Gravity Travels at the Speed of Light
Changes in gravity travel at the speed of light, and observations of a neutron star merger showed gravitational waves and light arriving together after 130 million years.
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Gravity does not act through space instantaneously. In general relativity, changes in a gravitational field propagate at the speed of light in vacuum, about 300 million meters per second. This is often described through gravitational waves, ripples in spacetime produced by accelerating massive objects. A useful thought experiment is to imagine the Sun suddenly disappearing. Earth would not immediately lose the Sun’s gravitational influence or see the sky go dark. Both changes would reach Earth after roughly eight and a half minutes, because that is how long light and gravitational effects take to cross the distance between the Sun and Earth. The prediction has also been tested directly. In 2017, astronomers observed gravitational waves from a neutron star merger along with a burst of gamma rays from the same event. The signals had traveled roughly 130 million years before reaching Earth only about two seconds apart, placing extremely tight limits on any difference between the speed of gravity and the speed of light. This result strongly supports the prediction of general relativity that gravitational waves propagate at light speed. It is also more precise to talk about the propagation of gravitational changes or gravitational waves than to imagine gravity as a conventional force carrying a message through space.
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