🚀 go-pugleaf

In article <1992Apr9.213832.1911@hellgate.utah.edu> 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

No you haven't; it's quite a good question.

>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.

In exactly the same way that a lot of quantum mechanics (viz. EPR) seems
pretty dubious.  ``Apocryphal yet true,'' as an old English teacher of
mine liked to say.  The simplest example for what you ask is an atom which
spontaneously decays, emitting one photon.  The photon is emitted over
a broad range of angles, e.g. in a characteristic ``dumbell'' dipole
radiation pattern.  The atom recoils, and its momentum is equally un-
certain, although the sum of the atomic and optical momenta is equal to
zero (in the rest frame of the initial, excited atom).  When a ``collapse''
occurs (or doesn't, I don't want to get into metaphysics in front of the
readers of this newsgroup any more) what is selected out is a state of
the entire atom+photon system, so if the photon is found to have gone off
in the +x direction, the atom will be found to have recoiled in the -x
direction, conserving momentum.  

aephraim

-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@garnet.berkeley.edu             | happen to me if I don't??"
                                         |       -- Eugene Wigner