🚀 go-pugleaf

In article <1992Apr10.212404.2528@math.okstate.edu>, gindrup@math.okstate.edu (Eric `'d'kidd' G..) writes:
>...The information that the second
> observer obtains does not travel between observers faster than light, because
> the second observer cannot know whether either observer was actually
> "there" when the photon collapsed.
> ...
> What's the problem?

That information is transferred instantaneously in singlet state quantum
collapse without being able to send a signal has been mentioned in this
discussion many times, but it is a subtle and confusing point. The best
intuitive explanation I have is based on cryptography. A perfect one time
pad in cryptography is generated by taking the exclusive or of each bit
in the message with a bit in a perfect random sequence. (I am ignoring
for the moment that I don't think there exists any such thing as a perfect
random sequence. :-) It is impossible to decrypt such a message without
knowing the pad or key because the encrypted message is also a perfect
random sequence.  Now suppose we have one observer transmitting a bit
sequence to a second distant observer. Some times the first observer sends
random bits and sometimes he sends a message encrypted with a perfect one
time pad. The receiver cannot know when a message is being sent. The sequence
he receives is equally random in either case. Now suppose the transmitter
gives the receiver the pads he was using. The receiver can use these pads
to determine when the messages were sent and to decrypt them. He can measure
precisely when the encrypted message was transferred even though he was
not able to decrypt the message at the time it was sent or even able to
know that a message was being sent at that time. 

This is precisely the situation with singlet state quantum collapse. The
perfect random pad is provided by quantum uncertainty. The transmitter in
such an experiment sends an encrypted message by varying the angle of a
polarizer. This is received as a series of detections. These detections
appear completely random. However they are precisely correlated with
an encryption key provided by quantum uncertainty. We know what this
encryption key is (it is the observed detections at the transmitting site).
We can communicate this key to the remote site (but not faster than the
speed of light) and this key can be used to both decrypt the message and
to measure precisely when the encrypted message or information was
transferred. Using this technique we can prove that the remote observations
were instantaneously correlated with the local observations and manipulations
of a local polarizer. This technique will allow us to experimentally verify
that information (the encrypted message) was transferred instantaneously
in the unlikely event that the instantaneous collapse postulate is true.

Paul Budnik