Gravitational Waves

The Prediction

Within months of completing general relativity, Einstein found that his equations allowed disturbances in the geometry of spacetime to travel outward at the speed of light, much as an accelerating electric charge sends out light. He published the idea in June 1916 and returned to it in a second paper in 1918.

A passing gravitational wave stretches space in one direction while squeezing it in the perpendicular direction, then reverses. Its strength is the strain $h$: the fractional change in a length. For any source we could hope to observe, the strain is fantastically small, and Einstein doubted the effect would ever be measured.

The Doubt

In 1936, by then in Princeton, Einstein and his assistant Nathan Rosen wrote a paper claiming to prove that gravitational waves do not exist. They submitted it to the Physical Review. The journal sent it to an anonymous referee, who found a mistake.

Einstein was not used to peer review. He withdrew the paper in irritation and never submitted another paper to that journal. But the referee, later identified as the cosmologist Howard Percy Robertson, was right. Before the paper appeared in the Journal of the Franklin Institute in 1937, Robertson had quietly shown the error to Einstein's assistant Leopold Infeld, who passed it on. Einstein, by Infeld's account, had just found a mistake himself. He rewrote the conclusion. The published version describes waves that do exist.

The physics community only settled the question in 1957, less than two years after his death, at a conference in Chapel Hill where it was shown that a gravitational wave could in principle heat up a physical object, and therefore carries real energy.

The Detection

On 14 September 2015 the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), in Washington State and Louisiana, recorded the same signal seven milliseconds apart. Two black holes, about 36 and 29 times the mass of the Sun, had spiraled together and merged roughly 1.3 billion light-years away. In the final fraction of a second, about three Suns' worth of mass was converted into gravitational waves.

The detectors are L-shaped, with arms four kilometers long. They are designed to sense a change in arm length of about one ten-thousandth of the width of a proton.

Rainer Weiss, Barry Barish, and Kip Thorne received the 2017 Nobel Prize in Physics for the detection.

Where It Stands

Gravitational waves are now an observing tool. In August 2017 detectors caught two neutron stars colliding, and telescopes around the world saw the flash that followed, the first event observed in both gravity and light. See Gravitational-Wave Astronomy.