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Gravity Probe B

Testing Einstein's Universe

NASA News Release No: 76-6

Space Probe to Test Einstein's "Space-Time Warp" Theory

Written by Joyce B. Milliner, Wallops Flight Center June 10, 1976

A clock-carrying space probe will be launched by NASA in late June to test an important part of Einstein's general theory of relativity. Known as Gravity Probe A (GP-A) or the Red Shift Experiment, the probe will test Einstein's "equivalence principle", which is the foundation of almost every theory of relativity.

The experiment will be a significant step toward a better understanding of gravitational effects, since GP-A is expected to be about 500 times more accurate than any previous measurement using ground-based instruments.

Einstein's 70-year-old relativity theory is of particular interest currently because of its significance to scientists studying astronomical concepts, such as "black holes", which have to do with gravitational phenomena. ("Black holes" are believed to be bodies of such tremendous gravitational magnitude that even light cannot escape them.)

According to the "equivalence principle", within a limited region of space, every form of acceleration is indistinguishable from a gravitational field. Newton showed that, in the absence of an acceleration, every object moves in a straight line at a constant speed. The theory of relativity generalizes this by stating that every body which is affected only by a gravitational field moves along a path in four-dimensional space-time - which is analogous to the motion along a straight line with constant speed in the absence of the field. Because (as viewed in three dimensions) the path of a body in the neighborhood of a large mass is not a straight line but is deviated towards this mass, physicists say that space-time is "warped" or "curved" in the presence of a massive body.

During GP-A's space flight, an extremely accurate clock inside the payload will always be in a weaker gravitational field than an identical clock on earth (because the clock in spacecraft will be farther from our massive planet). Hence, the frequency of the clock in the probe, as observed by signals sent from the clock to earth, will always appear to be greater than that of the identical clock on the ground. Moreover, as the payload clock rises from the Earth through the increasingly weaker field to its maximum altitude of 10,000 kilometers (6,200 miles), it will appear to run increasingly faster. Its rate will then progressively decrease as it returns to the stronger field at lower altitudes. During the flight, the difference between the clock rate in the probe as indicated by the received signal and that of the clock on the ground will be compared with the difference predicted by Einstein's theory. At GP-A's maximum altitude, Einstein's theory predicts that the red shift effect should be about seven parts in 10 billion.

To measure this small but significant effect, the Smithsonian Astrophysical Observatory (SAO), Cambridge, Massachusetts, has developed atomic hydrogen MASERs that function as clocks of extraordinary stability, or accuracy. The interaction of the electron and proton in the hydrogen atom generates a microwave signal (1.42 billion cycles per second) stable to one part in a quadrillion (1 x 10-15), or the equivalent of a clock that loses less than two seconds every 100 million years. For the GP-A mission, the reference clock on the ground will be located at the Merritt Island Launch Area of Kennedy Space Center, Florida. The clocks are expected to provide a measurement accuracy within five thousandths of one per cent (5 x 10-5) of the predicted effect. The primary ground station will be supplemented with experiment-peculiar equipment and ground-based comparator MASERs.