Quantum gravity in the lab

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Synopsis

It has long been assumed that gravity and quantum mechanics can only be confronted at very high energies ~ 10^28 eV (enough to boil 5 tons of water, and 15 orders of magnitude above the range of particle accelerators). However, recent theory indicates that gravity may cause a breakdown of quantum mechanics at much lower energies, for large masses. This has led to a new experimental field in which such a breakdown is sought in earth-based labs.  I will review the theory which has made such predictions, and their history, which begins with speculations by Feynman in 1957. I will focus on the Correlated Worldline (CWL) theory, which is the only one so far to make definite quantitative predictions. In this theory, the quantum-mechanical Feynman paths are coupled via gravity, causing a breakdown of the superposition principle.  I will then discuss some of the optomechanical experiments designed to test the theory, and where they are right now. These experiments are pushing the boundaries of our understanding of quantum mechanics at the macroscopic scale; and they have implications for everything from quantum computation to future gravitational wave detectors.