Re: The speed of light: no crackpots!

From: Gregory L. Hansen (glhansen_at_steel.ucs.indiana.edu)
Date: 06/04/04 

Date: Fri, 4 Jun 2004 17:20:18 +0000 (UTC)

In article <10c1684sqncnh55@corp.supernews.com>,
kenseto <kenseto@erinet.com> wrote:
>
>"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
>news:c9pvpk$f8p$1@hood.uits.indiana.edu...
>> In article <10c0shi3pfdto0a@corp.supernews.com>,
>> kenseto <kenseto@erinet.com> wrote:
>> >
>> >"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
>> >news:c9nnnu$ojm$1@hood.uits.indiana.edu...
>> >> In article <10bum2cjdm9eg54@corp.supernews.com>,
>> >> kenseto <kenseto@erinet.com> wrote:
>> >> >
>> >> >"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
>> >> >news:c9nc1n$k7l$3@hood.uits.indiana.edu...
>> >> >> In article <10bu855lq9gmtc5@corp.supernews.com>,
>> >> >> kenseto <kenseto@erinet.com> wrote:
>> >> >> >
>> >> >> ><carlip@no-physics-spam.ucdavis.edu> wrote in message
>> >> >> >news:c9l9as$enb$1@woodrow.ucdavis.edu...
>> >> >> >> Creighton Hogg <wchogg@hep.wisc.edu> wrote:

[...]

>> >> >> >> Basically, when light enters a medium, it excites atoms, which in
>> >turn
>> >> >> >> emit light. The newly emitted light has two components. One is
>> >> >exactly
>> >> >> >> out of phase with the incoming beam, and destructively interferes
>> >with
>> >> >> >> it, essentially eliminating it. The second looks like the
>incoming
>> >> >wave,
>> >> >> >> but has a slightly different phase. The overall effect of this
>> >phase
>> >> >> >> shift is to ``delay'' the propagation, reducing the phase
>velocity.
>> >> >The
>> >> >> >> same process continues as the new wave propagates through the
>> >medium,
>> >> >> >> giving a larger and larger phase shift/propagation delay. The
>> >> >existence
>> >> >> >> of this effect is called the Ewald-Oseen extinction theorem, and
>can
>> >be
>> >> >> >> used to quantitatively compute propagation speed, at least in a
>> >> >somewhat
>> >> >> >> idealized medium.

[...]

>>
>> Yes, it takes light longer to go through a thicker peice of glass. That's
>> because the light has more distance to travel to get from one end to the
>> other.
>
>You didn't read what I said. I said that if the measured slow speed
>of light in glass is due to the absorption and re-emission processes then
>the speed of light should measured to be slower in a thick piece of
>glass than a thin piece of glass.

I can't figure out why you think that. The light in the second half of
the glass will be doing pretty much the same thing it did in the first
half.

>>
>> >>
>> >> >2. Dr.Carlip's explanation suggests that the light should be
>> >> >scattered completely upon entering the glass.
>> >>
>> >> You might want to check in on what the emission looks like from a line
>of
>> >> oscillators excited in phase. Check into phased-array radar, too, for
>an
>> >> interesting macroscopic example. Light waves and radar waves, as well
>as
>> >> wavefunctions in quantum mechanics, interfere.
>> >
>> >So you are saying that the light is being absorbed and re-emitted
>> >in a straight line of glass molecules? This sound more like an assertion
>> >than
>> >an explanation.
>>
>> Of course not. I'm saying that if you explore some simple interference
>> effects on your own, you'll discover that the light *shouldn't* scatter
>> diffusely in all directions when it hits an optically smooth surface. I
>> didn't want to ask you to just trust an assertion, I wanted you to work it
>> out for yourself and become convinced.
>
>We are not talking about light hitting an optically smooth surface. The
>light is being absorbed aand re-emitted by the molecules of the glass. It
>would be very strange if these processes are carried out only in a linear
>direction.

Then do the full three-dimensional case, if you like. But start doing
some math. The punch-line is constructive interference ahead, destructive
interference elsewhere. The reason it's constructive interference ahead
is because all the atoms are radiating in phase. You can change the
direction of the radiated beam by adjusting the relative phases, e.g. by
hitting it with light from an angle. This is done electronically with
microwave emitters in the phased-array radar -- you can point the beam
wherever you like, with no moving parts.

You'll find the combination of the initial wave and the wave emitted from
the first line of atoms will hit the second line of atoms and do
basically the same thing it did before. But it's useful to start by
solving a simpler problem and then add complexity to it.

If the wavelength is about the same size as the interatomic spacing,
you'll get useful diffraction effects. X-rays are used to study
materials, on that principle.

If the wavelength is much smaller than the interatomic spacing, you'll
pretty much get diffuse scattering as you've suggested.

>> >>
>> >> No, curvature of spacetime is not an explanation of index of refraction
>or
>> >> the slower speed of light in glass. Run the numbers, see how big the
>> >> effect is.
>> >
>> >We don't know what the distortion of space inside the glass is like. For
>> >sure it is not just gravitational effect. It is mostly electromagetic
>> >effect.
>>
>> Electromagnetism is part of the stress-energy tensor. And your curvature
>> argument *is* an argument of just gravitational effect.
>
>NO. We know that the motions of the particles inside the glass will
>have an effect on the geometry of the space inside the glass.

Not much. Since the speed is much less than that of light, they're
quasi-stationary as far as gravity is concerned.

>>
>> >For
>> >example we can see the distortion of space by a magnet.
>>
>> No, we can't. We can interact with the field of a magnet. A magnetic
>> field doesn't act like a gravitational field.
>
>Yes we can. We can see the curved lines of force of a magnet using iron
>filings.

Sure. That's not curvature, that's the magnetic field.

>> >Again your arguement is based on gravtational effect alone.
>>
>> You dragged general relativity and curvature into this, you're the one
>> that brought up gravitational effects.
>
>I should have included the electromagnetic effect.

All right, let's include the electromagnetic effect.

"Basically, when light enters a medium, it excites atoms, which in turn
emit light. The newly emitted light has two components. One is exactly
out of phase with the incoming beam, and destructively interferes with
it, essentially eliminating it. The second looks like the incoming wave,
but has a slightly different phase. The overall effect of this phase
shift is to ``delay'' the propagation, reducing the phase velocity. The
same process continues as the new wave propagates through the medium,
giving a larger and larger phase shift/propagation delay. The existence
of this effect is called the Ewald-Oseen extinction theorem, and can be
used to quantitatively compute propagation speed, at least in a somewhat
idealized medium." 

-- 
"And don't skimp on the mayonnaise!"

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