Spotlight

Can a spot of light move faster than c, the speed of light? For example, if a lighthouse light beacon spins around at very high speed, will the spot of light seen far from the beacon cut across the sky with a speed greater than 3 × 10⁸ m/s?

Answer

Yes, a spot of light from the lighthouse beacon when moving across your field of vision can move faster than c. But the actual light itself (i.e., the photons in the beam) moves from the source to the reflecting spot in the sky at c, no faster.

A good example from astrophysics is the radio wave beam from the pulsar in the Crab Nebula, which sweeps across our Earth observatory thirty times a second from a distance of a few thousand light-years. The electromagnetic waves coming to us from the distant source travel at light speed, but the sweep across our planet moves faster than c.

Occasionally one encounters other suggested examples of spots traveling faster than light speed, such as the intersection edge in a very long pair of scissors progressing outward when closing. Unfortunately, this intersection edge’s speed is limited by the speed of sound in the metal of the scissors, which is quite slow compared to c. However, the spot of light on an oscilloscope trace can move across a screen faster than c even though this spot is produced by the slower-moving electrons striking a phosphor.