A tiny gold ball is the smallest object to have its gravity measured

Scientists measured the gravitational field of an object with a mass of just 90 milligrams

gold sphere experiment set up

A gold sphere on a horizontal glass beam responds to the gravitational pull of a neighboring sphere (right). The measurement confirms that gravity behaves as expected even for very small objects.

Tobias Westphal/University of Vienna

Even teeny objects obey the law of gravity.

A gold ball just 2 millimeters wide, with a mass of about 90 milligrams, is now the smallest object to have its gravitational pull measured. Observations of that gold sphere tugging on another similarly sized sphere confirm that gravity behaves as expected even for extremely weak gravitational fields, physicists report in the March 11 Nature. Previous experiments have involved objects with masses of hundreds of milligrams or more.

Newton’s law of universal gravitation states that the gravitational force between two masses is inversely proportional to the square of the distance between them. Double the distance between two objects and they’ll pull on one another with one-quarter the strength.

A team of physicists based in Vienna tested whether that relationship holds up for tiny masses. The researchers attached a gold sphere to a horizontally suspended beam that was free to rotate in response to the gravitational pull of another gold sphere just a few millimeters away. The experimenters measured the motions of the first sphere while repeatedly moving the second sphere closer and farther away. The results matched predictions of Newton’s law and of Einstein’s general theory of relativity, two theories of gravity that are equivalent under most everyday conditions (SN: 10/4/15).

Ultimately, researchers want to test how gravity behaves on far smaller scales (SN: 10/28/20) — for objects so minuscule that they can perform quantum feats, such as existing in multiple places at once. The gold spheres are too large to follow quantum rules, but the experiment takes scientists a small step closer to exploring gravity’s quantum side.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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