Shostak's optimism is based partly on the availability of the array, and partly on the fact that new planets are being discovered outside our solar system on about a monthly basis. They are not all capable of sustaining life as we know it, but some of them may be. That has lead many scientists to speculate that life, and possibly intelligent life, abounds throughout the universe.
But that contributes to one of the debates that have plagued SETI over the years. If there are lots of other planets out there like ours, with living creatures that are at least smart enough to build radio transmitters, why haven't we found them? SETI may not have had all the tools it needed, and funding has been scarce, but lots of very smart people have been looking very hard for ET for several decades.
Some astronomers dismiss those questions by noting that there's just zillions of places to look, and they are a long ways away, and no one is certain what to look for. So in the past, searchers have counted on finding radio signals that are beamed our way by someone on another planet who is looking for us.
It would probably have to be a directed beam because "leakage," like from television broadcasts, would be too weak. Besides, why would someone who is so much smarter than us broadcast over the air when fiber optics are more reliable and efficient? Our own electromagnetic signature is shrinking, Shostak noted in his talk, so we're not likely to stumble across ET's version of "ABC News with Charles Gibson."
But new tools, including the Allen Telescope Array, could open new ways to search for ET. Unlike other radio telescopes, the array can image a huge piece of the sky at once, or concentrate on a single star with an interesting planetary system. All objects in space emit radio waves, which can reveal much about their composition.
One way to narrow the search is to look for some of the things produced on Earth that would not be here if there had been no life. Methane, Shostak noted, would disappear from a planet's atmosphere in a relatively short time, so if it's there, something must be producing it.
"Much of the methane in this room is produced by what is politely called 'bovine flatulence,' and also by porcine flatulence, so this technique would at least allow you to find pigs in space," he added. But, of course, they would not necessarily be smart pigs.
It is widely believed that water would have to be present on another planet for it to support life, and scientists at the University of Washington have devised a technique that they say would determine if another planet has oceans. Using instruments aboard a NASA spacecraft, the scientists studied light intensities from Earth in seven bands of visible light from near ultraviolet to near infrared.
They found two dominant colors, blue and red, and surmised that the red came from land masses, and the blue from oceans. If they could detect the same colors from a distant planet, they would have evidence that the planet had huge areas of water, the scientists said in an article that is scheduled for publication in Astrophysical Journal.
"Liquid water on the surface of a planet is the gold standard that people are looking for," said Nicolas Cowan, a doctoral student in astronomy and lead author of the paper.
Of course, oceans do not necessarily mean life, especially intelligent life, but such a discovery would help focus the search on more promising planets. It will, however, require a new generation of space telescopes.
And if Shostak is right, we may already know by then.
"If we don't find ET within a generation, there is something very fundamentally wrong with our assumptions," he said in his talk, which was published in Caltech's quarterly, Engineering and Science.
It would make things a lot easier if ET would just give us a shout.