Einstein's Wave

Not since Newton had anyone devised a more precise explanation of gravity--the universal "glue" that pervades everything, from minuscule atoms to massive stars. Yet for all the significance to modern physics of Einstein's 1915 general theory of relativity, his complex equations have still not been proved beyond a shadow of a scientific doubt.

Last week, in an announcement that excited physicists round the world, University of Massachusetts Astronomer Joseph H. Taylor added new weight to the Einsteinian case. At a gathering of astrophysicists in Munich, Taylor reported indirect experimental evidence affirming a major tenet of general relativity: the existence of gravitational waves. Predicted by Einstein, but never positively detected, this elusive radiation is the carrier of gravity, just as light waves are the carriers of electromagnetism, another of the universe's basic forces.

The achievement of Taylor and his colleagues, Peter M. McCulloch and Lee A. Fowler, was a triumph of radio astronomy. In 1974, while scanning the heavens with the giant bowl-shaped radio telescope near Arecibo, Puerto Rico, the researchers detected rhythmic radio signals from the constellation Aquila. The bursts were coming from a pulsar, or rapidly rotating neutron star-the incredibly compressed cadaver of a giant star whose nuclear fires have died out. Some 15,000 light-years away, it apparently was in orbit around a second compact object, perhaps another neutron star or even a black hole, whose gravity is so strong that nothing, not even light, can escape its grasp.

As the unique pair whirled through space, they offered an ideal test of Einstein's theory. According to general relativity, their movements should be accompanied by an emission of gravity waves. That faint radiation would be impossibly difficult to detect from earth. Still, if Einstein were right, the energy drawn from the orbiting bodies by those waves would cause a predictable effect: the two bodies, which spin around each other about once every eight hours at a velocity of 1.06 million k.p.h. (660,000 m.p.h.), would move ever closer, causing a shortening in their orbital period. The loss, to be sure, would be infinitesimal: only one ten-thousandth of a second per year, as determined from the pulses picked up by the Arecibo "ear."

Last June, after four years of patient observing, the researchers finally made the crucial measurement. They employed a new, extremely sensitive computerized clocking device capable of detecting orbital timing changes of only one fifty-millionth of a second. This superaccurate timer revealed that in those four years the orbital period of the objects had decreased a total of four ten-thousandths of a second. That was exactly on the Einsteinian mark. Said Taylor: "We don't claim to have detected gravitational waves themselves, but simply proved they exist."

Other scientists were slightly more cautious. Larry Smarr of the Harvard-Smithsonian Center for Astrophysics pointed out that the orbital reduction could have been caused by other influences, perhaps the tug of another unknown massive object. Still, Smarr and other astrophysicists seemed generally impressed. Said the University of Rochester's David Douglass, who was handing out buttons in Munich saying GRAVITY WAVES DO EXIST: "It is quite unlikely that Taylor's claim will be disproved."

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