For 10 points, name this theoretical upper limit for the speed of any object, equal to about three hundred million meters per second.
Cherenkov ("CHAIR-en-kov") radiation exists when this value is exceeded. The ratio of this quantity in a vacuum to this quantity in a material is that material's index of refraction. It is squared and multiplied by mass to give energy in an equation derived by Einstein, E equals m c squared. For 10 points, name this theoretical upper limit for the speed of any object, equal to about three hundred million meters per second.
Cherenkov ("CHAIR-en-kov") radiation exists when this value is exceeded.
speed of {light} [accept {c} before mentioned]