Short #36 · Time & gravity

You Can Move Something Faster Than Light

Sweep a laser across the Moon and the spot it makes really does move faster than light. Flick the laser through a quarter turn in a tenth of a second and the spot sweeps at 15.7 radians per second × 384,400 km = 6.04 million km/s, or 20.1 times the speed of light.

0:35 · November 11, 2026

The science

Sweep a laser across the Moon and the spot it makes really does move faster than light. Flick the laser through a quarter turn in a tenth of a second and the spot sweeps at 15.7 radians per second × 384,400 km = 6.04 million km/s, or 20.1 times the speed of light. On the Moon itself it crosses the 3,474 km face in about 0.6 milliseconds.

But nothing travels across the Moon. Every photon flies straight from Earth at the speed of light; the spot is just where each one lands, a different photon every instant. Relativity limits the speed of causes and information, not the speed of patterns.

Observatories really do fire lasers at the Moon (lunar laser ranging, off the reflectors Apollo left behind).

Sources

  • NASA Goddard, Moon Fact Sheet (mean distance 384,400 km, diameter 3,474 km).
  • Speed of light 299,792.458 km/s (SI definition).
  • Superluminal spots and shadows: a standard result of special relativity.

Watch next