Einstein's Theory Put to the Test Gravity Probe-B and Laser-Beam Observatories Will Measure Unproved Relativity Predictions

By Robert C. Cowen, writer of The Christian Science Monitor | The Christian Science Monitor, August 7, 1990 | Go to article overview

Einstein's Theory Put to the Test Gravity Probe-B and Laser-Beam Observatories Will Measure Unproved Relativity Predictions


Robert C. Cowen, writer of The Christian Science Monitor, The Christian Science Monitor


FAR off in the constellation Aquila, a rare object called pulsar 1913 +16 beams a radio signal that challenges physicists to answer the question: Was Einstein right?

Is his theory of gravity - the general theory of relativity - right when it predicts the existence of ripples in the fabric of space and time called gravity waves?

Is the theory right when it predicts that a massive rotating object like Earth warps the space-time fabric in ways that change the orientation of gyroscopes?

These are two of the most important unproved aspects of Einstein's great theory. Now, after decades of experimentation and development, physicists have the technology to mount experiments to test these predictions.

With National Aeronautics and Space Administration (NASA) funding, a Stanford University team in Palo Alto, Calif., is constructing a satellite to test the gyroscope effect. Called Gravity Probe-B, it will carry the most precise gyroscopes ever made to perform what NASA has called "the most challenging test we'll undertake in this millennium." The project's principal investigator, Francis Everitt, says it will make "the most precise measurement ever made of an Einstein prediction."

Meanwhile, the National Science Foundation is supporting a team of scientists at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) in designing an observatory that uses laser beams to detect gravity waves. The observatory is to be part of a global complex that, after several decades of failure with smaller detectors, should have the sensitivity to pick up these elusive phenomena. Then, says project director Rochus E. Vogt of Caltech, "we can put Einstein's general relativity theory to the most stringent test ever."

The pulsar in Aquila spurs on both projects, because here, for the first time, radio astronomers have seen signs of both untested effects. It is a system in which two dense, compact stars are in a close orbit. One star emits a radio beam that, like a lighthouse beam, sweeps past Earth where it is heard as a series of pulses.

Princeton University radio astronomer Joseph H. Taylor and various colleagues have studied this system since 1974. They have found its members spiraling closer together, just as expected if energy is radiating away in the form of gravity waves. Their observations agree with the relativity theory's prediction to within 1 percent. Because of this, Dr. Taylor says, "you can say that the existence of gravity waves now has been accepted."

Recently, Taylor and his colleagues have seen evidence of the gyroscope effect as well. Taylor calls this a "qualitative confirmation" of Einstein's prediction, because the scientists can't pin this down to the point of making a precise comparison with theory.

That kind of test has to wait for Gravity Probe-B. "It's been a long time in coming and I hope it works," Taylor says. In fact, the late Stanford University physicist Leonard Schiff and his colleagues physicist William M. Fairbank and astronautical engineer Robert Cannon came up with the idea three decades ago.

IN relativity theory, space and time form a seamless four-dimensional fabric called space-time. Matter can warp this fabric. The theory predicts that the axis of a gyroscope moving through such warped or curved space-time will change direction, even though no force acts on it. It further predicts that a massive, spinning body will drag the space-time medium around with it, and this dragging will also move a gyro axis.

Dr. Schiff showed that our spinning Earth warps and drags space-time enough for a sensitive gyro in orbit to exhibit both effects. …

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