PLAYING sound waves backwards could help the US Navy set up secure communication links with submarines or unmanned submersibles. The system works by broadcasting messages in such a way that they can only be received at one point in the water-so no one else can intercept them.
Underwater communications are very primitive, says Geoff Edelmann at the Scripps Institution of Oceanography in San Diego. Most use simple underwater transducers called hydrophones to send and receive messages. But because sound waves travel in all directions in the ocean, reflecting off the seabed and the surface and also interfering with each other, acoustic signals are often too garbled to understand.
"They tend to be very unstable, very short-range and only good in deep water," says Edelmann. To communicate for any length of time, or over large distances, submarines either surface or use a floating antenna. But that's not always possible in covert ...
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