Scientists Create Artifical Blowhard

The device, called an acoustic circulator, runs counter to the principle that sound waves, and other types of waves, are a two-way street.

The findings, published in Friday's issue of 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.

There are some obviously stealthy ways to use the method, such as in spying devices. 

"I can listen to you, but you cannot detect me back, you cannot hear my presence," Alù told LiveScience.

...the principle could also potentially be used to create one-way light waves. (Ghosts?!?!) (more)