Scientists Measure Gravitational Pull Between 7.6 mg Wafers
Physicists led by Shan-Qing Yang of Sun Yat-sen University have measured the gravitational attraction between two silicon wafers of 7.6 milligrams each, the smallest pair yet. The result matched Newton's predictions and points toward tests of gravity at quantum scales.
Scientists measure gravitational pull between the smallest pair of masses yet, according to a team led by Shan-Qing Yang of Sun Yat-sen University in China. The objects were two silicon wafers, each weighing 7.6 milligrams.
The physicists reported the work in a paper in Nature Communications. They call it "a significant step toward bridging the classical and quantum regimes of gravitation."
Why tiny masses matter
Everything with mass pulls on everything else, however small. Physicists want to know what happens when gravity is measured on ever smaller scales.
General relativity describes gravity as the curvature of spacetime. Quantum mechanics describes matter and energy using probability. Both theories work well in their own domains, but their mathematics do not fit neatly together. That leaves no complete account of gravity once quantum effects matter.
A smaller Cavendish experiment
The team built on the experiment made famous by 18th-century English scientist Henry Cavendish. His torsion balance used lead weights on a suspended rod, with larger weights nearby. The attraction twisted the wire holding the rod, and the twist revealed the force.
In 2021, another team measured the attraction between two gold spheres of around 92 milligrams each. That was then the lightest version of the experiment, and the researchers challenged others to go further.
Yang's group used deep silicon etching to make a miniature torsion balance. One 7.6-milligram wafer sat at one end of the suspended rod, with a counterweight at the other.
How the signal was isolated
Instead of one nearby mass, the team built a rotating wheel with 16 identical silicon wafers around its edge. Each wafer passed close to the test mass in turn and gave it a tiny gravitational tug.
The wheel turned once every 10 minutes, so the tug repeated 16 times per rotation. That repeating pattern let the researchers separate the signal from background noise.
The whole apparatus sat inside a vacuum chamber. A thin, gold-coated silicon membrane separated the rotating masses from the balance to limit interference from electrostatic forces.
The balance was not allowed to twist freely. An electronic feedback system held it almost perfectly still, and the researchers measured the force needed to cancel the gravitational attraction.
The torque on the balance showed an attraction consistent with Newton's predictions.
