© David A. Hardy/Science Photo Library
An artist’s depiction of the Arecibo radio telescope.
An artist’s depiction of the Arecibo radio telescope.
One of nature’s most spectacular experiments involves a pair of stars, each about the size of a large mountain on Earth but with more mass than the Sun, locked in a gravitational embrace, orbiting around each other at speeds of hundreds of kilometres per second. Observations of this system, dubbed the binary pulsar, show that it is producing ripples in the fabric of space itself – gravitational waves.
This kind of gravitational radiation was predicted as long ago as 1916, by Albert Einstein. But the discovery confirming the accuracy of his prediction was not made until 1974. That year, Russell Hulse, a young astronomer working with the Arecibo radio telescope in Puerto Rico, noticed something odd about the behaviour of a radio star known as a pulsar. Pulsars are fast-spinning neutron stars, tiny but very dense objects (as dense as the nucleus of an atom), which emit beams of radio noise that sweep around like the beam of a lighthouse. A neutron star has a radius of about 10 kilometres, and the force of gravity at the surface of such a star is a hundred-thousand-million times more than on Earth. If the Earth happens to be in the path of the beam from a pulsar, radio telescopes pick up a regular pulse of radio waves, like the ticking of a clock. This particular pulsar, known as PSR 1913+16, spins on its axis once every 0.059 seconds, making it one of the fastest pulsars known.
Most pulsars are superbly accurate clocks, beating time with a precision measured to many decimal places. But during a series of observations of this pulsar in the summer of 1974 Hulse found that it has a period which changes by as much as 30 microseconds from one day to the next – a huge ‘error’ for a pulsar. This variation follows a rhythm of its own, changing the measured period over a regular cycle. He realized that this could be a result of the changing Doppler effect (see here) caused by the pulsar moving in a tight orbit around a similar star that was not emitting any detectable radio noise.
Hulse’s colleague Joseph Taylor (they both worked at the University of Massachusetts) joined Hulse in Arecibo to carry out a more detailed investigation. Together, they found that the pulsar zips round its companion once every 7 hours and 45 minutes, reaching a maximum speed of 300 kilometres per second and with an average speed of about 200 kilometres per second. The size of the orbit is about 6 million kilometres, about the same as the circumference of the Sun. So the whole binary system would fit inside the Sun. The orbital properties also told them that the combined mass of both stars added up to 2.8275 times the mass of the Sun.
Astronomers immediately realized that such an extreme system would provide a test bed for Einstein’s ideas about gravitational radiation, which were based on his general theory of relativity. According to the general theory, under these extreme conditions the orbiting stars should be producing ripples in space, like the ripples you might imagine being produced in a tank of water by a rotating dumbbell. This gravitational radiation would carry energy away from the system, altering the orbit. It would cause the orbital period (which in round numbers is 27,000 seconds) to increase by 75 millionths of a second per year – about 0.0000003 per cent per year. In 1978, after four years of observations, this change had been measured accurately enough to confirm that Einstein was right, that gravitational radiation really exists. By 1983 the change had been measured to an accuracy of 2 millionths of a second per year, giving a value of 76 ± 2 millionths of a second per year. Since then, the accuracy of the general theory has been confirmed to better than 1 per cent.
The extended observations also made it possible to work out the ratio of the masses of the two stars, partly from the time-dilation effect of the pulsar’s high speed on its timekeeping. Along the way, this confirmed the accuracy of the special theory of relativity. As they already knew the total mass, this ratio enabled Hulse and Taylor to work out that the mass of PSR 1913+16 itself is 1.42 times the mass of the Sun, while its companion has a mass of 1.40 times that of the Sun. These were the first accurate measurements of the masses of neutron stars.
Since the discovery of the binary nature of PSR 1913+16, other similar systems have been discovered, always confirming the accuracy of the general theory. But to astronomers, PSR 1913+16 is still known as ‘the’ binary pulsar. In 1993, Hulse and Taylor shared the 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.’