Our search for intelligent alien life depends on picking up the radio waves of a reasonably advanced extraterrestrial civilization. We had assumed these aliens would be biological life like us - but logic says that's probably not the case.
The only way we can probe for intelligent life in other star systems is to look out for radio waves that appear to be artificial in origin. That's the purpose of SETI (Search for Extra-Terrestrial Intelligence), a collective name for various alien-hunting activities that have been going of for about fifty years.
Of course, humanity itself hasn't been a detectable civilization for very long, as it's only in the last century or so that we've gained the necessary technology to become a noticeably artificial source of electromagnetic radiation. When you compare that to the thousands of years of human civilization that came before, not to mention the millions of years of evolution that preceded humanity's arrival, it's fairly obvious that we'd have to get very lucky to "catch" extraterrestrial life at the exact moment in its history (particularly its evolutionary history) in which it has radio technology.
And then there's the thorny problem of a civilization's survival once it develops that technology - for humanity, we gained the ability to communicate with the universe at almost precisely the same moment we became able to destroy the planet in one fell swoop. So far, we've done all right, but that's no guarantee a radio-capable human civilization will endure for even a fraction of the length of the rest of human history. If we only have any given radio-capable alien civilization has only a few centuries to make contact with us before they (or we) are wiped out, then the odds are stacked almost impossibly against us.
That's part of the reason why SETI experts are rethinking what sort of life we might expect to find out there. SETI astronomer Seth Shostak thinks it's entirely likely that it won't be biological aliens we make contact with, but their artificially intelligent successors. He reasons that, based on the current arc of human technology, the time gap between the invention of radios and the development of artificial intelligence is probably quite short:
"If you look at the timescales for the development of technology, at some point you invent radio and then you go on the air and then we have a chance of finding you. But within a few hundred years of inventing radio - at least if we're any example - you invent thinking machines; we're probably going to do that in this century. So you've invented your successors and only for a few hundred years are you a 'biological' intelligence."
If he's correct, then the majority of intelligence in the universe might not be biological life, which takes millions of years to evolve and can easily wipe itself out, but sentient machines that would have progressed beyond their creators in just a generation. These machines wouldn't need to be tethered to a planet, would be effectively immortal, and as a "species" be nearly indestructible. As such, their long-term survival odds seem a whole lot better than those of their fragile creators, at least if humans are any indication.
So what difference does this make? Shostak admits that, if he's right, SETI's task just got a whole lot more difficult. Biological alien life could be almost unrecognizable to us, but at least they would have to think in a way that's governed by the rules of biochemistry, just like us. Thinking machines, on the other hand, could have cognitive processes that are almost entirely incomprehensible to us, which would make the job of finding and understanding a potential alien radio signal that much more difficult.
However, Shostak thinks there might be a way for us to tell the difference between biological and mechanical radio sources. Applying a bit of rudimentary robot psychology to the problem, Shostak suspects that these machines would head towards the most highly concentrated sources of matter and energy, because those are the only things that would really interest machines.
That means they would most likely move towards hot, young stars or the center of galaxies, where both matter and energy are available in nearly limitless supplies. Neither of those are thought of as good candidates for biological life, which may mean SETI needs to start dividing its time between looking at likely biological sources and likely AI sources, as Shostak explains:
"I think we could spend at least a few percent of our time... looking in the directions that are maybe not the most attractive in terms of biological intelligence but maybe where sentient machines are hanging out."