Scientists have created a one-way sound machine.
The device, called an acoustic circulator, runs counter to the principle that sound waves are a two-way street.
The findings, published in the journal Science, could lead to the sound equivalent of a one-way mirror. With such a device, people can hear someone talking, but they themselves cannot be heard. [The 9 Biggest Unsolved Mysteries in Physics]
All waves have a physical property known as time reversal symmetry. What that means is that a wave sent one way can be sent back. "If I am able to talk to you, you should be able to talk to me back," said study co-author Andrea Alù of the University of Texas at Austin. For radio waves, researchers figured out how to break this rule using magnetic materials that set electrons spinning in one direction. But they hadn't figured out how to complete the trick for other types of waves.
To accomplish the feat with sound waves, Alù and his colleagues created a cavity loaded with tiny fans that spin the air with a specific velocity. As sound waves go through the cavity, they are routed through one of three pipes, each of which has a microphone at the end. The air spins in one direction, so the flow of air "feels" different to the wave in one direction versus the other, preventing backward transmission.
The technique could be used to create the sound equivalent of one-way glass, perhaps for spying devices. But it could also be used to create one-way light waves.