Invisible wrote:
> "In quantum field theory a Feynman diagram is an intuitive graphical 
> representation of a contribution to the transition amplitude or 
> correlation function of a quantum mechanical or statistical field theory."

Basically, it's QED - Quantum ElectroDynamics. Which is to say, the behavior 
of electrons and photons.

There's two basic operations that happen: An electron absorbs a photon, and 
an electron emits a photon.  Each of these has a certain probability.

Since you can only measure things at the end of an experiment, your 
experiment winds up with questions like "What's the probability of an 
electron at event[1] A and an electron at event B turning into an electron 
at C, an electron at D?"

[1] Event: A point in spacetime, i.e., a place and time.

The way you calculate this is proabilities. Except the probabilities are 
complex numbers, two-dimentional. They're called "amplitudes". The way you 
get a probability from amplitudes is this:

For each possible path, you multiply together the individual amplitudes that 
could make that path. So if the electron could go from A to X, while the 
other goes from C to Y. It could then emit a photon that goes from X to Y, 
be absorbed by the other electron, and the electrons wind up at B and D. 
(You'd get something like that diagram without the green). That's one 
possibility, and you multiply the amplitudes of an electron going from A to 
X, emitting a photon, going from X to B, the photon going from X to Y, 
getting absorbed by the electron, an electron going from C to Y, and from Y 
to D.

Or the "second" electron could go from C to Y to B the the first could go 
from A to X to D. Or C->X->B and A->Y->D. Or emitting two photons. And so on.

Add up all those amplitudes, take the absolute value of the result, and 
that's the probability that you get from experiment start to experiment end. 
There's an infinite number of different collections of amplitudes to add up, 
so it gets messy. But figuring out all the possible amplitudes is aided by 
drawing pictures of the interactions like that. Fortunately, amplitudes are 
always < 1 (being probabilities altho in 2D), so as you multiply more 
together, the probability they affect your result gets smaller.


-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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