SDRAWKCAB sevaworcim! Physicists have recorded microwaves in enough detail to replay them backwards for the first time, a feat that has never been achieved with electromagnetic waves before. The trick is based on a technique pioneered for acoustic waves more than a decade ago.
The idea of time-reversed signals is simple. A detector analyses the signal and rebroadcasts it in reverse order 鈥� so the last signal to arrive is the first to return. The result is that time-reversed waves converge on the original source. 鈥淚t鈥檚 like playing a movie backwards 鈥� if a fountain is shooting out water, the water comes back,鈥� says Arnold Kim, a mathematician who studies potential applications of the technique at Stanford University in California.
But the difficult part is measuring the incoming waves in enough detail to recreate them in reverse. Since each wave needs to be measured in about five places, you need a sampling rate of 10 gigahertz to analyse a 2.4-gigahertz microwave signal, something that is well beyond current abilities.
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Now physicists led by the technique鈥檚 founder, Mathias Fink of the University of Paris VII, France, have found a way round this problem. They broadcast a 1-microsecond microwave pulse at 2.4 gigahertz in a copper-walled room. A second antenna detected the wave after it had been smeared into an 8-microsecond pulse by being bounced round the room some 6000 times. This antenna then 鈥渢ime-reversed鈥� the pulse, so a near-replica of the original wave arrived back at the first antenna (DOI: 10.1103/PhysRevLett.92.193904).
The trick Fink and his team have perfected to perform this feat is to strip away the 2.4-gigahertz carrier signal and measure only the low frequencies making up the pulse鈥檚 envelope. So the sampling could operate at the slower rate of 40 megahertz 鈥� which itself has only become possible in the last few years.
The technique could have applications both in medicine and communications. Time-reversed microwaves with a frequency of 10 gigahertz could be focused to a spot about 1 centimetre wide, which could be used to burn away tumours without an incision, Fink says. The same technique could also help mobile phone base stations cope with scatter from buildings in built-up areas. 鈥淚t is a very simple way to create an information channel that refocuses only on the phone,鈥� he says.