Hulse-Taylor Binary Pulsar (1975): The First Gravitational Wave Clue

The Hulse-Taylor binary pulsar, PSR 1913+16, was discovered by Russell Hulse and Joseph Taylor in 1974 with the Arecibo radio telescope. A pulsar locked in a tight orbit with another neutron star, it later showed its orbit shrinking exactly as general relativity predicts for energy lost to gravitational waves: the first evidence that they exist.

The question before the paper

Pulsars, rapidly spinning neutron stars that sweep beams of radio waves across the sky, had been discovered only a few years earlier, by Jocelyn Bell and Antony Hewish. Their pulses arrive with clock-like regularity, which makes them extraordinary timekeepers. Hulse and Taylor were running a systematic search for new pulsars. One of them refused to keep time.

What the paper found

• A fast pulsar. PSR 1913+16 spins with a period of 59 milliseconds.
• A period that drifts. Its pulse period shifted from day to day, revealing that the pulsar is moving in an orbit, alternately approaching and receding.
• A very tight orbit. The orbital period is only about 7.75 hours.
• A dark, compact partner. With no eclipses, the companion must also be compact; it is now known to be another neutron star.

How they did it

The pulsar turned up in July 1974 during a systematic search with the 305-meter Arecibo radio telescope in Puerto Rico. Noticing that the pulse period changed in a regular, repeating way, Hulse and Taylor recognized the Doppler effect of orbital motion: pulses arrive slightly faster when the pulsar moves toward us and slightly slower when it moves away. Tracking the changes mapped out the orbit.

Why it mattered

A pair of neutron stars in a tight orbit is a near-perfect laboratory for gravity. General relativity predicts that such a system radiates gravitational waves, losing energy so that the two stars slowly spiral together. Follow-up timing of PSR 1913+16 over the following years showed the orbit shrinking at exactly the predicted rate, agreeing to better than 0.5%.

That was the first, indirect evidence that gravitational waves exist, four decades before LIGO heard them directly. Russell Hulse and Joseph Taylor received the 1993 Nobel Prize in Physics “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation”.

What we know now

Gravitational waves were detected directly in 2015, from two merging black holes. The binary pulsar also showed that neutron star pairs eventually merge, and in 2017 the LIGO-Virgo-KAGRA network caught exactly such a neutron star merger, GW170817. Pulsars now detect gravitational waves in another way too: the NANOGrav 15-year data set used 67 of them as a galaxy-sized detector.

Who wrote it

Russell Hulse, then a graduate student at the University of Massachusetts Amherst, found the pulsar; Joseph Taylor was his supervisor. Their short letter announcing the discovery appeared in 1975.

Read the original

Hulse, R. A., and Taylor, J. H. (1975). “Discovery of a pulsar in a binary system.” The Astrophysical Journal Letters 195, L51. doi:10.1086/181708. Free scanned PDF at the NASA Astrophysics Data System.

Questions people ask

What is the Hulse-Taylor binary pulsar?

The Hulse-Taylor binary pulsar, PSR 1913+16, is a pulsar orbiting another neutron star every 7.75 hours. Discovered in 1974, it became the first system to show the effects of gravitational waves: its orbit shrinks as the pair loses energy, exactly as general relativity predicts.

How did the binary pulsar prove gravitational waves exist?

Years of precise timing showed the pulsar’s orbit shrinking at the rate general relativity predicts for energy carried away by gravitational waves, agreeing to better than 0.5%. That was indirect evidence: the waves themselves were not detected until LIGO’s first observation in 2015.