🚀 go-pugleaf

In article <s4htvINNc7f@agate.berkeley.edu>, aephraim@physics (Aephraim Steinberg) writes:
|> 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.  

Actually, if you calculate the 'in' and 'out' states, and the
transfer matrices, and calculate the cross-sections for various
events, the cross-section for events that violate conservation are
zero...

You have a distribution of possible events in the shape of the
dumbell, but the recoil that is ultimately detected in any one of
those directions depends on the direction the photon was detected
in on an event-by-event basis.


Dan