Saturday, June 07, 2008

Isn't The Great Filter something in the Whatman catalog?

Twice in the last week the Globe has run pieces on a concept called 'The Great Filter', once on the Op-Ed page and now in the Star Watch astronomy column. I've read both, and the pseudo-statistical thinking in them just irks me.

The headline on the star watch column suggests the hubris that is perhaps what is goading me: "Why a microbe on Mars would change humanity's future". I'd completely agree that discovering microbial life on Mars would be exciting, but where it goes from there is bizarre.

The gist of the argument can be found in this quote
If life arose independently twice in just one solar system, it would mean that the life formation process is easy and common. Life would be abundant everywhere. Most starts have planets, os the entire universe would be teeming with living things.
Good news? No. The chance for humanity's long-term survival would immediately look worse.
Follow carefully now. Whether or not simple life is common, we know that intelligen, technological life -- like us -- is probably rare. Otherwise, goes the arugment, it would have noticed such a good planet as Earth and come here to colonize as early as hundreds of millions of years ago
Given that we haven't yet found signs of other advanced life (or any life) elsewhere

If life is common, something apparently stops it from developing to the point of gaining interstellar travel and settling the galaxy...Apparently, some kine of "Great Filter" preveents life from evolving to the point of getting starships. If the Great Filter lies early in evolution -- such as if the origin of life itself is a rare fluke -- then we, humanity, have already gotten through it. If the Great Filter lies ahead of us -- such as, for instance, if technological civilizations always destroy themselves as soon as they get to power -- then we have no more chance of making it than all the others who have failed and left the cosmos silent.
The more advanced the fossils of living things that Mars may hold, the greater the chance that the Great Filter lies not behind us but ahead.

Okay, just where to start. First, the current Mars mission finding life on Mars is a far cry from finding that life arose independently on Mars. We know that rocks make the transit occasionally, and while we think we sterilized all the probes, the possibility that any life form found really shares a common heritage must first be ruled out. Gary Ruvkun has suggested an experiment for a future probe to look for & sequence ribosomal RNA (if I remember correctly); that would be an appropriate follow-up.

There's also the problem of an N of one: Mars is one planet. Maybe you count an N of 2 with Earth as the second case, though since you're trying to predict on it that's a case of training on your test set. Mars is hardly an independent sample; the same solar system, which may or may not have some unusual properties.

But perhaps more irksome is conflating the reasonable idea that there are difficult barriers against spacefaring species to arise with the rather silly one that there is a single "Great Filter". Mars is a particularly poor example, as we would have a good guess what the filter is there: the planet quit being a nice place to live.

How improbable is life? How often do planets get life but it stays unicellular? How often multicellular but never ambulatory, sentient beings? How often do those sentient beings come up with some way to prevent travel to the stars -- a religion that forbids it, self-extermination (which our species has toyed with). Perhaps some inhabited planets have a super Van Allen belt which dissuaded their residents from becoming star travelers. Perhaps there are intelligent cultures far away -- but with a timing such that their signals can't yet reach us.

The fact is, any estimates of the probability of any one of these (or anything else you can imagine) are nothing but personal priors, wild guesses without much basis in fact. Feel free to make them, but spare us the headlines about predicting doom and gloom.

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