Re: Does "c" loose some velocity after leaving a dense medium back into space?

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 03/22/05 

Date: Tue, 22 Mar 2005 02:09:33 GMT

"Tom Roberts" <tjroberts@lucent.com> wrote in message
news:d1nk0n$96k@netnews.proxy.lucent.com...
> richard miller wrote:
> > "Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> > news:VvG%d.6356$C7.5337@news-server.bigpond.net.au...
> >>Each photon when created travels at the velocity of light until it
> >> interacts
> >>with something. In a medium it is the interaction that causes it to
slow
> >>down. The photon undergoes absorbtion and reemission in the medium
during
> >>which it may loose energy.
>
> That's one way of looking at it, but not a verry good one -- that
> essentially enshrines the perturbation APPROXIMATION to QED in your
> description.
>
> It is better, and more faithful to QED itself (rather than an
> approximation), to say that in a medium the interference between a
> traveling light ray and the charges in the atoms cause the light ray to
> be measured as traveling with speed c/n relative to the rest frame of
> the medium; here n is called the index of refraction and characterizes
> the density and masses of the charges in the atoms of the medium.

Yes Tom. I should have been awake to that because you have mentioned it
before. Since the question comes up every now and then I really should file
the answer away for future reference.

>
> Outside the medium, of course, the density of charges is VASTLY less, so
> there is correspondingly less interference affecting the phase of the
> light ray, so it is measured to travel with speed c.
>
> Note I am discussing the theoretical predictions of QED. Real
> experiments, however, agree with these predictions to within
> experimental resolutions, which are typically a few parts per billion
> for such measurements.
>
> [Such measurmeents must use the pre-1984 definition of the
> meter.]
>
>
> > Fair enough, doesn't a photon interact with the quantum vacuum?
>
> Sure (well, the virtual charges in it). But in vacuum the density of
> charges is VASTLY less than in any optical medium.

Tom I have recently been reviewing QFT from Zees book. He develops QFT from
a simple mattress model with springs and masses. Of course he points out
physicists do not really think it is a mattress - simply that it leads to
theories in agreement with nature. He considers particles a disturbance
from the undisturbed or ground state of the system ie the vacuum. First do
you consider this a reasonable view or is Zee being a little inexact for
easy introduction of the ideas to students? And if it is a reasonable view
then I find it conceptually difficult differentiating a particle from the
vacuum for the idea that it interacts with it to actually mean something.

>
>
> > I know I'll get panned for asking about permittivity of free space etc.
But
> > I would like to know where we get a finite speed from. As a
mathematician,
> > one would rather have 0 or infinity.
>
> Light is observed to have a speed that is neither 0 nor infinity. So
> sensible physicists use units with the value 1 for the speed of light (1
> is just as good a number as your 0 and infinity, and has the virtue of
> being both nonzero and finite, as observed for the speed of light).
>
>
> > And I guess that makes inifinity the
> > chosen one.
>
> No. Experimentally light clearly has a finite speed in vacuum.

For some reason, I do not know why, we have a lot of people who think there
must be a reason why light travels at the same speed in all inertial frames.
The answer that is just how nature works never seems to be good enough for
them and represents a problem for SR. It never seems to occur to them that
if you come up with an answer you could level exactly the same criticism
against its premises.

Thanks
Bill

>
>
> Tom Roberts tjroberts@lucent.com

Relevant Pages