Intergalactic messages 'in a bottle' are best

Rather than transmitting radio messages, extraterrestrial civilisations would find it far more efficient to send us a "message in a bottle" - some kind of physical message inscribed on matter. And it could be waiting for us in our own backyard.

That is the conclusion of a new analysis of interstellar communications by Christopher Rose of Rutgers University in New Jersey and Gregory Wright, a physicist with Antiope Associates also in New Jersey.

Assuming the aliens do not care how long it takes for their message to arrive, beaming a radio signal that can be detected 10,000 light years away, for instance, would take a million billion times as much energy as just shooting out matter in which data is embedded. "If energy is what you care about, it's tremendously more efficient to toss a rock," Rose says.

Radiation loses out to rocks over long distances because it spreads out as it travels through space, diluting the signal below detection levels unless the beam is extremely powerful to begin with.

Bottle throwing aliens

If aliens are using a transmitter the size of the dish that astronomers on Earth use to look for their signal - the 305-metre radio telescope in Arecibo, Puerto Rico, which is the largest single-dish telescope on Earth - they would have to be closer than Saturn for the transmission to be more energy efficient than just flinging a bottle at us.

If the Voyager space probes, which are now at the edge of the solar system, were each carrying three DVDs' worth of data, they would be a more energy-efficient way of sending information to someone 2000 light years away than an Arecibo-to-Arecibo radio communication.

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Also, once radio signals pass they are gone for ever. So aliens would have to beam signals continuously, or other civilisations might blink and miss them. Physical objects stay where they land.

Not everyone is convinced by Rose and Wright's analysis. "Their conclusion is that we should be looking for the Encyclopaedia Galactica within the Solar System," says Fred Walker, an astronomer at Stony Brook University in New York who is interested in the search for extraterrestrial intelligence. "I'm not sure I would expend any effort on this."

However, Don Yeoman at NASA's Jet Propulsion Laboratory in Pasadena, California, who co-ordinates a project looking for asteroids on a collision course with Earth, accepts that their project could conceivably spot such alien objects. "It's not completely out of the question."

Journal reference: Nature (vol 431, p 47)

Jenny Hogan