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Re: LASERS

Referring to:

It turns out that photons produced by stimulated emission travel in
the same direction as those which trigger the decay. Furthermore,
they are perfectly synchronized with the original photons.

William Beaty wrote:

...is a bit misleading, since it implies that individual photons are like
baseballs with positions and trajectories, and that atoms somehow know
to eject their photons so they travel in line with incoming photons. From
my understanding of QM, this does not occur. Instead, if we explain the
process of stimulated emission using waves, and then use the waves to
predict the distribution of photons, then we explain why the atoms SEEM
to send out photons with a trajectory parallel to the stimulating photons.

I agree that the "perfect synchronization", in stimulated emission of
light by atoms, is usually presented as an experimental fact rather as
a statement which students are expected to explain. The phrase
"it turns out" is commonly used to hide this kind of situation.

I would also welcome somebody's explanations, even a semi
quantitative one. The matching of f is easy to justify on the basis of
conservation of energy. By why is polarization of "induced photons"
exactly the same as the polarization of the incoming wave? And why
do we have this perfect match of phases? It suggests that moments
of emission always correspond to an exactly well defined distance
between the incoming photon and the atom to be deexcited? I know
that the classical terminology of "exact moment and exact distance"
are not appropriate but I am using it to ask questions, not to answer
them.

I realize that my explanation above is a bit too longwinded for Ludwik's
small handout. Mostly this is my response to a little niggling issue that
caused me confusion as a student (and which in later years I never saw
discussed in any textbook.) I suspect that I am probably not unique,
and others besides myself have wondered about this issue.

No, you are not alone, William. Perhaps somebody will help us. By
the way, my original handout was distributed today, together with
several other messages of this thread. I assume that what is posted
on phys-L can be shown to others without asking for a permission.

Ludwik Kowalski