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.
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