Re: What is the velocity of a relativistic electron?

Juan R. wrote;

(QED continues to
work only for infinite time scattering processes between free fields
over a classical space-time).

This is not true. One can compute - nonrigorously, in renormalized perturbation theory - many time-dependent things, namely via the Schwinger-Keldysh (or closed time path = CPT) formalism; see, e.g.,
http://theory.gsi.de/~vanhees/publ/green.pdf


See also the entry
S9c. What about relativistic QFT at finite times?
in my theoretical physics FAQ at
http://www.mat.univie.ac.at/~neum/physics-faq.txt


Arnold Neumaier

.