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How do we know that gravity has the same speed as light? And why is this the case?

Ron Dippold
San Diego, California, US

In Newtonian physics, gravity was thought to be instantaneous. But in Albert Einstein鈥檚 general theory of relativity, the speed of light, c, is the fastest possible speed for any interaction that carries information, not just light. Gravitational waves carry a lot of information, which we measure with detectors like LIGO and Virgo.

Unfortunately, gravity is by far the weakest force, so the speed of gravity hasn鈥檛 yet been precisely measured in a laboratory. Instead, we have to turn to nature鈥檚 laboratory. In 1859, astronomer Urbain Le Verrier showed that Mercury鈥檚 orbit precesses (changes) in a way that couldn鈥檛 be fully explained by Newtonian gravity. This orbital puzzle was only solved after Einstein published his general theory of relativity in 1915.

Gravitational wave detector LIGO spotted two neutron stars colliding 2 seconds before telescopes saw the gamma rays

For a more precise measurement of the speed of gravity, we can turn to binary pulsars. A pulsar is a rotating neutron star that shoots a beam of electromagnetic radiation from each of its poles. Every time a pulsar鈥檚 rotation sweeps a beam across Earth, we see a spike of radiation. The rotation of pulsars is extremely regular, making them excellent clocks.

When one of these is rotating around another neutron star or white dwarf, their gravity fields drag on each other, slowing them both down. The Hulse-Taylor binary pulsar was the first discovered and has been studied extensively. The rate of slowing is directly related to the speed of gravity, and for this system (and other binary pulsars) it is within 1 per cent of the speed of light.

Since LIGO came online, we can also compare when telescopes see major astronomical events with when we 鈥渟ee鈥� them with LIGO. In 2017, LIGO detected two neutron stars colliding 2 seconds before telescopes saw the gamma rays, in the GW170817 event. This happened 130 million light years away, so if gravity waves travelled at anything other than the speed of light, there would have been a huge time difference.

Again, the 鈥渨hy鈥� is because, in general relativity, the 鈥渟peed of light鈥� is really the conversion factor between the three dimensions of space and the dimension of time in space-time. Nothing real can travel faster than it. Of course, in quantum theory, entangled particles appear to violate this, which is one reason why a theory of quantum gravity has been elusive so far.

But at this point, we are very sure the speed of gravity is near the speed of light and is likely to be the same.

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