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.
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 true for all cases where the observer is not noticeably entangled in a causal manner with the event they are trying to observe. Otherwise, the Observation Selection Effect can contribute false evidence. If we presumed that earth is typical, then there should also be life on Mars, and in most other solar systems. However, we wouldn't ever have asked the que...
Our sun appears to be a typical star: unremarkable in age, composition, galactic orbit, or even in its possession of many planets. Billions of other stars in the milky way have similar general parameters and orbits that place them in the galactic habitable zone. Extrapolations of recent expolanet surveys reveal that most stars have planets, removing yet another potential unique dimension for a great filter in the past.
According to Google, there are 20 billion earth like planets in the Galaxy.
A paradox indicates a flaw in our reasoning or our knowledge, which upon resolution, may cause some large update in our beliefs.
Ideally we could resolve this through massive multiscale monte carlo computer simulations to approximate Solonomoff Induction on our current observational data. If we survive and create superintelligence, we will probably do just that.
In the meantime, we are limited to constrained simulations, fermi estimates, and other shortcuts to approximate the ideal bayesian inference.
The Past
While there is still obvious uncertainty concerning the likelihood of the series of transitions along the path from the formation of an earth-like planet around a sol-like star up to an early tech civilization, the general direction of the recent evidence flow favours a strong Mediocrity Principle.
Here are a few highlight developments from the last few decades relating to an early filter:
The Future(s)
When modelling the future development of civilization, we must recognize that the future is a vast cloud of uncertainty compared to the past. The best approach is to focus on the most key general features of future postbiological civilizations, categorize the full space of models, and then update on our observations to determine what ranges of the parameter space are excluded and which regions remain open.
An abridged taxonomy of future civilization trajectories :
Collapse/Extinction:
Civilization is wiped out due to an existential catastrophe that sterilizes the planet sufficient enough to kill most large multicellular organisms, essentially resetting the evolutionary clock by a billion years. Given the potential dangers of nanotech/AI/nuclear weapons - and then aliens, I believe this possibility is significant - ie in the 1% to 50% range.
Biological/Mixed Civilization:
This is the old-skool sci-fi scenario. Humans or our biological descendants expand into space. AI is developed but limited to human intelligence, like CP30. No or limited uploading.
This leads eventually to slow colonization, terraforming, perhaps eventually dyson spheres etc.
This scenario is almost not worth mentioning: prior < 1%. Unfortunately SETI in current form is till predicated on a world model that assigns a high prior to these futures.
PostBiological Warm-tech AI Civilization:
This is Kurzweil/Moravec's sci-fi scenario. Humans become postbiological, merging with AI through uploading. We become a computational civilization that then spreads out some fraction of the speed of light to turn the galaxy into computronium. 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.
One of the very few reasonable assumptions we can make about any superintelligent postbiological civilization is that higher intelligence involves increased computational efficiency. Advanced civs will upgrade into physical configurations that maximize computation capabilities given the local resources.
Thus to understand the physical form of future civs, we need to understand the physical limits of computation.
One key constraint is the Landauer Limit, which states that the erasure (or cloning) of one bit of information requires a minimum of kTln2 joules. At room temperature (293 K), this corresponds to a minimum of 0.017 eV to erase one bit. Minimum is however the keyword here, as according to the principle, the probability of the erasure succeeding is only 50% at the limit. Reliable erasure requires some multiple of the minimal expenditure - a reasonable estimate being about 100kT or 1eV as the minimum for bit erasures at today's levels of reliability.
Now, the second key consideration is that Landauer's Limit does not include the cost of interconnect, which is already now dominating the energy cost in modern computing. Just moving bits around dissipates energy.
Moore's Law is approaching its asymptotic end in a decade or so due to these hard physical energy constraints and the related miniaturization limits.
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%.
From Warm-tech to Cold-tech
There is a way forward to vastly increased energy efficiency, but it requires reversible computing (to increase the ratio of computations per bit erasures), and full superconducting to reduce the interconnect loss down to near zero.
The path to enormously more powerful computational systems necessarily involves transitioning to very low temperatures, and the lower the better, for several key reasons:
Assuming large scale quantum computing is possible, then the ultimate computer is thus a reversible massively entangled quantum device operating at absolute zero. Unfortunately, such a device would be delicate to a degree that is hard to imagine - even a single misplaced high energy particle could cause enormous damage.
Stellar Escape Trajectories
The Great Game
If two civs both discover each other's locations around the same time, then MAD (mutually assured destruction) dynamics takeover and cooperation has stronger benefits. The vast distances involve suggest that one sided discoveries are more likely.
Spheres of Influence
Conditioning on our Observational Data
Observational Selection Effects
All advanced civs will have strong instrumental reasons to employ deep simulations to understand and model developmental trajectories for the galaxy as a whole and for civilizations in particular. A very likely consequence is the production of large numbers of simulated conscious observers, ala the Simulation Argument. Universes with the more advanced low temperature reversible/quantum computing civilizations will tend to produce many more simulated observer moments and are thus intrinsically more likely than one would otherwise expect - perhaps massively so.
Rogue Planets
Although the error range is still large, it appears that free floating planets outnumber planets bound to stars, and perhaps by a rather large margin.
Assuming the galaxy is colonized: It could be that rogue planets form naturally outside of stars and then are colonized. It could be they form around stars and then are ejected naturally (and colonized). Artificial ejection - even if true - may be a rare event. Or not. But at least a few of these options could potentially be differentiated with future observations - for example if we find an interesting discrepancy in the rogue planet distribution predicted by simulations (which obviously do not yet include aliens!) and actual observations.
Also: if rogue planets outnumber stars by a large margin, then it follows that rogue planet flybys are more common in proportion.
Conclusion
SETI to date allows us to exclude some regions of the parameter space for alien civs, but the regions excluded correspond to low prior probability models anyway, based on the postbiological perspective on the future of life. The most interesting regions of the parameter space probably involve advanced stealthy aliens in the form of small compact cold objects floating in the interstellar medium.
The upcoming WFIST telescope should shed more light on dark matter and enhance our microlensing detection abilities significantly. Sadly, it's planned launch date isn't until 2024. Space development is slow.