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Why Does Gravity Travel at the Speed of Light?

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130 million years ago, in a galaxy not too far away, the dead cores of two stars hit each other.

The impact was so strong that it made a ripple in space-time, called a gravitational wave. The light from the star’s explosion and that gravitational wave both went all the way across the universe. They both got to Earth at 6:41 a.m. Eastern time on August 17.

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The event was called the start of “multimessenger astronomy” in news stories around the world. Astronomers had been waiting for this moment for a hundred years. But it was also the first time that gravity was shown to move at the speed of light.

How Fast Gravity Moves

Light has a set speed, which is about 186,000 miles per second. Nothing moves more quickly. But why should the speed of gravity stay the same?

To answer that question, we need to take a quick look at Albert Einstein’s theory of gravity, or general relativity. This is the same theory that predicted gravitational waves 100 years ago.

Isaac Newton’s idea of “absolute time” was thrown out by Albert Einstein. Newton thought that time moved forward at the same rate everywhere, no matter how we humans saw it. It didn’t back down. If he thinks that way, one second on Earth is the same as one second near a black hole, which he didn’t even know existed.

Newton also thought that gravity worked right away. Distance didn’t matter.

It Doesn’t Matter

But Einstein proved that time is not a fixed thing. It changes as it moves and as gravity pulls on it. One effect of this is that you can’t do things at the same time from far away. So, the speed of any kind of information is limited, whether it’s a photon, which carries light, or a graviton, which carries gravity.

“In relativity, there is something called the’speed of information,'” says Jolien Creighton, a physicist at the University of Wisconsin-Milwaukee and an expert on general relativity who was part of the LIGO team that first found gravitational waves.

Creighton says that in electromagnetism, shaking an electron makes a change in the electric field that spreads at the speed of light. The same thing happens with gravity. When you move a large mass, the change in the gravitational field, or gravitational wave, moves at the same speed.

“So the reason why the speed of gravitational waves and electromagnetic waves are the same is that they both travel at the speed of information,” says Creighton.

There’s also a simple way to imagine this. Picture that the sun went away right now. Earth wouldn’t just start floating off into space all at once. After eight minutes, the sun would go out and Earth would move straight away.

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