What are Feynman diagrams? Could you give good bibliography about QED?

Asked by: Miguel Angel de Blas

Answer

Feynman diagrams are a visual summary of a quantum mechanical calculation. When quantum
field theory was still in its infancy in the 1930s and 1940s the principal technique for
computing the probability for a certain particle interaction to occur was time-dependent
perturbation theory in relativistic quantum mechanics. The basics of this calculational
technique can be found in any advanced quantum mechanics textbook. Feynman, Schwinger and
Tomonaga (and many others) improved this old technique and developed the modern version of
quantum field theory we use today (they won the Nobel prize for this, see
http://www.slac.stanford.edu/library/nobel.html).

Feynman realized that the somewhat lengthy sums of integrals can be conveniently summarized
as a sum of diagrams. The diagram acts like a recipe: take these incoming particles, have
them annihilate into some intermediate particle, and then have this intermediate particle
decay into some final state particles. Integrate over the intermediate particle's momentum
and impose energy/momentum conservation. The graphical shorthand is very convenient and
appeals to our intuitive sense that physics is local -- interactions occur when particles
are at the same spacetime point (the vertices in a Feynman diagram), like billiard balls
bouncing off one another. A nice graphical description of these things is provided at:
http://www2.slac.stanford.edu/vvc/theory/feynman.html.