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.Skip to next paragraph
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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.
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.