[net.physics] Simple QM question

MJackson.Wbst@XEROX.ARPA (05/23/84)

Your experiment does not violate the Uncertainty Principle, which
governs the *simultaneous measurement* of position and momentum.  Note
that you have not simultaneously determined the position and momentum of
your particle to closer than h-bar over two (the absolute minimum, which
holds for the standard deviations of Gaussian wave packets).  Although
you know the position precisely at the moment of release, the momentum
at release is only known after detection.  There was no way to predict
the capture point, since the momentum was unknown at release.  Further,
the general behavior of subatomic particles is such that one cannot
infer from such an experiment that the particle "followed a trajectory"
in the classical sense from release to capture.

See /The Feynman Lectures on Physics/, vol. 3, early chapters, for a
clear exposition of the essentials of QM.

Mark

gwyn@Brl-Vld.ARPA (05/24/84)

From:      Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA>

Mel Lax & I decided that the propagation of the computed momentum
at the detecting sphere "backward in time" to the point of
emission would necessarily involve a spread in the particle wave
packet so that the Uncertainty Principle would be obeyed at the
place/time of emission of the particle.

This conclusion was the result of intuition about these things
and could perhaps be mistaken.  We didn't do any calculations.