Among the fundamental forces of nature, gravity is the one that holds the universe together, connecting distant galaxies in a vast cosmic web. Some people wonder if, like light, gravity has speed which can be precisely measured.
Does Gravity Have Speed?
Suppose at this very instant, the Sun suddenly disappeared. It would take about eight minutes before our sky gets dark and we realize that the Sun has vanished. This was calculated using the speed of light and the known distance between the Earth and the Sun.
Right after disappearing, the Sun would stop emitting light and also the gravity that holds the planets in their orbit. But how soon are we going to find out that it has lost its gravity? If gravity is infinitely fast, it would also disappear as soon as the Sun vanished. We will still see the Sun for almost eight minutes, but our planet will start wandering off due to loss of gravity.
If gravity travels at the speed of light, the Earth would continue to orbit around the as usual for eight minutes, after which it would stop following its familiar path. If it travels at some other speed, the interval between the time a regular skywatcher noticed the disappearance of the Sun and when astronomers observed that the Earth is heading in the wrong direction would be different.
Throughout the history of science, different answers have been proposed in this thought experiment. Isaac Newton, the one who developed the first sophisticated theory of gravity, believed that the speed of gravity was infinite. He predicted that the path of the Earth through space would change before humans even notice that the Sun was gone.
On the other hand, Albert Einstein believed that gravity traveled at the speed of light. According to him, humans would simultaneously notice the disappearance of the Sun and the change of the Earth's path through space. He also built this assumption into his famous theory of general relativity.
What Is the Speed of Gravity?
In 2015, the first observation of gravitational waves occurred when two merging black holes were located more than 1 billion light years away from Earth. It did not answer the question of the speed of gravity, so a different observation was needed.
Determining the speed of gravity would require seeing the merging of two neutron stars, and in 2017, astronomers finally got their chance. The merging of two neutron stars emits both light and gravitational waves at the same time. If gravity and light have the same speed, they should be detected on Earth at the same time. Indeed, the astronomers got this observation as predicted. They detected a gravitational wave; and after two seconds, they also detected gamma radiation which is a form of light. The gravitational wave and the gamma radiation were detected from the same location in space that originated in a galaxy 130 million light years away.
It is now accepted that gravity and light travel at the same speed which can be determined by a precise measurement. It also validates Einstein's theory and provides clues about the nature of our universe.
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