How fast is gravity, exactly? [View all]

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Of all of the fundamental forces known to humanity, gravity is both 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, a fascinating question to ponder is whether gravity has a speed. It turns out that it does, and scientists have precisely measured it.

Let’s start with a thought experiment. Suppose at this very instant, somehow the Sun was made to disappear — not just go dark, but vanish 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 would know the Sun had disappeared. It would take about eight minutes and 20 seconds before the noon sky would go dark. But what about gravity? If the sun disappeared, it would not only stop emitting light, but also stop exerting the gravity that holds the planets in orbit. When would we find out?

If gravity is infinitely fast, gravity would also disappear as soon as the Sun poofed into nonexistence. We’d still see the Sun for a little over eight minutes, but the Earth would already start wandering off, heading for 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 20 seconds, after which it would stop following its familiar path. Of course, if gravity traveled at some other speed, the interval between when beachgoing Sun worshipers noticed the Sun was gone and when astronomers observed that the Earth was going 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 the speed of gravity was infinite. He would have predicted that the Earth’s path through space would change before Earth-bound humans noticed 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 humans would simultaneously notice the disappearance of the Sun and the change of Earth’s path through the cosmos. He built this assumption into his theory of general relativity, which is currently the best accepted theory of gravity, and it very precisely predicts the path 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 need to think of a way to directly measure it. And, of course, since we can’t just “disappear” the Sun for a few moments to test Einstein’s idea, we need to find another way.

Read more: https://bigthink.com/hard-science/speed-of-gravity/