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From: don@allegra.UUCP (D. Mitchell)
Newsgroups: net.physics
Subject: Nature of Photons
Message-ID: <2367@allegra.UUCP>
Date: Wed, 28-Mar-84 01:39:40 EST
Article-I.D.: allegra.2367
Posted: Wed Mar 28 01:39:40 1984
Date-Received: Wed, 28-Mar-84 07:14:31 EST
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 55

Our best understanding of the nature of photons is through quantum
field theory.  To give the flavor of this view of reality, let me
describe how a laser works in terms of very fundamental physical
concepts.  First, consider an atom in an excited state.  At any moment,
there is a probability of seeing the atom in a lower energy and a
photon coming out, or one may just see the exited atom with no photon.
In a sense, the true state of the system (when no one is looking!) is a
SUPERPOSITION of these two states.

				      ~~~~~>

		(( o ))                 o

The probability of emission is derived from a low level axiom, namely
the coupling between the photon field, and the electron field.
Excitations and de-excitations of these fields correspond to what we
mere mortals understand as the CREATION AND DESTRUCTION OF PARTICLES
(photons or electrons).  So in a crude sense, the exited atom is
destroyed, and a photon and a lower energy atom are created at some
instant, and a probability for that happening is some fundamental
constant.  That is what quantum field theory (sort of) says.

Now, suppose we have these two possible states:

				      ~~~~~>
		~~~~~>                ~~~~~>
		~~~~~>                ~~~~~>
		~~~~~>                ~~~~~>
		~~~~~>                ~~~~~>

		(( o ))                 o

All particles are Bosons or Fermions, and photons are Bosons.  Bosons
like to be in the same state, Fermions loath being in the same state.
Why?  Well, if you have a bunch of IDENTICAL PARTICLES, quantum
mechanics says the state of the system is really the superposition of
all possible permutations of these particles.  With bosons, these
states INTERFERE CONSTRUCTIVELY, so the amplitude of the state is much
higher than classical physics predicts.  With fermions, half the
permutations cancel the other half, so the amplitude is zero unless
there is just one particle.  I know that sounds confusing.  I just say
it to show how strange things are.

So, in the second picture, there is much higher probability of the
second state (emission of a photon) than there was in the first
picture.  We call this STIMULATED EMISSION, but that is somewhat
misleading.  The emission is more probable because it results in a big
group of bosons in the same state.

Inversely, if you have a big herd of bosons traveling along, it is very
hard to make one of them break off and do something else.  At low
temperatures, electricity is carried by bosons called "Cooper Pairs",
and that is why one sees SUPERCONDUCTIVITY.  Once the current starts,
individual cooper pairs don't want to scatter off the metal atoms, and
so there is no resistance.  The herd just keeps moving along.