OUR galaxy may be teeming with intelligent civilisations, but none of them wants to talk to us. That鈥檚 the only obvious way to understand years of radio silence, say ET researchers.
Today鈥檚 searches for extraterrestrials are based on the idea that advanced civilisations will communicate with a radio beacon somewhere close to the frequency naturally emitted by hydrogen atoms. Although we aren鈥檛 yet trying to attract the attention of alien intelligences in this way, the rationale is that this would be the best way for an advanced civilisation to get noticed because observers are more likely to be looking there than at some randomly chosen frequency. From 1988 until 1993, the Megachannel ET Assay (META) based at Harvard University scanned the sky and investigated more than 10 trillion signals of this type.
META researchers have ruled out almost all the signals鈥攅ither as random noise, or because they varied as the Earth rotated, suggesting that they originated somewhere on Earth. That left 11 calls that couldn鈥檛 be explained, each lasting at least 20 seconds.
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The researchers couldn鈥檛 explain why, if the calls were from ET, they had apparently faded out. But in 1997 Jim Cordes at Cornell University in Ithaca, New York, and colleagues suggested that twinkling caused by interstellar plasma passing between natural radio sources and Earth鈥攁 process called interstellar scintillation鈥攃ould temporarily make a steady signal strong enough for us to see.
Independent astronomer Joseph Lazio, along with Jill Tarter and Peter Backus of the SETI Institute in Mountain View, California, have now tested this idea by taking a closer look at the areas of the sky where the candidate ET signals had appeared to originate. They saw nothing, they will report later this year in the Astronomical Journal.
The null result has allowed researchers to reduce their limits of the maximum number of steady and deliberate beacons there could possibly be in our Galaxy. They calculate that at most there is just one civilisation in the Galaxy capable of producing at least as much power as we can, and deliberately trying to talk to us. 鈥淵ou can鈥檛 say categorically that there is nothing,鈥� says Tarter, because interstellar scintillation is a random process.
Despite this blow, she says we can鈥檛 rule out the presence of aliens which haven鈥檛 spotted us, and are trying to communicate merely with whoever might be out there. In this case, they鈥檇 have to share their available power over the entire sky, so the beacon in our direction would be probably too weak for us to see. The researchers鈥� negative result still allows for as many as 10,000 civilisations in the Galaxy to be broadcasting this way.