Fifty years ago, human beings took the first steps in the search for other intelligent civilizations in the universe when a young astronomer named Frank Drake aimed a radio telescope at a nearby, sun-like star known as Tau Ceti. Other astronomers have followed in Drake's footsteps, and to date they've examined several thousand stars, out to a distance of 100 light years or so (still relatively nearby in astronomical terms), listening for any sign of intelligence above the static hum of the cosmos. But all we've heard so far, as the title of Paul Davies's latest book captures succinctly, is an "eerie silence."
The quest is known as SETI - the Search for Extraterrestrial Intelligence - and Davies is probably the earthling most qualified to tackle the subject. He's an Australian physicist, astrobiologist and prolific science writer now based at Arizona State University, where he heads the Beyond Center for Fundamental Concepts in Science.
Davies also chairs the "Post-Detection Taskgroup," a committee charged with figuring out "what to do next" if ET does happen to call. (The committee has no legal powers, but even so, it's a heck of a title to put on your business card, and Davies takes the responsibility seriously.)
If we one day detect such a signal from the depths of space - and of course it is a huge "if" - we would know we are not alone in the cosmos. The consequences "would be truly momentous," Davies writes early in the book, "having a greater impact on humanity than the discoveries of Copernicus, Darwin and Einstein put together." He quickly adds: "But it is a needle-in-a-haystack search, without any guarantee that a needle is even there."
As Davies explains, it's difficult to even surmise how many alien civilizations might be out there - to estimate how many (if any) needles are in the cosmic haystack. It depends on a number of probabilities, most of which we can only guess at.
The good news involves planets: Astronomers have now tallied several hundred planets orbiting stars beyond our solar system; most are giant-sized, but the discovery of Earth-like planets seems almost inevitable. Of course, that is just the starting point. Given an Earth-like planet, what is the likelihood that life will take root? At this stage, we simply don't know, because we only have data for one such planet: our own. (Yes, life evolved here. But was it inevitable or was it a fluke?)
However, Davies presents a bold idea that could potentially improve the odds: The prevailing view is that life arose just once here on Earth, but for all we know, he says, there could have been a "second genesis" (or a third or fourth, for that matter). While Darwin imagined a "tree of life," there could in fact be multiple trees, from multiple origins.
The result would be a "shadow biosphere" composed of creatures (probably microbes) that are hiding in plain view, so to speak. These creatures wouldn't interact with "regular" life, and would therefore be hard to detect; indeed, they might not even be DNA-based.
"There could literally be alien organisms right under our noses (or even in our noses!), as yet unrecognized for what they are," he says. If life did arise more than once on our planet, he argues, it would vastly increase the odds that the universe "is teeming with life." Davies is surely right to urge biologists to be on the lookout for such organisms.
But what of intelligence? Given a planet on which life has appeared, what is the likelihood that it will eventually evolve into intelligent life, the sort that might eventually use radio technology, or seek out other like-minded beings? The development of such life on Earth - that is, us - seems to rest on contingency upon contingency. If we could "rewind the tape" of Earth's history, so to speak, would we have the plow, the pyramids and the iPod once again? The answer is far from clear, Davies says.
Moreover, he says, when it comes to intelligent life, it's possible that we've been looking for the wrong signs. We think of creatures using technology to build machines, but the galaxy may well be populated by "post-biological" beings that are essentially mechanical in nature. Such artificial intelligences would be far better equipped to populate the cosmos than their squishy biological predecessors, although whether they would have any interest in contacting backward folk like us is anybody's guess.
Ultimately, there may be a "spectrum of intelligence" in the galaxy, Davies says, from pre-technological civilizations to those in which biological and mechanical entities co-exist, to "full-blown cyber-intellects."
Of course, one explanation for the "eerie silence" could be that ET simply doesn't exist, and beyond our Pale Blue Dot (to use Carl Sagan's phrase) the universe may be a lonely place indeed. Of course, even that would be a profound discovery - and a sobering thought, to say the least.
Davies is a scientist, but some of his most provocative arguments involve the human condition at large. Consider religion: If we find ET, he says, our belief systems may need an overhaul; after all, it would suddenly be clear that the universe was not "about us." The discovery of aliens "would deal a severe blow not only to Christianity, but to all mainstream religions."
From a less-qualified writer, some of these bold assertions would seem a tad presumptuous, but for the most part I found myself willing to go along for the ride. To his credit, Davies is careful to distinguish established science from speculation, and speculation from opinion, at every turn. The Eerie Silence is honest yet provocative, and enormously entertaining.
Dan Falk's most recent book is In Search of Time: Journeys along a Curious Dimension.