Well, there is no such thing as an absolute speed. An object may be at
rest according to one observer, but be moving according to another
observer, if the two observers are moving with respect to each other.
If an object's emitting light, then all observers should agree it's
emitting light.
Electrically charged objects will emit light if they pass through
a material quickly enough. The space around an electrically charged
object has an electric field in it, and if the object is moving, also a
magnetic field. These fields cannot propagate any faster than the speed
of light in whatever material they happen to be in. If the object is
moving faster (with respect to the material) than the speed of light in
the material, it will emit light which is commonly called "Cherenkov
Radiation" after the Russian physicist who first observed it. The speed
of light in a material is the speed of light in vacuum divided by the
index of refraction of the material. Glass, for example, has an index
of refraction of around 1.4. The refrence frame in which the material
is stationary provides the reference by which to compare the speed of
the object.
In a vacuum, light travels at c, which is the maximum speed
anything can travel at. So an object will not emit Cherenkov radiation
in a vacuum. In a vacuum, there is no unique choice of reference frame
as there is with a material.
Objects emit light for all kinds of other reasons (warm them up
for example, and they will emit radiation. Even a tiny amount of
radiation gets emitted by any object with a temperature just above
absolute zero). There is no lower limit on how fast something can go
for it to glow if it's hot.
Tom
(published on 10/22/2007)
