From: Hal Finney (hal@finney.org)
Date: Sat May 11 2002 - 12:31:14 MDT
Scerir writes:
> But the ***main*** point, of course, is this. If one of the
> photons (ph.1) is measured with a certain value (of positon,
> of momentum, of energy, [of time]) its twin brother (ph.2)
> must have been propagating with a well defined value.
>
> Btw in the case of two entangled photons (bi-photon, actually
> one particle) Popper suggested that also Heisenberg's principle
> should break.
>
> And Dr. Shih performed the experiment:
> http://arxiv.org/abs/quant-ph/9905039
Actually if you read this closely, you find that it does not show the
signalling phenomenon. The key point is that to see a difference,
you must look at only "coincidence" events. These are cases where the
detector at the left (beyond the slits) triggered simultaneously with
the detector at the right. If we restrict our attention to that subset
of the photons, then indeed changing the width of the slit at the left
changes the pattern at the right. If you look at figure 4 you can see
the coincidence detector, connected to the photon detectors at both sides.
Likewise figure 5 displays the "coincidence pattern", meaning it is only
showing events for which photons were captured by both sides.
But the point is that we can only identify coincidence events via ordinary
communication. Once the left side has told the right side which photons
it detected as passing through its slit, and hence which had their
vertical position measured, the right side can restrict itself to that
subset of the photons and see some effects. But without that (slower
than light) information transfer, the right side sees no difference
from what the changes at the left side do. (Actually the experiment
does not directly try changing the left side, but the results imply
that there would be a change in the right side pattern in that case,
again if we restrict ourself only to coincidence events.)
Therefore this does not allow for remote communication, any more than
with other similar EPR setups.
Hal
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