The above dealt primarily with the first half of your post, but let me also address the 2nd half. You've assigned several probability estimates to various outcomes of our civilization:

1. Collapse/Extinction: "in the 1% to 50% range." I'm inclined to agree with you on this one, as described in the last paragraph of my above post.

2. Biological/Mixed Civilization: “This scenario is almost not worth mentioning: prior < 1%” I think you've defined this a bit too narrowly. We don't yet see any limiting factor for AI advancement besides physics, but that doesn't mean that one won't make itself apparent. Maybe this factor will turn out to be teraFLOPS (aka limited by Moore's law) or energy (limited by our energy production capacity) or even matter (limited by the amount of rare earth elements necessary to make computronium). But it could also happen that we fail to make a super-intelligence at all, or that AI eventually achieves most, but not all, of humans mental abilities. The livelihood of a general intelligence increases asymptotically with time, but I think it would be a mistake to assume that it is increasing asymptotically toward 1. It could easily be getting closer and closer to 0.8 or some other value which is hard to calculate. The existence of the human mind shows that consciousness can be built out of atoms, but not necessarily that it can be built out of a string of transistors, or that it is simple enough that we can ever understand it well enough to reproduce it in code. There's also the existential risk of developing a flawed AI. We only have 1 shot at it, and the evidence seems to be against developing one correctly on the first try. I suspect that the supermajority of civilizations that develop AI's develop flawed AIs. Even if 90% develop an AI before going to the stars, perhaps >99.9999% are wiped out by a poorly designed AI. This would lead to many more “Biological/Mixed Civilizations” than AI civilizations, if the flawed AI's tend to wipe themselves out or not to spread out into the universe.

3. PostBiological Warm-tech AI Civilization: “I assign a prior to the warm-tech scenario that is about the same as my estimate of the probability that the more advanced cold-tech (reversible quantum computing, described next) is impossible: < 10%.” This seems slightly low to me, but not by much. “This particular scenario is based on the assumption that energy is a key constraint, and that civilizations are essentially stellavores which harvest the energy of stars.” Although this state doesn't flow from energy being a limiting factor (aka biological/mixed civilizations may also be energy limited) I agree that such a civilization would eventually become energy limited. I see 2 ways of solving this: better harvesting (aka Dyson swarms, since Dyson spheres are likely mass-limited) or broader civilization (if it takes less energy to send a colony to the nearest star, then you do that before you start building a Dyson swarm).

4. From Warm-tech to Cold-tech: This seems to be where you are putting the majority of your probability mass. I'd probably put less, but that's not actually my main contention. I don't buy that this is sufficient reason to travel to the interstellar medium, away from such a ready energy and matter source as a solar system. You list 3 reasons: lower energy bit erasures, superconductivity, and quantum computer efficiency. Bit erasure costs seem like they would be more than made up for my a surplus of energy available from plentiful solar power, materials for fusion plants, etc. Only a few superconductors require temperatures below ~50 Kelvin, and you can get that anywhere perpetually shaded from the sun, such as the craters on the north and south poled of the moon (~30 Kelvin). If you want it somewhere else, stop an asteroid from spinning and build a computer on the dark side. I'm not sure that quantum computers need to be below that either. Anywhere you go, you'll still be heated be cosmic microwave background radiation to ~4 K. Is an order of magnitude decrease in temperature really worth several orders of magnitude decrease in energy/matter harvesting ability? In order to expand exponentially, such a system would still need huge amounts of matter for superconductors and whatever else.

Thanks for writing this up, I'll add a direct link from the main article under the historical model/early filter section.

So, if there is a filter, it probably lies in the future (or at least the new evidence tilts us in that direction).

You mentioned several possibilities for a great filter in the past, but that was by no means a comprehensive list.

Yes. The article was already probably too long, and I wanted to focus on the future predictive parts of the model.

Before responding to some of your specific points, I will focus on a couple of key big picture insights that favor "lots of aliens" over any filter at all.

Bayesian Model Selection.

Any model/hypothesis which explains our observations as very rare events is intrinsically less likely than other models that explain our observations as typical events. This is just a simple consequence of Bayesian inference/Solonomoff Induction. A very rare event model is one which has a low P(E|H), which it must overcome with a high prior P(H) to defeat other hypothesis classes which explain the observations as typical (high probability) outcomes.

