This award allows the University of Washington Eot-Wash Group to develop ultra-sensitive torsion-balance instruments capable of detecting the ultra-weak forces suggested by modern ideas in theoretical physics, especially attempts to unify gravity with the other three fundamental forces. In particular, instruments will be developed that: 1) probe for ``large'' extra dimensions by testing Newton's inverse-square law down to length scales substantially smaller than the diameter of a human hair, 2) test Einstein's Weak Equivalence Principle with a sensitivity fifty times better than previously published results, 3) look for quantum gravity effects by testing Lorentz symmetry or charge-parity (CP) and charge-parity-time (CPT) violation, 4) search for the force produced by axion-like particles with masses around 1000 micro-eV, 5) operate at temperatures close to absolute zero to study factors that limit torsion-balance sensitivity.
The Eot-Wash Group has consistently provided the most precise studies of the broadly interesting issues they investigate. This required technical innovations that have found applications in other areas such as ground and space-based gravitational-wave detection. The Group's results are used by elementary-particle, gravitational and cosmological theorists to constrain interesting new ideas, are widely disseminated through publications, review papers, colloquia, invited talks, lab tours and public lectures, and are featured in magazine and newspaper articles in this country and in Europe. The variety of scientific issues addressed by the Group and the many experimental challenges involved provide an excellent education in experimental physics for graduate students and postdoctoral fellows, as well as unusually attractive opportunities for undergraduate research projects.
Unifying gravity with the other three forces in nature is one of the major unsolved mysteries in science. Attempts at such unification, such as through string theory, lead to predictions for new spatial dimensions, new particles yet to be observed. These effects lead to the breakdown of the inverse square law of gravity (the classical observation that the gravitational attraction between objects increases by a factor of 4 when the distance between the objects is halved), and the Principle of Equivalence that asserts that all objects have precisely the same acceleration in a uniform gravitational field. Only extremely sensitive experiments can determine whether any of these ideas truly describe the universe we inhabit. Gravitational experiments are notoriously difficult because gravity is so much weaker than the other forces in nature for laboratory scale objects. We use specialized, and unprecedently sensitive, torsion balances to test the current ideas for unifying gravity with the other 3 fundamental forces. A torsion balance is a mass distribution suspended from a long thin tungsten fiber (thinner than a human hair), that is essentially free to rotate when an external force, such as gravity, acts on the mass distribution. Our flagship projects are unbiased tests of Einstein's Equivalence Principle (this is a broad-gauge search for any new spin -independent forces with ranges greater than a few centimeters) and testing the gravitational inverse-square law at the shortest attainable length scale (we are currently able to do this at length scales of about half the diameter of a human hair). In addition, we are using electon-spin polarized test bodies and attractors to search for new spin-dependent forces that are suggested by some current ideas in particle physics. To pursue these objectives we also develop new technologies ranging from autocollimators to gravity gradiometers to beam balance rotation sensors. With these instruments, we have discovered that beryllium, aluminum, and titanium test bodies have the same gravitional acceleration to within 3 tenths of a part per trillion, which provides the most sensitive test of Einstein's law of gravity. We have also found that the inverse square law of gravity is valid down to 40 microns, about half the diameter of a human hair. This result also provides a strong constraint on the chemeleon mechanism, a speculation designed to hide the missing particles predicted by string theory. Our spin sensitive measurements provide the tightest constraints on new interations mediated by bosons suggested by contemporary ideas in elementaty particle physics. For example, we have shown that any new spin-spin force between electrons is a hundred million million times weaker than the magnetic force between electrons at laboratory scale length ranges. The photographs that accompany this report show the mass distributions used to test the Principle of Equivalence and the inverse sqaure law of gravity.
