Monday, August 3, 2009

Drake's equation

I think that one's given far too much credit.
Parameters in it just do not correspond to any real, measurable quantities, and there's far too many assumptions.

Reality is very complicated. There is a wide variety of stars, some of them more variable than others (and thus more of hazard to life), with a range of lifetimes. There's wide variety of planetary systems possible. Some systems may be slow to develop intelligent life and may require long-lived, old stars - and some systems may develop intelligent life relatively quickly.
It could be that unusually large moon is a requirement - Earth got one, and we do not even know if irregularity of combination of moon&sun tides is absolutely essential or unimportant. Nor do we know how common are planets with large moons. Most curiously, looking from Earth, moon has almost same disk size as Sun - we don't know if that doesn't matter or is very important to development of culture.
It could be that Jupiter is very important for diverting comets and protecting earth from constant asteroid bombardment - if there was a dinosaur killing sized meteorite every 20 millions years, intelligent life might never have evolved on Earth (that's what Stephen Hawking thinks about it, and it seems quite reasonable). Some stars are poor in heavy elements, and life as we know it requires heavy elements. Position of star within the galaxy may matter a lot.

But, of course, Drake's equation has nothing of that. No, it just has abstract "planets within continuously habitable zone". Err. Our own orbit is not 'continuously habitable' for most animals larger than rat - because of asteroids and comets that intersect it once in a while - look at what happened to dinosaurs, and exact same thing would happen to us, should dino-killing meteorite strike tomorrow. Not to mention that solar variability results in ice ages. Such somewhat hostile but not too hostile environment may be yet another requirement for intelligent life.
It's not hard to imagine a habitable world where survival is too hard for anything big, and nothing intelligent ever evolves. It is not hard to imagine Eden without the apple, a world that is too perfect for life, where apes are never forced off the trees then forced to survive ice age. Would the competition between species compensate, leading to intelligence even on such very hospitable planets? Or would it not? We do not know. Worse yet we don't even know what factors we do not know. All what we know is that a planet with history identical to that of Earth's would result in human society - with no knowledge whatsoever as of how closely similar a planet needs to be to develop something else that also sends radio signals.

There is far more variables in reality than in Drake's equation, and those variables are not merely multiplied together, but are all inter-linked in very nonlinear ways (and its not probabilities that vary, but probability distributions). There could be probability of intelligent life on planets with huge moon, and another, lower probability for planets without huge moon - or it may just take longer for intelligent life to emerge without moon, requiring much older, lighter, earlier star - which may be poorer in heavy elements. There can be varying size moons. And then probability of having a planet with huge moon within habitable zone may depend to mass of star (which is linked to lifetime). You cannot meaningfully dissect that mess into half dozen abstract parameters that are just multiplied together. It's like dissecting a hologram - each piece will still be as complex as whole, unless you get to *really* tiny pieces.


  1. Agreed. I had a very similar reaction when I first saw the equation, which not suprisingly, was on a popular science show.

  2. It's very similar in terms of trying to "predict" the number of planets in the galaxy etc.

    Personally, I consider calculations of this sort as nothing but a glorified guess. Like they don't know anyway, but pretend to do so with fancy mathematics.