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What is the speed of gravity?
Category: Physics
Published: April 30, 2013
Updated: November 28, 2023
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Gravity travels at the speed of light in vacuum. Let's look at a hypotheical situation to see what this means. The earth is held in its orbit about the sun by the sun's gravity. What if the sun suddenly disappeared, along with its light and gravity? As should be obvious, the earth would go dark and would slingshot out of the solar system. But when would the earth stop feeling the sun's gravity and tumble out of its orbit? Would it happen at the exact same instant that the sun disappeared? Would it happen some time after the sun disappeared but before the sun is seen to go dark? Or would the earth leave orbit after the sun was seen to go dark?
The answer to these questions depends on the speed of gravity, which is more accurately called "the speed of gravitational change propagation" or "the speed of gravitational waves". Einstein's theory of general relativity predicts that the speed of gravitational waves exactly equals the speed of light in vacuum. This is not a coincidence. All massless particles travel at the speed of light in vacuum. This includes gravitational waves which propagate changes in the gravitational field, light waves which propagate changes in the electromagnetic field, and gluons which propagate changes in the strong nuclear force field. In fact, the phrase "speed of light" can be misleading as it seems to make light look special. A better name would be "the universal speed limit", or "the speed of massless particles".
Returning back to our hypothetical situation, we can work out what would happen to the earth. The moment the sun disappeared, it would stop generating light and gravity. Both of these changes would propagate out to the earth at the speed of light in vacuum, which is about 300 million meters per second (671 million mph). The earth sits about 150 billion meters (93 million miles) distant from the sun, so it takes about eight and a half minutes for light and gravitational effects from the sun to reach the earth. This means that earth's sky would go dark and earth would leave its orbit at the same time, eight and a half minutes after the sun had disappeared. (Note that the sun can't really suddenly disappear.)
The theoretical prediction that gravitational waves travel at the speed of light in vacuum has been confirmed experimentally. LIGO has been directly detecting gravitational waves since 2015 and various groups have been directly detecting bursts of gamma ray light since 1967. A particular neutron star merger event in the NGC 4993 galaxy created both a burst of gamma ray light and chirped gravitational waves at almost exactly the same time. These light waves and gravitational waves then traveled through space for 130 million years. They then reached earth's LIGO and gamma ray detectors at almost exactly the same time. These measurements showed that the speed of light and the speed of gravity are the same, out to the fifteenth digit.