Renewal of “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, its connection with quantum mechanics and other fundamental interactions remains elusive. Indeed, the development of a quantum theory of gravity that can be incorporated into the Standard Model of fundamental interactions is a central goal of 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 gravitation and the presence of residual electromagnetic interactions. This grant funds the continuation of an experimental program in this area that has just started providing results.Previous measurements at these distance scales have employed techniques derived from human-size devices in which mechanical springs are used as part of force sensors. In the last few years, with the support of the NSF, the PI has developed a drastically new technique, using the light field of a laser to confine and measure the motion of micron-size quartz microspheres. In the last year the first full search for deviations from the inverse square law in the 10 micron range has been completed. While not yet competitive with the most sensitive measurements, this represents the first demonstration of the new technique that more directly measures forces at the range of interest, instead of extrapolating with objects that are larger than such range. In addition to this, the systematics are expected to be very different from those of other measurements and there are indications that background force models will soon be within reach. Additionally, the new technique used has been found to have many other applications in other areas of fundamental as well as applied physics. In the first category are searches for millicharged particles or tests of neutrality of matter, while inertial navigation and vacuum technology are examples of the second.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.