This is not a quite a knockdown argument against the entire class of rare earth models, but it is close.

Observational Selection Effects due to the Simulation Argument

Some physical universes tend to produce tons of simulated universes containing observers such as ourselves. This acts a very large probability multiplier that strongly favors models which produce tons of simulations. The class of models I propose where there are 1.) lots of aliens and 2.) strong motivations to simulate the history of other alien civs are exactly the types of conditions that maximize the creation of simulations and observers.

Now on to the potential early filter stages:

(1. Habitable stars are abundant (20 to 40 billion suitable candidates in the GHZ of our galaxy)

(2. Habitable planets are rare/abundant. Water is common - mars and many other bodies in our system have significant amounts of water.

We have an oddly large moon,

This is true. Our moon is unusual compared to the moons of other planets we can see. However, from the evidence in our system we can only conclude that our moon is roughly a 1 in 100 or 1 in 1000 event, not a 1 in a billion event. Even so, it is not at all clear that a moon like our is necessary for life. There are many other means to the same end.

Even if our planet is a typical draw, it is likely to be an outlier in at least a few dimensions.

(3. Panspermia / Abiogenesis

Recent evidence seems to favor panspermia. For example - see the "Life Before Earth" paper and related.

Also, there's a weird coincidence where the formation of the first life on earth seems to coincide well with the end of the late heavy bombardment,

That's only a weird coincidence if one assumes abiogenesis on earth. Panspermia explains that 'coincidence' perfectly.

(5. Prokaryotic -> Eukaryotic

(6. Multicellular

(7. "Complex Land Life"

Again any model that explains these evolutionary developments as rare events is intrinsically less likely than models which explain the developments as likely events. Systemic evolutionary theory - especially its computational and complexity theory variants - explains how variation and selection over time inevitably and automatically explores the genetic search space and moves through a series of attractors of escalating complexity. The events you describe are not rare - they are the equivalent of the main sequence for biology.

(8. Complex life is common, but is regularly wiped out before it can become intelligent.

Of all your points, I think this one is perhaps the most important. Large extinctions have also acted as key evolutionary catalysts, so the issue is somewhat more complex. To understand this issue in more detail, we should build galaxy simulations which model the distribution of these events. This would give us a better understanding of the variance in evolutionary timescales, which could give us a better idea concerning the predicted distribution over the age of civilizations. On worlds that have too many extinction events, life is wiped out. On worlds that have too few, life gets stuck. We can observe only that on our world the exact sequence of extinction events resulted in a path from bacteria to humans that took about 5 billion years. It is intrinsically unlikely that our exact sequence was somehow optimal for the speed of evolution, and other worlds could have evolved faster.

(9. Complex life is common, but intelligent life is rare.

I addressed this point specifically. Chimpanzees have about 5 billion cortical neurons, elephants have a little more, some whales/dolphins are comparable. All 3 creatures display comparable very high levels of intelligence. Chimpanzees are very similar to the last common ancestor between ourselves and other primates - essentially they are right on the cusp of evolving into techno-cultural intelligence. So complex intelligence evolved in parallel in 3 widely separated lineages.

This is actually some of the strongest evidence against an early filter - as it indicates that the trajectory towards high intelligence is a strong attractor.

What is rare, however, appears to be the capacity for abstract thought.

This is basically nonsense unless you define 'abstract thought' as 'human language'. Yes language (and more specifically complex lengthy cultural education - as feral humans do not have abstract thought in the way we do) is the key to human 'abstract thought'. However, elephants and chimpanzees (and perhaps some cetaceans) are right on the cusp of being able to learn language. The upper range of their language learning ability comes close to the lower range of our language learning ability.

If you haven't seen it yet, I highly recommend the movie "Project Nim", which concerns an experiment in the 1970's with attempting to raise a chimp like a human, using sign language.

In short, chimpanzee brains are very much like our own, but with a few differences in some basic key variables (tweaks). Our brains are both larger and tuned for slower development (neotany). A chimpanzee actually becomes socially intelligent much faster than a human child, but the chimp's intelligence also peaks much earlier. Chimps need to be able to survive on their own much earlier than humans. Our intelligence is deeper and develops much more slowly, tuned for a longer lifespan in a more complex social environment.

