Could there be a faster way to discover interesting galactic neighbors? Is there some scheme for detecting aliens that might work quicker than tuning in their radio transmissions or hunting down their laser pulses?

There might be, and for a simple reason. The cosmos is three times as old as Earth. During most of creation's 14 billion year history, our solar system wasn't around. Nonetheless, the early universe still had the right stuff for life, and contained worlds that were just as suitable for spawning biology and intelligence as our own.

Humans have existed only for the last 0.001 percent of cosmic time. All of which says that -- unless the Homo sapiens brain is the one-and-only instance of cogitating machinery -- nearly all the intelligence that's out there is beyond our level.

And that intelligence is more than just a little bit beyond. Clearly, unless thinking beings inevitably wipe themselves out soon after developing technology, extraterrestrial intelligence could often be millions or billions of years in advance of us. We're the galaxy's noodling newbie's.

That suggests an oft-overlooked approach to finding them. Advanced thinkers might be advanced tinkerers. Perhaps the really ambitious aliens have "disturbed the universe" (to use a phrase from British physicist Freeman Dyson) in ways that are directly visible.

This disturbance might take the form of deliberate engineering, such as the construction of mammoth artificial structures. Perhaps they've rearranged some stars into easily recognizable patterns. Maybe there's left-over debris from massively destructive wars. You can consult science fiction for other examples of what an enormously advanced society might have strewn about. It's as good a predictor as any, because obviously we don't know.

Indeed, the fact that we can't easily foresee clues that would betray an intelligence a million millennia farther down the road suggests that we're like ants trying to discover humans. Ask yourself: Would ants ever recognize houses, cars, or fire hydrants as the work of advanced biology?

The best we can do is to carefully look at the images collected (for whatever reason) by our telescopes, and decide if any of them show something that looks funny -- something that appears as if it were put together by deliberate design, rather than natural processes.

To some extent, this exercise is taking place, albeit as part of conventional astronomy research and not as a deliberate attempt to uncover alien astro-engineering. A decade ago, two Austrian astronomers, Ronald Weinberger and Herbert Hartl, reported on the results of spending more than two dozen years poring over telescopic photos of northern hemisphere skies. They discovered 12 thousand objects that were previously unknown. But they were also on the lookout for things that gave the appearance of being artificially constructed. Alas, they didn't find any.

More recently, the Galaxy Zoo project has attracted more than 150 thousand volunteers to the task of looking at astronomical photographs from the Sloan Digital Sky Survey (as well as from other telescopes). The goal is to classify the morphology of the myriad galaxies visible on these pictures -- a job that astronomers were able to handle themselves until modern telescopes began disgorging billions of high-grade galaxy images. And while unexpected things have turned up in this cosmic cornucopia, so far they all seem understandable as natural objects.

Richard Carrigan, a physicist from Fermilab in Illinois, makes no bones about it: he's deliberately searching for alien hardware. Carrigan pores through astronomical data looking for star systems that emit excessive amounts of infrared light. Infrared would be the signature of waste heat from, for example, massive swarms of solar-cell satellites, placed in orbit around a star by advanced societies eager to collect as much energy as possible. He's teased out some interesting cases, but as yet, no sure-fire solar swarms.

Do these failures to find a galactic equivalent of the Great Wall of China or the interstate highway system mean anything? Is there some deep truth being revealed by the lack of any obvious, large-scale engineering in a vast and ancient universe?

Well, it might be telling us that no one's out there. On the other hand, truly sublime intelligence may not build structures large enough to show up in our pictures. Perhaps high-tech societies veer in the other direction: preferring to think small and miniaturize, rather than scale up.

Alas, such speculation -- while interesting -- is not terribly informative. After all, we've been making truly detailed telescopic images for less than a century. It's far too early to declare that space is litter free -- we haven't checked carefully. Not yet.

In addition, searching for extraterrestrial artifacts is hobbled by the fact that we don't know what we're looking for. In that regard, it's less appealing than our SETI experiments, which pick through the radio and optical spectrum looking for unambiguous, engineered signals.

Nonetheless, artifacts are available for discovery 24/7. As Weingerger and Hartl wrote, they're 'frozen messages.' You don't have to hope for an alien transmission hitting the Earth just as you swing an antenna or a telescope to the sky. And finding artifacts -- which might be the unexpected outcome of everyday astronomy -- benefits from a large labor force. There are at least a thousand times as many astronomers as there are SETI researchers.

So don't rule out serendipity. Sometimes the greatest discoveries are made by those who aren't even looking.