DIVIDING a falling cloud of frozen atoms sounds like an exotic weather experiment. In fact, it鈥檚 the latest way to probe whether tiny objects obey Einstein鈥檚 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鈥檚 effects are not well understood, isn鈥檛 clear. Figuring it out could help create a quantum theory of gravity, one of the biggest goals of modern physics.
聯Creating a quantum theory of gravity is one of the biggest goals of modern physics聰
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Creating a quantum equivalent of Galileo鈥檚 test isn鈥檛 easy. In 2010 a team led by of the University of Hannover in Germany monitored a quantum object in free fall, by tossing a Bose-Einstein condensate (BEC) 鈥� a cloud of chilled atoms that behaves as a single quantum object and so is both particle and wave 鈥� down a 110-metre tall tower. Now they have split and recombined the wave 鈥� 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 (). 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 , where the waves can be split for longer. By showing that a matter-wave can be split and recombined while falling, Rasel鈥檚 result is a 鈥渕ajor step鈥� towards the space version, says of the University of Aberdeen, UK.
This article appeared in print under the headline 鈥淨uantum skyfall tests Einstein鈥檚 gravity鈥�
