# The Path Integral

1. Jul 16, 2010

### Pants

I'm a bit confused about how the path integral for, say, a spin-0 photon is calculated. My understanding of quantum mechanics is somewhere above Feynman's book QED, but somewhere below actually figuring out what every part of the technical definition means. Right now the main sticking point for me is grokking the Hamiltonian, but I don't think I have to figure that out in detail just yet to get the concept.

Anyways, as Feynman describes it in the first chapter of QED, the path integral represents each path that's possible from the source to the detector, and the phase of that path is determined by the energy of the photon and how long it takes to get from source to detector. (Please correct me if I'm misinterpreting this). Here's what I don't get: if the particle is emitted at time 0, and measurement occurs at time T, do we only look at paths that take T seconds to get to the detector travelling at velocity c, or are superluminal paths included in the calculation as well?

Thanks!

-Vince