PM: Measuring Gravity at the Micron-Scale with Laser-Cooled Trapped Microspheres: A Renewal Proposal

Gravity is the least well understood of the four known fundamental forces in Nature. Its weakness when compared to the other three Standard Model forces makes gravity particularly challenging to measure precisely in experiments. There have been several predictions from theories beyond the Standard Model of particle physics, including string theory and supersymmetry, that the Newtonian gravitational inverse square law will break down at some distance below the millimeter scale.

To put these theories to the test, a method using an optically-trapped laser-cooled glass bead as a test mass and a microfabricated silicon and gold device as a source mass has been developed. When surrounded by a high-vacuum environment, the glass bead experiences very little friction and becomes an ultraprecise force measurement instrument, needed to measure feeble gravitational interactions between objects at such close ranges.

At the same time, scanning and screening methods are employed to eliminate systematic effects from undesired electromagnetic background forces. It is estimated that the method can improve the search for corrections to the gravitational inverse square law at the micron length scale by more than three orders of magnitude. One graduate student and one postdoctoral researcher will be broadly trained in experimental physics and nanofabrication.

By participating in this highly interdisciplinary research project, students will be well equipped for scientific careers, and efforts to include researchers from under-represented minorities will be undertaken. The fundamental nature of this project can instill a sense of wonder about the natural world in the general public. The nation will benefit from an improved understanding of high-energy physics related to gravitational physics at the micron length scale, at a fraction of the cost of particle-collider experiments.

In this project, an experiment will continue to be developed which makes use of laser-cooled trapped microspheres to test for Yukawa-type deviations from Newtonian gravity at the micron length scale. This new technique can advance the understanding of gravity at this length scale by over three orders of magnitude and may lead to ground-breaking discoveries.

Building on previous results, including calibrated zeptonewton force sensitivity and the development of techniques to reliably maneuver nanospheres in three-dimensions within micron-distances from a source mass surface, the next phase of the project is conceptually divided into two tasks: (1) investigation of systematic errors in preliminary gravity measurements, with a goal of acquiring millions of integrated data in a dedicated Yukawa-force search at the ∼ 1 μm-scale, and (2) in-parallel development of novel methods for trapping and cooling the levitated nanoparticles, including sympathetic cooling with cold atoms.

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.