In our everyday world, we observe all sorts of waves, including sound waves, water waves, and radio waves. But what about gravitational waves?

In the antenna of a radio station, electrons surge back and forth and produce electromagnetic waves. Suppose a very massive body like a star moved back and forth. Would it produce gravitational waves?

Artist’s drawing of gravitational waves emitted by a rotating system of two massive, tightly bound stars (image courtesy of Caltech/LIGO)

It may seem far-fetched to think of a star moving like this, but in fact many of the stars we see are members of binary star systems—two stars that circle around each other in a dance of mutual gravitational attraction. If the stars are massive and close together, the stars swing around each other pulled by enormous forces. Einstein, in 1915, predicted that such a system would emit gravitational waves, which would move at the speed of light, but he suspected that the radiation would be too feeble to be detected. The drawing shows a representation of these waves.

If two such stars did give off gravitational radiation, their mechanical energy would decrease, they would draw closer together, and their rate of mutual rotation would increase. So if physicists could measure the period of a binary system very precisely, they could look for evidence of gravitational radiation.

The physicists who found the evidence, Russell Hulse and Joseph Taylor, were studying pulsars—neutron stars that spin rapidly and emits sharp pulses of radio energy with extraordinary regularity. They found a pulsar whose rate of pulsing oscillated, first speeding up and then slowing down. Careful analysis showed that this variation was a Doppler shift, and that the pulsar was alternately moving towards and away from Earth as it and an unseen companion star orbited each other. The two objects rotate around each other about every eight hours, at a separation of from one to five solar radii. Since they are so close together, the gravitational forces between them are extraordinarily strong.

Painstaking, long-term measurements of the pulse arrival times revealed that the rate of rotation of the system was increasing, and in just the way that Einstein predicted 63 years before. In 1993, Hulse and Taylor were awarded the Nobel Prize in physics for this work. They had identified a source of gravitational waves. The next step would be to build a detector to observe these waves directly.