The reason that we are the only species to evolve language/technology is simple. Language leads to technology which quickly leads to civilization and planetary dominance. It is a winner take all effect.

(10. Technological civilization

Once you have language, technology and civilization follows with high likelihood.

We haven't evolved noticeably over the past 200,000 years, and yet we only developed agriculture and colonized the planet 10,000 years ago.

Hunter gatherers expanded across the globe and lived an easy life, hunting big dumb game until such game became rare, extinct, or adapted defenses. This led to a large extinction of the megafauna about 10,000 years ago, and then agriculture follows naturally once the easy hunting life becomes too hard.

We didn't invent bronze or written language until 5,000 years ago

Follows directly from agriculture leading to larger populations and warring city-states.

Given all this, I wouldn't be so quick to assume that the great filter is in front of us.

I wouldn't be so quick to assume that there is a filter at all - that is the much larger assumption.

Parrots and other birds seem to be about that intelligent, and octopi are close.

Perhaps that's an argument for the difficulty of the chimp to human jump: we have (nearly) ape-level intelligence evolving multiple times, so it can't be that hard, but most lineages plateaued there.

Er, a few species of placental mammal are hardly "widely separated lineages".

Sure they are - given that the placental clade contains most of the extant mammal diversity.

Trying to draw conclusions for completely alien biologies by looking at convergent evolution inside a clade with a single common ancestor in the last 2-or-3% of the history of life on Earth is absurd.

Hardly. Using the "last 2-or-3% of the history of life on Earth" is perhaps disingenuous, as evolution is highly nonlinear. The entire period from the cambrian explosion to now is what - 15% of the history of life?

More importantly - elephants, cetaceans and primates occupy widely diverse environments and niches.

And the fact that the Placentalia start with an unusually high EQ among vertebrates-as-a-whole

EQ is a rather poor indicator of intelligence compared to total synapse count.

The common placentilia ancestors are believed to be small rodent like insectivores which had small brains - presumably on the order of 21 million neurons in the cortex, similar to rats. The fact that brains increased by a factor of 2 to 3 orders of magnitude in 3 divergent branches of placentilia is evidence to me for robustness in selection for high intelligence.

Now of course, it's fairly easy for evolution to just make a brain bigger. The difficulty is in scaling up the brain in the right way to actually increase intelligence. Rodent brains scale better than lizard brains, and elephant, cetecean, and primate brains scale even better still. So evolution found increasingly better scaling strategies over time, and in some occasions in parallel.

Our universe might be fined-tuned for life because there are a huge number of universes each with different laws of physics and only under a tiny set of these laws can sentient life exist and we shouldn’t be surprised to live in one of these fine-tuned universes.

I was assuming Solonomoff Induction over the full space of computable universes, which is a more principled take on fine tuning and selection effects. We should expect to find ourselves in the universe described by the simplest theory (TOE) which explains our observations.

Our universe might also be fine-tuned for the Fermi paradox, especially if advanced civilizations often create paperclip maximizers.

Paperclip maximizers are a specific absurdity with probability near zero, and I find that discussing them sucks insight out of the discussion.

Perhaps if you look at the subset of all possible laws of physics under which sentient life can exist,

This full set is infinite, complex, and irrelevant - for all we know much of this space could have life radically different than our own. It is more productive to focus on the subset of the multiverse with physics like ours - compatible with our observations. In other words - we are hardly a random observer sampled from the full set of 'sentient life'.

in a tiny subset of these you will get a Fermi paradox because, say, some quirk in the laws of physics makes interstellar travel very hard or creates a trap that destroys all civilizations before they become spacefaring.

Interstellar travel does look pretty hard, but not hard enough to prevent slow colonization. For this argument to apply to our section of the multiverse, it would need to involve new unknown physics. This is one possibility - but it seems low probability compared to other options as discussed in my post.

creates a trap that destroys all civilizations before they become spacefaring. Civilizations such as ours will constantly arise in these universes.

But then they are destroyed. I didn't describe this in my post, but from an observational selection effect, it is crucial to consider the effect of deep simulations. The universes that produce lots of deep simulations with sentient observers will tend to swamp out all other possibilities, such as those where civilizations arise but do not produce deep simulations.

