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Researchers at the University of Texas at Austin are reporting that they’ve built the first ever one-way acoustic device — a device that lets you hear something, without being heard in return. This breaks a fundamental rule in physics, called time reversal symmetry, that stipulates that if waves can travel in one direction, they must be able to travel in the opposite direction. In short, if you can hear someone, they can always hear you — except, with this new device, that’s no longer the case.