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Q:
It’s about laser theory. How could it be explained, physically speaking, that when radiation interacts with an excited atom, the emitted photon is in-phase with, has the polarization of, and propagates in the same direction as the stimulating radiation? Why not in any direction, with different phase?
- Nilton (age 36)
Brazil
A:
We aren't going to be able to give a full explanation, but maybe a few pointers will be useful.
The first is that there is absolutely nothing in classical physics
which could help you make sense of this. If someone claims to give you
a classical explanation, don't buy a laser from them.
The second point is that although it is hard to say why nature is
the way it is, we can argue from other known phenomena that it must be
this way. The basic effect- that the rate of photon emission into a
mode is proportional to a constant plus a term proportional to the
number of photons already in the mode- was essentially derived by
Einstein, and is referred to under the name "Einstein A and B
coefficients". The main ingredient used in his argument was the
knowledge that atoms which can emit or absorb electromagnetic radiation
have to stay in thermal equilibrium in radiation baths of varying
temperatures.
Of course, once the structure of quantum mechanics was built up,
the equality of the absorption rate (obviously proportional to light
intensity) and the induced emission rate (the part which lasers use)
could be derived from a set of formal assumptions.
Mike W.
(published on 10/22/2007)
Follow-up on this answer.
