A Victory for Relativity

After Mariners 6 and 7 photographed Mars last year and went into perpetual orbits around the sun, scientists at Pasadena's Jet Propulsion Laboratory conducted an important test. Using NASA's giant 210-ft. Goldstone antenna in the California desert, they beamed powerful radio signals past the sun toward the little unmanned spacecraft. When they reached the Mariners some 250 million miles away, the signals were automatically amplified on board and transmitted back to earth. The entire round trip took only about 43 minutes, but the results may be momentous for all of physics. Last week, at a conference on gravity at Caltech, the experimenters reported that they had gathered dramatic new evidence in support of Einstein's 1916 General Theory of Relativity.

Such support was needed. Although Einstein's theory offers the most comprehensive explanation of gravity since Newton formulated his gravitational laws, it has recently encountered its most serious challenge. One consequence of the theory is that light and other electromagnetic waves should be measurably bent when passing through a strong gravitational field. Contesting Einstein's equations, Physicists Robert Dicke of Princeton and Carl Brans of Loyola University (New Orleans) argued that such waves are bent to a lesser extent than Einstein had predicted. Though subtle and wrapped in complex mathematics, the differences in the two theories are extremely important. As Einstein himself once admitted, if only one part of his theory was proved wrong, the entire edifice would come tumbling down.

By Einsteinian calculations, a radio signal traveling past the sun to the Mariner 6's position at the time of the test should take about 200 millionths of a second longer than if it did not pass through the sun's gravitational field; that is because the signal's path would be curved, not straight. The Brans-Dicke theory, on the other hand, predicts less curvature and a slowdown of only 186 millionths of a second. While such bending has been measured before, the tests have never been accurate enough to make a firm case for either the Einstein or Brans-Dicke theory.

But the J.P.L. experimenters reduced the margin of error to 4% or less by locating the distant spacecraft within 100 ft. of their actual position. Thus, when they calculated that the signal to Mariner was slowed down by 204 millionths of a second on its round trip, they dealt the Brans-Dicke theory a sharp if not decisive blow. Their measurement was only 4 millionths of a second off the Einsteinian prediction, but 18 millionths of a second off the Brans-Dicke figure.

Despite the odds stacking up against him. Physicist Dicke was not yet ready to surrender. "If this were a poker game," he said, "I would be staying with my hand." If Einstein were still alive, however, he would certainly be ready to raise.

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