sci.physics.research

I find different answers:

((Classical electrodynamics))

v < c

((Relativistic Quantum Mechanics))

v = c using Dirac equation for |PSI>.

((Space-time approach to QED))

v = c because the spacetime kernel K_+ is derived from the Dirac equation
for |PSI>.

Feynman even writes in his well-known textbook on QED [1] that

{BLOCKQUOTE
This result is sometimes made pausible by the argument that a precise
determination of velocity implies precise determinations of position at
two times. Then by the uncertainty principle, the momentum is completely
uncertain and all values are equally likely, this is seen to imply that
velocities near the speed of light are more probable, so that in the limit
the expected value of the velocity is the speed of light. }

Then in a footnote, Feynman recognizes that argument is invalid since v
commutes with p. Thus, an incorrect value is justified using an incorrect
argument!

((Field approach to QED))

I have computed the value of v using also the field approach to QED.
the value for the v corresponding to a Dirac field PHI(x,t) is again c.

Conventional thinking says that position is not an observable in field QED
(just a parameter) and thus v cannot be indentified with the velocity of a
particle.

This argument is difficult to accept because is if x and v are not
observables at some fundamental level of theory, then,

i) why there exist observables x and v at level of non-relativistic
quantum mechanics?

ii) How does one obtain classical observables x and v?

Already Dirac warned about QED but his quote is almost ignored (only his
related quote on renormalization is usually cited). The quote on
incompatibility of QED with previous theories is

{BLOCKQUOTE
Most physicists are very satisfied with this situation. They argue that if
one has rules for doing calculations and the results agree with
observation, that is all that one requires. But it is not all that one
requires. One requires a single comprehensive theory applying to all
physical phenomena. Not one theory for dealing with non relativistic
effects and a separate disjoint theory for dealing with certain
relativistic effects. [...] For these reasons I find the present quantum
electrodynamics quite unsatisfactory. }