From: scerir (scerir@libero.it)
Date: Thu Aug 07 2003 - 10:41:28 MDT
[Jeff]
> Detector number one is closer than detector number
> two. [...]
> Can it be determined, at detector number two, whether
> the wave function has already--presumably at detector
> number one--been "collapsed", whether the photons have
> already been "disentangled"?
I'm not sure I understand your question. As far as I remember
there is no way to know if a particle is entangled or not.
But if you perform experiments with entangled particles
you must look at both detectors.
Anyway, Bell experiments with time-like separated impacts at
the detectors (splitters) have been done demonstrating
the same correlations as for space-like separated ones.
(The first one was performed by Tapster, Rarity, Owens,
in Phys. Rev. Lett, 73, 1994, p. 1923.)
Thus there is a non-locality (better, non-separability)
in space, but also in time. And that is the important point.
The violation of Bell's inequalities seems to indicate that
if the quantities involved are physical 'directions', a value
of the system can be fixed by some physical rotation of
a distant apparatus. But if the quantities involved are 'times',
we can (maybe) think that the value possessed by the system
depends on whether we perform a measurement on a distant
apparatus at time t1, or at time t2. (This is the so called
'changing the remote past'! Which can be an extreme implementation
of Wheeler's 'delayed choice principle'. Which, in turn, can be
the quantum form of Weyl's 'block universe', according
to Ashmead.)
Hence, not only quantum correlations cannot be explained
by local causes, or by local common causes (Reichenbach
causality), but one cannot suggest any 'causal' explanation
in which an earlier event influences a later one. And, as
somebody wrote, "quantum entanglement cannot be cast into
any relativistic scheme".
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