Physics Colloquium: Testing Gravity at 100 AU With a Drag-free Spacecraft

Our current experimental tests of the inverse square law of gravity greatly depend on the distance scale, and very few tests exist at the scale of the Solar System and beyond. I will describe a possible space mission optimized to test the inverse square law with great accuracy at scales of up to 100 AU. This is the largest scale that can be reached with a direct probe using current technology. Sensitivity to part-in-10⁷ modifications of the inverse square law at 100 AU is well within the reach of a relatively modest space mission, improving the current state of the art by two orders of magnitude. Such an improvement would provide a powerful test of long-range modified gravity theories including many theories motivated by explaining dark energy or dark matter. I will also describe some in-progress work on possible tests of relativistic gravity at long distances using precision atomic detectors.