New J. Phys. (in the press); preprint at http://arxiv.org/pdf/1503.01213 (2015)

One of the first scientific tests of gravity was Galileo Galilei's experiment, in which he allegedly dropped balls of different masses from the Leaning Tower of Pisa. The idea that inertial and gravitational masses are identical was more rigorously formulated three hundred years later by Albert Einstein, as the equivalence principle. The universality of free fall — that in a gravitational field, in the absence of friction, two bodies of different masses will drop at the same rate — would be taken for granted by most people, but even tiny violations can provide clues for quantum gravity models.

A modern take on Galileo's experiment uses matter-wave interferometry instead of balls and feathers. Jonas Hartwig and colleagues have proposed an experiment involving the preparation of an ultracold rubidium and ytterbium mixture in a magneto-optical trap, which is then released into a ten-metre free fall. They analysed the experimental conditions and found that in this atomic fountain, a new level of accuracy for ground tests can be reached — providing tighter bounds on the violation scenarios.