Einstein's crucial breakthrough about the nature of light, made in 1905, can be summed up in a deceptively simple statement: the speed of light is constant. So what does this sentence really mean? Surprisingly, the answer has nothing to do with the actual speed of light, which is 186,000 miles per second through the "vacuum" of empty space. Instead, Einstein had an unexpected insight: that light from a moving source has the same velocity as light from a stationary source. For example, beams of light from a lighthouse, from a speeding car's headlights and from the lights on a supersonic jet all travel at a constant rate as measured by all observers—despite differences in how fast the sources of these beams move.
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