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Einstein's critical gravity theory still being plumbed

According to the United States National Bureau of Standards, Albert Einstein's description of gravity recently passed yet another experimental test. That seems a fitting way to celebrate the 70th anniversary year of the general theory of relativity and the 80th anniversary of its forerunner, the special theory of relativity. It also is nice to know that Einstein's work continues to hold up as a Stanford University team prepares the most elaborate test of it yet conceived.

General relativity predicts a coupling between the Earth's rotation and the spin of an orbiting gyroscope. The effect is tiny. It should amount to a drift in the gyroscope's axis of only 0.044 arc-seconds per year. Stanford's satellite-borne equipment must function for 14 months at liquid helium temperatures (around -450 degrees F.) with a precision equivalent to measuring the angle subtended by the width of a human hair several miles away.

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After some three decades of effort and with help from the NASA Marshall Space Flight Center, the Stanford team appears to have licked the formidable technical problems this involves. It should have an instrument assembly ready for testing on a shuttle mission in 1988. It then hopes to have the experiment in polar orbit some 300 miles high in time for the university's 1991 centennial celebration.

The sustained dedication that is going into this experiment and the enthusiasm of the space science community to carry it out reflect the critical role that Einstein's gravity theory has come to play in modern science. Indeed, the Space Science Board of the National Academy of Sciences has said that the Stanford experiment -- called Gravity Probe B -- addresses the ``highest priority science objective in gravitational physics.''

This has not always been the case with Einstein's theory of gravity. Its complexity and mathematical difficulty have obscured many of its implications. Research on the theory was peripheral to mainline physics until after Einstein's death.

Special relativity, in contrast, fairly quickly won its way after Einstein introduced it in June 1905. It deals with physical processes as described in different reference frames which move with uniform velocity with respect to each other. It says nothing about gravity. Its basis seems ethereal. Yet it has yielded such potent insights as the transformation of material mass into energy which powers the sun and nuclear reactors.

Einstein postulated that the basic laws of physics should have the same mathematical form in any of these reference frames. However, certain common-sense notions have to go by the board if these laws are to retain their form and if any physical constants they contain are to retain their numerical values as they are mathematically translated from one reference frame to another.

First, space and time can no longer be considered separate. They meld into a single, four-dimensional entity -- manifold, to use the geometer's term -- called space-time. In space-time, events are specified by their location in three-dimensional space, and four-dimensional time.

Second, distances and clock rates do vary from reference frame to reference frame, although the effects are usually only significant at speeds near that of light. The clock on a fast-moving spaceship appears to an earthly observer to run more slowly than earth clocks. Distances within the spaceship appear to that observer to be foreshortened. There is the famous ``twin paradox'' in which an astronaut on a fast spaceship returns home to find she is younger than her twin sister who stayed behind.

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All these and other aspects of special relativity have been abundantly demonstrated. The theory is thoroughly integrated into modern physics.

This is not true with general relativity, first introduced by Einstein in November 1915. Here, Einstein dealt with gravity. He described gravitational effects in terms of the ``warping'' of space-time by material mass and energy.

He specified that, over a fairly small volume of space, there is no difference between the effects of gravity and of an acceleration. The two are equivalent. He extended the principle of relativity to state that physical laws should retain their form in whatever coordinate system they may be expressed even if one coordinate system is accelerated with respect to another. Again, these abstract requirements produced a theory which has very practical consequences.

For example, the theory predicts the bending of light near massive bodies, describes how clocks run slower as gravity becomes stronger, and generally accounts for much of the action of gravity throughout the cosmos. The recent Bureau of Standards experiment tested the implication that the relative rates of two atomic clocks located near each other should not depend on their orientation with respect to our galaxy. Many of the theory's predictions have been verified to a precision of from 1 to 10 percent.

This is good, but not good enough to firmly establish Einstein's formulation.

Physicists suspect the theory needs refinement. No one has been able to integrate it with the quantum theory, which deals with subatomic processes. Referring to this in supporting Stanford's Gravity Probe B project, Chen Ning Yang of the State University of New York at Stony Brook said a needed refinement may involve spin and rotation.

Whatever the fate of Einstein's theory, physicists agree that he made a major contribution to human enlightenment in getting us to view the natural world with more discernment. He showed that humans' inborn sense of space and time is illusory. Space and time are not separate and absolute entities. They are a single, integrated fabric which forms the tapestry of the cosmos.

There may seem to be something transcendent, even mystical, about this revelation. Actually, the opposite is true, for Einstein demystified the subject. As Nobel laureate Steven Weinberg has observed, ``the most remarkable achievement of Einstein in developing relativity theory is . . . that he for the first time made space and time a part of physics and not of metaphysics.''

Robert C. Cowen is the Monitor's natural science editor. A Tuesday column.

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