[DennisP]

We know a lot more than the simple fact that civilization exists on our planet. See section 3 for how they estimated the parameters.

One of the most uncertain parameters is the rate of abiogenesis events per planet. For that one they used a log-normal with a standard deviation of 50 orders of magnitude. They discuss specific theoretical limits from biology for both ends of the range.

Compared to this approach, the usual method is to just pick particular values out of a hat. This paper at least improves that by directly representing our vast uncertainty for some of the values.

It doesn't tell us how many alien civilizations there are. But it does tell us the range of possibilities, given what we know and don't know.

[lazide]

Except we don’t actually know the real ranges of values here, so our actual level of uncertainty is still infinite.

[DennisP]

If your level of uncertainty is infinite then you're suggesting that abiogenesis could be happening every day in your back yard. I think you might admit we're a little more certain than that.

[lazide]

In the same sense that we could be certain that life never generated and we’re in a simulation, sure.

[IAmBroom]

Really?

Life, once established, is about competition for niche resources. Established life would kick the polypeptides out of a protocell quite easily (with certainty > 99%).

Protocells could be evolving right now at vents in the ocean, with zero of them managing to escape their birthplace due to being outcompeted by things with fully developed organelles.

[DennisP]

Fine, let me put it another way. Fill a test tube with simple chemical building blocks of life. Sterilize everything. If our uncertainty is infinite then we aren't willing to say whether metabolizing, reproducing life arises from scratch within five minutes every time we do it.

If you're willing to concede that in fact, that doesn't happen, then you're putting a limit on that parameter, just like the paper did.