First off, I apologize for not having posted a blog entry for several weeks. I was on vacation, and have just caught up with things after returning last week.
Yesterday, several colleagues and I caught wind of some rumors over the Internet that the SETI Institute was going to make a "major announcement" today. While Sky & Telescope editors are quite used to possible big stories that turn out to be letdowns, my ears perked up briefly, until I heard that there would be no announcement that humanity has picked up a signal from ET. And that got me to thinking about the age-old question of whether or not we're alone in the universe. The only intellectually honest answer to that question is "We don't know." But that should not stop us from trying to make educated guesses.
While I applaud the efforts of privately funded SETI programs and the multifaceted research conducted by SETI Institute scientists, and support SETI through my membership in the Planetary Society, the admittedly limited evidence at our disposal indicates that technological civilizations are extremely rare, and it wouldn't surprise me if we're the only one in our galaxy right now. (I certainly do not think we’re alone in our visible universe.)
Both ET optimists and pessimists agree that the term L (the average lifetime of a communicating civilization) in the Drake equation is crucial. If L is a low number relative to the age of our galaxy, then no matter what values one assigns to the other terms in the equation, N (the number of communicating civilizations) will be very small. Given the 13-billion-year history of our Milky Way Galaxy, if L is small, it means (to paraphrase Arthur C. Clarke) that civilizations are flickering in and out of existence like fireflies in the night. Unless some civilizations endure for millions of years, only one civilization will exist in our galaxy at a given time.
So let's assume that the Milky Way Galaxy is teeming with civilizations right now, which necessarily implies that some of them have existed for a very long time. It is shockingly naive to think they're all going to sit around in their home system for millions of years beaming radio waves into space, as many SETI proponents would have us believe. As UCLA astronomer Ben Zuckerman has effectively argued, intelligent beings are going to go out and remake the universe around them. At least some technological civilizations are going to build giant space interferometers to identify all the life-bearing planets within several hundred light-years, and when they find them, they'll have an insatiable desire to explore them up close, and once they get there, they might as well remain there. They'll build large-scale astro-engineering projects. With millions of years at their disposal, and technology beyond our wildest imaginations, they'll figure out how to traverse interstellar distances. After all, we've already launched five probes (Pioneers 10 and 11, Voyagers 1 and 2, and New Horizons) that will leave our planetary system, and we've only been able to fly in our own atmosphere for about 100 years — a pitiful timespan with respect to the age of our galaxy.
In fact, the ability to travel vast distances, and establish multi-planet societies, is probably a requirement to guarantee that a civilization (and its offshoots) will be long-lived. For L to be large, civilizations will need to secure themselves against local disasters and the evolution of their host stars. If there are large numbers of civilizations out there, does anyone seriously think that Earth would have escaped colonization long ago, especially given that it's hosted life for at least 3.5 billion years and resides in a planetary system with vast resources and a stable star? Yet there's not a shred of credible evidence that our planet or solar system has ever been visited.
The Fermi paradox (i.e. the question "Where are they?"), the nondetection of radio signals, the lack of any evidence of astro-engineering projects — all of this is admittedly limited and inconclusive, but it's the only direct evidence we have that bears on the question. And all of it is indicating that technologically capable life is extremely rare.
Another important point is that if all the SETI programs were to be shut off (something I would not support!), that doesn't mean our search for ET will come to an end. Many of the great discoveries in astronomy were made serendipitously: the Galilean moons of Jupiter, Uranus, quasars, the cosmic microwave background, pulsars, gamma-ray bursts, the first extrasolar planets (which orbit a pulsar, not 51 Pegasi)…the list goes on and on. The history of astronomy suggests that if we ever do find evidence for extraterrestrial civilizations, it will not come through a directed SETI program, but from a telescope built for another purpose that passed some critical threshold that makes alien civilizations detectable. I hope to live to see that day, but I'm not holding my breath.