Re: Light slows down in glass?

[William Beaty <billb@eskimo.com>]

On Fri, 6 Feb 1998, LUDWIK KOWALSKI wrote:

> As I wrote several days ago, I am no longer satisfied with the "absorbed,
> reemitted, absorbed again" explanations. Any comments on what is below?
>
> > I would suggest that it is important to stress that electromagnetic
> > radiation always propogates at the same speed (speed of light) but that 
> > when light enters a transparent medium it is absorbed, reemitted, 
> > abosorbed again, reemitted again etc. and that this process makes 
> > it _seem_ that the light has moved slower in 'passing through' the glass.  


My own opinion is that there are various models for what light does, and
if we insist that light is emitted and reabsorbed, then we really are
saying that the particle model for light is correct and the wave model is
false.  After all, it is *photons* which are absorbed and re-emitted. 
Light wavelength is far larger than the spacing between atoms, and I
cannot see how "absorbed/emitted" applies to the EM fields or to EM waves. 
Instead, the transparent material acts as a non-vacuum medium, having an
altered propagation velocity.  If we insist on "absorbed/emitted", then we
fly in the face of wave/particle duality.

Perhaps the real issue is a collision between classical and QM models? 
For example, if I send an EM pulse down a pair of plastic-coated wires,
the pulse doesn't travel at c, it moves slower depending on the dielectric
constant of the plastic.  This effect is fairly independent of frequency.
It is normally explained by ignoring photons.  This is obviously not
QM physics.  Yet is certainly is not "wrong", and if it is wrong, then any
reference to EM fields is also "wrong."

A separate issue: the original message mentioned Feynman.  I'm curious. 
Do most physicists believe Feynman's interpretation of QM, that the wave
nature of EM is a consequence of the actions of vast numbers of virtual
particles which take all possible paths and only sum to "real" particle
events sometimes?  I was under the impression that this was a minority
viewpoint in physics, and most physicists still thought in terms of
"instantaneous collapse of wavefunction."  Feynman seems to say that
photons are real and waves are not, while "Copenhagen" viewpoint says that
probability waves are real, and they act to "cause" the appearance of
photon particles.  And if light is "really" virtual particles, how can we
explain polarizatino and other field phenomena?

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