A Drop of Quantum Matter

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Science  18 Jun 2010:
Vol. 328, Issue 5985, pp. 1491-1492
DOI: 10.1126/science.1191666

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Physics in the 20th century witnessed two major revolutions, relativity and quantum mechanics. General relativity relies on the equivalence principle. When an object in a gravitational field undergoes free fall, it is indistinguishable from the same object in an inertial reference frame—it acts as if it were weightless in outer space. A popular account of a free-fall environment was given by the thought experiment of “Einstein's elevator” (1) (see the figure, panel A). General relativity is mainly formulated in terms of classical objects. On page 1540 of this issue, van Zoest et al. (2) describe an intriguing experiment that brings together fundamentals of general relativity and quantum mechanics. They follow the evolution of a prototypical quantum object, a Bose-Einstein condensate (BEC), under free-fall conditions (see the figure, panel B). The use of BECs in atom interferometers should allow for more sophisticated tests of general relativity.