All Alone In The Night

I pity doctoral candidates in liberal arts fields. Completing a Ph.D. in anything requires a dissertation, and that dissertation must be an original contribution to the state of the art. For a budding literature scholar, that's a fairly daunting prospect. There's a limit to how thin you can slice Shakespeare without getting patently silly. I admire the ingenuity it takes to do that well. On the other hand, for a scientist or engineer, it's easy. Just pick a question no one's figured out yet. No shortage of those. Even if you don't answer it, you can eliminate a few blind alleys.

So ... having been a doctoral candidate, I tend to wonder about unanswered questions. Metaphysically speaking, the biggest question still on Science's plate is, "Are we alone?" People have wondered about this for ages, but Frank Drake was the first to take up the search in earnest, at the National Radio Astronomy Observatory in Green Bank, West Virginia in 1960. In 1961, the National Academy of Sciences asked Drake to convene a meeting on the subject. In his preparations for that meeting, Drake came up with a formula to estimate the number of civilizations we might expect to talk to:

N = R* x Fp x Ne x Fl x Fi x Fc x L

where:

N = number of civilization in the Galaxy with which communications might be possible
R* = Rate of star formation
Fp = Fraction of stars that have planets
Ne = Average number of planets that can support life per star with planets
Fl = Fraction of Ne that go on to develop life
Fi = Fraction of the above that develop intelligence
Fc = Fraction that go on to develop radio, and announce their existence
L = Lifetime of that civilization

Drake originally estimated N to be equal to 10. But most of the numbers Drake used were wild, if educated, guesses. A lot of those numbers are still wild guesses. But, we know a lot more about extrasolar planets now than we did then ... Missions like Kepler and COROT have begun to hit paydirt.

In particular, there's evidence to suggest that the nearby star Gliese 581 has a planet, Gliese 581g, that could have liquid water on its surface. It's at the right distance from its parent star, it's about the right size. Someone might just be living there. But the real point is, we now have a lower bound of sorts for the frequency of vaguely Earthlike planets: 1 in 500. Knowing that, we can have a go at recranking the numbers.

R*: The current best estimates are that seven stars are formed in our Galaxy every year.

Fp: Scientific opinion has gone back and forth on this like you wouldn't believe. At one point, I was about to make my dissertation topic a numerical study on the stability of hypothetical planetary orbits in binary star systems. At the time, many people thought that binary star systems couldn't have planets at all. Now, we've discovered several extrasolar planets in such systems. Drake originally estimated this fraction to be about one-half. Evidence suggests it's somewhere between 40% and 60%, with one-third being a fairly confident lower bound. So, going with 0.5 seems a safe enough bet.

Ne: Here's where our newest information comes in: out of eight known planets in our own solar system, and 505 known elsewhere, there is one known Earthlike world (Earth, obviously) and one suspected (Gliese 581g). So, according to our best information available, we can guess this number to be about 0.04.

Fl: Once it was thought that life was abundant in the cosmos. Then it was thought that life was rare. Now we're swinging back the other way. Everywhere we have found liquid water, nutrients, and an energy source, we've found life. Extremes of temperature don't matter. Acidity doesn't matter, neither does alkalinity. Presence or absence of salt, ditto. We've even discovered a microbe that can substitute arsenic for phosphorus. I'm fairly convinced this is as near to 1 as makes no difference.

Fi: This, on the other hand, is a trickier question. Single-cell life is probably ubiquitous. It arose on Earth almost the moment conditions allowed it. Multi-cell organisms, on the other hand ... The Rare Earth hypothesis isn't anything like proven, but I do find its reasoning fairly sound. The leap from single-cell to multi-cell life is the one big step that you probably ought not take for granted. It might require finicky conditions that are rare, or it might be easy. We just don't know. But, once you cross that barrier, evolution does tend to favor complex organisms. That complexity tends to give a creature a richer toolbox for dealing with its world. And intelligence, once it arises, is evolution's killer app. Simple life adjusts to its environment by altering its internal chemistry. Complex life adjusts to its environment by changing shape or behavior. Intelligent life adjusts to its environment by building new tools. Drake guessed 0.01, for reasons unknown. I'm going to guess 0.5, because I think that the leap to multi-cell life is the rare part. Once you clear that hurdle, the rest is gravy.

Fc: Now we devolve into hand-waving. We have no idea how to estimate this. Drake guessed 0.01, pretty much by pulling a number out of his hat. From our own history, we know of cultures that just never got around to fiddling with machines. And we also know that writing was only independently invented a handful of times. I think 0.01 might be too low, but I do think that 0.1 might be about right.

L: Marconi's experiments in radio date from approximately 1900, so we know this is at least 110 years. Frank Drake estimated this to be 10,000 years, and more recently Michael Schermer estimated this number at 420 years, based on an analysis of civilizations in our own past. I've been thinking on this myself lately, and I'm wondering if L actually measures what we think it measures. It doesn't necessarily measure how long a civilization lasts. It measures how long they use analog AM or FM signals as their primary means of communication. A civilization won't necessarily "go dark" because they blow themselves up. They'll more likely "go dark" because they move to more tightly-beamed transmissions, digital communications, fiber optics, frequency-hopping, or any of a dozen other similar technologies. Back in 1960, they thought radio was forever. It may instead be a phase civilizations go through. I'm thinking that window of opportunity is two centuries wide, at most.

So:

N = 7 x 0.5 x 0.04 x 1 x 0.5 x 0.1 x 200 = 1.4

If these estimates are valid, it means that we're probably not alone, but we're not likely to find anyone interested in interstellar ham radio anytime real soon.

But we may as well keep looking. It's not like we have anything better to do.