— Despite its 300-year history, Newton's gravitational constant, *G*, is the least well measured of all the fundamental constants. Now quantum mechanics may help pin down the strength of gravity more precisely.

Traditionally, various kinds of torsion balance have been used to calculate *G* by measuring the twist in a wire induced by the gravitational force between two masses. But this can only provide rough estimates.

Mark Kasevich of Stanford University in California and colleagues split a beam of caesium atoms into two using an interferometer, then recombined them to produce interference fringes. A 540-kilogram lead weight placed near the beams shifted their paths and the interference pattern. The team used the shift to calculate a value for *G*, which matched that found by traditional methods (*Science*, vol 315, p 74).

This technique could one day yield the most accurate value of *G*. "We're seeing the gravity field in a purely quantum mechanical way, so we're free of the errors that limit the accuracy of traditional methods," says Kasevich.