Call it Einstein meets Einstein. A new experiment using a form of matter Albert Einstein predicted to exist might someday pave the way for fine-scale tests of general relativity, the famed physicist's phenomenally successful theory of gravitation.
In a paper in the June 18 issue of Science, a group of European researchers reports creating a type of matter called a Bose–Einstein condensate, only to drop it down a 120-meter shaft to watch its evolution in free fall. In a Bose–Einstein condensate (BEC), a cloud of ultracold atoms all occupy the same quantum state, acting like one supersize quantum particle. (The condensates are co-named for Indian physicist Satyendra Nath Bose, who along with Einstein theorized their existence in the 1920s, although such atomic gas BECs were not observed until 1995 with the Nobel Prize–winning experiments of Eric Cornell, Wolfgang Ketterle and Carl Wieman.)
Having cooled a gas of rubidium atoms to nine billionths of a degree above absolute zero to create a Bose–Einstein condensate, the researchers let the experimental capsule drop down the shaft inside a drop tower built for microgravity research at the University of Bremen in Germany. The idea was to get the quantum object in free fall to watch it expand freely during a second or so of weightlessness. (The first part of free fall is used to control residual vibrations in the experimental capsule; at the bottom of the drop an eight-meter-deep pile of polystyrene balls breaks the capsule's fall.) Such timescales of BEC evolution, the researchers report, are difficult to achieve in conventional laboratory setups in which gravity tugs the atoms preferentially in one direction.
The experiment could lay the groundwork for BEC experiments in the weightless environment of space, where the quantum wave nature of the condensates might be used to create ultrasensitive matter interferometers, in much the same way that atoms are already used in such devices to probe minute physical effects. In the optical realm, interferometry usually relies on lasers; BECs are often compared to lasers in the way that they each represent a coherent collection of quantum objects—photons for lasers, atoms for BECs. Interferometers based on BECs could one day reach orbit to probe the intricacies of spacetime curvature predicted by general relativity and perhaps shed some light on the interface between quantum mechanics and gravity.
Photo of Bremen drop tower: ZARM/FAB - Drop Tower Operation/Service Company mbH/University of Bremen