[bee_rider]

It might just feel that way because it is a boring answer, but boring answers are usually a good sign. We all want the emergence of life to be special, but there’s every reason to believe it was just random. You can fit some pretty unlikely events in (billions of years)*(however many planets). It is really more space, than time, I think, though. I’m actually not sure how many planets there are in the whole universe (not just the observable universe; if we happened to have sprouted up on another planet we’d just be asking this question from inside a different light cone, nothing special about ours).

[pfdietz]

No, it's a copout. The handwaving about "a long time/many planets" is very weak, when confronted with the complexity barrier between non-living matter and the simplest known system capable of Darwinian evolution. All that time, and all those planets (that we can see in the observable universe), give a number that is very small compared to the unlikelihood of just randomly surmounting this barrier.

[bee_rider]

Is there an actual mathematical model of the complexity barrier that I can look up?

I don’t see any reason to limit things to the observable universe. What prevents life from emerging outside of our light cone?

[pfdietz]

Nothing prevents life from originating outside the region we will ever be causally connected to, but life that can never be observed doesn't present much of a handle for conducting science.

As for complexity barrier: consider the number of possible stable configurations with N atoms. It grows exponentially in N. You need to argue either that the chance of a random molecule being alive is high enough (which doesn't seem consistent with the structure we see in life) or that the sampling of these molecular configurations is extremely biased toward some that can start evolution (which is just begging the question.)

[bee_rider]

> Nothing prevents life from originating outside the region we will ever be causally connected to, but life that can never be observed doesn't present much of a handle for conducting science.

It is a mathematical argument, and not a complicated one, so I’m confused as to how we could be talking past one another here. If we model the chance that abiogenesis occurs on a planet as a random event for that planet, then the probability that it doesn’t occur on any planets is

1 - product(from i=1 to n_planets, 1-P_i), where P_i is the probability that it occurred on the i’th planet. (Sorry for the ugly notation, it is a limitation of basic text, hopefully it makes sense).

These are independent random events, there’s no need to have them linked causally.

> As for complexity barrier: consider the number of possible stable configurations with N atoms. It grows exponentially in N. You need to argue either that the chance of a random molecule being alive is high enough (which doesn't seem consistent with the structure we see in life) or that the sampling of these molecular configurations is extremely biased toward some that can start evolution (which is just begging the question.)

So this seems like an argument from first-principles in chemistry, I don’t know enough about chemistry to say much about it, but I don’t think enumerating every possible configuration of atoms is how they typically do it. I could be wrong though, it is outside my wheelhouse. I would be more comfortable with this argument if it was some well-sourced/well-known thing.

Anyway, this sort of argument seems like it will just shift the individual P_i’s in the equation up or down. We don’t really know enough about abiogenesis to make good estimates there anyway as far as I know, so I don’t see any reason to write off this explanation as a cop-out.

[pfdietz]

When I mean "causally connected to" I meant "we could send signals to each other". Most of the universe we can currently see is causally disconnected, in that we can never signal them (the nonzero cosmological constant means they will recede faster than a signal could reach them.)

There could be unlimited volumes of spacetime beyond this horizon. Whether life originates there or not we can never know. There is a finite amount of matter within this horizon, and finitely many chances for life to originate. If OoL is sufficiently unlikely it is unlikely to have happened twice in the volume we can reach.

[bee_rider]

I definitely would agree that we don’t know enough about those probabilities to say much about the density of life.