starship-design: I have many questions...

[Steve VanDevender <stevev@efn.org>]

kyle writes:
 > Greetings,
 > 
 > I am working on something and wish to get my underlying physics correct
 > (Steve, you taught me to do this) and have some questions I hope some of
 > you can answer:
 > 
 > 1: If you were placed at creation beyond lightspeed, could you continue
 > being in an FTL state?
 > 
 > 2: If you are beyond lightspeed, can you continue accelerating?
 > 
 > 3: Can light be slowed down in a medium to less than 99.99c?
 > 
 > 4: If you are beyond lightspeed in a dense medium, what happens? (it has
 > happened)
 > 
 > 5: If you are travelling FTL, can you ever slow down below c?
 > 
 > Kyle Mcallister

>From what I've read on the topic:

If you start out superluminal, you stay superluminal, just as if you
start out subluminal, you have to stay subluminal.

Analyzed simply by relativistic kinematics, a superluminal particle
accelerates by _losing_ energy, and has to be raised to asymptotically
infinite energy to slow down to c, the unreachable minimum speed for it.

Even in a transparent medium, hotons travel at c between atoms, but the
slight delay between their absorption and re-emission by atoms in the
material reduces their average speed in the medium.  A medium is
transparent if it tends not to want to absorb photons of the energies to
which it is transparent; those photons are only absorbed briefly but
push electrons to unwanted energy levels that cause the photons to
quickly get re-emitted with their original energies.

Charged particles traveling faster than the average velocity of photons
in a medium emit Cerenkov radiation in their interaction with charged
particles in the medium.  If you've seen photographs of the blue glow in
water-cooled nuclear reactors, that glow is Cerenkov radiation from
particles traveling faster than the average speed of photons in water.
(The sci.physics FAQ explains why the glow is blue.)  Presumably
superluminal particles would also emit Cerenkov radiation in a vacuum,
but since they would lose energy as a result this would make them go
faster.  Since the universe is not filled with a blue glow from a
quantity of near-zero-energy nearly-infinitely-fast tachyons we might
conclude that either a) tachyons don't exist or b) there's something
wrong with this analysis.

The quantum-mechanical treatment of tachyons in the sci.physics FAQ is
somewhat more interesting.  Either tachyons move faster than light, but
can't ever be localized (that is, you can't measure them) or they can be
localized but then move slower than light.  Either way, their
superluminal properties are inaccessible.