Caltech Question of the Month
Question of the Month Submitted by John Propst, Fullerton, California.
Answered by Ken Libbrecht, Caltech professor of physics.
Light travels at the speed of light, and is created traveling at light speed. When Einstein invented the theory of special relativity, he postulated that the speed of light was a constant.
If you carry your flashlight on a moving train, the photons travel out from it at a constant speed, whether you measure their speed from the ground or measure their speed from the train. You can't derive this fact from anything, so it becomes a fundamental law of physics.
We don't know why nature chooses to operate this way, but many measurements have shown that Einstein's postulate is very accurately followed.