Indirect Detection of Gravitational Radiation

[Source: J.M. Weisberg, J.H. Taylor: Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis, arXiv: astro-ph/0407149v1 Figure 1.]

One of the predictions of Einstein's General Theory of Relativity is the existence of gravitational waves: Two large masses in orbital motion will create small, wavelike distortions in spacetime that propagate like ripples on a pond, and carry away energy.

Despite big efforts and huge detectors, no gravitational waves could be measured so far. But direct discovery notwithstanding, physicists are confident that gravitational waves are real, since there is very compelling indirect evidence for their existence, and for their compliance to the rules of General Relativity.

A typical source of gravitational waves would be a couple of very massive, compact stars in close orbits. Binary neutron stars are good candidates for sources of gravitational radiation. The energy oss by the emission of gravitational waves would result in a slowly decaying orbit: the stars would et ever closer the more energy would be radiated away as gravitational waves. And if one of the stars is a pulsar, there are chances that one can measure the orbit, and establish the indirect consequences of the emission of gravitational waves.

That is exactly what has happend with the binary pulsar Binary Pulsar PSR 1913+16: In December 1973, astronomy student Russell A. Hulse was at the Arecibo Radio Observatory in Puerto Rico, collecting data on pulsars for his Ph.D. thesis. One of the pulsars he was observing showed a curious, periodic variation in the pulsation frequency. It soon became clear that this variation could be best understood as the periodic Doppler shift of the pulsar in an elliptic, Keplerian orbit around another star. This allowed for the very precise reconstruction of the orbit of the pulsar.


Schematic picture of the binary pulsar PSR1913+16: Two neutron stars circle each other closely in elliptical orbits. Their mean separation is only a few times the distance Earth-Moon, and one orbit takes only about eight hours. When the stars pass close to each other, they emit large amounts of gravitational radiation. [Source: Nobel Foundation, 1993.]

The orbital parameters of this binary system, however, are so extreme - masses on the order of the Sun orbiting within eight hours at 1 permille of the speed of light at a distance on the order of the distance Earth-Moon - that Kepler's laws for the motion of two masses under the influence of gravitation are not sufficent anymore: As Hulse's advisor Joseph Taylor noted, the full-scale apparatus of General Relativity was necessary to describe the orbit, instead of the simple Newton law of gravitation, For example, the binary pulsar shows a big motion of the periastron - the equivalent of the perhelion shift of Mercury, whose share not accounted for by the perturbations of the other planets in the Solar System was explained by Einstein as the first "postdiction" of his new theory of gravitation. What's more, the time evolution of the orbit can be measured precise enough to check for the consequences of the emission of gravitational waves!

The data points in figure show the observed change in time of periastron over the last 30 years, since the first data on pulsar PSR 1913+16 have been available. The parabola illustrates the theoretically expected change in periastron time for a system emitting gravitational radiation, according to general relativity.

Not only the Nobel committee in Stockholm considered this an extremely important result - but they awared to Russell A. Hulse and Joseph H. Taylor, Jr., the Nobel Prize in Physics 1993 for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.

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Taylor's Nobel Lecture is on Binary Pulsars and Relativistic Gravity (PDF file), while Hulse in his Nobel Lecture talks about The Discovery of the Binary Pulsar (PDF file).

The Binary Pulsar PSR 1913+16 is a nice short introduction that explains in more detail how the pulsar data are analysed, and how the periastron shift can be extracted.

For the latest data about the "Taylor-Hulse" binary pulsar PSR1913+16, check out Wikipedia.

For two recent related rviews, see Duncan R. Lorimer: Binary and Millisecond Pulsars, Living Rev. Relativity 8, (2005), and Ingrid H. Stairs: Testing General Relativity with Pulsar Timing, Living Rev. Relativity 6, (2003).

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This post is part of our 2007 advent calendar A Plottl A Day.