Re: Carroll on general relativity, poll questions

From: Mike Helland (mobydikc_at_gmail.com)
Date: 09/08/04 

Date: 7 Sep 2004 20:20:22 -0700

Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<chh69j$p7l$2@news.urz.uni-heidelberg.de>...
> Mike Helland wrote:
> > Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<chfa7c$bla$3@news.urz.uni-heidelberg.de>...
> >
> >>mobydikc@gmail.com wrote:
> >>
> >>>Bjoern Feuerbacher wrote:
> >
> >
> >>>>No, not at once. Can *you* provide a cite that supports "QED says
> >>>>observable photons don't travel always at c"? Remember, *you* made a
> >>>>claim first. It is *your* responsibility to support it.
>
> >>>>From Feynman:
> >>>
> >>><quote>
> >>>It may surprise you that there is an amplitude for a photon to go at
> >>>speeds faster or slower than the conventional speed, c. The amplitudes
> >>>for these possibilities are very small compared to the contribution
> >>>from speed c; in fact, they cancel out when light travels over long
> >>>distances. However, when the distances are short...these other
> >>>posibilities become vitally important and must be considered.
> >>></quote>
> >>>
> >>>He doesn't make the distinction here between real or virtual, which is
> >>>why I asked if you had something else to look at.
> >>
> >>He does not make this distinction because for anyone who reads this
> >>and knows what he is talking about, it is clear that he *means*
> >>virtual photons!
> >
> >
> > So would you mind providing a cite?
>
> No one bothers to state the obvious. Open a textbook on Quantum
> Electrodynamics and look at the equations for yourself.

In QED virtual photons are the photons that don't appear in the inital
or final states of the system. They are these things that are never
observed but we need to factor in their role to get the right results
out of the theory.

To get the right results you also need to take into consideration that
photons may move slower or faster than c.

This applies to all photons as far as I know.

Quantum electrodynamics and special relativity do have an
incompatibility. SR says the speed of light is constant. QED says
thats not true. 

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