NOVA Online | Time Travel | Think Like Einstein (2)

Think Like Einstein
Part 2: The Speed of Light

OK. Everything so far makes sense. Let's move on to the speed of light for a moment.

In 1887 two American scientists performed a now-famous experiment. The experiment seemed to show that the speed of light was independent of motion. In other words, that light always travelled at the same speed: 186,000 miles per second. It didn't matter if the source of the light was moving or if the observer was moving.

There was another indication that the speed of light was constant, too—one that Einstein found especially difficult to ignore. James Clerk Maxwell, the mind behind electromagnetic theory, had developed equations that described the nature of electricity, magnetism, and even light. These equations, the predictions of which were confirmed by experiment, by the way, implied that light always travelled at the same speed.

Which brings us to the next question...

flashlight & train animation

Again, you're on a train. This time, though, the train is moving much faster—at half the speed of light, or 93,000 mps (miles per second). And instead of throwing a ball, you turn on a flashlight.

Question: How fast is the light travelling relative to the observer standing alongside the tracks?