Science and Reason: General relativity passes cosmic test - Einstein's theory holds in extreme gravitational fields.

Chalk up another win for "orthodox" scientific theories. General relativity has again passed stringent tests.

General relativity survives gruelling pulsar test — Einstein at least 99.95 percent right

An international research team led by Prof. Michael Kramer of the University of Manchester's Jodrell Bank Observatory, UK, has used three years of observations of the "double pulsar", a unique pair of natural stellar clocks which they discovered in 2003, to prove that Einstein's theory of general relativity - the theory of gravity that displaced Newton's - is correct to within a staggering 0.05%. Their results are published on the 14th September in the journal Science and are based on measurements of an effect called the Shapiro Delay.

The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It consists of two massive, highly compact neutron stars, each weighing more than our own Sun but only about 20 km across, orbiting each other every 2.4 hours at speeds of a million kilometres per hour. Separated by a distance of just a million kilometres, both neutron stars emit lighthouse-like beams of radio waves that are seen as radio "pulses" every time the beams sweep past the Earth. It is the only known system of two detectable radio pulsars orbiting each other. Due to the large masses of the system, they provide an ideal opportunity to test aspects of General Relativity.

The large mass of the pulsars and their proximity to each other is the key thing, resulting in a very strong gravitational field. The binary pulsar system provides an opportunity to check the validity of general relativity under conditions that are more extreme than any studied before.

Shapiro delay can be described as an apparent change in the speed of light in a strong gravitational field. It occurs because spacetime itself is warped in the field, which effectively forces light to travel a larger distance. Since the radio-frequency beam from each pulsar sweeps across Earth at a very precise frequency (22.8 milliseconds for one, 2.8 seconds for the other), like an exceptionally accurate clock, it is possible to predict exactly when the beam should be seen. Any departure from this prediction would be due to the Shapiro delay. The orbital period of the two pulsars around each other is about 2.4 hours. During this period, the distance between the pulsars varies, so the mutual gravitational fields vary correspondingly. This allows the theoretical delay time to be calculated, and the observations match the prediction very well.

This effect is distinct from the time dilation which occurs in a large gravitational field. The dilation causes time intervals to appear to lengthen. So the period of rotation of each pulsar appears to change, and the spectrum of radio waves from each object is redshifted, as the pulsars experience a change in the gravitational field. Here again, the observations match the predictions of general relativity very well.

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Additional information:

Pulsars' Gyrations Confirm Einstein's Theory: News article about the research from Science. (Subscription required for full access.)
Tests of General Relativity from Timing the Double Pulsar: The actual research paper from Science. (Subscription required for full access.)
Millisecond Pulsars as Tools of Fundamental Physics: A review paper by Kramer posted to the arXiv in May 2004. It explains the underlying physics in some detail. It also describes the binary pulsar system, which had only recently been discovered.
The Confrontation between General Relativity and Experiment: An expository paper by Clifford M. Will that reviews the status of experimental tests of general relativity and of theoretical frameworks for analyzing them.
General relativity passes cosmic test - Einstein's theory holds in extreme gravitational fields.: News article at Nature.com news. (Subscription required)