[net.physics] More About Photons

don@allegra.UUCP (D. Mitchell) (04/01/84)

I wanted to say a few words about waves and particles.  This is an a
further attempt to answer the question "WHAT IS A PHOTON?".

You have a conducting box.  It is possible to have electromagnetic
standing waves in this box, but not just any frequency is allowed.  As
you know, only wavelengths that are integer divisions of the length of
the box are allowed, so you have a discrete sequence of frequencies.
This is FIRST QUANTIZATION.  The discrete energy levels of an atom are
the same type of phenomena; the electrical attraction of the nucleus
confines the electrons and yields a discrete sequence of allowed modes
of the wave function.

Getting back to our box, each standing wave mode has both a frequency
and an amplitude, and quantum electrodynamics (QED) says the amplitude
is also quantized!  This is called SECOND QUANTIZATION.  The intensity
of some given standing wave goes up or down by discrete amounts, and
this is called creation or destruction of a photon.

OK, now things get weird, but I will try to introduce the weirdness
step by step as we go from a quantum mechanical to a quantum field
view.  Let's add some atoms to the box, and say that some of them are
in excited states.  Keep in mind, that I am also getting confused by
this because my poor brain is the product of millions of years of
evolution all aimed at making me better at throwing stones at rabbits.
So, here is a description of a simple event:

	An excited atom falls into a lower energy level, and emits
	a photon.  The photon travels through the box and hits
	another atom and is absorbed, exciting it.

Now let's recognize that photons are quantum levels in a field:

	An excited atom couples with the photon field and falls into
	a lower energy level, raising the number of photons in the
	field.  A little later, the photon field couples with another
	atom and a photon is destroyed, exciting the atom.

Now let's recognize that atoms  are quantum levels in a field.  I am
bending the rules a little in talking about atoms like this, but the
flavor of the theory is preserved.  Obviously, it is the electrons in
the atom that are coupling and changing state:

	The excited-atom field couples with the photon field and
	the nonexcited-atom field.  The excited-atom field goes down
	one level (an atom is "destroyed"), the photon field goes up,
	and the nonexcited-atom field goes up (an atom is created).
	Later, photon field couples with the nonexcited-atom field
	and the excited-atom field to create an excited atom and
	destroy a photon and a nonexcited atom.