Nearly thirty-two years ago, a giant radio telescope in Ohio working on a SETI (Search for Extraterrestrial Intelligence) project made a curious discovery: an unidentified signal that remains to this day the best candidate for a genuine ET transmission. This discovery—dubbed the Wow! signal due to the enthusiastic scribblings of its finder—was a spike in radio emissions to over 30 times the natural background levels coming from somewhere in the constellation Sagittarius and possessing the “expected hallmarks of [a] potential non-terrestrial and non-solar system origin.” Moreover, it came at a frequency of about 1420 MHz, the so-called “water hole” of the radio spectrum—-likely the most natural frequency for members of the galactic radio wave-transmitting community to favor. Unfortunately, despite repeated searches for it, the signal was never detected again and remains unexplained to this day.
But how likely is it to have been the product of an extraterrestrial intelligence? To even attempt to answer this—and the bigger questions of how likely there is to be someone out there to transmit and how likely SETI is to someday be successful—requires a very large range of scientific and social knowledge. That’s why this will be the first of several posts on a range of subjects germane to these cosmic questions and supplied by each of the contributors to this blog (in our first collaborative effort). Posts sharing their unique expertise should trickle in over the next week or so.
I’m going to open up the discussion with a very brief overview of a staple of any SETI discussion: the Drake equation. Astronomer Frank Drake launched SETI in 1960 with Project Ozma, a search for ET radio signals emanating from two nearby Sun-like stars. (On a geeky sidenote, I’ve seen up close the telescope Drake used—an 85-foot radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia.) A year later, Drake devised the equation that bears his name to help organize the discussion at the first conference held to discuss the nascent SETI. Though the Drake equation ostensibly yields a value for the number of radio-transmitting civilizations in the galaxy, it is far from being a hard-and-fast formula; rather, it is a tool for sharpening our understanding of what factors are key to the existence of civilizations like ours. The Drake equation is a chain of reasoning, from the physical to the biological to the social, in which each new link of the chain cuts down our estimate of the number of civilizations in the galaxy. Let’s break it down:
● The Physical Factors. We start by considering the rate of star formation in the galaxy (or, more simply, the total number of stars). Now, what percentage of stars have planetary systems? And, in those systems, how many planets (on average) will be physically capable of supporting life?
● The Biological Factors. Once we’ve zoomed in on potentially life-supporting planets, we face the hardest question yet: what fraction of those go on to actually develop some kind of life? And once we’ve estimated that: what fraction of those life-sustaining planets go on to eventually develop intelligent life?
● The Social Factors. Now that we’ve gotten this far, we need to consider what percentage of planets with intelligent life go on to develop civilizations that eventually create radio telescopes or something similar. The final factor is to consider is how long such civilizations last, either in terms of actual longevity (i.e. do they destroy themselves?) or in terms of a communication window bounded by the technological point where ETIs are undetectable to us in our SETI efforts.
Each of these steps is a discussion unto itself and each is crucial. Solutions to the Fermi paradox (put simply, if the naïve assumptions that humans are typical and that life like us is common in the universe are correct, why have we seen absolutely no trace of anyone else?) can be found by making a very pessimistic estimate for virtually any of the factors found in the Drake equation. The simple and inexorable logic of the Drake equation has guided discussions on SETI for the past forty-eight years. So shall it guide ours as we post our thoughts on different aspects of the problem over the next few days.
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