The First Binary Pulsar and Einstein's General Theory of Relativity

The First Binary Pulsar and Einstein's General Theory of Relativity

Joel Weisberg, Carleton College

The first binary pulsar, i.e., a pulsar orbiting another star, was discovered at Arecibo by Joseph Taylor and Russell Hulse in 1974. The discovery earned Hulse and Taylor the Nobel Prize in Physics because the object is so exotic and so well suited to testing modern theories of gravitation such as Einstein's General Theory of Relativity. The pulsar is so weak that it is very difficult to make meaningful observations of it from any radiotelescope in the world except the Arecibo dish. The companion star is almost surely another neutron star. The two stars orbit each other every eight hours at speeds up to 300 kilometers per second, approaching each other to within a distance equal to the Sun's radius! See this animation of the life of a double pulsar system created by John Rowe Animations to help visualize the processes leading to the rapidly spinning and orbiting pair of stars. (While this video shows two pulsars, we actually only see one beam from the first binary pulsar system.)

The two stars come so close to each other that Newton's theory of gravitation is not adequate to describe their motion. For example, the stars' elongated orbits do not remain fixed, but rather rotate themselves by over four degrees per year, in a process called "advance of periastron." The rotation is some 35,000 times larger than a similar effect in the orbit of the planet Mercury because the two stars' gravity is so strong! The animation below shows a top view of the orbit exhibiting this phenomenon.

The most exciting measurement in this system is the observation that the two stars' orbits are shrinking at a rate of 1 cm/day. This shrinkage is caused by the loss of orbital energy due to gravitational radiation, which is a travelling ripple in spacetime that is predicted by Einstein's General Relativity Theory but never previously verified (see this animation showing gravity waves from a binary star system as moving undulations in the spacetime grid). Arecibo observations show that the pulsar orbit is shrinking at exactly the rate that general relativity predicts it should, if gravity waves exist and are carrying away the expected amount of energy.

Figure 1: The evidence that Binary Pulsar B1913+16 emits gravitational radiation. As gravitational radiation carries energy away from the binary system, the orbit loses energy, the stars spiral in toward each other, and the pulsar runs "early" in its orbit. The dots are measurements of how early the pulsar is in its orbit, while the curve represents the expected behavior if gravitational waves are carrying energy away from the system at the rate predicated by Einstein's Theory of General Relativity. The excellent agreement between observation and theory represents the strongest current evidence for the existence of gravitational radiation. (Graph from J.M. Weisberg, D.J. Nice, and J.H. Taylor.)

These observations are the first to show that gravity waves exist. As a result, astrophysicists currently searching directly for gravity waves with immense detectors such as LIGO are secure in the knowledge that their quarry exists. (Fellow department member Nelson Christensen and his students are making major contributions to LIGO.)

Another interesting process first seen in this pulsar is a slow wobble of its spin axis. The wobble, called "geodetic spin precession," is caused by the curvature of spacetime induced by the companion. The wobble enables us to observe different parts of the pulsar's "lighthouse beam" than would ordinarily be seen. These measurements will produce a two-dimensional map of the beam until it precesses away from Earth, probably in a few decades.

These essential tests of general relativity are especially suited to Arecibo Observatory, with its great sensitivity and advanced instrumentation. Exciting measurements of this and other binary pulsars continue to be made with the telescope today.

Sources of further information:

General level, early article on this pulsar and its use to show that gravitational waves exist:

"Gravitational Waves from an Orbiting Pulsar," J.M. Weisberg, J.H. Taylor, & L.A. Fowler, Scientific American, 245, 74 (1981).

Scientific Review Articles on Binary and (related) Millisecond Pulsars:

"Testing General Relativity with Pulsar Timing," I.H. Stairs, Living Reviews in Relativity , 6, 5 (2003); http://www.livingreviews.org/lrr-2003-5

"Binary and Millisecond Pulsars," D.R. Lorimer, Living Reviews in Relativity , 8, 7 (2005); http://www.livingreviews.org/lrr-2005-7