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.
Brent
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