<mmeron@cars3.uchicago.edu> wrote in message
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In article <vsvMe.890$FA3.277@news-server.bigpond.net.au>, "Bill Hobba"
<rubbish@junk.com> writes:

<mmeron@cars3.uchicago.edu> wrote in message
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In article <6OuMe.837$FA3.133@news-server.bigpond.net.au>, "Bill Hobba"
<rubbish@junk.com> writes:

"Igor" <thoovler@excite.com> wrote in message
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Nick wrote:

What is it about matter that
causes light to slowdown? So far
the answers that are forthcomming
are inadequate. So recognise it
if you post one.

When light enters a material it
slows down. When light comes out
the opposite end it speeds back up.
This implies light's propulsion.
Something pushes light and I know
what it is.

It is time. A kind of aether.
Dynamic dimensions. Catch up to time
and what happens?
It goes slower and space shrinks.
Sound familiar?
Yes. It should. It's relativity.

What is it about matter that slows
light?

For the umpteenth time, matter does not cause individual photons to
slow down. They're just being absorbed and re-emitted as they move
through the material. A finite amount of time is spent in this
process,
so there is a delay for a single beam to appear to travel through the
material. So the average speed ends up being < c.

Yes and no - depending on if one is using a pertubative treatment of QED
or
not. I have often fallen into there same trap and Tom has had to
correct
me
a couple of times. In a non pertubative approach it is more like the
treatment of classical EM where the properties of the medium cause the
EM
waves to slow down.

You mean "slow down" in what sense? Phase speed, group speed, signal
speed?

Signal speed - the speed of the wave front - in the sense of Maxwell's
equations in linear media. Classically the equations governing the EM
field
are altered by the media so that the solutions represent waves traveling
not
at the speed of light but at a lower speed. I do not know of a
corresponding treatment for the quantum case - all the treatments I have
seen rely on perturbation theory which does predict a photon is absorbed
then reemitted. But Tom has pointed out on a number of occasions
(correctly
IMHO) that such an approach enshrines the pertuabative treatment which is
a
mathematical device that allows us to handle the equations rather than an
essential part of the physics. The physics is contained in the
interaction
between the photon state and the state of electrons bound to the nucleus.

Indeed, though this goes beyond what I had in mind. The point I mean
is the following: Strictly speaking, a signal isn't slowed down (in
classical EM, that is). A signal (EM pulse of limited time extent)
must have an unlimited frequency domain extent and, regardless of the
details of the specific material, at the limit of frequency going to
infinity the dielectric function goes to 1.

Simple Fourier analysis. But you may be forgetting one thing - the old QM
issue Bohr always managed to hang Einstein on - you must consider the total
set up. If you are considering a wave localized in space outside a
different medium then what you say is true. Extend that to cover the medium
it is propagating into then AFAICS the analysis is no longer applicable -
the longer the 'impulse' you are considering then the less out of band info
in the Fourier transform.

And when we consider waves in the medium then
Maxwell's equations need modification.

But even considering that you still
have an interesting point.