Albert Einstein's theory of general relativity has been upheld by results thus far of Jet Propulsion Laboratory radio experiments with two Mariner spacecraft as far as 250 million miles out in space.
Dr. John D. Anderson, chief investigator of the JPL experimental team, said today that time delays in the round-trip radio signals caused by the Sun's gravity verified Einstein's 54-year-old theory.
These delays thus far cast doubt on more recent theories, such as that of Drs. Charles Brans and Robert H. Dicke, holding that Einstein's formulations about the effects of gravity on radio and light signals could be in error by 7 per cent.
"Our experiments to date show that Einstein's predictions are right, to within 2 to 4 per cent," said Dr. Anderson. "The best measurements we have thus far indicate a maximum delay of about 204 microseconds for Mariner VI, as compared with an expected 200 microseconds using Einstein's theory.
"If the Brans-Dicke theory were correct, the time delay would be much less than that, perhaps about 186 microseconds. On the other hand, if the much older Newtonian theory were correct, there would have been no time delay at all."
A microsecond is one millionth of a second. The effective slow-up in the signal at maximum delay corresponded to about two-tenths of a mile per second. The maximum elapsed roundtrip time of the radio signal was 43 minutes--transmitted from JPL's Goldstone Tracking Station on the Mojave Desert to either Mariner VI or VII.
The JPL experimenters have taken several hundred radio measurements involving the two Mariners, now in wide-ranging orbits around the Sun after completing their successful Mars flyby missions in the summer of 1969.
Dr. Anderson reported the findings to a Conference on Experimental Tests of Gravitational Theories, held at the California Institute of Technology. Caltech operates JPL for the National Aeronautics and Space Administration.
Dr. Anderson headed a radio science team which included Dr. Duane O. Muhleman, Caltech, Dr. Pasquale B. Esposito, JPL, and Warren L. Martin, JPL.
Martin and Dr. Richard Goldstein, JPL, devised the precise distance-measuring system which, together with the 210-foot Goldstone antenna, enabled these first spacecraft tests of Einstein's theory.
Einstein's general theory of relativity is actually a geometrical theory of gravitation. It predicts that the velocity of light (186,000 miles per second) should apparently be slower in the gravitational field of the Sun. Hence the same should be true of radio signals.
Analysis of the Goldstone-to-Mariner signals began shortly after the spacecraft finished their Mars tasks in August, 1969, and will continue through 1971, Anderson said. The JPL investigator added there is no reason to believe further analysis will reverse the apparent vindication of Einstein.
The Mariner signal readings were processed by JPL computers, with particular stress placed on the observations obtained during the spring of 1970 when the radio signals passed within one million miles of the Sun's surface.
The experiments represent a fourth test of general relativity proposed as early as 1964 by Muhleman and Dr. Irwin Shapiro of the Massachusetts Institute of Technology. In 1916 Einstein advanced his theory correcting the earlier theory of Sir Isaac Newton.
Einstein himself proposed three methods of testing his theory. One was by measuring spectral line frequency (red) shifts in the light from massive stars. Another was by measuring the bending of star light rays by the Sun's gravitational field.
The third test, measuring the slight change in Mercury's orbit around the Sun, was considered valid until 1966. Then Dicke and co-investigators at Princeton University determined that the Sun is slightly oblate, not round.
Dicke argued that this could be sufficient to throw Einstein's prediction of Mercury's orbital shift off by 10 per cent. Brans and Dicke also theorized that, if such error existed in the orbital prediction of Einstein's theory, his light or radio time delay prediction would also be in error from 7 to 10 per cent.
The Mariner data indicate that this is the case, Anderson said flatly.
The year-and-a-half series of experiments has been the farthest-out scientific test yet conducted by JPL. From the 210-foot antenna at Goldstone, the radio signals are shot in a narrow beam with up to 200,000 watts of power. As they strike the spacecraft antenna far out in space, the signals trigger a transponder which amplifies and returns the signal to the desert tracking station.
The ranging performance of NASA's Deep Space Network has been improved so elapsed time can be measured to within a millionth of a second, or less. Radio astronomers and trackers thus can establish the actual distance to each spacecraft to within 100 feet over a distance of a quarter of a billion miles.
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