From: John K Clark (johnkc@well.com)
Date: Tue Sep 17 1996 - 22:40:07 MDT
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On Tue, 17 Sep 1996 Suresh Naidu <snaidu@undergrad.math.uwaterloo.ca> Wrote:
>Okay, somebody clarify this for me. From my understanding
>of Paulis Exlusion principle and rudimentary quantum
>mechanics, couldnt you use electron spin states for FTL
>communication, if not travel. By spreading two electrons
>really far apart, and changing the state of one, shouldnt
>the other one shift instantly, enabling us to communicate
>digitally faster than light.
If 2 electrons in a atom are in a very large orbit they must have a very small
velocity. According to the uncertainty principle, the better you know the
velocity of something the less you know its position. In this case we know a
lot about the electron's velocity (very close to zero) so we know little about
its position. The effective size of these electrons would be huge, they would
overlap, when we measure the spin of an electron we can't be certain which
electron we are measuring so the "message" would be random.
It has been shown experimentally that it is possible to change things far
away faster than light, instantly in fact, but that's not enough to sent a
message. To send a message I must not only change something but the receiver
must be able to compare that change with something, otherwise the message
will look random to him.
Some physical processes produce 2 photons that have the same polarization but
move in opposite directions. A billion years before I was born somebody in
the Virgo Cluster started making pairs of photons that have identical but
unknown polarization. He sent one stream of photons to the earth, a billion
light years away, he sent another stream of photons to the Coma cluster in
the opposite direction from the earth a billion and one light years away.
A billion years later on Earth I spin my polarizer to a random direction,
record the position, observe if the photon made it through the detector or
not, then record that too. Now I spin the polarizer again and do the same
thing for the next photon and then for the next several thousand photons.
I know that a year from now a friend of mine in the Coma Cluster will
perform the exact same experiment on his stream of photons and I decide to
visit him. I get in a space ship with my records and blast off for the Coma
Cluster at 99% of the speed of light.
After 2 billion years I arrive in the Coma Cluster and compare notes with my
friend. I notice that the direction I had my polarizer turned to and the
direction my friend had his turned to were different, not very surprising
since both were picked at random, but then I find something astounding.
The square of the cosign of the angle between the 2 detectors for each photon
pair is proportional to the probability that a photon will make it through my
friend's detector.
I have instantly changed something that is 2 billion light years away and was
made long before dinosaurs walked the earth. Pretty weird. Unfortunately this
effect can not be used for faster than light communication because before I
arrived with my records the results of my friend's experiment looked random
to him, it's only when he compared his results with my records, and that can
only be done at the speed of light or less, did it become obvious that turning
my polarizer and observing the results instantly changed his photons far away.
Nevertheless this does have a practical benefit, you can use it to make a
communication link that is absolutely secure, it's already been done, but I
wrote about Quantum Cryptography in another post.
John K Clark johnkc@well.com
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