Reversing time to talk underwater

Communications on land have come on in leaps and bounds over the last 20 or 30 years; for relatively little money you can buy a phone which will transmit hundreds of thousands of characters every second, or you can buy a satellite phone which means you can talk to your mum from the middle of the Gobi desert.

Conversing underwater though, isn't so easy. The problem is that, because seawater conducts electricity, it absorbs radio waves, which means that once you get more than a few metres below the surface there is no point trying to tune into your favorite radio station.

Sound waves, on the other hand, will travel through water very well, for example whales use them to communicate over thousands of kilometres. The problem is the echos, sound waves will bounce off the surface of the water, the seabed, shoals of fish, even layers of warm and cold water.

This means that if I shouted to you you would hear a whole series of overlapping versions of my conversation and it would sound like gobbledy gook. This means that you have to communicate very slowly to wait for the echos to die down.

However William Kuperman and colleagues at the Scripps Institution of Oceanography have used a very neat method to get around this problem. If I wanted to send a message to you, first he would get you to play me 2 noises one to represent a 1 and another to represent a zero. I would then record how that sounds to me. Then I would reverse the tape and play them back to you, this means that the echos will cancel out the time reversal and you will hear a loud clear version of what you sent me with much less overlapping. This means that I could play the two sounds in a pattern to represent a message. This is what William Kuperman has done, and he can now transmit 15kbit/s over 5km and 5kb/s over 15km which is 3-4 times faster than normal links.

This is not just important for talking to submarines, but the bottom of the ocean is less well known that mars, and it will probably be explored by robotic rovers which will of course need to communicate with the surface.