Understanding Gravity at the Smallest Scale

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This award is supported by the Gravitational Physics and the Atomic, Molecular and Optical Experimental Physics programs. 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 be consistently combined with the Standard Model of particle physics to provide a description of gravity at small scales. The development of a quantum theory of gravity that can be incorporated into the Standard Model 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 distances shorter than a mm. Such deviations are extremely difficult to measure experimentally due to the small strength of gravitational interactions at microscopic distances. The studies described here attempt to gain insights into the realm of microscopic gravitational forces with a novel experimental setup. Additionally, the research will enhance the training of students in STEM areas which are vital for the future of the nation.Previous measurements at these distance scales have employed techniques derived from human-size devices in which mechanical springs are used as force sensors. This group has developed a drastically new technique, using the light field of a laser to confine and measure the motion of micron (or, eventually, submicron)-size quartz microsphere. Previous studies undertaken by this group include the most sensitive search to date for fractional charges over 4 orders of magnitude smaller than the charge of an electron. This is a by-product of having to discharge the microspheres and check that they are really neutral. The group has completed the construction of a new trap in which the entire microsphere position readout is carried out interferometrically. This is a first in this area of research and will be applicable to other areas, e.g. biology or polymer science. The studies supported by this grant include exploring the possibility of spinning the microspheres, something that could reduce the background of the gravity measurements and may also lead to the development of nanogyroscopes.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.