Why I think the Drake equation is a crock of shit.

in I had a brief discussion with a good friend and colleague this evening about the Drake equation. He has recently come to think that the equation can provide a good handle on understanding how many communicating civilizations exist in the universe, and how that reflects on the fragility of life on earth. A long time ago I had dismissed the Drake equation, feeling that it had very little to tell us about these matters. In face of his enthusiasm I thought it prudent to look again. I still feel that there is little of analytic value in this tool.

Before getting on, I will say that there are two very good aspects to the conversation about the equation. Firstly the question being tackled is a difficult one, and the idea of breaking it down into smaller more manageable components is an appropriate way to tackle such a complex issue. Secondly life on earth, all life, is fragile and precious, and taken far too much for granted by us, the only apparently self-reflecting species on the plant, as we sleepwalk our way from one human caused disaster to another. Anything at all that can bring the attention to the loneliness of our position in the cosmos, and perhaps through that engender a sense of solidarity in the face of a silent universe, is a good thing. Even if that thing is chimera, a vision beyond substance, an ideal.

That out of the way, I continue to feel that there is little informational value in the Drake equation. My sense is that our current limited understanding of many of the core components that make up the equation render it no better than guesswork. Moreover, the components could equally be divided in other groups without affecting how well the principle functions. I may well be out of date in my knowledge of what science can say about these questions. That would be wonderful. Let's go through the equation term by term. I'll try to argue that for each an order of magnitude difference in either direction could hold in the answer, and as such the range of possible answers provided by the equation as it is constituted is too broad to support much.

The equation is


N = R^{*} \times f_{p} \times n_{e} \times f_{l} \times f_{i} \times f_{c} \times L \\
R^{*} -- the rate of galactic star formation
f_{p} -- fraction of stars with planets
n_{e} -- average number of life supporting planets
f_{l} -- fraction of these that go on to support life
f_{i} -- the fraction of these that go on to create intelligent life
f_{c} -- fraction of those civilizations that become communicative
L -- the length of time they hang around sending signals


OK, let's go through these one by one:


R^{*} -- the rate of galactic star formation
-- usually interested in sun like stars, as big start
-- dwarfs, which have a long incubation period, can capture/create planets

-- g type stars


This is given importance because the assumption here is that life has to be supported by planets. We could argue that life could arise in nebula where stars are almost formed, which are rich in heavy elements. I admit that this would still be functionally dependent on the star formation rate, but it would change the dependence of the final figure on the star formation rate by a considerable amount. It is quite far fetched to think of life like this, and it is clear that the bias in the equation is to find DNA based life like ours, liquid water, probably an oxygen atmosphere. I'm not sure that this is fair. We know from extremophiles on earth that it is possible for sulphur based life to exist on earth. It may be that what is needed is not a specific set of chemical elements, but rather the proper available energy transfer paths in a system so that a sufficient degree of local order can emerge. Of all the elements in the equation this is the one that I think is strongest candidate, and yet I'm not 100% convinced that it's effects can be simply determined.


f_{p} -- fraction of stars with planets

increase - binary systems can support more planets than we assumed
there are more - tight binaries



n_{e} -- average number of life supporting planets
f_{l} -- fraction of these that go on to support life

I'm going to roll these three into one discussion here. As far as I can tell, we don't know how life originated on earth. We do know the relative abundance of the building blocks of carbon based life in the universe. From studying line emissions we can see that relatively complex molecules, such as alcohol, and the bases that form DNA are pretty common, so the soup that formed life could be expected to be in quite a lot of places. I remember recalling that self-replicating forms of molecules were often low-energy states, boosting the likelihood of the emergence of such forms in a random process than one might naively think, but without a model for how to make the kind of life that we know we are yet to be able to put a lower bound on this figure. Then there are all of the potential non-carbon based life's that might emerge, driving this number upwards.



f_{i} -- the fraction of these that go on to create intelligent life
f_{c} -- fraction of those civilizations that become communicative


Another approach might be to look at the cross section of what
I haveMy feeling is that the underlying assumption of what could life subcomponents

In discussing this briefly with Adam this afternoon I thought it would be good to go over my thoughts on this again.

OK, it's not that bad.