From: "Michael D. Crawford" 
To: atm{at}shore.net
Reply-To: "Michael D. Crawford" 


There is a phenomenon that I think is called phase conjugation, that gives
the effect of having crossed beams of light interacting.

But that's not really what is happening.

An intense beam of light passes through a nonlinear material, and the
material's reaction to this can have the effect of modulating another beam
of light.

I don't understand it very well.  A friend of mine tried to work on it for
his undergraduate thesis in physics at UC Santa Cruz, and got bogged down
in the math and gave up on it.

This doesn't happen in a vacuum because free space is a linear medium.

You can understand how photons can interact if you know about feynman diagrams.

In quantum electrodynamics there is no such thing as a force that acts at a
distance.  The paths of particles are deflected as they emit and absorb
other particles.

Two electrons interact via the electomagnetic field by exchanging a virtual
photon.  This is a particle that is created by borrowing energy from the
Universe but gets away with it because its energy is returned before anyone
notices (except the two electrons).

The Feynman diagram is like this:


      \     /
       \ ~~/
       /   \
      /     \

the straight lines are electrons and the ~~ is the virtual photon.

But you can rearrange Feynman diagrams in various ways to predict other
phenomena, as long as you obey the various conservation laws.  So you can
have two photons interact by exchanging a virtual electron:

~~~~~~~~~
     |
~~~~~~~~~

If I could draw it on paper the photons' wiggly lines would bend at the
points where they touch the electron's straight line.

You can understand the basics of how various particle phenomena work by
learning how to draw Feynman diagrams.  But to get quantitative values like
the probability that such interactions occur, you need to work the
mathematics of quantum electrodynamics.

Unfortunately things weren't working out for me financially in graduate
school, so I dropped out just before I was about to learn all that.  I hope
to go back someday.  I still have the books, if I can work up the gumption
I may try to learn it on my own.

Feynman and Tomanaga got their Nobel prize for working out QED in such a
way that it is apparently able to predict the behavior of any
electromagnetic interaction with complete precision.

QED doesn't work for particles that obey the strong force, like quarks. 
One problem is that quarks are only found in bound states, where there are
swarms of virtual particles and you can't observe the isolated behaviour of
a single virtual particle exchange.

Another problem is that the binding energy of the states is so high that
the equivalent mass becomes significant - the particles travel at
relativistic speeds.

You can draw Feynman diagrams that deal with the strong force, but the
mathematical laws that govern it have not been quite worked out.

For a long time the Standard Model looked like a good approximation, but no
one believed that it was the fundamental law every physicists seeks.  And
the recent observation of neutrino oscillation demonstrated that the
Standard Model isn't even a good approximation anymore.

I knew Feynman when I was a CalTech student.  He was a wonderful man.  Each
week he taught an informal class called "Physics X".  It wasn't
an actual class, you couldn't get credit and there was no homework.  But
what you could do is ask a Nobel Prize Winner pretty much any question you
wanted, and he'd do his best to answer.

The one exception is that you couldn't ask questions that required him to
work out any mathematics.  This from a man who can do numerical simulations
in his head - he was the head of the computing section at Los Alamos.  The
"computers" used on the a-bomb project were people with log
tables and adding machines.

Physics X only allowed conceptual questions.

Each week I would come and tell him yet again that I didn't believe quantum
mechanics.  I was an entrenched Newtonian.  I believed in a clockwork
universe.

It wasn't that I didn't understand quantum mechanics, I just thought it was wrong.

And each week he would sketch out diagrams of the great physics experiments
on the chalkboard, and patiently explain away each of my objections.

I finally accepted quantum mechanics around the end of my freshman year.

Yours,

Mike
--
Michael D. Crawford
GoingWare Inc. - Expert Software Development and Consulting http://www.goingware.com/
crawford{at}goingware.com

      Tilting at Windmills for a Better Tomorrow.

--- BBBS/NT v4.00 MP