Why does gravity travel at the speed of light?

We all know light obeys a speed limit — roughly 186,000 miles per second. Nothing travels faster. But why should gravity travel at the same speed?


That question requires a quick dive into Albert Einstein’s general relativity, or theory of gravity — the same theory that predicted gravitational waves a century ago.

Einstein overthrew Isaac Newton’s idea of “absolute time.” Newton thought time marched onward everywhere at an identical pace — regardless of how we mortals perceived it. It was unflinching. By that line of thinking, one second on Earth is one second near a black hole (which he didn’t know existed).


Newton also thought gravity acted instantaneously. Distance didn’t matter.

Gravity does not travel at the speed of light just like electromagnetism does not travel at the speed of light.

Changes in the gravitational field, far from sources, traveling in the vacuum, do travel at the vacuum speed of light as gravitational waves. This is just like changes in the electromagnetic field, far from sources, traveling in the vacuum, do travel at the vacuum speed of light as electromagnetic waves.

If changes in these fields traveled slower than the vacuum speed of light, you’d be able to catch up with them in principle. So you have to ask yourself what you’d see if you traveled alongside with them. Relative to you, these changes would now be at rest. Yet they’d still have to have energy, i.e., mass. In short, you’d see a static gravitational field (or electrostatic field) with mass.

Therefore, the statement that changes in the gravitational (or electromagnetic) field travel at the vacuum speed of light is equivalent to stating that these fields are massless. As it turns out, this is quite important to the way these fields work. If they were massive, they would have a finite range: the range would be inversely proportional to mass. Being massless, the range of these fields is infinite. Yes, gravity gets weaker over distance, but even over cosmological distances, it is still there, affecting cosmic expansion, for instance.

In contrast, an example for a massive field is the weak nuclear interaction: it really isn’t weak (at the subatomic scale, it is just as powerful as electromagnetism) but because this field is very massive, its range is extremely short.