The attraction and repulsion of gravity.

The attraction and repulsion of gravity

Newton and Einstein together did not settle the question of gravity; it continues to weigh on physicists' consciousness. The latest of several attempts to amend or extend the two historic theories comes from three physicists at the Los Alamos (N.M.) National Laboratory, Terry Goldman, Richard Hughes and Michael Nieto. Working from what hughes calls generic characteristics of recently formulated quantum theories of gravity, they have concluded that gravity can be partially repulsive, that its strength may vary for different substances and that gravity may be weaker for moving bodies than it is for the same bodies when they are still.

Early this month their proposal was accepted by the management of the CERN laboratory in Geneva, Switzerland, for an experiment with falling (or rising) anti-protons. The procedure will not only test their ideas but will also be the first experiment to directly measure the earth's gravitational force on antimatter.

The quantized theories are called supergravity, and though they differ among themselves, they all come up with a three-component gravity, whereas Newton and Einstein had only one.

Two of these components are attractive for all things, but the third depends on the number of neutrons and protons in a given substance (its baryon number) and so may differ for different chemical elements and may be repulsive between matter and matter. The strengths of two of the components are altered by motion in different ways, leading to a complicated relationship between velocity and gravity. Although the earth always attracts, in the net result antiprotons should weigh more than protons, and bodies may weigh less when moving than when still. Furthermore, the weights of different elements will not accord exactly with the number of atomic mass units they possess.

This last provision violates the classic principle of equivalence that both Mewton and Einstein adopted, Hughes told SCIENCE NEWS. The principle of equivalence states that a body's inertial mass, which determines how it responds to forces, is the same thing as its gravitational charge, the quality that determines the size of the gravitation force it exerts. A good deal of the science of dynamics and the philosophy of physics is based on the equivalence principle. The inertial mass of an given atomic nucleus is the atomic mass units it possesses, but this atomic mass is not exactly equivalent to the total of neutrons and protons. The forces that hold the neutrons and protons together make a contribution to the inertial mass -- that is, the atomic mass -- but for at least one component of supergravity, they make no contribution to the gravitational charge -- that is, the weight. The difference is about 1 part in 1,000, but it's enough to kill the equivalence principle.

Thus, Hughes stresses, the CERN experiment will test both the quantum theories of supergravity, which are difficult to reach experimentally, and the principle of equivalence.