Is gravity left-handed?

London: Is gravity left-handed? For years, physicists have been looking for a definitive answer that may provide a clue to a long-sought theory of quantum gravity. Now British scientists say it could be known by 2013.

General relativity describes gravity's actions at large scales. For tiny scales, however, a theory of quantum gravity, incorporating quantum mechanics, is needed.

General relativity doesn't distinguish between right and left, so one might expect gravity to be transmitted by both varieties. But the quantum world may play favourites.

When it comes to the ghostly particles known as neutrinos, the weak force only interacts with the left-handed variety.

To find out whether gravitons fall into the "ambidextrous" camp of general relativity or exhibit quantum asymmetry much like a neutrino, a team at Imperial College London suggests looking to cosmic microwave background, relic radiation from the big bang, the 'New Scientist' reported.

During inflation, the faster-than-light expansion of the nascent universe, powerful gravitational waves may have rippled through space-time, polarising the CMB's photons in a telltale pattern.

The team calculates that if gravity depended on just left or right-handed gravitons, that would have skewed the polarisation pattern in an obvious way.

What's more, inflation would have stretched these effects to astronomical proportions, making them easily visible to astronomers, according to Jojo Magueijo and Dionigi Benincasa, who led the research team.

The European Space Agency's Planck telescope will image CMB's polarisation and will release the data in 2013.

A theory called loop quantum gravity, an attempt to unite quantum mechanics and general relativity, suggests that an asymmetry might be embedded deep into the laws of the universe and that this should render gravity left-handed.

Evidence of left-handed gravitons in the CMB would be "a triple discovery", according to Lee Smolin of the Perimeter Institute in Waterloo. "It would confirm inflation, that gravity is quantum mechanical and that there is left-right asymmetry in quantum gravity," he said.

The findings are to appear in an upcoming edition of the 'Physical Review Letters' journal.