Light - Relation between speed and source

The speed of light doesn't change when you boost your light source. Imagine throwing a ball as fast as you can. Depending on what sport you're playing, you might get all the way up to 100 miles per hour (~45 meters/second) using your hand-and-arm alone. Now, imagine you're on a train (or in a plane) moving incredibly quickly: 300 miles per hour (~134 m/s). If you throw the ball from the train, moving in the same direction, how fast does the ball move? You simply add the speeds up: 400 miles per hour, and that's your answer. Now, imagine that instead of throwing a ball, you emit a beam of light instead. Add the speed of the light to the speed of the train... and you get an answer that's completely wrong.

Really, you do! This was the central idea of Einstein's theory of special relativity, but it wasn't Einstein who made this experimental discovery; it was Albert Michelson, who's pioneering work in the 1880s demonstrated that this was the case. Whether you fired a beam of light in the same direction that Earth moved, perpendicular to that direction, or antiparallel to that direction made no difference. Light always moved at the same speed: c, the speed of light in vacuum. Michelson developed his interferometer to measure the motion of the Earth through the aether, and instead paved the way for relativity. His 1907 Nobel prize remains the world's most famous null result, and the most important one in scientific history.