Is Gravity Weakening?

Elephants may never fly, but they may have quite a load off their feet in some far distant day. Though scientists generally assume that gravitation is as unchanging as any of nature's absolute laws, Physics Professor Robert H. Dicke of Princeton has a different theory. A firm believer in the theory of the expanding universe, he also believes that gravitation gets weaker as the universe expands and permits gravitational forces to penetrate more space. By his guess, the force of gravitation is about 13% less today than when the earth was formed 4 1/2 billion years ago.

Why gravitation should be weakened by its own expansion into space is something that only mathematicians can discuss intelligently, but Professor Dicke believes that a practical test of the theory will soon be possible. Atomic clocks, which depend in no way on gravitation, are already accurate to one part in 1010 (one second in more than 300 years). If a clock that depends on gravitation can be made anywhere near as accurate, its rate can be compared with the rate of atomic clocks. If the two rates become increasingly different, it will mean, says Dr. Dicke, that the strength of gravitation is changing.

Dr. Dicke does not propose to use any ordinary gravitational clock, such as a clock with a pendulum. He thinks that an earth satellite can be made to move in such a way that gas drag and light pressure will not affect its orbit. Such a satellite will, in effect, be a gravitational clock, its period of revolution around the earth governed by gravitational pull.

If the two clocks get out of step, proving that gravitation itself is weakening, it will bring great changes in many branches of science. Dr. Dicke points out that gravitation is what holds the earth together. If it is weakening, the earth must be expanding, and this may be the cause of the cracks that were recently found in the ocean floor (TIME, Sept. 14, 1959). Gravitation also determines the size of the stars, which are balls of hot gas. If gravitation was stronger in the past, the stars must have been smaller. They were probably brighter, too, because their denser interiors generated more thermonuclear energy than they do now. The sun, a typical star, must have been bright enough 2 billion years ago to make the surface of the earth much warmer than it is now. Perhaps this is why the oldest fossils found in ancient rocks are remains of algae, some of whose modern descendants still thrive in hot water.

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