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THE silence has been deafening. After almost 50 years of combing the skies, the search for extraterrestrial intelligence (SETI) hasn鈥檛 heard a peep from any alien neighbours.

Now SETI researchers and physicists are debating whether the programme needs a radical rethink. This week, they met at a conference called 鈥淪ound of Silence鈥� at Arizona State University (ASU) in Tempe to work out what they could do better. 鈥淗ave we been looking in the wrong place, at the wrong time, in the wrong way?鈥� asked conference organiser Paul Davies of ASU.

The roots of the programme go back to 1959, when Philip Morrison and Giuseppe Cocconi at Cornell University in Ithaca, New York, realised that we ought to be able to pick up radio signals transmitted across interstellar space. SETI researchers have been scanning radio-telescope data ever since, looking for potential alien messages (see 鈥淎 brief history of alien-spotting鈥�).

Many argue that waiting for a signal is not enough, however. SETI should be broadcasting too, they say, despite the potential risks (see 鈥淗ey, over here!鈥�). 鈥淪ETI鈥檚 big mistake is that it鈥檚 relying on ET to do all the heavy lifting,鈥� says Richard Gott, an astrophysicist from Princeton University. What if aliens have the same attitude as us, he asks. 鈥淲e鈥檒l all just be sitting round listening, but nobody鈥檚 doing any talking.鈥�

Gott also has clear ideas about the best frequency to transmit on. Any advanced civilisation, he argues, will probably be aware of the cosmic microwave background (CMB), the relic radiation left behind by the big bang. So they will already be 鈥渢uned鈥� to the radio channel set by the frequency of photons in this radiation band.

鈥淚f aliens have the same attitude as us, we鈥檒l all just be sitting round listening, but nobody doing any talking鈥�

A reply to our message would take decades to reach us, but that is no reason not to make a cautious try, says Douglas Vakoch of the SETI Institute, based in Mountain View, California. 鈥淚t鈥檚 a case of inter-generational ethics. If we were living in the year 2058, what would we have wanted our predecessors to have done?鈥�

The SETI endeavour might also have a more fundamental flaw than its failure to broadcast. Researchers think the signature of an engineered message would be a pulsed signal, transmitted within a narrow bandwidth, which changes its period of pulsation in a regular way. But this assumes that the aliens communicate in exactly the way that humans would, argues Davies: 鈥淲e鈥檙e making a lot of assumptions about aliens based on human 20th-century western society.鈥�

Charley Lineweaver of the Australian National University in Canberra thinks that we are being vain when we assume that any advanced alien civilisation will have evolved into a species similar to our own. We only look on our type of intelligence as the height of evolution because it鈥檚 humanity鈥檚 defining feature, he says. 鈥淚f we were elephants, we would think that animals with the longest nose are the goal of evolution.鈥�

Many want SETI鈥檚 search to be broadened. In practice, this would mean removing everything that can be explained by background noise or known astrophysical sources, such as pulsars, and sifting through everything else for something unrecognised. 鈥淓ither you鈥檝e found a new astrophysical phenomenon, which would be staggering, or you鈥檝e got a candidate for extraterrestrial intelligence,鈥� says Paul Shuch of the SETI Institute.

But a search this broad is easier said than done. 鈥淚f you want to hunt for buried treasure in Australia, sure, the best strategy is to take a shovel and dig up the whole country, but in practice you just can鈥檛 do that,鈥� says Seth Shostack, also of the SETI Institute.

鈥淚鈥檓 not going to waste time trying to think of strategies to detect something that by definition I can鈥檛 conceive of,鈥� says Jill Tarter, another SETI Institute researcher. She reckons it鈥檚 far too early to abandon conventional detection methods.

For most of the time since its inception in 1984, the SETI Institute has had to scrounge time on telescopes. This has allowed it to examine just a few thousand star systems in detail. 鈥淲e haven鈥檛 begun to scratch the surface,鈥� says Tarter. But last year, the first of the 42 dishes that will make up the Allen Telescope Array began operation. This will be dedicated entirely to SETI. What鈥檚 more, the proposed Square Kilometre Array telescope will be sensitive enough to pick up signals leaked into space 鈥� such as alien TV and radio 鈥� and not just directed messages.

Improvements in computing power should also help SETI to speed up. 鈥淚n the next two years we will have collected and analysed more new data than we have over the past 50 years combined,鈥� says Shostack. He estimates that the SETI effort will have surveyed more than a million star systems by 2028. If there鈥檚 anything to be heard, he says, we should hear it by then. 鈥淚f it remains silent after that, then that will be the time to rethink.鈥�

Astrobiology 鈥� Learn more in our out-of-this-world .