Which Theory of Gravity Fits?

Traditional theories of gravitation (Newton's and Einstein's) are based on the assumption that there is one characteristic, called mass, that determines both the size of the gravitational forces a body exerts and its inertial response to any forces exerted on it. This is called the principle of equivalence because it equates what might be called gravitational charge with mass. Galileo is supposed to have proved the principle of equivalence by dropping balls from the campanile at Pisa, though modern science historians say that if he did a proof, he probably used balls rolling down inclined planes. Since his time, many tests of the principle have been made, none of which have shown significant deviance. Nevertheless it is possible to disbelieve in the principle of equivalence. Mentally, the concepts of gravitational charge or gravitational (responsible for gravitational forces) and inertial (responsible for all inertial responses) can be separated. Indeed experience with other natural forces makes this easy to do. In electricity, for example, electric charge is quite a separate characteristic from mass. It is charge that determines the size of electric forces, and inertial that governs the bodies' response to them. The equivalence principle sets gravity apart from the other natural forces and appears to give it a special connection with the very being of material objects, the that measures the quantity of matter they contain. Some physicists dislike the uniqueness, and theories that deny the principle of equivalence have been put forward. Of these the most widely quoted is one by Carl H. Brans of Loyola University and Robert H. Dicke of Princeton University. Although the Brans-Dicke theory would make only minute changes in current measurements, it makes important philosophical and mathematical alterations. One of the most striking changes is that the relative strength of the forces in Brans-Dicke gravitation depends on the distribution of matter in the universe and declines as the universe expands. Some cosmologists would prefer to have this kind of link between cosmology and gravitation rather than the Newtonian and Einsteinian gravitation in which the forces retain a constant strength unto ages of ages regardless of what happens to the universe. There is thus a continuing series of experiments attempting to decide between the rival theories. This can be done by testing the principle of equivalence directly or by measuring other consequences of BransDicke and similar theories: The motions of the planets, the gravitational redshift of light, and the gravitational delay or deflection of a light beam should all differ from the Einsteinian prediction. Experiments aimed at all these points were described at the Fourth Cambridge Conference on Relativity held at the Massachusetts Institute of Technology in June. One way to test the equivalence principle directly is to compare accelerations induced by gravitational and nongravitational forces on the same bodies. This is called an Eotvos experiment after the Hungarian physicist who first did it with modern precision. The National Aeronautics and Space Administration's Manned Spacecraft Center has plans for doing it in orbit. Philip Chapman described the scheme at the meeting. It will take a hollow aluminum sphere with a gold proof mass inside it. The apparatus will be put into orbit at a distance from the earth where the earth's gravitational attraction and the centrifugal force are equal and oppositely directed. If the principle of equivalence is not exact there is likely to be a difference between the ratios of gravitational to inertial for aluminum and for gold. That inequality would produce a small acceleration of one substance with respect to the other. The experiment hopes to measure the acceleration if there is one. Another orbital experiment will put a pair of gyroscopes into orbit. A characteristic of a rotating gyroscope is that if an outside force like gravity or centrifugal force due to the orbital motion of the spacecraft is imposed on it, its rotation axis will change orientation in a cyclic way. The experiment will have two gyroscopes. One will be oriented so that it suffers precession from the earth's gravity; the other so that it suffers motional precession only. In the July 8 Science News (p. 30), physics editor Dietrick Thomsen reported on experiments being established to try to confirm the detection of gravitational radiation. In this article, he reports on experiments in another frontier area of gravity research-attempts to determine which of two competing theories of gravitation (Einstein or BransDicke) is correct.