No matter how you measure it, the speed of light is always the same.

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 300,000 kilometers per second (186,000 miles per second) through the "vacuum" of empty space. Instead, Einstein had an unexpected—and paradoxical—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.

Light in Motion

The Special Theory of Relativity is based on Einstein's recognition that the speed of light does not change even when the source of the light moves. Although it might seem logical to add the speed of the light source and the speed of the light beam to determine the total speed, light does not work this way. No matter how fast Einstein rides his bike, the light coming from his headlight always moves at the same speed.

Stationary Light

Light from a stationary source travels at 300,000 km/sec (186,000 miles/sec).

Moving Light

Light from a moving source also travels at 300,000 km/sec (186,000 miles/sec).

Say that Einstein's bike travels at 10% the speed of light (30,000 km/sec): the speed of light from Einstein's headlight does NOT equal 330,000 km/sec.

The speed of light is constant and does not depend on the speed of the light source.