From: utzoo!decvax!harpo!seismo!hao!hplabs!sri-unix!jackson.PA@PARC-MAXC.ARPA
Newsgroups: net.physics
Title: Re: Faster than a Speeding Photon
Article-I.D.: sri-arpa.365
Posted: Fri Feb  4 17:33:07 1983
Received: Mon Feb  7 01:51:23 1983

Re;"Quantum mechanics seems to be more tangled with causality.  In the
Bell
paradox causation appears to travel instantaneousy (faster than light).
It's not clear, however, that this could be used to send signals.  The
event
that is caused at the far end seems to be unobservable (a change in the
state function, which is destroyed by observation)."

Grumble...as I understand it the Bell Paradox involves a pair of
particles that are constrained to occupy distinct states. When the
particles are seperated and the state of one measured, the experimenter
knows the state of the other particle, instantly without bothersome
measurement.  While this *seems* to imply FTL transmission of causality
there is no way to signal in this fashion.

For signalling to occur, the experimenter must be able to modulate the
electron source (to send the recipient a spin up or spin down electron)
which will happen at lightspeed or less.  Then the electron has to
travel from the electron source to the receiver, at lightspeed or less.
Total time, assuming an instantainious modulator, is equal or greater
than the signaling time for light.

Of course, if the experimenter does not bother to modulate the source,
and measures incoming electrons, then she knows instantly the state of
the electrons at the receiver.  No information is exchanged between the
receiver and the experimenter though.  In a sense, they each posess the
same information, which traveled at lightspeed from the source.

Stephen Jackson
Xerox PARC
(415) 494-4226