Of all the fundamental forces known to mankind, gravity is both the most familiar and the one that holds the Universe together, linking distant galaxies in a vast, interconnected cosmic web. With that in mind, a fascinating question to ask is whether gravity has velocity. It turns out that it is, and scientists have measured it precisely.
Let’s start with a thought experiment. Suppose that at this precise moment, somehow, the Sun was caused to disappear – not only to darken, but to disappear entirely. 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 would take before we here on Earth knew the Sun was gone. It would take about eight minutes and 20 seconds before the midday sky darkened.
But what about gravity? If the sun disappeared, not only would it stop emitting light, but it would also stop exerting the gravity that keeps the planets in orbit. When will we know?
If gravity is infinitely fast, gravity would also vanish as soon as the Sun falls into non-existence. We would still see the Sun for just over eight minutes, but the Earth would already begin to move away, heading into interstellar space. On the other hand, if gravity were moving at the speed of light, our planet would continue to orbit the Sun as usual for eight minutes and 20 seconds, after which it would stop following its familiar trajectory.
Of course, if gravity were traveling at any other speed, the interval between when Sun worshipers on the beach noticed the Sun was gone and when astronomers observed the Earth was heading in the wrong direction would be different. So what is the speed of gravity?
Different 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 is said to have predicted that the Earth’s trajectory through space would change before Earthbound humans noticed the Sun was gone.
On the other hand, Albert Einstein believed that gravity moves at the speed of light. He is said to have predicted that humans would simultaneously notice the disappearance of the Sun and the change in the trajectory of the Earth through the cosmos. He incorporated this hypothesis into his theory of general relativity, which is currently the most accepted theory of gravity, and it very accurately predicts the trajectory 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’t. If we want to measure the speed of gravity, we have 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. Most importantly, he realized that the familiar gravity we feel 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 malleable, similar to the surface of a trampoline, which deforms when a child steps on it. Moreover, if this same child jumps on the trampoline, the surface changes: it bounces up and down.
Similarly, space can metaphorically “bounce up and down”, although it is more accurate to say that it compresses and expands in the same way that air transmits sound waves. These spatial distortions are called “gravitational waves” and they will travel at the speed of gravity. So if we can detect gravitational waves, maybe we can measure the speed of gravity. But warping space in a way that scientists can measure is quite difficult and well beyond current technology. Fortunately, nature helped us.
Measure gravitational waves
In space, planets revolve around stars. But sometimes stars orbit other stars. Some of these stars were once massive and have lived out their lives and died, leaving a black hole – the corpse of a dead, massive star. If two of those stars are dead, then you may have two black holes orbiting each other. As they orbit, 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 to merge. This violent process releases huge amounts of gravitational waves. During the split second the two black holes come together, the merger releases more energy in gravitational waves than all the light emitted by all stars in the visible Universe during the same time.
While gravitational radiation was predicted in 1916, it took nearly a century for scientists to develop the technology to detect it. To detect these distortions, scientists take two tubes, each about 2.5 miles (4 kilometers) long, and angle them at 90 degrees, so they form an “L”. They then use a combination of mirrors and lasers to measure the length of both legs. The gravitational radiation will change the length of the two tubes differently, and if they see the correct pattern of length changes, they have observed gravitational waves.
The first observation of gravitational waves took place in 2015, when two black holes located more than a billion light-years from Earth merged. Although this was a very exciting time in astronomy, it did not answer the question of the speed of gravity. For this, another observation is in order.
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. Neutron stars are also burnt stars – similar to black holes, but slightly lighter. Moreover, when neutron stars collide, they not only emit gravitational radiation, but they also emit a powerful burst of visible light throughout the Universe. To determine the speed of gravity, scientists had to observe the merger of two neutron stars.
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In 2017, astronomers had their chance. They detected a gravitational wave and just over two seconds later orbiting observatories detected gamma radiation, which is a form of light, coming from the same place in space and coming from a galaxy 130 million of light years. Finally, astronomers have found what they need to determine the speed of gravity.
Merging two neutron stars emits both light and gravitational waves, so if gravity and light have the same speed, they should be detected on Earth at the same time. Given the distance from the galaxy that housed these two neutron stars, the two types of waves are known to have 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 precise measurement. This once again validates Einstein and hints at something profound about the nature of space. Scientists hope to one day fully understand why these two very different phenomena have identical speeds.