🚀 go-pugleaf

 tolman%asylum.utah.edu@cs.utah.edu (Kenneth Tolman) writes
.   In QM there is a probability that a photon will end up "hitting" a
.certain spot.  If there is only one photon, and it has a nondetermined
.end point, then some quantity is not being conserved.  Consider if I threw
.a ball, and then it appeared randomly.  How can there be equal and opposite
.reaction unless it is going to a specific place?
.
.   Obviously I have something wrong.  Does a photon have absolutely zero
.momentum?  I thought that the idea behind solar sailing was to take
.advantage of incredibly small momentums.
.
.  The other possibility is that it retroactively changes the transmitter.
.This seems pretty dubious.


Momentum is conserved, but quantum mechanically (that does not mean "only on
average")
If you threw the ball classically in a random direction, there will be a
reaction on you, random in direction, but precisely correlated with the
actual direction of the of the ball.  You can in fact determine (in principle)
the direction of the ball by measuring the reaction. The randomness can only
be a matter of ignorance of the well defined values for the components of
momentum.

If, on the other hand, you threw the ball  quantum mechanically, and the
randomness was a result of a quantum mechanical superposition, things
change bit.  It may be that no amount of idealization (for the particular
quantum interaction which caused you to throw the ball) can tell you which
direction the ball went, even in principle.  However, conservation laws
dictate that the correlations observed classically will still be found.
That is to say, if we observe (measure) the direction of the ball, we know
the direction of the reaction on you, even before measurement (100%
correlation).  If you measure the reaction, then you know the direction of the
photon, er ball.  You will never be able to make a _pair_ of measurements
which indicate that momentum was not conserved.

An idle comment: It is in the nature of the choices of what measurements
to make which give rise to such issues as the EPR "paradox" and Bells's
theorem.


regards
-Mike Gallis