Wednesday, September 07, 2016

Why I Don't Believe in ET

Is there life on other planets? The naive answer is "of course, why should Earth be special." What makes a lottery winner special? We could have just won the life lottery and the losers aren't around to complain.

The more scientific approach uses variants of the Drake Equation. I'll use the variant from the recent paper A New Empirical Constraint on the Prevalence of Technological Species in the Universe by Frank and Sullivan.
We define the ‘‘A-form’’ of the Drake equation, which describes the total number of technological species that have ever evolved anywhere in the currently observable Universe:
A  = Nfpnpflfift
where N is the total number of stars, fp is the fraction of those stars that form planets, np is the average number of planets in the habitable zone of a star with planets, fl is the
probability that a habitable zone planet develops life, fi is the probability that a planet with life develops intelligence, and ft is the probability that a planet with intelligent life develops technology (of the ‘‘energy intensive’’ kind such as that of our own civilization).
I first encountered the Drake equation in high school from Carl Sagan's book Cosmos. The conclusion that there must be life out there from the equation never satisfied me because of our inability to measure the f values.

Frank and Sullivan's computations show that we expect there to only be life on Earth, say A=0.01 then f = flfifmust be at most 2.5 x 10-24.

Seems small but is it really? 2.5 x 10-24 is roughly the probability of flipping 78 coin tosses and having them all come up heads. Maybe life requires 78 50/50 independent events to occur. Or 1060 independent events each with 95% probability. 1060 doesn't seem that big.

Our attempts at finding life, whether by SETI or Mars soil or UFOs have so far turned up nothing substantial. We just might be the only ones out there.

By no means am I arguing that we give up the search. I could be wrong, f could be much larger than 2.5 x 10-24. Let's keep looking, perhaps exploring the ice caps of Mars or the moons of Jupiter, keep listening the the stars, explore new ways to probe the galaxy. It would be incredible to find extraterrestrial life, but just don't be surprised if we don't.


  1. An interesting complement:

  2. Amazing - I was going to write a blogpost today with almost exactly the same title and argument. I may still, but it feels more redundant now.

    I think the only piece of the argument that you are leaving implicit is the anthropic one - that the fact that we're here should not be taken to be evidence of a large f. It tells us that f > 0 but not anything else about it.

  3. Below a certain probability (roughly 1/1000), humans are terrible at estimating odds because they simply can't grasp them. 1/1000 = roughly once every three years something that's graspable.

    Here's an experiment that scientists actually have trouble with - compare million with billion with trillion.

    Not the number of zeroes - what does it truly mean? You'll find that even though these numbers differ by six orders of magnitude, most scientists and mathematicians don't have a firm grasp of million v/s trillion.

    When you are dealing with such small odds and an insane amount of uncertainty in those estimates, our intuitions fail us completely.

  4. Or maybe we are just the first ?
    See paper: Abraham Loeb, et al. Relative Likelihood for Life as a Function of Cosmic Time.
    also on

  5. Carl Sagan was intrigued by the idea of extraterrestrial intelligence and the possibility to get in touch with "it." Just think about his novel (and the later film) "Contact." Probably it will not be as straightforward as he thought that "they" contact us in the immediate future. We have to wait a few centuries and see what else science and technology have in store and if our age of booming intelligence and rationality is more than a fleeting star.

  6. In his scifi trilogy starting with "The Three Body Problem", Cixin Liu proposes the following:

    Advanced civilizations are threats to one another (even if a discovered civilization 100 lightyears away seems less advanced than you now, technological progress can go so quickly that you can't be sure it won't be an existential threat by the time you make physical contact).

    So there is strong incentive to try and destroy any civilization as soon as you detect it. Everyone out there has either figured this out, and is staying quiet, or no longer exists to keep broadcasting strong signals.

    We, presumably, have not yet developed the technology to produce noticeable signals from far away. (We're not quite a "Type 1 civilization" )

  7. I believe that f is much less than 2.5 x 10^(-24), and we are alone in the OBSERVABLE universe.

    However, I believe that the space is infinite, and N = infinity. Because our existence proves that f>0, this implies that A = infinity. There are infinitely many civilizations, they are just too separated to observe each other.

  8. The biggest problem is that most civilizations wouldn't spend that much time in the energy intensive part of communication. So thereafter we would have to wait for a civilization of advanced enough means to create and broadcast a powerful signal for the sole purpose of reaching another civilization. Since we ourselves are not close to achieving this we do not know yet the challenges involved in the process.

  9. Well, the fact that we exist can be taken as some evidence that A is not that small. The right way to think about it would be: If you flip billions of 0/1 coins (each coin heavily biased towards 0) and sum up the result, what are the chances that the sum is exactly 1.

  10. Frankly, the fact that SETI hasn't seen anything is fairly meaningless. All we've really ruled out was the (highly unlikely) possibility that there are civilizations within our observable sphere creating hypothetical megastructures like Dyson spheres. Under the assumption that civilizations have a limited energy budget, it quickly becomes apparent that we'll have to get extremely lucky to detect a signal even if someone very nearby is (briefly) pointing it right at us; among other things, they'd have to have some reason to think we were worth beaming at, and we don't give much of one to anyone far enough away not to be able to detect our atmosphere. A friend has done some energy budget calculations on this and the costs are really enormous even for a machine that does short, intense beams for a few seconds at likely planets over a period of many years.

    In the meanwhile, as the paper noted, all the other probabilities in the Drake equation people used to point to as reasons we might be alone have done little but rise. I'd say that whether you believe there are other life forms in our galaxy or not, you pretty much have to be less pessimistic about it now than you were, say, twenty years ago.

  11. One problem is that we as humans simply don't know what N is. It could be a finite number, or it could be infinite. But, as long as f(i) and f(p) are greater than zero, I'll keep my hopes up, even if A is still only between 0 and 1...

  12. Our "outreach" to our intelligent ET brothers and sisters has only been happening for about 100 years or so (considering the fact that we have been using radio waves only for that long). Assuming that SETI is scanning the entire 100 Light years sphere 24x7, we have just ruled out life in a ball of 100 LY. The radius of our galaxy 100,000 LY. Is it a surprise that we have not found one?

  13. I think that the ‘‘A-form’’ of the Drake equation should have a function of time associated with it. The real question is if there is ET life NOW that we could observe it - No? So there is a factor in there to rule all those magnificent life forms that eliminated themselves 276 years ago before we were looking up.