An optical probe of quantum gravity?

An optical probe of quantum gravity? In conventional quantum mechanics you can measure position with arbitrary precision, provided you are willing to pay the cost of growing uncertainty in momentum. In some theories of quantum gravity, though, spacetime has a certain coarseness—a fundamental limit to how precisely you can measure position—enforced with a modified uncertainty relation. But if the uncertainty relation is modified, so too must be the position–momentum commutator that gives rise to it; the equation shown here gives a typical alteration, characterized by a deformation parameter β. For a system of mass m, the modifications to the conventional relation are suppressed by powers of m/M_P, where the Planck mass, M_P =22 µg, is the natural mass scale for quantum gravity. Despite that suppression, reports an international collaboration of physicists from the University of Vienna and Imperial College London, quantum-gravity effects on commutators could be experimentally accessible. The trick is to couple a macroscopic oscillator to the radiation field in a highly reflective cavity. The Vienna–London team engineer the matter–light coupling so that, during the course of a mechanical oscillation, the cavity Hamiltonian alternately couples to the oscillator’s position and momentum. As a result, the position–momentum commutator becomes imprinted onto the phase of the radiation field. An oscillator mass of 0.01 µg is heavy enough for subsequent interferometric measurements to set bounds on the value of β or analogous deformation parameters that arise in other models. (I. Pikovski et al., Nat. Phys., in press, doi:10.1038/nphys2262.)