Researchers at the University of Texasat 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.
The new device is called a one-way acoustic circulator. If you look at the image above, the colored arrows show the path of the acoustic signals. So, if you imagine three people sitting around the device, the person at the top can only be heard by the person on the right, and the person on the right can only be heard by the person on the left, and the person on the left can only be heard by the person at the top. A bit like a game of telephone (Chinese whispers for our non-US readers). If time reversal symmetry was still in play, the sound waves would be able to travel bidirectionally, and everyone would be able to hear each other.
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