What is an Amplitude in Quantum Electrodynamics (QED)?

After looking at some Richard Feynman material on QED I'm putting down some thoughts on amplitudes so that (a) I remember and (b) I remember - yes, I did type that twice.

An amplitude can be described as a line on a plane.  The line can be of any particular length and angle on that plane.  We can use an amplitude to get the probability of an event - to do this we simply square the amplitude, i.e.:

   probability = (ampl)^2

QED is concerned with and can be summarised at a very high level by the following 3 points:

• the probability that an electron goes from place to place
• the probability that a photon goes from place to place
• the probability that an electron will 'emit' a photon

The dimension of time is added to the above so that probabilities can be calculated for electrons and photons to be in a certain place at a certain time.

(more to follow - it's 3am after all ..)