Quantum rumbles may change the way we look at the universe. A device for detecting sound-like vibrations in ultra-cold gas might confirm the last major untested prediction of Einstein's general theory of relativity.
According to Einstein, violent events in the universe, such as two black holes merging, should cause the very fabric of space-time to ripple, analogous to water when a stone is dropped in a pond. Astronomers are anxious to find these gravitational waves because they would offer a new way to study the cosmos. But the waves are extremely weak by the time they reach Earth, and so far no one has directly detected them.
Current efforts involve huge detectors like the ones at the Laser Interferometer Gravitational Wave Observatory. There are two LIGO sites in the US, where lasers bounce between mirrors inside twin perpendicular tunnels, or arms, each 4 kilometres long. A passing gravitational wave is expected to slightly change the length of one arm relative to the other and create a detectable signal in the laser light.
Initial LIGO observations between 2002 and 2010 came up empty, and the experiment is now being upgraded to improve sensitivity and eliminate noise. LIGO scientists are even searching for a third location to house another pair of giant tunnels. Having three would allow them to pinpoint the sources of gravitational waves. Other existing detectors and planned projects for hunting gravitational waves on Earth also need tunnels spread over kilometres.
Instead, we might be able to achieve the same thing using a tabletop device that holds a cloud of ultra-cold atoms, says a team led by Ivette Fuentes at the University of Nottingham, UK.
Her team has shown that, in theory, a gravitational wave would create noticeable vibrations in a Bose-Einstein condensate, a collection of atoms cooled down to almost absolute zero. At this temperature, the atoms behave as one quantum object, and fluctuations can generate "particles" of vibrational energy called phonons.
Bose-Einstein condensates are held in place with traps made of lasers. Previous experiments showed that changing certain properties of the laser trap, such as its size, can create extra phonons in the cloud. Sabin and his colleagues think that gravitational waves should have the same influence (arxiv.org/abs/1402.7009).
"In this case it is space-time that is changing, but that generates a similar effect, which is the creation of phonons," says team member Carlos Sabin, who is also at the University of Nottingham. The team calculates that a detector using a Bose-Einstein condensate would be four orders of magnitude more sensitive to gravitational waves than LIGO.To read more, click here.