
What makes a history "accessible"? You have offered only magic to rule out histories that violate the basic conservation rules.
I don’t see this at all. (I assume QM here, not mechanism).
I don't know how I can make you understand that just multiplying the number of "worlds", and appealing to some obscure notion of "histories", does nothing towards providing a coherent local causal account of the observed correlations.
It is not the multiplication of worlds per se which solves the problem, it is the fact that the "multiplication of worlds” is itself a local phenomenon. When Alice measures her particle, and Bob measure his particle, they both “multiply” their accessible worlds, in which both will met eventually their correlated counterparts, whatever the results their could be said to have obtained initially (if we could give some sense for this).
The magic comes only from the idea that there is one Alice and one Bob, which would make this reasoning obviously invalid, or introduce faster than light physical influence.
The argument does not depend on any "one world" assumption. The problem is clearly present even in the Everettian setting, when there are copies of Alice and Bob who see each result. These always exist, since both up and down results are always possible for any measurement on the separated singlet particles.
I don’t see this. Alice and Bob have prepared the particle together (or by some entanglement swapping technic), the state ud-du require the correlations, in all base. Once separated, they can only access to their correlate parts, which requires the “creation” of “new” Bob and Alice.
Any copies of Alice and Bob that there might be are created at the time they make their measurements and observe (record) their results. This happens at space-like separations, so any correlations are necessarily non-local in origin. All else is magic or mysticism.
The splitting or differentiation of the “world” is a local phenomenon.
FTL action at a distance seems more magic to me than a local differentiation of histories.
It is up to you, if you think that some FTL influence occur, to explain why and how.
Don't try and divert attention from your own failings by claiming that it is all my responsibility. This is about you, and your failure to provide the advertised local causal account through many worlds, that is at issue.
The wave propagate locally. Everything is local at all “space-time” situation. You continue to talk like if the wave collapsed when we do measurement, but that does not occur once we abandon the idea that any collapse ever occur.
A quantum state does not describe a world, but is only an indexical map, for a subject, of the histories that he/she can access to. ud-du means only, to Alice and Bob, that they will always means their corresponding correlated counterpart, whatever they found.
Given the miracle that occurs at this step in your account.
Which miracle? I just use the fact that once a superposition is there, it never collapse. It is the collapse which would be magical.
Collapse is irrelevant.
I don’t see this. If the wave collapse, it has to be non-local. Without collapse, the multiplication of worlds is a local phenomenon spreading at the speed of the possible local interaction with the environment.
This is just another of your diversionary tactics. You claim that there is "some collapse" if you have no answer to the points being made. I have not made any collapse assumption; all along I have been working in a many-worlds setting. I maintain that this does not result in a local account. You claim different, so prove it by providing this local account.
My feeling is that you interpret the multiplication of worlds like if that was an instantaneous process, but it is a local one: like a front wave made of many single waves. The singlet sate is for personal use only: it says to Bob and to Alice which types of Alice and Bob they can meet in their future. In a sense, once separated, the “original” will never meet again: only their counterparts in the relevant histories with a measure given from the state.
Even if we give sense to “they both find u”, each of us will meet they counterparts, which they can interact with when meeting again, having found d.
You are not convincing anyone other than yourself here, Bruno.
I don’t think so. I am not sure if you are not the only one, perhaps with one other, who believes that the violation of Bell’s inequality entails physical action at a distance (which have no meaning for me in a relativistic context).
Don't change the subject, and ascribe to me views that I have never held and have never advocated. There is no FTL physical action. The no-signalling theorems guarantee this.
Existence of FTL is weaker than No-signalling, but with some amount of physical realism, it is about the same.
If you agree that there is no FTL when there is no collapse, then again, I am no more sure we disagree on anything here.
Both EPR and Bell assumes that "Alice and Bob” are well defined and keep their identity throughout the experience, which indeed would require some FTL influence, but I don’t see that FTL when we keep all branch of the superposition into account.
Your conclusion does not follow from your premises. I have given a clear account of how Alice and Bob can maintain their identities, even though they split according to the results obtained, and still meet to exchange data and calculate non-local correlations, even though there are no physical FTL effects.
In absence of collapse.
This account explicitly keeps all branches of the superposition in play, even though that does not really alter anything.
If there is a collapse, it has to be non local, as Einstein understood very early. Only differentiation of worlds at lower speed of light can maintain the localness of the actions together with maintaining the quantum statistics right in the mind of all Alices and Bobs.
Bruno
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