Gravity measured between two 7.6 mg silicon wafers
· BehindLines newsroom
A team headed by Shan-Qing Yang, from China's Sun Yat-sen University, says it has measured the gravitational attraction between two silicon wafers of 7.6 milligrams each. According to the Nature Communications paper, this is a measurement between objects of roughly a milligram, which the authors describe as a step toward bringing the classical and quantum regimes of gravitation closer. ScienceAlert presents this pair as the smallest measured so far.
Why measure so small
General relativity and quantum theory each describe their domain very well, but their mathematical frameworks do not fit together. To explore this frontier, physicists repeat the Cavendish experiment with ever lighter masses. An earlier test, according to the paper, measured the attraction between two gold spheres of about 90 milligrams.
The setup
The researchers etched a tiny torsion pendulum into silicon. At one end sits a 7.6-milligram wafer; facing it, a wheel carries 16 identical wafers that pass close to it in turn. According to ScienceAlert, the wheel turns once every 10 minutes, producing an attraction that repeats 16 times per turn and can be told apart from background noise. Everything sits in a vacuum, and a thin gold-coated silicon membrane limits electrostatic forces. An electronic system holds the pendulum almost still and measures the force needed to cancel the attraction.
The results
For distances from 0.654 to 1.554 millimeters, the measured forces agree with Newton's predictions, with a maximum uncertainty of 50 attonewtons (50 × 10⁻¹⁸ N, at 2 standard deviations), according to the paper. ScienceAlert cites a force of 471 attonewtons at the shortest distance. The paper derives the gravitational constant G from them, 6.67 × 10⁻¹¹ in SI units; ScienceAlert reports an uncertainty of 5.3% and a value consistent with the internationally recommended one. The data also allowed limits to be set on possible Yukawa-type deviations from Newton's law.
What this means
The result remains far from quantum gravity, ScienceAlert stresses: it shows that Newton's law holds at this scale. In the paper, the authors write that the progress of several complementary approaches, including torsion balances and objects held in suspension by light or magnetic fields, gives hope of one day observing gravity between objects in a quantum state. ScienceAlert notes that each of these routes has its own difficulties, in particular removing disturbances.