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Invisible <voi### [at] dev null> wrote:
> Bill Pragnell wrote:
>
> > http://en.wikipedia.org/wiki/Feynman_diagram
>
> "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."
>
> ...the HELL?! o_O
Yes, I was expecting a slightly "lighter" article too! I think in common usage
it's not as complicated as it seems. I'm sure others here can (and will)
explain properly - I'm not even going to try, it's been a long time since I
last drew one...
I was just impressed with myself for remembering what they were called. :-)
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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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