On 10/1/2019 4:38 AM, Bruno Marchal wrote:
On 1 Oct 2019, at 07:37, 'Brent Meeker' via Everything List
<[email protected]> wrote:
On 9/30/2019 9:52 PM, Bruce Kellett wrote:
I haven't read Carroll's book, it isn't released in Australia until November. I would be interested to see if
he has a better account of Bell non-locality than Wallace. About the spread of "splitting":
decoherence is a local physical interaction -- photons interacting with walls and the like. This clearly
spreads at the speed of light (or less). But splitting is not really just decoherence. The trouble with Bell
non-locality is that the splitting of worlds is not a physical interaction like decoherence. Bruno and others
speak about a "spread of entanglement" as being associated with the splitting. But again,
entanglement is the result of physical interaction, and the interaction of looking at a pointer to see a
result is not really an entanglement interaction. I think that there is a lot of loose thinking about this
"splitting" process.
The absence of disallowed branches (Alice and Bob both seeing spin-up with
aligned polarisers) is not a matter of worlds (branches) cancelling by
destructive interference, because there is no interaction -- the light carrying
information from the space like separated measurements is not coherent, so it
can't interfere. If it were coherent, allowing interference, then that
coherence itself would indicate non-locality.
But the copying of information as to the measurement result, quantum Darwinism,
is a physical interaction that writes the information into the environment. So
that we can imagine that both UP and DOWN information spreads from Alice and
also separately from Bob. Where they overlap in the future they must correlate
per QM. Why can't we suppose that the inconsistent worlds cancel out. You say
the light carrying the information isn't coherent, but it's not just the light
that carries the information; it's information encoded in the wave function of
the environment. So no small part of the environment (like the light) is going
to appear coherent, but it's still going to be inconsistent with the opposite
result and zero out cross terms in the density matrix. That's essentially what
the mathematical process of taking the reduced trace does.
Right. Then the non locality has disappeared from the wave equation at the
start.
No, Bruce's point is that it must be present at the start. Otherwise
Bell's inequality couldn't be violated.
Brent
The Wave act locally in the Hilbert space (or the von Neumann algebra, and we
see non locality only from a branch/term perspective. But without collapse, the
non locality does not involved neither FTL communication, nor any FTL influence
(which, for a realist on a unique world would be as much embarrassing). That is
why the violation of Bell’s inequality is a quasi-confimartion of the “other
histories” being as real as our’s.
Bruno
Brent
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