Measuring Gravity at the Microgram Level

Background noise is the primary concern in a recent push to measure gravity at the smallest scales. PhysicsWorld reports on a recent experiment that measured the gravitational field between gold spheres with a radius of 1 mm!

The torsion balance technique has long been employed to study gravity at small scales, but these Austrian physicists have gone out of their way to isolate the system with a vacuum and a Faraday cage!

By using a laser to observe the vibrations in balance fiber, a frequency analysis revealed a signal consistent with the gravitational field between the gold spheres. Aside from thermal noise on the fiber, what kind of background noise were the experimenters worried about? The gravitational attraction of a human standing too close to the experiment, or the tram driving down the street outside. The experiment is literally able to sense gravitational fields generated by humans socially distancing. That's outrageously sensitive!

And yet, it's not. By changing the separation of the bar and the bead, the experiment was able to reproduce Newton’s famous 1/r^2 force law between 3-5mm. While not a major surprise, such low resolution on the gravitational interaction affords a lot of models for exotic physics with “large” (~1 mm) extra dimensions. For these reasons alone, precision gravity experiments are always worth keeping an eye on.