A team of physicists in Austria has measured the gravitational force between two gold spheres of 1.07 millimeter radius.
Gravity is the weakest of all known fundamental forces and poses some of the most important open questions to modern physics: it remains resistant to unification within the Standard Model of physics and its underlying concepts appear to be fundamentally disconnected from quantum theory.
Testing gravity at all scales is therefore an important experimental endeavor. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond.
“During the time of Isaac Newton, it was believed that gravity was reserved for astronomical objects such as planets,” said Dr. Jeremias Pfaff from the Faculty of Physics at the University of Vienna and colleagues.
“It was not until the work of Cavendish — and Nevil Maskelyne before him — that it was possible to show that objects on Earth also generate their own gravity.”
“Using an elegant pendulum device, Cavendish succeeded in measuring the gravitational force generated by a lead ball 30 cm tall and weighing 160 kg in 1797.”
“A so-called torsion pendulum — two masses at the ends of a rod suspended from a thin wire and free to rotate — is measurably deflected by the gravitational force of the lead mass.”
“Over the coming centuries, these experiments were further perfected to measure gravitational forces with increasing accuracy.”
In the team’s miniature version of the Cavendish experiment, the gravitational source is a nearly spherical gold mass with a radius of 1.07 mm and a mass of 92.1 mg. A similarly sized gold sphere acts as a test mass of 90.7 mg.
The idea is that a periodic modulation of the position of the source mass generates a time-dependent gravitational potential at the location of the test mass, the acceleration of which is measured in a miniature torsion pendulum configuration.
The experiment is conducted in high vacuum, which minimizes residual noise from acoustic coupling and momentum transfer of gas molecules.
“We move the gold sphere back and forth, creating a gravitational field that changes over time,” Dr. Pfaff said.
“This causes the torsion pendulum to oscillate at that particular excitation frequency.”
“The largest non-gravitational effect in our experiment comes from seismic vibrations generated by pedestrians and tram traffic around our lab in Vienna,” added Dr. Hans Hepach, a researcher at the Institute for Quantum Optics and Quantum Information (IQOQI) Vienna at the Austrian Academy of Sciences.
“We therefore obtained the best measurement data at night and during the Christmas holidays, when there was little traffic.”
The team’s work, published in the journal Nature, opens the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions and provide a path towards exploring the quantum nature of gravity.
T. Westphal et al. 2021. Measurement of gravitational coupling between millimetre-sized masses. Nature 591, 225-228; doi: 10.1038/s41586-021-03250-7
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