* Astronomy

A rare cosmic phenomenon - a double pulsar - has enabled the Albert Einstein's theory of general relativity to be tested under extreme conditions, the British Association was told by Dr Michael Kramer of the Jodrell Bank Observatory.
The two exotic stars orbit each other at speeds of 621,000 mph, and each pulsar is about 7.5 miles across and emits radio beams as it rotates.

The 23-millisecond pulsar was first detected in April 2003 using the 64-m Parkes radio telescope in New South Wales, Australia. Subsequent observations were made both at Parkes and with the 76-m Lovell Telescope of the University of Manchester's Jodrell Bank Observatory in Cheshire.



General relativity describes gravity as a warping of space and time.
"General relativity is difficult to test in laboratories. Instead, one needs to involve large celestial bodies to compare its predictions in observations and experiments" - Dr Michael Kramer.

Whenever the rotating radio beam comes across Earth, radio astronomers have found that each is so regular that the precision of these "pulsar clocks" rival the best atomic clocks on Earth.

Orbiting each other in less than 150 minutes, these two clocks offer an ideal way to test Einstein. The pulsar "clocks" tick more slowly when gravity rises as they move closer together, as Einstein predicts.
The measurements match the predictions of general relativity, providing the "best agreement ever found", down to three decimal places.

The 23-millisecond pulsar PSR J0737-3039A and the companion, PSR J0737-3039B, rotates every 2.8 seconds and orbits PSR J0737-3039A in only 2.4 hours. Both stars in this remarkable binary system have masses greater than that of our Sun and have an orbital separation which is less than the size of the Sun.

the also astronomers say they have measured another very significant phenomenon - the stars' orbit is shrinking because of a loss of energy caused by the emission of gravity waves. The orbit is currently shrinking by 7 millimetres per day, and this decay will accelerate in the future. This means that the two superdense neutron stars will collide in 85 million years.

"The measured decay of the orbit is exactly what is predicted by GR, so this is another important victory for Einstein's theory" - Dr. Ingrid Stairs, assistant professor at the University of British Columbia.

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