How fast is gravity?

This article was first published on Big Think in October 2022. Updated December 2023.

Of all the fundamental forces known to man, gravity is the most familiar, the force that holds the universe together, connecting distant galaxies in a vast and interconnected cosmic web. With this in mind, an interesting question to ponder is whether gravity has a speed. It turns out it does, and scientists have measured it precisely.

Let’s start with a thought experiment. Suppose that at this very moment, for some reason, the sun disappeared—not just darkened, but completely disappeared. We know that light travels at a fixed speed: 300,000 kilometers per second, or 186,000 miles per second. Based on the known distance between the Earth and the Sun (150 million kilometers, or 93 million miles), we can calculate how long it would take for us on Earth to know that the Sun has disappeared. It takes about eight minutes and 20 seconds for the sky to darken at noon.

But what about gravity? If the sun disappeared, it would not only stop shining, it would also stop exerting the gravitational pull that keeps planets in their orbits. When will we find out?

If gravity were infinitely fast, then once the sun disappeared, so would gravity. We’ll still see the Sun for a little more than eight minutes, but the Earth has already begun its drift, heading toward interstellar space. On the other hand, if gravity traveled at the speed of light, our planet would continue to orbit the sun as usual for eight minutes and twenty seconds, after which it would stop following its familiar path.

Of course, if gravity traveled at other speeds, the time interval between when sun worshipers on the beach noticed the sun disappearing and astronomers observed the Earth moving in the wrong direction would be different. So, what is the speed of gravity?

Different answers have been proposed throughout the history of science. Sir Isaac Newton developed the first sophisticated theory of gravity, believing that the speed of gravity was infinite. He could have predicted that Earth’s path through space would change before humans on Earth noticed the sun’s disappearance.

Albert Einstein, on the other hand, believed that gravity travels at the speed of light. He predicted that humans would notice both the sun’s disappearance and the Earth’s changing path through the universe. He incorporated this assumption into general relativity, currently the most accepted theory of gravity, which predicts the paths of planets around the sun with remarkable accuracy. His theory made more accurate predictions than Newton’s. So, can we conclude that Einstein was right?

No, we can’t. If we want to measure the speed of gravity, we need to find a way to measure it directly. Of course, since we can’t just make the sun “disappear” for a while to test Einstein’s idea, we need to find another way.

Einstein’s theory of gravity made testable predictions. Most importantly, he realized that the familiar gravity we experience can be explained by distortions in the fabric of space: the greater the distortion, the higher the gravity. This idea has significant ramifications. It shows that space is malleable, like the surface of a trampoline, which deforms when a child steps on it. Furthermore, if the same child jumps on a bouncer, the surface changes: it bounces up and down.

Likewise, space can metaphorically “bounce up and down,” although it would be more accurate to say that it compresses and relaxes, similar to the way air travels sound waves. These distortions in space are called “gravitational waves” and they will propagate at the speed of gravity. So if we can detect gravity waves, we might be able to measure the speed of gravity. But warping space in a way that scientists can measure is quite difficult and well beyond the scope of existing technology. Fortunately, nature comes to our rescue.

Measuring gravity waves

In space, planets orbit stars. But sometimes stars orbit other stars. Some of these stars were once massive and have lived out their lives and died, leaving behind a black hole – the corpse of a dead, massive star. If two such stars die, then there will be two black holes orbiting each other. As they orbit, they emit trace amounts of (currently undetectable) gravitational radiation, which causes them to lose energy and move closer to each other. Eventually, the two black holes got close enough that they merged. This violent process releases massive gravity waves. At the moment when two black holes come together, the merger releases more gravitational wave energy than all the light emitted by all the stars in the visible universe at the same time.

Although gravitational radiation was predicted as early as 1916, it took scientists nearly a century to develop the technology to detect it. To detect these distortions, scientists took two tubes, each about 2.5 miles (4 kilometers) long, and oriented them at 90 degrees, forming an “L” shape. They then used a combination of mirrors and lasers to measure the length of both legs. Gravitational radiation changes the length of the two tubes in different ways, and if they see the right pattern of length changes, they’ve observed gravitational waves.

The first observation of gravitational waves occurred in 2015, when two black holes merged more than 1 billion light-years away from Earth. While this is a very exciting time in astronomy, it doesn’t answer the question of the speed of gravity. To do this, different observations are required.

Although two black holes emit gravitational waves when they collide, that’s not the only possible cause. Gravitational waves are also emitted when two neutron stars collide. Neutron stars are also burned-out stars—similar to black holes, but slightly lighter. Additionally, when neutron stars collide, they not only emit gravitational radiation, but also emit powerful light that can be seen throughout the universe. To determine the speed of gravity, scientists need to observe the merger of two neutron stars.

In 2017, astronomers got their chance. They detected gravitational waves, and a little more than two seconds later, the orbiting observatory detected gamma radiation, a form of light coming from the same location in space, originating in a galaxy 130 million light-years away. Finally, astronomers have found what they need to determine the speed of gravity.

The merger of two neutron stars emits both light and gravity waves, so if gravity and light have the same speed, they should be detected on Earth at the same time. Taking into account the distance of the galaxies hosting the two neutron stars, we know that these two types of waves have been traveling for about 130 million years and arrived within two seconds.

So, here’s the answer. Gravity and light travel at the same speed, which is determined through precise measurements. It reaffirms Einstein’s ideas and hints at profound implications about the nature of space. Scientists hope to one day fully understand why these two very different phenomena have the same speed.