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Timothy replies to Steve:
>I'd like to present a similar idea framed in terms ofw a laser and
>a sheet of "slow glass" -- a material that was the main gimmick
>in a few science fiction stories by an author whose name I can't
>remember right now (darn it).  Slow glass transmits light, but
>with a long time delay between when it comes in one side and goes
>out the other.  In the stories, it was used for things like
>scenery windows -- a sheet of slow glass was set in a field for a
>few years, then put on the wall of a house, where it would
>release the years of scenery.

The slowest glass would be diamond with a speed of light 2.4 times slower
than vacuum. But maybe there are other material with a higher refractive
index for other wavelengths.

>At least to the first analysis, the glass must pick up momentum
>from the laser beam while it is absorbing the photons, and
>accelerates for as long as the laser is beamed at it.  Then, when
>the light begins coming out the other side, the glass loses that
>momentum and decelerates.  The end result seems like it ought to
>be that the glass ends up with some amount of spatial
>displacement from its original position, and the laser light
>passes through effectively unchanged, so that energy and momentum
>are conserved throughout.

This is a really fascinating idea, I wish I had thought of it.

>However, assuming a laser that emits constant power in its rest
>frame, the light intensity from the laser seen by the slow glass
>won't be constant.  As the slow glass accelerates, the incident
>power from the laser will decrease from doppler shifting.
>Getting the glass to accelerate to high relativistic speeds will
>also cause the system of the glass and the photons absorbed to
>that point to increase significantly in mass (this isn't the same
>as what Timothy calls "relativistic mass increase" -- why?).

Yes why, I don't understand, the glass is just a fast moving mass, so it
will increase in mass due to a higher velocity. Of course the photons add
some extra weight (m=Ephoton/c^2)

>So I'm a little troubled about the overall physics -- if the glass
>emits exactly the light it saw, then at the end of the first year
>of its proper time, if it emits the same light it saw at the
>beginning of the year, an observer in front of it would see that
>light tremendously blue-shifted if the glass was accelerated to
>high relativistic speeds.  On the other hand, by the end of the
>second year of proper time of the glass, the light it emits is
>the very red-shifted light it was absorbing just before the laser
>turned off.  I'm not sure, without doing the math, whether this
>comes out such that the energy emitted during the second year is
>the same as the energy absorbed during the first year.

As far as I can tell this is indeed what happens the first light that comes
out is blue shifted. But the last light that comes out isn't more
red-shifted than when it entered, because when the last light comes out the
glass isn't moving anymore.
Of course this slow glass should be very "clear" otherwise most of the light
would be really absorbed before it could leave.

Now that we are talking about slow-glass, did you know that you could make a
photon stand still relative to you.
Just shoot it in a bar of glass, then move the glass with about 0.7c in the
opposite direction. The photon will then not move relative to you.