On Fri, Apr 29, 2016 at 8:32 PM, Bruce Kellett <bhkell...@optusnet.com.au>
wrote:
>
> That is a semantic matter. There is a problem if one insists that
> "non-local" means the propagation of a real physical influence (particle of
> wave) faster-than-light. But "non-locality" in standard quantum usage means
> the above -- the entangled state acts as a single physical unit even when
> its components are widely separated.



I agree it's a semantic matter, but your description of the "standard
quantum usage" doesn't seem to be accurate. Among physicists, the standard
understanding of "local" and "non-local" in the context of Bell's theorem
and relativity is the one I cited earlier--a theory is "local" if and only
if the function that gives you the value of local variables at any given
point P in spacetime (or gives the best possible probabilistic prediction
about their values, in the case of a non-deterministic theory) only
requires as input the values of local variables at other points that lie
within P's past light cone, whereas a "non-local" theory would be one where
the function requires knowledge of variables at a spacelike separation from
P to generate the best possible prediction. As I mentioned, I think this is
explained most clearly in Bell's paper "La nouvelle cuisine" which you can
find in the collection "Speakable and Unspeakable in Quantum Mechanics",
and you can also find it discussed in other sources,
http://arxiv.org/abs/0707.0401 for example. As for "acts as a single
physical unit", that seems like a decidedly non-mathematical definition
which physicists would steer clear of, unless you can provide a
mathematical formalization or what you mean, or cite a mainstream source
that provides one.

Bruno should be aware that in the discussion you and I had earlier, you
used this sort of qualitative non-standard definition to argue even if the
function giving values of physical variables at each point *does* depend
solely on data from the past light cone, that is "irrelevant" to deciding
whether the theory is "local" in your sense, presumably because you think
there can be qualitative features of the function itself that can make it
"non-local" for reasons unrelated to the question of what data the function
takes as input.

Jesse

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