What exactly is the velocity of gravity?

Of all the fundamental forces known to mankind, gravity is the most common that holds the universe together, linking distant galaxies into a vast, interconnected cosmic web. With that in mind, a great question to think about is whether gravity has velocity. It turns out that it does, and scientists have precisely measured it.

Let’s start with a thought experiment. Suppose that at this very moment, the sun has somehow disappeared – not only to darkness, but completely fading away. We know that light travels at a constant speed: 300,000 kilometers per second, or 186,000 miles per second. From the known distance between the Earth and the Sun (150 million kilometers, or 93 million miles), we can calculate how long it will take before we know here on Earth that the Sun is gone. It would take eight minutes and 20 seconds before the sky turned dark.

But what about gravity? If the sun were to disappear, not only would it stop emitting light, but it would also stop exerting the gravitational pull that keeps the planets in their orbit. When will we find out?

If gravity were infinitely fast, gravity would also disappear once the sun was no longer present. We’d still see the sun for a little more than eight minutes, but the Earth would already begin to wander, heading out into interstellar space. On the other hand, if gravity traveled at the speed of light, our planet would continue to revolve around the sun as usual for eight minutes and 20 seconds, after which it would stop following its familiar path.

Of course, if gravity moved another speed, the interval between when the sun worshipers noticed the sun had disappeared, and when astronomers noticed the Earth was going in the wrong direction would be different. So what is the velocity of gravity?

Various answers have been proposed throughout scientific history. Sir Isaac Newton, who invented the first complex theory of gravity, believed that the velocity of gravity is infinite. He was predicting that Earth’s path through space would change before Earth-bound humans would notice the disappearance of the Sun.

On the other hand, Albert Einstein believed that gravity travels at the speed of light. He predicted that humans would simultaneously notice the disappearance of the sun and the change of the Earth’s path through the universe. He built this assumption into his theory of general relativity, which is currently the best accepted theory of gravity, and it predicts very accurately the paths of the planets around the sun. His theory makes more accurate predictions than Newton’s. So, can we conclude that Einstein was right?

No we can not. If we want to measure the velocity of gravity, we have to think of a way to measure it directly. And of course, since we can’t “hide” the sun for a few moments to test Einstein’s idea, we need to find another way.

Einstein’s gravitational theory made testable predictions. Most importantly, he realized that the familiar gravity we experience can be interpreted as distorting the fabric of space: the greater the distortion, the greater the gravity. This idea has dire consequences. It indicates that the space is elastic, like the surface of a trampoline, which deforms when a child steps on it. Moreover, if the same child jumps on the trampoline, the surface changes: it bounces up and down.

Likewise, space can metaphorically “bounce up and down,” although it is more accurate to say that it compresses and relaxes similar to the way air transmits sound waves. These spatial distortions are called “gravitational waves” and will travel at gravitational speed. So, if we can detect gravitational waves, maybe we can measure the velocity of gravity. But distorting space in ways that scientists can measure is very difficult and goes beyond current technology. Fortunately, nature helped us.

Gravitational wave measurement

In space, planets revolve around stars. But sometimes stars revolve around other stars. Some of these stars were once huge and lived their lives and died, leaving a black hole – the corpse of a massive dead star. If two such stars died, you could have two black holes orbiting each other. As they spin, they emit tiny (and currently undetectable) amounts of gravitational radiation, causing them to lose energy and get closer to each other. Eventually, the two black holes get close enough that they merge. This violent process releases huge amounts of gravitational waves. For the split second that the two black holes come together, the merger releases more energy into gravitational waves than all the light emitted by all the stars in the visible universe during the same time.

While gravitational radiation was predicted in 1916, it took scientists nearly a century to develop the technology to discover it. To detect these deformations, scientists take two tubes, each about 2.5 miles (4 kilometers) long, and orient them at a 90-degree angle, so that they form an “L.” Then they use a combination of mirrors and lasers to measure the length of both legs. Gravitational radiation would change the length of the two tubes differently, and if they saw the correct pattern of length changes, they would have noticed gravitational waves.

The first observation of gravitational waves occurred in 2015, when two black holes merged more than a billion light-years from Earth. While this was a very exciting moment in astronomy, it did not answer the question of gravitational speed. Therefore, a different note was needed.

Although gravitational waves are emitted when two black holes collide, this is not the only possible cause. Gravitational waves are also emitted when two neutron stars collide with each other. Neutron stars are also burning stars – similar to black holes, but slightly lighter. Moreover, when neutron stars collide, they not only emit gravitational radiation, but also emit a powerful burst of light that can be seen across the universe. To determine the speed of gravity, the scientists needed to see the merger of two neutron stars.

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In 2017, astronomers got their chance. They detected a gravitational wave and just over two seconds later, orbiting observatories detected gamma radiation, a form of light, from the same location in space that originated in a galaxy located 130 million light-years away. Finally, astronomers have found what they need to determine the speed of gravity.

The merger of two neutron stars produces light waves and gravitational waves at the same time, so if gravity and light had the same speed, they should be detected on Earth at the same time. Looking at the distance of the galaxy that housed these two neutron stars, we know that these two types of waves have traveled for 130 million years and arrived within two seconds of each other.

So, this is the answer. Gravity and light move at the same speed, and they are determined by precise measurement. It validates Einstein once again, and hints at something profound about the nature of space. Scientists hope one day to fully understand why these two completely different phenomena have identical speeds.

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