The discovery of the binary pulsar

During 1974 Joseph Taylor and Russell Hulse were searching for new pulsars with the Arecibo telescope. They discovered 40, one of which was to be very important.
    When Hulse was observing the new pulsar, which has been named PSR1913+16, he found that the pulses arrived sometimes more often and sometimes less. The simplest interpretation was that the pulsar was orbiting another star very closely and at high velocity: Here one "pulsar year" is only about eight hours.
    By observing the shift in the pulses, Hulse and Taylor found that the stars were equally heavy, each weighing about 1.4 times as much as the Sun. Since they were not visible on any photographs either, it was concluded that the other body, somewhat unexpectedly, was also a neutron star. Seen from Earth, however, it does not show up as a pulsar.

Pulsars and neutron stars

In 1967 radio astronomers Jocelyn Bell and Antony Hewish* discovered an object in the heavens that was emitting rapid radio pulses – a pulsar. It was soon realized that the pulsars were neutron stars, which had long been the subject of speculation.

Neutron stars: When a heavy star has used up all its nuclear fuel it is destroyed in what is called a supernova explosion. A small, compact star may be left. Its gravity is then so strong that the electrons have been forced into the protons in the atomic nuclei and formed neutrons. The star has enormous density; it is only about 20 km in diameter, yet it weighs at least as much as our Sun.

Pulsars: A pulsar is a rapidly rotating neutron star with a strong magnetic field. The fastest pulsars spin several hundreds of revolutions per second. From their magnetic poles radio waves are emitted in two cone-shaped beams. As the star rotates, these sweep round in space like a lighthouse beam. Each time one of them points towards the Earth we can record a radio pulse.

*Nobel Prize 1974

Einstein's theory of relativity

In 1916 Albert Einstein presented his General Theory of Relativity, which was to succeed Isaac Newton's theory of gravity.

Curved space: Time and space are united in Einstein's theory to form "space-time". Gravity no longer appears as a force. Instead, heavy bodies will curve space around themselves. The Earth orbits the Sun simply because it follows the easiest path - a straight line in space-time which the Sun has curved around itself.

Gravitational radiation: If an electrical charge is accelerating, it will emit radiation, e.g. as radio waves from an antenna. Correspondingly, according to General Relativity, accelerating masses should emit gravitational radiation, a propagating deformation of space-time. An object exposed to strong gravitational radiation would become alternately longer and thinner, shorter and broader. The variations are, however, so small that it has not yet been possible to measure them.

Measuring gravitational radiation

Since the two neutron stars in PSR1913+16 are moving so fast and close together they should, according to General Relativity, emit large amounts of gravitational radiation. This makes them lose energy: Their orbits will therefore shrink and their orbiting period will shorten.

Indirect evidence: The binary pulsar has been observed continuously since its discovery, and the orbiting period has in fact decreased. Agreement with the prediction of General Relativity is better than 1/2%. This is considered to prove that gravitational radiation really exists. This in turn is currently one of our strongest supports for the validity of the General Theory of Relativity.

The future: Hulse's and Taylor's discovery has provided us with an excellent laboratory for testing different theories of gravity. So far General Relativity has brilliantly passed all tests. Instruments are now being planned that should be able to detect and measure gravitational radiation directly on Earth. Hopefully these new "gravitational telescopes" will offer us an entirely new way of studying the Universe, even perhaps its most dramatic events.

Einstein smiles in his heaven

Albert Einstein* (1879-1955) presented his General Theory of Relativity in 1916. It was to revolutionize our view of gravity. The indirect evidence for gravitational radiation afforded by binary pulsar PSR 1913+16 is one of many supports for the validity of the theory.

*Nobel Prize 1921

A pulsar is a rapidly rotating neutron star with a strong magnetic field. In this field electrically charged particles move along the field lines. At the magnetic poles, particles can escape and give rise to radio emission. Since the magnetic axis (red arrow) is tilted with respect to the axis of rotation (blue), the two beams of radio emission will sweep space like a lighthouse beam.

Schematic picture of binary pulsar PSR1913+16. Two neutron stars circle each other closely in elliptical orbits. Their mean separation is only a few times greater than that of the Earth and the Moon. One orbit takes only about eight hours. When the stars pass close to each other they will predictably emit large amounts of gravitational radiation. Studies of the changes in orbital motion have shown that this also appears to be the case.