Exploring a Low-energy Frontier of Particle Physics Using Methods of Experimental Gravitation

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The goals of this project are to make more precise and more general tests of the Universality of Gravitational Free Fall. The standard, classical theory of gravity, General Relativity, incorporates the Universality of Free Fall as an exact statement about gravity. It is an experimental question, however, if this principle holds exactly. There are two strong reasons for testing the principle. First, General Relativity is not a quantum mechanical theory and some current ideas about possible quantum-mechanical theories of gravity predict violations of the Universality of Free Fall arising from additional `gravitational` scalar or vector fields. Second, by searching for violations of a symmetry that classical gravity holds to be exact, one can probe for new, ultra-weak, non-gravitational forces. Because it is impossible to shield a gravitational field, such ultra-weak forces would normally lie hidden `underneath gravity`. But any new force is expected to violate the Universality of Free Fall and so can be detected even if it is much weaker than gravity. The proposed tests require the continued development of improved instruments for measuring tiny differences in the accelerations of different materials attracted toward the earth, toward a massive Uranium laboratory source, or to astronomical structures. Three different torsion-balance instruments will be employed. Two of them, developed under previous NSF support, will be upgraded for the proposed tests, and a new more sensitive instrument will be constructed. The combination of these instruments will permit tests of the Universality of Free Fall over length scales ranging from 1 cm to infinity. In particular, it will be possible to test whether gravity is the dominant long-range force between ordinary mater and the cosmological dark matter; and, in conjunction with existing data on lunar laser-ranging, to make a `loop-hole free` test of the gravitational properties of gravitational binding energy. This last test will be performed by comparing the accelerations toward the sun of `scale models` of the Moon and the Earth's crust.