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From: FIRTH@TL-20B.ARPA
Newsgroups: net.space
Subject: Re: SPACE Digest V6 #38
Message-ID: <8512050225.AA26412@s1-b.arpa>
Date: Wed, 4-Dec-85 21:16:02 EST
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Posted: Wed Dec  4 21:16:02 1985
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Well, I'm also not sure how much of the original post was a joke,
but decided to say something about the parts that seem to make sense.

The simplest experiment that illustrates the "instantaneous action"
problem is the Wu experiment:

Take a source of photon pairs, eg an electron/positron annihilation.
This emits two photons in opposite directions (conservation of
momentum), and orthogonally polarised.  Now put a polarising filter in
the path of one of the streams of photons.  Of course, half get through.
Do the same for the other stream - again, half get through.

Now try this: what fraction of PAIRS get through BOTH filters?  (You
detect that a pair gets through because your two detectors ping at the
same time).  Well, if the photons were randomly polarised, the probabilities
multipl;y, and the answer would be 1/4.  Since they are orthogonally
polarised (correlated), the calculation is a bit harder, and comes to

	filters parallel: 1/8
	filters perpendicular: 3/8

all by good classical probability theory.  Now try the experiment.  The
result is

	filters parallel: 0
	filters perpendicular: 1/2

One explanation is this: the photons are not only initially correlated bu
remain correlated.  If one is perturbed by a filter (eg has its direction
of polarisation rotated) then the other changes ITS direction of polarisation
to remain orthogonal.  The photons are coupled.

Now put the filters, and detectors, VERY far apart (eg one in Pittsburgh and
the other in the Imperial library on Trantor).  We have no reason to suppose
we won't see the same result: the photons are still correlated.  So, in the
old, particle way of thinking, photon A (in Pittsburgh) has just got through
the filter and says "Hey, sister on Trantor, you better rotate left pi/8 to
stay orthogonal!".

The assumption that the photons become uncorrelated when they are some distance
apart is the assumption of "locality" (incidentally, one of the key assumptions
in the Einstein-Podolsky-Rosen thought experiment), and experiments such as
the above lead one to suspect that this assumption is not always true.

Well, you could say that something - even if only a perturbation in a wave
function - "travels faster than light", though I find that model unhelpful.
But, as far as I know, nobody has been able to imagine a device that would use
this effect to send specific SIGNALS faster than light.

Robert Firth

PS: should you care, I happen to believe it IS possible to move information
faster than light, we just haven't found out how, because our model of reality
is defective.
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