NOVA Black Hole Apocalypse Astronomy Practice Test

Gravitational waves propagate through empty space at the same speed as light because they are ripples in the fabric of spacetime itself. In general relativity, disturbances in the spacetime metric—gravitational waves—are massless excitations that travel at the universal light speed, c. This means they carry information and energy through spacetime at that same speed, just like how electromagnetic waves (light) do.

A key real-world check comes from observations of a neutron-star merger where gravitational waves were detected almost simultaneously with gamma rays from the same event. The tiny difference in arrival times, over a vast cosmic distance, shows that gravitational waves travel at essentially the speed of light, within extremely tight experimental bounds.

So the speed of gravitational waves matches the speed of light, not sound, not gravity as a separate speed, and not the universe’s expansion rate.