Understanding Gravity at the Smallest Scale

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Understanding the nature of gravity at microscopic distances is one of the most important open problems in fundamental physics. Although General Relativity provides an extremely well-tested framework for describing gravitational effects at large distances, it cannot provide consistently a description of gravity at small scales where quantum effects are prevalent. The development of a quantum theory of gravity is a central goal of fundamental physics, with broad implications for our understanding of particle physics and the mysterious nature of the "dark energy" that appears to permeate the universe. Many theories attempting to provide a consistent microscopic framework for gravity (e.g., those involving extra dimensions) predict that gravity could deviate from the familiar inverse square law at sub-millimeter distances. Such deviations are extremely difficult to measure experimentally due to the small strength of gravitational interactions at microscopic distances. This project represents an attempt to do this. At the same time, while the direct scientific goals of this program are clearly central to the development of modern physics, the general investigation of the technique should enrich many other fields of science and technology. The ability to trap and control small objects in vacuum using laser beams is being explored for applications in quantum control, quantum computing and in the general area of detection of small forces. In addition the work will require the detailed understanding of residual electromagnetic interactions between the microspheres and the materials composing the attractors, and it is conceivable that progress in this area may enable new techniques for measuring properties of surfaces that are not yet accessible by probes such as Atomic Force Microscopes. Finally, the students (graduate and undergraduate) exposed to the project will receive a very complete training in many areas of science and technology.Previous measurements at these distance scales have employed techniques derived from human-size devices in which mechanical springs are used as force sensors. We propose in this project to develop a drastically new technique, using the light field of a laser to confine and measure the motion of micron (or, eventually, submicron) size quartz nanosphere. This technique takes advantage of the modern development of optical tweezers, which has produced significant advances in biology and polymer science. By confining the nanospheres in vacuum and cooling them to low temperatures through active feedback of the trapping laser, the nanospheres can be decoupled from the room temperature environment, significantly reducing thermal and vibrational noise sources. The nanosphere oscillates in the harmonic potential of the optical trap, and its interaction with attractor masses positioned several microns away can be measured by studying the motion of the microsphere. The use of a light field in lieu of a mechanical spring affords much greater flexibility. Backgrounds can be mitigated through careful selection of the materials used for the attractors and the coating of the attractors with appropriate shielding layers. We have already cooled 5 micro-meter diameter microspheres to mK temperatures and demonstrated force sensitivities of 10^-17 N/sqrt(Hz). We have recently published a paper in Phys Rev Lett showing that the nanospheres can be easily discharged and setting a new limit on the existence of particles with very small fractional charges. In the course of this project we expect to be able to study the Casimir effect and, in general, residual electromagnetic interactions between the nanospheres and the attractors and perform a first competitive gravity measurement.