Gravity Measured with Milligram masses

https://www.science.org/doi/10.1126/sciadv.adk2949

Abstract​

Gravity differs from all other known fundamental forces because it is best described as a curvature of space-time. For that reason, it remains resistant to unifications with quantum theory. Gravitational interaction is fundamentally weak and becomes prominent only at macroscopic scales. This means, we do not know what happens to gravity in the microscopic regime where quantum effects dominate and whether quantum coherent effects of gravity become apparent. Levitated mechanical systems of mesoscopic size offer a probe of gravity, while still allowing quantum control over their motional state. This regime opens the possibility of table-top testing of quantum superposition and entanglement in gravitating systems. Here, we show gravitational coupling between a levitated submillimeter-scale magnetic particle inside a type I superconducting trap and kilogram source masses, placed approximately half a meter away. Our results extend gravity measurements to low gravitational forces of attonewton and underline the importance of levitated mechanical sensors.

This is a very interesting experiment, but just to be clear, it is not actually probing any quantum aspects of gravity itself. It is using quantum effects to control the test mass so that its response to an ordinary Newtonian gravity field from a source mass in the laboratory can be accurately measured.

One pernicious wrong idea is that gravity experiments need to be big. The Cavendish experiment is scale invariant.

If I double the length scale, the masses go up by 8, so the force goes up by 16 and the torque goes up by 32. The moment of inertia also goes up by 32 , so the angular acceleration is constant.

This is a good chunk of the reason its hard to do better measuring G.

The real limitation in "going small" is making all the non-ideal parts of the experiment - wires and screws and attachments and whatnot - smaller.

jedishrfu said:

https://www.science.org/doi/10.1126/sciadv.adk2949

Indeed important experiment.

"However, gravity has never been tested for small masses and on the level of the Planck mass. Measurements of gravity from classical sources in laboratory table-top settings is contrasted by an increasing interest to study gravitational phenomena originating from quantum states of source masses, for example, in the form of the gravitational field generated by a quantum superposition state (15–19). The effort ultimately aims at directly probing the interplay between quantum mechanics and GR in table-top experiments. Because quantum coherence is easily lost for increasing system size, it is important to isolate gravity as a coupling force for as small objects as possible, which in turn means to measure gravitational forces and interactions extremely precisely"

"At the same time, massive quantum sensors are especially suited for tests in a regime with appreciable gravitational influences, which is favorable in probing fundamental decoherence mechanisms related to gravity (20, 21) or proposed physical models of the wave function collapse (22–24) featuring the system mass explicitly, such as the continuous spontaneous localization model (25) and the Diósi-Penrose model of gravitationally induced collapse (26–28)."