Planets in the habitable zone, the Drake Equation, and the Great Filter

Your post got me thinking on a completely different tangent: How much of the filter might be at a high tech level for most species but we managed to escape it based on what resources we actually lacked?

The most obvious example is the amount of U-235. If humans had arose 2 billion years ago there would be about six times as much U-235 on the planet (since U-235 has a half-like of around 700 million years), making it much easier to develop nuclear weapons. That could have a substantial negative impact on a species chance of not wiping themselves out.

I'm not sure how much this would matter in that fission weapons are a lot easier already to produce than fusion weapons. Moreover, this would also make it easier to use nuclear power for productive purposes. Even a Fermi style pile would be much, much easier to construct (Fermi's original pile did not use enriched uranium). So the ability to use nuclear power in this way would help for simple electricity generation a lot as well as nuclear rockets (which would directly help beating the Great Filter). So overall this seems like a wash without a lot more data.

Is there some other isotope that could have a similar impact? Possibly something that now is very rare so we aren't paying much attention to it? The other obvious candidates don't seem to work. For example, tritium has a very short half life but natural processes produce more of it so that shouldn't matter. Similarly, plutonium 239 has much too short a half-life so that any species that arose even after the first billion years wouldn't see any substantial amounts of it. Maybe Plutonium-244? It is primordial, has a half-life around 80 million years, so would be around in larger quantities on a young planet. But I don't know of any obvious fission chain for it, and the quantities produced would be very little, since it is not easily produced in supernovae.

Duncan seems to be just talking about the Earth related constraints. It is possible to have those constraints as well as other constraints. I don't see Duncan as directly claiming that the Earth filtration issues are the entire filter. His point seems to be more that being in the habitable zone is not the only filtration issue for a planet.

It is pretty easy to imagine lots of smallish filters as responsible. If I am doing my math correctly, then 9 filters that each weed out only 9 out of 10 solar systems would reduce the expected population of intelligent life in the Milky Way to just .1 systems. Since we know so much more about star formation I am a lot more comfortable saying "habital planet" will weed out 90% of systems, that is not nearly a big enough filter to account for what we see by itself but as you stack them filters of this size are quite substantial.

edit: er no that's wrong, off by a few orders of magnitude. So yeah, gosh where is everyone?

First, yes, the unit of selection is of course the individual. But a species-wide feature that causes each individual's chance of survival and propagation to go down does, in fact, imperil the survival of the species when it is selected against in every individual's case. When every individual dies, so does the species; similarly, if merely enough individuals die that there's a genetic bottleneck, the species also risks extinction because of inbreeding-related unfitness of individuals. Next time try actually thinking through what is being said, instead of partial-pattern-matching in an effort to score points.

And yes, runaway selection pressure in general happens a lot. Now, how often does it happen specifically on the characteristic of intelligence, instead of all the other possible features it could happen on? Looking at the only paleontological record we have available, it seems to be awfully rare.

Hasn't Multicellularity evolved independently many times on Earth?

Well, I started to cache-dump on colonies versus multicellular organisms, but when I got to the bit on the origin of mutlicellular plants/animals/fungi, I noticed I had a lot of what sure looks like special pleading. And then I noticed myself "rationalizing" the arguments instead of engaging them, because of my emotional dislike of the idea that a Great Filter might be in the future instead of safely in the past.

I am currently raising the probability of a strong filter still in our future on the basis of having one of my two strong past filters seriously weakened. I still have the desire to argue that plant/animal/fungi multicellularity is single ancestral cause (either a single evolution with branches that went back to unicellular life, or "parallel" evolution from a single origin instead of "independent" evolution), but I'm finding things like, for example, "oxygen levels finally got high enough to efficiently support multicellular life" coming in with higher probability.

I would suggest, against the number of species that actually exist, tool use is actually fairly rare.

(An old hat response that your comment still seems to warrant.)

The fact that we are still here reduces the surprise significantly. If intelligence did develop, we probably won't be still here, and we know that we are. It's not just an empty galaxy, it's an empty galaxy where we live. It is not surprising in the same intuitive sense as observing the sequence 0101100100010 of coin flips is not surprising, once you take it as data you have (specification of which phenomenon you are dealing with) and not the property to be explained.

Given the information that we are in the second or third time span required for evolution

There's a twist that actually reinforces your point. The first stars (Population III, all dead now) were metal-free. The next generation, Population II, had low metal content. Our Sun belongs to Population I, with higher metal content. So it looks like our Sun belongs to the first, maybe the second, generation of stars that could possibly support intelligent life.

Science has taught us that we're usually not special

Maybe, but due to anthropic effects, this is one of the times in which we definitely cannot use the we're-not-special rule of thumb. Noting that we happen to have developed gives us absolutely no evidence about the rarity of observers that can notice that they've developed (except that it rules out theories that make it so rare that even 1 observer is unlikely).

if the probability of intelligent life developing in a 3 billion year period is 1/50 we might just need to wait a couple more billion years for the next group

Without more information about The Great Filter, most of the probability density does not reside in such perfectly balanced orders of magnitude (like 2% per 3Gyr-galaxy) to make us happen to be first. Though it's an open possibility that we're the first but not the only life that will develop, it's extremely unlikely that two huge numbers that could conceivably be orders of magnitude apart, happen to line up so closely.

Not necessarily. As Wikipedia says, "According to the Great Filter hypothesis at least one of these steps - if the list were complete - must be improbable." That is, if "Great Filter" means anything, it's that one or more of the steps to achieving a technological civilization that can expand throughout the galaxy is very difficult ("improbable").

What I'm talking about goes like this: suppose that none of the steps are very difficult. Of course, that doesn't mean they're instantaneous - each step takes time. You need elements other than hydrogen and helium for life, so you have to watch the supernova clock ticking until Population I stars form (maybe Pop II, but remember that this is hypothetical - let's assume that only Pop I stars have enough "metals"). Then you need planets - but we're seeing planets everywhere as the limits of our vision increase. Once the first replicator forms, you have to wait for evolution to grind its way up the complexity ladder (mandatory disclaimer: evolution doesn't "prefer" higher complexity, but there is a complexity lower bound - there is literally nowhere to go but up). In this scenario, where none of the steps are improbable, but they do take time, what would the first intelligent species in a given lightcone see?

They would arrive on the scene, and they would see a young-looking universe. Their star would be among the first stars capable of supporting life. Their planetary formation would have been almost immediately followed by the first replicator (and returning to reality for a moment, we see fossilized life as far back as 1 billion years after the Earth's formation). Their evolutionary history, while marked by giant impacts and mass extinctions, would appear relatively free of long reigns of nothing happening complexity-wise. And they would see an empty universe, and wonder where everyone else is.

Obviously, I don't know the answer to the Fermi Paradox. But if anyone exists, someone's got to be first. Maybe it's us, and maybe that's why the paradox is so baffling.

(Instead of "where are they", I think the LW way of phrasing the paradox is "why aren't we paperclips?".)

Part of me tires of people phrasing their agreement or extension as an objection.

The original Fermi Paradox, "where are they", seems sufficiently explained by general relativity and its implications for target selection. Unless you think there is an alien civilization very close by, you'd have no reason to expect a visit since you're not just far away, but unlikely to be even the closest interesting object among the those which are considered "far away".

Obviously the aliens don't use photons to communicate. They use muonic neutrinos, much faster that way.

Could there be an inhabitable planet without distinct climates?