From: Hal Finney (hal@finney.org)
Date: Sat May 11 2002 - 15:10:01 MDT
Googling about, I have found a good article describing an analogous
experiment. However it is not by a professional physicist; the author
is one Erann Gat, http://www.flownet.com/gat/, a polymath presently
developing award-winning software for autonomous spacecraft at JPL,
member of the Caltech flying club, founder of networking company Flownet,
solving quantum mysteries in his spare time.
His article at http://www.flownet.com/gat/QM.pdf describes a similar
paradox. We have EPR entangled photons allowed to separate, and each one
passes through a double-slit experiment. In a double-slit experiment,
we see an interference pattern on the screen if we can't tell which
slit the photon went through. The interference pattern vanishes if we
have some way of distinguishing the paths through the two slits.
However, with EPR photons, because of conservation of momentum, if
one photon goes through the top slit then the other goes through the
bottom slit. So it seems that by measuring one photon's position,
we can tell which slit the other went through, and the interference
pattern should vanish at the far end. If we refrain from doing that
measurement, we don't find out which slit either photon used, so we hope
to see interference at both ends. The paradox is again that by deciding
whether to measure or not at one end, we change the results at the other
end, contrary to relativity among other problems.
Gat goes through an analysis similar in complexity to what I described
for the interferometer experiment, and comes up with the same result:
the EPR photons do not show interference at either end, regardless of
whether you measure them or not. Basically this is because the paths
can be distinguished in principle, and that is enough to destroy the
interference, whether you peek or not.
He takes it farther though and looks at whether a "quantum eraser"
could be used to restore far-end interference and allow signalling, and
again shows that it won't work. He then discusses quantum information
theory and measurement and concludes that we are all made of information,
but I didn't look at that part too closely yet.
This is the closest I have found so far to an existing analysis of this
particular paradox, and it seems to agree that in fact the signalling
won't work.
Hal
This archive was generated by hypermail 2.1.5 : Sat Nov 02 2002 - 09:14:00 MST