Q & A: Cherenkov Radiation speed

Q:
What speed do you have to travel at, in order to start emmitting light?
- Socrates (age 13)
SFS (Seoul Foreign school), South Korea
A:
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)