Physics: Going for gold in small-scale gravitational measurements

The smallest known gravitational field measured so far is reported in Nature this week. This measurement, achieved using two 1-mm-radius gold spheres, could pave the way towards experiments to explore new areas of fundamental physics, such as probing dark matter or the interplay between quantum physics and gravity.

Gravity is a fundamental force, but our understanding of this force remains incomplete; it does not fit into the standard model of physics and seems to be disconnected from quantum theory. Testing the coupling force of gravity in very small objects may shed some light on some of the mysteries of this force, such as deviations from the predictions of Newtonian gravity theory. However, such tests are challenging and require a tightly controlled environment to minimize perturbations from other sources and gravity itself.

Markus Aspelmeyer and colleagues design an experiment to isolate gravity as a coupling force between two tiny gold spheres with masses of about 90 mg. Their tightly controlled set-up minimizes the influences of external perturbations; for example, a Faraday shield is used to block electrostatic forces, and seismic and acoustic effects are minimized by connecting one of the gold spheres to a vacuum chamber. The other sphere is periodically moved closer to the grounded sphere, allowing the isolation of gravitational coupling, which can be detected in a change in the rotational signal./p>

The experiments confirm what is already expected from classical Newtownian physics, in which the gravitational force between the two spheres depends on their masses and their distance. The authors suggest that the sensitivity of their experiment has potential to be further improved, which could enable the measurement of gravitational forces for even smaller objects. Such experiments may allow tests of fundamental physics that have so far remained elusive, including gravitational effects of dark matter and gravitational coupling between quantum systems. However, incorporating quantum physics in such tests still remains challenging, the authors conclude.

After the embargo ends, the full paper will be available at: https://www.nature.com/articles/s41586-021-03250-7