Gravity over macroscopic distances is well understood. The simple inverse-square law, proposed centuries ago by Isaac Newton, continues to accurately describe the force at scales across the nonrelativistic regime, from laboratory-scale torsion balance experiments to the motions of stars and galaxies. It’s been especially well tested at the scale of the distance from Earth to the Moon.

Short distances—microns or less—are another matter. In microscopic experiments, electromagnetic forces are so overwhelmingly dominant that the force of gravity at small scales has never been directly measured. All we have are upper bounds on its strength, some of which are astonishingly large. According to the best experimental constraint so far, the gravitational attraction between two objects 1 nm apart is no more than 1021 times what Newton’s law says it is.1 

It’s not so outlandish to imagine that the force of gravity could follow the inverse-square law over large distances...

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