Ripples in spacetime are what gravitational waves are, and they travel through space at the speed of light in all directions. Although the constants of electromagnetism never appear in the equations for Einstein’s General Relativity, gravitational waves undoubtedly move at the speed of light. Here’s why. (EUROPEAN GRAVITATIONAL OBSERVATORY, LIONEL BRET/EUROLIOS)

Ask Ethan: Why Do Gravitational Waves Travel Exactly At The Speed Of Light?

General Relativity has nothing to do with light or electromagnetism at all. So how to gravitational waves know to travel at the speed of light?

There are two fundamental classes of theories required to describe the entirety of the Universe. On the one hand, there’s quantum field theory, which describes electromagnetism and the nuclear forces, and accounts for all the particles in the Universe and the quantum interactions that govern them. On the other hand, there’s General Relativity, which explains the relationship between matter/energy and space/time, and describes what we experience as gravitation. Within the context of General Relativity, there’s a new type of radiation that arises: gravitational waves. Yet, despite having nothing to do with light, these gravitational waves must travel at the speed of light. Why is that? Roger Reynolds wants to know, asking:

We know that the speed of electromagnetic radiation can be derived from Maxwell’s equation[s] in a vacuum. What equations (similar to Maxwell’s — perhaps?) offer a mathematical proof that Gravity Waves must travel [at the] speed of light?