Testing times
Testing times

The plans are being drawn up by Tim Sumner of Imperial College London, who intends to submit them to the European Space Agency (ESA) later this year. If approved, the mission would cost about Euro 250m and could take off by 2015. Dubbed the GrAnd Unification and Gravity Explorer (GAUGE), the mission also involves researchers from elsewhere in the UK and Europe. It builds on earlier proposals called HYPER and STEP, which would also have carried out tests of fundamental physics but were not selected by ESA for launch.

Now, however, Charles Wang – a theorist from Aberdeen University – has given added scientific motivation to the GAUGE mission. He has shown that an atom interferometer could detect distortions in space–time caused by “gravitons” – particles that are believed to mediate gravity at the “Planck scale” at which all the laws of physics are unified (Class. Quant. Grav. 23 L59). Studying physics at the Planck scale, which occurs at lengths of about 10–35 m and times of about 10–43 s, would be impossible with current particle accelerators.

According to Wang and his collaborators from the CCLRC's Centre for Fundamental Physics near Oxford, gravitons constantly stretch and squash the geometry of space–time, a bit like the way in which pollen or smoke particles in air have a random Brownian motion as they are buffeted by much smaller molecules. By observing these tiny distortions in an atom interferometer, Wang and his collaborators think it will be possible to extract information on the gravitons and understand their underlying physics.

The experiment aboard GAUGE will involve sending beams of ultracold atoms down two identical arms of an interferometer. Fluctuations in space–time caused by the gravitons will randomly modulate the time it takes for the beams to travel down the arms. This will then create a slight fuzziness in the fringe patterns that are created when the beams interfere. Such decoherence experiments are much harder on Earth, where the effects of gravity are tricky to eliminate.

“Our work suggests signatures of quantum gravity are very close to being observed in the laboratory,” says Wang. “This makes a space mission very worthwhile.”

Giovanni Amelino-Camelio, a quantum-gravity phenomenologist from the University of Rome "La Sapienza", thinks that Wang's paper is an important step in transforming an idea first raised by Ian Percival of Queen Mary, University of London, into something that can be experimentally tested. "This work should provide additional motivation for matter-interferometric studies in space," says Amelino-Camelio. "It is not obvious that the effect should go excatly as described in this paper, but the fact that final some valuable new tools of analysis are being introduced for this type of matter-interferometric studies is encouraging."