This article was first published on Big Think in October 2022. It was updated in December 2023.
Of all the fundamental forces known to mankind, gravity is the most familiar and the one that holds the Universe together, connecting distant galaxies in a vast and interconnected cosmic web. With that in mind, an interesting question to ponder is whether gravity has speed. It has been done, and scientists have measured it accurately.
Let’s start with a thought experiment. Suppose that this time, somehow the Sun was made to disappear – not just darken, but disappear completely. We know that light travels at a fixed 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 be before we here on Earth know that the Sun is gone. . It takes about eight minutes and 20 seconds before it gets dark at noon.
But what about gravity? When the sun disappears, it not only stops emitting light, but also stops exerting the gravity that holds the planets in orbit. When will we know?
If gravity is so strong, gravity will also disappear when the Sun ceases to exist. We will still see the Sun for more than eight minutes, but the Earth will begin to wander off, into interstellar space. On the other hand, if gravity travels at the speed of light, our planet will continue to orbit the Sun as usual for eight minutes and 20 seconds, after which it will stop following its usual path.
Of course, if gravity traveled at a different speed, the interval between when the Sun worshipers on the beach noticed that the Sun was gone and when the astronomers observed that the Earth was moving in the wrong direction would be different. So, what is the speed of gravity?
Various answers have been proposed throughout scientific history. Sir Isaac Newton, who invented the first sophisticated theory of gravity, believed that the speed of gravity was infinite. He would have predicted that the Earth’s path through space would change before people orbiting the Earth would notice that the Sun was gone.
On the other hand, Albert Einstein believed that gravity traveled at the speed of light. He would have predicted that people would simultaneously notice the disappearance of the Sun and the change in the path of the Earth through the universe. He built this hypothesis into his theory of general relativity, which is currently the best accepted theory of gravity, and it very accurately predicts the path of the planets around the Sun. 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 think of a way to measure it directly. And, of course, since we can’t just “disappear” the Sun for a few moments to test Einstein’s idea, we have to find another way.
Einstein’s theory of gravity made testable predictions. The most important thing is that he realized that the familiar gravity that we experience can be explained as a distortion of the fabric of space: the greater the distortion, the higher the gravity. And this idea has important consequences. This suggests that the space is soft, similar to the surface of a trampoline, which distorts when a child steps on it. In addition, when the same child jumps on the trampoline, the surface changes: it jumps up and down.
Similarly, space can figuratively “bounce up and down,” although it is more accurate to say that it compresses and relaxes like how air transmits sound waves. These spatial distortions are called “gravitational waves” and they travel at the speed of gravity. Therefore, if we detect gravitational waves, we can measure the speed of gravity. But warping space in ways scientists can measure is extremely difficult and beyond current technology. Fortunately, nature helped us.
Measurement of gravitational waves
In space, the planets orbit the stars. But sometimes stars revolve around other stars. Some of the stars were once massive and lived out their lives and died, leaving behind a black hole – the body of a dead, massive star. If two such stars die, then you can have two black holes orbiting each other. As they orbit, they emit a small (and currently undetectable) amount of gravitational radiation, which causes them to lose energy and move closer to each other. Eventually, the two black holes get close enough that they merge. This violent process releases massive gravitational waves. In the fraction of a second that two black holes merge, the merger releases more energy in gravitational waves than all the light emitted by all the stars in the visible Universe at the same time.
While 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 take two pipes, each about 2.5 miles (4 kilometers) long, and orient them at 90 degrees, so that they form an “L .” Then they use a combination of mirrors and lasers to measure the length of both legs. Gravitational radiation changes the height of the two tubes differently, and when they see the correct pattern of height changes, they observe gravitational waves.
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The first observation of gravitational waves occurred in 2015, when two black holes located more than 1 billion light years away from Earth merged. While this is a very exciting moment in astronomy, it does not answer the question of the speed of gravity. For that, another observation is necessary.
Although gravitational waves are emitted when two black holes collide, that is not the only possible cause. Gravitational waves are also emitted when two neutron stars collide. Neutron stars are also burnt out stars – similar to black holes, but lighter. Furthermore, when neutron stars collide, they not only emit gravitational radiation, they also emit a powerful burst of light that can be seen throughout the Universe. To determine the speed of gravity, scientists need to see the merger of two neutron stars.
In 2017, astronomers got their chance. They detected a gravitational wave and a little more than two seconds later, the orbital observatories detected gamma radiation, which is a form of light, from the same location in space coming from a galaxy located 130 million which is light years away. Finally, astronomers found what they needed to determine the speed of gravity.
The merger of two neutron stars emits light and gravitational waves at the same time, so if gravity and light have the same speed, they should be detected on Earth at the same time. Because of the distance to the galaxy these two neutron stars reside, we know that the two types of waves traveled about 130 million years and arrived within two seconds of each other.
So, that’s the answer. Gravity and light travel at the same speed, determined by a precise measurement. It proved Einstein once again, and it showed something profound about the nature of space. Scientists hope to one day fully understand why these two very different phenomena have the same speed.
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