From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: sci.physics
Subject: Re: My son would like to know about photons...
Date: 27 Aug 92 00:30:49 GMT
Article-I.D.: husc8.mcirvin.714875449

phys169@csc.canterbury.ac.nz writes:

[Quite nice summary of answers to the question "can you split a photon
like you can an atom?" deleted]

>But if that gamma photon happens to hit another photon, and you get e+ and e-,
>why can't you also say these were constituent bits of the photon, like we said
>with an atom? 

In a sense you can say this.  After all, photons can scatter 
(the effect is very small) off of the virtual electrons and positrons
created by the "vacuum polarization" of another photon, and in this
sense one can call those virtual particles constituents of the
photon.  In renormalized quantum field theory, no particle is an island. 

In other senses, though, they're not constituents.  There would be
no atoms as we know them without the existence of electrons, protons,
and neutrons.  But light would behave in *almost* the same way in free
space if there were no electrons or positrons.  Another reason not
to call them constituents is that you can't model the photon as a
bound state of a *real* electron and positron; that's positronium,
and it's a very different thing.

The best way to think of this is in terms of fields.  In modern
quantum field theory, each fundamental particle is associated
with a field, which can be excited in various ways.  You can
(VERY roughly, particularly for the matter particles) think of
it as vibrations in giant bedsprings, following Heinz Pagels
(I am swiping the idea from his excellent book _The Cosmic
Code_, written on a lay-adult level, but probably accessible to
older children interested in such things; I read it as a
teenager).

There are separate, interlocking bedsprings for electromagnetism,
electrons and positrons, quarks, etcetera.  All of the bedsprings
fill all of space, and some of the bedsprings are connected 
to each other locally by small hooks.  Particles are vibrations,
which can be localized or they can be traveling waves.

The proton and neutron consist of vibrations in the quark
springs, held together by the quarks' hooks to the gluon springs.
The nucleus consists of proton and neutron vibrations held together
by complicated spring-hook interactions; and this lump of vibrations
is yoked to vibrations in the electron springs by mutual hooks to
the photon springs.  This mess of bedspring excitations is an atom.
So we say that the atom is a composite particle composed of all
sorts of things.
 
But the photon isn't just made of vibrations in the electron and
positron springs.  It's a vibration in the photon springs, which
does a little dragging, by the hooks, of the bedsprings associated
with electrons and positrons.  When two photons collide to produce
an e+ and e-, it's as if the hooks have communicated the vibration
entirely from the photon to the electron and positron springs.
They weren't in those springs in the first place, to any
large extent.

The mathematical headaches in quantum field theory are the result
of making the weave of the springs infinitely tight and the hooks
infinitely short!  You can see that the way the bedsprings drag
each other around in that case might be hard to visualize. 

This loose metaphor actually has a certain amount of validity when
you look at the behavior of bosons, like photons, since there the
field can actually be represented as a set of harmonic oscillators.
For fermions, like electrons and positrons, it's less valid, 
but still useful for thinking about things in a general way.
 
>I extended the bank-account analogy there, suggesting an account
>for positrons and electrons, but that was stretching things a bit far.

It's actually not that bad an analogy.  Think of an electric-charge
bank account and energy and momentum, etc. bank accounts.  The energy-
momentum bank accounts have a nonzero balance because of the
incoming photons, but to make an electron you have to make a
positron as well, because the electric-charge books won't
balance otherwise.
 
>Thanks especially for the mention of some suitable books.  A lot aren't really
>geared to younger readers interested in fundamental particles that have their
>own theories about black holes and the universe, and questions whether photons
>go exactly at the speed of light!

Quite right.  Books for kids on physics seem to be uncommonly geared
toward simple stuff, which I suppose is fine, since you can't understand
the more complicated phenomena without learning about the simpler ones.
But it's still frustrating that in other fields, like paleontology or
planetary astronomy, it's easy for a writer who does the requisite
homework to turn out an enjoyable and accurate book for nine-year-
olds that describes recent developments, whereas in physics it's
uncommon to see a book that goes beyond 17th-century ideas without
hopelessly garbling the material.  The first book I read that 
mentioned relativity did little more than quote the old "only
ten or twelve scientists in the world understand it" canard, and
say something vague about E = mc^2.  And special relativity, at
least, is a set of ideas that perhaps could be explained to a bright
nine-year-old with a fair amount of detail. 

-- 
Matt McIrvin, Cambridge, Massachusetts, USA 
Xref: sparky sci.physics:14110 sci.environment:11086 misc.consumers:16155