Quantum skyfall puts Einstein's gravity to the test

DIVIDING a falling cloud of frozen atoms sounds like an exotic weather experiment. In fact, it's the latest way to probe whether tiny objects obey Einstein's theory of general relativity, our leading explanation for gravity.

General relativity is based on the equivalence principle, which says that in free fall, all objects fall at the same rate, whatever their mass, provided the only force at work is gravity. That has been proven for large objects: legend has it that Galileo did it first by dropping various balls from the Tower of Pisa. Whether equivalence holds at quantum scales, where gravity's effects are not well understood, isn't clear. Figuring it out could help create a quantum theory of gravity, one of the biggest goals of modern physics.

Creating a quantum equivalent of Galileo's test isn't easy. In 2010 a team led by Ernst Rasel of the University of Hannover in Germany monitored a quantum object in free fallMovie Camera, by tossing a Bose-Einstein condensate (BEC) – a cloud of chilled atoms that behaves as a single quantum object and so is both particle and wave – down a 110-metre tall tower. Now they have split and recombined the wave – all before the BEC, made of rubidium atoms, reached the bottom. This produces an interference pattern that records the path of the falling atoms and can be used to calculate their acceleration (Physical Review Letters, doi.org/km6). The next step is to do the same experiment on a different kind of atom, with a different mass, to see if the equivalence principle holds.

The BEC can only be split for 100 milliseconds in the tower before hitting the bottom, so to allow tiny differences between the atom types to emerge, the work must be repeated in space, where the waves can be split for longer. By showing that a matter-wave can be split and recombined while falling, Rasel's result is a "major step" towards the space version, says Charles Wang of the University of Aberdeen, UK.

This article appeared in print under the headline "Quantum skyfall tests Einstein's gravity"

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