We still don’t know if there have been alien civilizations

Pedants (a group in which I have occasionally been included) often complain that nobody uses the phrase “beg the question” correctly anymore. It’s supposed to refer to the logical fallacy of circular reasoning — that is, of assuming the very conclusion for which you are arguing. Because the phrase is so often used to mean other things, you can’t use it in this traditional sense anymore, at least not if you want to be understood.

I’ve never found this to be a big problem, because the traditional meaning isn’t something I want to talk about very often. Until today.

The article headlined Yes, There Have Been Aliens in today’s New York Times is the purest example of question-begging I’ve seen in a long time. The central claim is that “we now have enough information to conclude that they [alien civilizations] almost certainly existed at some point in cosmic history.”

The authors use a stripped-down version of the Drake equation, which is the classic way to talk about the number of alien civilizations out there. The Drake equation gives the expected number of alien civilizations in our Galaxy in terms of a bunch of probabilities and related numbers, such as the fraction of all stars that have planets and the fraction of planets on which life evolves. Of course, we don’t know some of these numbers, particularly that last one, so we can’t draw robust conclusions.

The authors estimate that “unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first” such civilization. Based on this number, they conclude that ” the degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.”

Nonsense. It’s not the least bit irrational to believe that this probability is so low. We have precisely no evidence as to the value of the probability in question. Any conclusion you draw from this value is based solely on your prior (evidence-free) estimate of the probability.

I mean the phrase “evidence-free” in a precise Bayesian sense: All nonzero values of that probability are equally consistent with the world we observe around us, so no observation causes us to prefer any value over another.

They’d revoke my Bayesian card if I didn’t point out that there’s no problem with the fact that your conclusions depend on your prior probabilities. All probabilities do (with the possible exception of statements about pure mathematics and logic). But it’s absurd to say that it’s “irrational” to believe that the probability is below a certain value, when your assessment of that probability is determined entirely by your prior beliefs, with no contribution from actual evidence.

This sort of argument is occasionally known as “proof by Goldberger’s method“:

The proof is by the method of reductio ad asburdum. Suppose the result is false. Why, that’s absurd! QED.


Published by

Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

4 thoughts on “We still don’t know if there have been alien civilizations”

  1. I am surprised you don’t think we have evidence that’s relevant to assessing this probability. I do.

    For example, 100 years ago, abiogenesis seemed like it might be a very low probability event (since it seems to have happened only once on earth), but after a century of biology, it has become reasonable to think it might be quite likely on an earth-like planet. And the discoveries of various extremophile species suggest they might not even have to be all that earth-like.

    Also, there is some evidence to suggest that life can hop from one planet to another; if so, wouldn’t you say that increases the probability of evolving civilization, even if abiogenesis is rare?

    One other likely requirement for evolving a civilization is for at least one species to have some kind of positive feedback on brain size. That one is still a source of uncertainty, but I just read about a theory this morning that has some plausible support. I think it’s reasonable for me to use this theory to update my belief in the probability of evolving civilization. Don’t you?

  2. I admit that I’m not an expert, but I know of no sense in which “a century of biology” in general or the existence of extremophiles in particular bear on the question at hand. As far as I can tell, every experiment or observation is 100% consistent with the hypothesis that the probability of life arising on an Earth-like planet is, say 10^-100 or even smaller. If life can hop from system to system, then the probability of life arising on a planet is correlated with similar events on other planets, but once again, I know of no significant evidence that that probability is not exponentially tiny.

    I’ll grant that the phrase “no evidence” is an exaggeration (as it nearly always is), but not in a way that alters the fundamental point: to say that belief in a probability of 10^-22 is “irrational” is not justified.

  3. This begs the question (sorry Ted, couldn’t resist) of why life exists at all. I don’t mean that in a philosophical/religious sense. Just that it seems like the universe could go on very well creating solar systems, galaxies, and all the rest of it even if some chemicals hadn’t somehow combined into organic matter capable of self-replication, so we’d still have a universe even if no one was around to see it. So if you’ll forgive me as a non-scientist for asking silly questions, why should life of any kind have formed?

  4. It’s not a silly question at all, in my opinion, although it may not be a question we can answer very well.

    In some multiverse theories, there are lots of different universes with different physical laws. In many (perhaps most) of them, there’s no way for complex, replicating, information-processing systems (like us) to come into being, so they don’t. In some of them, systems like that can come into being, and so (with some probability) they do. We live in one of the latter, naturally.

    The sort of thing I just said is often called invoking the “anthropic principle”, and it’s enough to get some physicists enraged, but as far as I can tell it’s a perfectly reasonable hypothesis to entertain. There’s no great way to test it at the moment, although in my opinion it’s not out of the question that some specific versions of this hypothesis could be tested some day.

    Personally, I’m not interested in doing any actual work on hypotheses like these, because the prospect of observational test is remote at the moment. But I think it’s on the whole a good thing that some other people do spend time thinking about them.

Comments are closed.