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Kevin C. Houston writes:
 > [ . . . ]
 > Now, consider the the following engine design:
 > Sol sends out a maser beam, and the "Asimov" absorbs it, turning it into 
 > electricity (let's say 80% eff)  This gives the Asimov a nice 1 G accel.  
 > The energy is stored (if I knew how, I wouldn't need you guys  ;) ) and 
 > later, at the halfway point, the beam from Sol stops.  The "Asimov" gains 
 > some deceleration out of friction with the ISM (until you get down to 
 > about .90 C) and then releases the stored energy in the form of a maser 
 > beam generated by the "Asimov" and directed toward TC.  Various transfer 
 > losses would probably leave you with some velocity even after you 
 > expended all of the stored energy.  But I think that the velocity you 
 > would have left would be within the stopping range of a fusion engine.
 > Once the antenna array was done absorbing the maser beam, it could be 
 > used for ISM drag chute, and we don't really care that it erodes away 
 > because by the time it's gone, we're below the speed where we get any 
 > useful drag. 
 > [ . . . ]
 > eagerly awaiting responses. 
 > (esp. Steve and Tim -- renowned throwers of ice water)   :)
 > Kevin

You'll probably die of shock, Kevin, but this idea, at least in
concept, could actually work.  Collecting and storing the immense
energy required is likely to be almost impossible, but assuming
that a method existed, this doesn't seem to violate the laws of

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.

Let's pick a sheet of slow glass that has a year-long delay
between when it absorbs light on one side and re-emits it on the
other.  If we shine a laser at a sheet of slow glass, it will
effectively appear to absorb the laser light, and the light's
momentum, for the first year of its proper time.  At the end of
that year of proper time for the glass, the laser is turned off,
and the absorbed laser light is emitted from the other side of
the glass.  Light continues to be emitted for another year of
proper time for the glass, until all of the laser light absorbed
has been emitted.

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.

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?).  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.