[gbhn]

I read it as saying something a little more subtle, which is that there is a lot of uncertainty in a couple of our critical estimates, and given that uncertainty what we see (no other life in the galaxy) is not that surprising. (That is, if you take the mean and multiply, you get misleadingly-high intuition.)

Of course if we do some more experiments and find that in fact the probability of abiogenesis is better approximated by 10^{-3,-2} that would change the expectations dramatically. This happened recently: we now have much better science around the incidence of earth-ish planets in solar systems, and a better understanding of what seems to contribute to good life-favorable (at least as we understand it) conditions on those planets. So this uncertainty can change with good work.

It's just that at this point it's really quite fair (per the authors) to expect that with more research we might equally well discover that P(abiogenesis) < 10^-100, in which case it would be astounding if we weren't alone in the observable universe.

(Good talk slides: http://www.jodrellbank.manchester.ac.uk/media/eps/jodrell-ba...)

[danShumway]

This was helpful. That wasn't the way that I was reading the original paper, but I think I see what you're saying.

[thaumaturgy]

There's one remaining subtlety that I don't see in his comment or in almost any of the comments in this thread, and it's the entire novel point of the paper.

If you give any particular term in the Drake Equation a value of some magnitude, say, P(abiogenesis) < 10^-100, then you're still treating it as a point estimate, and this is the flaw at the heart of all arguments about the Fermi Paradox.

What the authors do instead is give each term of the Drake Equation a range of magnitudes. When you multiply those ranges of magnitudes together, you get a ton of different possible outcomes. If you then look at those outcomes together, you find that in over half of them, we are alone in the universe.

This is the "multivariate probability" that a top comment referred to, that seems to be problematic for a lot of people (including me).

The authors point out that with this approach, even if we nail down the range of probabilities for some of the terms, the uncertainties of the remaining terms will still lead towards a > 40% chance of being alone in the universe.

It's a little bit like: in a poker game, I draw five cards. Each of my cards is a 2, 3, 4, 5, or 6. What is the probability that I drew a straight? To calculate that, you'd count the number of different possible card combinations, and then count the number of those combinations that resulted in a straight.

What everyone has been doing previously in this analogy is calculating the odds of drawing each individual card.

In the case of the Drake Equation, using a probability distribution approach to estimating the chances of being alone in the universe leads to a very different result than we expect.

I hope I didn't add to the confusion.

[leeeeech]

> If you then look at those outcomes together, you find that in over half of them, we are alone in the universe.

Since we do not really know, anything but P=0.5 should imply incorrect prior estimates. That might be more insightful than whether alien life exists or not.