The models I discussed in my post all tend to produce enormous amounts of computation applied to deep simulation - which creates enormous numbers of observers such as ourselves and vastly outweighs universes where civilizations are destroyed.

In contrast, imagine that in universes fine-tuned for life but not the Fermi paradox civilizations often create some kind of paperclip maximizer that spreads at the maximum possible speed making the development of further life impossible.

Right - we don't live in that kind of universe.

Our universe might be fined-tuned for life because there are a huge number of universes each with different laws of physics and only under a tiny set of these laws can sentient life exist and we shouldn’t be surprised to live in one of these fine-tuned universes.

So what you're saying here is that we can conclude we live in one of these fine-tuned universes simply by updating on the fact of our existence, correct?

Computing near the Sun costs more because it's hotter, sure. Fortunately, I understand that the Sun produces hundreds, even thousands of times more energy than a little fusion reactor does, so some inefficiencies are not a problem.

Every practical computational tech substrate has some error bounded compute/temperature curve, where computational capability quickly falls to zero past some upper bound temperature. Even for our current tech, computational capacity essentially falls off a cliff somewhere well below 1,000K.

My general point is that the really advanced computing tech shifts all those curves over - towards lower temperatures. This is a hard limit of physics, it can not be overcome. So for a really advanced reversible quantum computer that employs superconduction and long coherence quantum entanglement, 1K is just as impossible as 1,000K. It's not entirely a matter of efficiency.

Another way of looking at it - advanced tech just requires lower temperatures - as temperature is just a measure of entropy (undesired/unmodeled state transitions). Temperature is literally an inverse measure of computational potential. The ultimate computer necessarily must have a temperature of zero.

You say that the reversible brains don't need that much energy.

At the limits they need zero. Approaching anything close to those limits they have no need of stars. Not only that, but they couldn't survive any energy influx much larger than some limit, and that limit necessarily must go to zero as their computational capacity approaches theoretical limits.

If it's energy, then they will want to pipe in as much energy as possible from their local star.

No. There is an exact correct amount of energy to pipe in based on their viable operating temperature of their current tech civ. And this amount goes to zero as you advance up the tech.

It may help to consider applying your statement to our current planet civ. What if we could pipe in 10000x more energy than we currently receive from the sun. Wouldn't that be great? No. It would cook the earth.

The same principle applies, but as you advance up the ultra-tech ladder, the temp ranges get lower and lower (because remember, temp is literally an inverse measure of maximum computational capabillity).

OK, but more computing power is always better, the cold brains want as much as possible, so what limits them?

Given some lump of matter, there is of course a maximum information storage capacity and a max compute rate - in a reversible computer the compute rate is bounded by the maximum energy density the system can structurally support which is just bounded by its mass. In terms of ultimate limits, it really depends on whether exotic options like creating new universes are practical or not. If creating new universes is feasible, there probably are no hard limits, all limits becomes soft.

So you should get a universe of Dyson spheres feeding out mass-energy to the surrounding cold brains who are constantly colonizing fresh systems for more mass-energy to compute in the voids with

Dyson spheres are extremely unlikely to be economically viable/useful, given the low value of energy past a certain tech level (vastly lower energy need per unit mass).

Cold brains need some mass, the question then is how the colonization value of mass varies across space. Mass that is too close to a star would need to be moved away from the star, which is very expensive.

So the most valuable mass that gets colonized first would be the rogue planets/nomads - which apparently are more common than attached planets.

If colonization continues long enough, it will spread to lower and lower valued real estate. So eventually smaller rocky bodies in the outer system get stripped away, slowly progressing inward.

The big unknown variable is again what the end of tech in the universe looks like, which gets back to that new universe creation question. If that kind of ultimate/magic tech is possible, civs will invest everything in to that, and you have less colonization, depending on the difficulty/engineering tradeoffs.

I agree this is a possibility - a special subtype of the collapse. It seems unlikely to be a convergent enough high probability outcome that it could explain the fermi paradox.

You can't get rid of the waste heat without it being visible. You can't even sequester it - you always need to dump it to a location of lower temperature.

most of the universe's mass was already used up by alien civs.

But then why not all of it? Why leave anything for civs like ours?

I like this quote from Next Big Future: " looking on planets and around stars could be like primitives looking into the best caves and wondering where the advanced people are."