Exploring a Frontier of Experimental Gravitation

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The Universality of Gravitational Free Fall, the underlying symmetry of general relativity (Einstein's theory of gravity), will be tested with unprecedented precision and generality. There are two strong reasons for testing this symmetry. First, general relativity is not a quantum theory and current ideas about quantum gravity naturally produce violations of the Universality of Free Fall that may be experimentally accessible. Discovery of such fields would have profound consequences for our understanding of gravity and perhaps of cosmology. Second, by searching for violations of a symmetry that general relativity holds to be exact, one can probe for new, ultra-weak, non-gravitational fields. Current ideas in cosmology suggest that much of the energy in the universe may be tied up in such fields. Although such ultra-weak fields would normally be obscured by gravitational forces, they are detectable because they generically violate the Universality of Free Fall. The proposed tests require the continued improvement of instruments for detecting tiny differences in the accelerations of test bodies attracted toward the earth, the sun, large-scale cosmological structures, or toward a massive uranium laboratory source. The test bodies include Be, Al, Ti, Cu, Pb, plus bodies having compositions close to the earth's core and the moon respectively. Three different torsion-balance instruments, initially developed under previous NSF support, will be upgraded and employed in this work. The combination of these instruments will test the Universality of Free Fall for Yukawa length scales from 1 cm to infinity with differential acceleration resolutions down to 10-13 cm/s2. The torsion balance instruments will be adapted to probe for spin-dependent forces by using an electron-spin polarized pendulum and attractor. In addition, the Newtonian constant G will be measured using a new technique that avoids important sources of error in previous measurements. Recent results for G have scattered widely, demonstrating the necessity of several independent measurements of this fundamental constant. The technique should be capable of determining G to a precision of 1 part in 105.