Posted: Sep 25, 2011 7:59 pm
twistor59 wrote:zaybu wrote:
But the "something happens" is not as nebulous as your diagram implies, it's more like: SEE ATTACHMENT
By considering all possible interactions that we can get very precise agreement between theory and experimental observation in QED.
No, those diagrams are only pictorial representations of a bunch of integrals you need to compute the scattering amplitude of the incoming and outgoing electrons. They're only a way of organising the mathematics. You only have to do it this way because you're using perturbation theory. We're only doing perturbation theory because we don't know how to set up and solve the equations of QED exactly.
The scattering amplitude is given by the sum of the contributions of all the diagrams to a given order. Each diagram (in configuration space) requires you to integrate over all the spacetime locations of each vertex. Only the sum has meaning. The electron never really emits a virtual photon.
Let me ask a question:
The diagram (a) in the picture represents a contribution of order α2, (where α is the fine structure constant), diagram (b) of order α4 etc...
Your objection is counter science. We build a model, and on that basis, we make calculations, which allows us to make predictions. If these predictions are verified experimentally, then we say our model is close to what is real. Your objection is: it's an approximate calculation, therefore the pictures (model) doesn't represent reality.
???
Einstein's E=mc2 also comes from an approximation. We nevertheless see that as something describing a real phenomenon. Most of the stuff we get from theory were reached through approximations. Very, very few results were ever obtained through an exact solution, and science would be the poorer if we demand that only exact solutions are valid.
The fact is: each of the Feynman diagrams requires a precise formulation base on the number of straight lines, vertices, wiggling lines, incoming and outcoming lines. If those interactions don't represent the real McCoy, I wonder what does, certainly not the wave model!
To what order must we go in order to represent reality ? (just sticking to QED here - assuming no new physics, quantum gravity etc)
Each order has a smaller and smaller probability to happen. It doesn't mean that they don't happen. Remember QM, and by extension QFT, is a theory about probabilities from the start. Are we to say that then, being just probabilities, it doesn't describe reality?