Is gravity left-handed? An answer could provide a clue to a long-sought theory of quantum gravity - and might be within our grasp 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. But first physicists need to understand gravitons, hypothetical quantum particles that mediate the gravitational force. These likely come in left and right-handed varieties: in the former, the particle's spin would be aligned with the direction of its motion; in the latter, the spin would be the opposite.
General relativity does not distinguish between right and left, so you 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, for example, 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, João Magueijo and Dionigi Benincasa of Imperial College London suggest looking to the cosmic microwave background, relic radiation from the big bang. 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 pair calculate 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, write Magueijo and Benincasa in an analysis to appear in Physical Review Letters. The European Space Agency's Planck telescope will image the 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, already 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", says Lee Smolin of the Perimeter Institute in Waterloo, Ontario, Canada, who has worked with Magueijo and Benincasa on the subject. "It would confirm inflation, that gravity is quantum mechanical and that there is left-right asymmetry in quantum gravity."