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From: AI.Mayank@MCC.ARPA
Newsgroups: net.physics
Subject: Re: Faster than light
Message-ID: <315@sri-arpa.ARPA>
Date: Tue, 25-Jun-85 20:35:43 EDT
Article-I.D.: sri-arpa.315
Posted: Tue Jun 25 20:35:43 1985
Date-Received: Tue, 2-Jul-85 06:12:25 EDT
Lines: 57

From:  Mayank Prakash 



> I just read "In Search Of Schrodinger's Cat," a book by John Gribbin,
> intended to introduce laymen to the subject of quantum mechanics.  Does
> anybody have any comments on the following excerpt, with respect to
> info. travelling faster than the speed of light?  The first (long)
> paragraph gives technical details on how the experiment works, the
> second (short) paragraph gives the results of the experiment: that
> information was transmitted instantaneously, i.e., faster than the speed
> of light.

There is nothing wrong with faster than light speeds, as long as no information
is transmitted at those speeds. In this particular experiment, there is no
paradox for the following reason - this setup cannot be used to communicate at
super-luminal speeds. To see this, imagine two people sitting at opposite sides
of the ring. Let's call them John and Mary. Suppose they use the different
states of the system as the letters of an alphabet. For simplicity, assume that
the system has two states, denoted by 0 and 1. To send the message,say 11001,
to John, Mary would have to successively put the system in the states 1, 1, 0,
0, 1. Let us assume that the system starts out in the state 0. Then, Mary has
to change the state of the system to 1. To do this, she will have to apply an
external influence to the system for a certain duration. However, due to
uncertainty principle, she cannot be certain that the system is an the state 1
at the end of this duration. She can only be sure that it is in state 1 with
probability, say 98%, and in state 0 with probability 2%. To decipher this
message, John will now have to make a measurement on the system, and he will
find it in state 1 with probability 98%, and in state 0 with probability 2%.
But, no matter which state he finds the system in, he does not know what the
probabilities are (if he did, he wouldn't have to make the measurement, as he
already knows what state Mary INTENDED the system to be in, i.e., what she was
trying to communicate). Therefore, from John's point of view, the measurement
of the state of the system does not tell him anything at all. Result: no
meaningful messages can be transmitted between John and Mary using this system
at ANY speed, let alone faster than light.

This experiment reminds me of the so-called EPR (Einstien-Podolsky-Rosen)
paradox - imagine a bound system of two spin half particles with net spin zero.
We hit it with a spin zero particle to seperate the two particles, so that they
start moving in opposite directions. The net spin of the system was zero when
we started, so due to conservation of angular momentum, it will remain zero.
Suppose after the two particles have been seperated by a large distance, we
make a measurement of the spin of one of the particles. Before the measurement
was made, the spin state of each particle was inderminate. After the
measurement is made, the spin state of the measured particle is determined (say
it is up). Since the total spin must be zero, the other particle is now forced
to chang its state from an inderminate state to one with spin down. This must
happen instantaneously, no matter how far apart the two particles are. The
important thing here is that no information can actually be transferred in this
manner, since what state the first particle will be found cannot be determined
in advance. This famous paradox caused a big debate between Einstein and Niels
Bohr in the early 30's, as  Einstein did not believe in the probabilistic
interpretation of QM.

- mayank.
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