From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
We both agree that there is FTL signalling. What I say is that there
is no FTL influence at all, in EPR, when developed in the Everett
theory. I don’t see it. When they are space-separated, given they
don’t know in which branch they are, they can find result which would
violate Bell’s inequality, but this means that they will never met.
Each Alice and Bob will only met those correlated to them. All
interactions are local. No mysterious magic forcing the result of Bob
or Alice to influence the outcome of the others. We need to take into
account the many numbers of Alice (Bob), because none knows in which
branch they are. They know only that they are correlated, and that
means only that when they come back they will observe those
correlations, but as there has not been any collapse, that is
explained entirely by local interactions, as they were in the right
branch at the start, due to the preparation of the singlet state.
I think you mean 'no' FTL signalling in the first sentence.
It is not really a matter of branches when looked at from the point at
which Alice and Bob meet. They bring to this meeting a world line -- a
personal history, a path through these branching events. They are each
like someone who has undergone multiple duplications, ending in either M
or W each time. Each diary will contain some sequence of WWMWMM.... and
so on. There are 2^N such sequences for N duplications, each of the 2^N
copies at the end of this has one sequence. It is exactly the same for a
sequence of binary quantum measurements in MWI. At the end, when an
Alice and a Bob meet, they carry with them their world lines -- their
particular sequences of '1' and '0' results.
By starting from the final meeting point, we can unravel the chain of
events without getting confused as to which branch anyone is in. After N
trials, there are 2^N such meetings. The recording in lab books and
walking to the meeting between the experimenters is all local. No
question about this. And in every such meeting, the comparison of lab
books will reveal results that /always/ agree with the quantum
correlations and violate the Bell inequalities. There are no surplus
branches that have to be discarded by some mechanism. All branches at
this stage are good.
Where you are going wrong is in saying that they were in the right
branch at the start -- due to the properties of the singlet state. This
is misleading you, because you are not explaining how the correlations
arise at spacelike separations when the polarizer angles are set at
random. Sure, this is a property of the singlet state in standard QM,
because Alice's measurement collapses the state so that only the
correlated part is available to Bob. It is that part of the explanation
that is lacking in your account. You do not see any non-locality,
basically because you are assuming it with being aware of what you are
doing. Don't despair -- many other highly trained physicists do exactly
the same thing. But this does mean that you have not explained anything
-- you have simply assumed the result. The individual measurements of
Alice and Bob do influence each other, or else no correlation could ever
arise. This a logical consequence of a correlation between two
independent events. /Independent/ means /no correlation/. Here we have
spacelike separated events that do show a correlation. Consequently, the
assumption of locality is not tenable, even though we appear to have
only local interactions. Whatever you say about branching or Everett is
not going to alter the basic logic of this situation.
I think that after all these exchanges it is unlikely that you are ever
going to be able to accept this fact. But I do assure you that it is a fact.
Bruce
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