But two physicists have come up with an intriguing solution. They suggest a way in which aliens could send messages to each other across space that not only disguises their locations but also makes it impossible for a casual observer to even distinguish the messages from background noise. Messages sent by this method could be criss-crossing our Galaxy without us ever knowing.
At first glance, sending a message without giving away your location appears impossible. If a signal - a stream of photons - comes from a single source, its origin can always be determined by measuring the direction of recoil of a detector struck by the photons. But Walter Simmons and his colleague Sandip Pakvasa from the University of Hawaii at Manoa have come up with a cunning way around this.
The signaller splits the message into two parts, so that the photons are sent in opposite directions to mirrors located far from the home planet. The mirrors redirect the signals to the intended receiver, who recombines the photons to reconstruct the message (see Graphic).
The key idea is that the message is encoded not by the pattern or sequence of photons sent over time, but by their positions in space. For example, this can be done by shining the light beam through a stencil.
If the image is tiny enough, Heisenberg's uncertainty principle - which limits the amount of information that can ever be known about a microscopic particle - means that measuring the position of the photons makes it impossible to gain accurate information about the direction in which they are travelling. So if you detected the message, it would be impossible to determine the origin of the two beams. "The mere act of reading the message introduces enough uncertainty to make it useless for direction-finding," explains Simmons.
Neither the intended receiver nor any eavesdropper would be able to locate the home planet of the sender. What's more, it would be impossible to detect the message at all without extremely sophisticated technology. In order to recombine the beams and recreate the message you would need to detect the arrival time of the photons extremely accurately to identify pairs of photons split by the sender. "Such photons are distinguishable from the background of stellar photons because they arrive very close together in time," says Simmons. "But any eavesdropper, like us, might not realise this and see only the background."
"The proposal is ingenious," says Jonathan Rosner, a physicist at the Enrico Fermi Institute in Chicago, although he says it is hard to tell if the method could work in practice. Paul Shuch, director of the SETI League in New Jersey, also points out that being able to disguise a sender's location would be extremely useful for secure military communications here on Earth. "In a few decades, when it's declassified, we may well find that such a technique is already in use."
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New Scientist issue: 10 May 2003
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