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Einstein's Theory Passes a Space 'Laboratory' Test

A RECURRING type of science news about tests of Einstein's general relativity theory - his theory of gravity - reports the (so far) recurring result that Einstein wins. It's happened again. But this time, it's an especially stringent set of tests using natural "laboratories" in space that produce the strong gravitational conditions needed to put the theory through its paces.

In spite of its fame, general relativity has not been thoroughly tested since Einstein introduced the theory nearly 90 years ago. Yet it is a key guide in scientists' present understanding of our universe. They would like to pin it down.

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There are two major aspects of Einstein's theory. Special relativity deals with the physics of bodies moving at a substantial fraction of the speed of light - that is, at speeds where Newton's laws need modification. General relativity also brings in a new theory of gravity that works under strong gravitational conditions where Newton's theory fails. For example, both theories predict how much the sun's gravity will deflect light that passes close to its surface. Newton's theory gives the wrong answer wh ile the Einsteinian prediction is right.

What challenges physicists is the realization that there are other non-Newtonian theories that also give the right answer.

Special-relativity theory routinely demonstrates its validity as particles swirl through accelerators at near-light speeds in laboratories on Earth. But gravity is too weak throughout the solar system to give any of the non-Newtonian gravity theories a clear edge. So physicists have been on the lookout for places in the universe where they could make definitive tests.

They may have found such a cosmic laboratory in the binary pulsars. A pulsar is a star that has condensed to a very high density. It sends out radio pulses with a regularity that rivals that of atomic clocks. While astronomers have located many pulsars, they have found only a few binaries - pulsars with nonpulsing companions.

Reporting their research recently in Nature, Joseph H. Taylor of Princeton University in New Jersey, Alex Wolszczan of the Arecibo Observatory in Puerto Rico, Thibault Damour of the Institut des Hautes Etudes Scientifiques in Bures sur Yvette, France, and J. M. Weisberg of Carleton College in Northfield, Minn., note that binary pulsars "provide nearly ideal laboratories for testing of strong field gravity." Predicted effects show up in changes in the timing of the radio pulses as the companions of a bina ry pair move and interact under gravitational conditions hundreds of thousands of times stronger than those known in our solar system.

Looking at predicted effects with several such binary pulsars, the researchers say that "Einstein's theory has passed several new, deep and sensitive experimental tests."

They report, for example, that 10 years' observations of the binary pulsar astronomers call PSR1913+16 show it is losing energy as it radiates gravitational waves - ripples in space-time - just as Einstein's theory predicts.

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This doesn't mean Einstein's relativity is home free. The new tests do not completely rule out competing theories. But they have made it much tougher for such theories to stake their claims.

Dr. Taylor and his colleagues say they look forward to the discovery of more binary pulsars. Physicists had been frustrated by their inability to throw Einstein's theory up against really strong gravitational fields. It seems that nature has been kind to them after all.