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jacob_cannell comments on Astrobiology, Astronomy, and the Fermi Paradox II: Space & Time Revisited - Less Wrong
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It's the exact opposite.
If the earth was rare, this rarity would show up in the earth's rank along many measurement dimensions. Rarity requires selection pressure - a filter - which alters the distribution. We don't see that at all. Instead we see no filtering, no unusual rank in the dimensions we can measure. The exact opposite is far more likely true - the earth is common.
For instance, say that the earth was rare in orbiting a rare type of star. Then we would see that the sun would have unusual rank along many dimensions. Instead it is normal/typical - in brightness, age, type, planets, etc.
This overstates the case. If planets like Earth were very rare in ways that didn't change much with time you'd still see a time that was typical. One can imagine some things we have a sample size of one for being rare in ways that don't have anything to do with star order - origins of eukaryotes, plate tectonics, oxygenic photosynthesis...
This being said, I think the sheer DEGREE of rare Earth being implied by turchin and others is still very unlikely, even though there's a whole lot that we have little information on. It remains a not fully excluded possibility, but there are a hell of a lot of others.
The time measurement is not the only rank measurement we have. We also can compare the sun vs other stars, and it is mediocre across measurements.
Rarity requires an (intrinsically unlikely, ala solomonoff) mechanism - something unusual that happened at some point in the developmental process, and most such mechanisms would entangle with multiple measurements.
At this point in time we can pretty much rule out all mechanisms operating at the stellar scale, it would have to be something far more local.
Tectonics as rare has been disproven recently. Europa was recently shown to have active tectonics, possibly pluto, and probably mars at least at some point.
For later evolutionary development stuff, it will be awhile before we have any data for rank measurements. But given how every other measurement so far has come up as mediocre . . ..
We can learn alot actually from exploring europa, mars, and other spots that could/should have some evidence for at least simple life. That can help fit at least a simple low complexity model for typical planetary development.
Dear gods yes. We are finally at the point where we can start asking the intelligent questions. We have learned so much about these places and about life on Earth that we forget how little we do know.
If earth is really very rare, it may imply some natural process which kills habitable planets very often. We were just lucky to survive until now, but nothing garantee future survival that said natural disaster will not happened very soon.
For example, the real reason for "rareness" may be intrinsic instability of earthlike planets atmosphere which tend to become either iceball or greenhouse (Mars or Venus). In this case we overestimate stability of our athmosphere and small push could move it into another state. Think about runaway global warming connected with clathrate gun https://en.wikipedia.org/wiki/Clathrate_gun_hypothesis
So rare earth is rising probability of human extinction soon. It doesn't "save" us from technological great filter of self-extermination, which may be another solution of Fermi paradox, if not rare Earth.
Life on Earth has existed for billions of years without experiencing a terminal snowball or greenhouse scenario. It also recovered from several snowball periods once they ended.
So the fact that intelligence took this long to evolve - 4-5 billions of years after biogenesis, and 600-700 million years after the first multicellular animals - must be important.
If it were the case that the Great Filter was the short average lifespan of habitable planets before they became iceballs or greenhouses, then we should expect to appear much much earlier in our planet's history.
We don't know if speed of evolution was maximum possible speed or not.
We could derive some limitations on ice ball probability from the fact that we are late, using the same line of reasoning as was used by Bostrom and Tegmark in their article : http://arxiv.org/pdf/astro-ph/0512204.pdf
If it was the maximum possible speed, then it must have involved very unlikely events that took billions of years to happen maybe just once, and that's evidence of a Great Filter in our past.
If it wasn't the maximum possible speed, then there should be many planets where intelligence evolved much earlier in the Universe's lifetime, and the fact we don't see aliens is evidence of a Great Filter in the future.
Most of the space of possible great filters in the past have been ruled out. Rare planets is out. Tectonics is out. Rare bio origins is out. The mediocrity of earth's temporal rank rules out past disaster scenarios, ala Bostrom/Tegmark's article.
Mediocrity of temporal rank rules out any great filter in the future that has anything to do with other civs, because in scenarios where that is the filter, surviving observers necessarily find themselves on early planets.
Furthermore, natural disasters are already ruled out as a past filter, and thus as a future filter as well.
So all that remains is this narrow space of possibilities that relate to the timescale of evolution, where earth is rare in that evolution runs unusually fast here. Given that there are many billions of planets in the galaxy in habitable zones, earth has to be 10^10 rare or so, which seems pretty unlikely at this point.
Also, 'seeing aliens' depends on our model of what aliens should look like - which really is just our model for the future of post-biological civs. Our observations currently can only rule out the stellavore expansionist model. The transcend model predicts small, cold, compact civs that would be very difficult to detect directly.
That being said, if aliens exist, the evidence may already be here, we just haven't interpreted it correctly.
Really? Why?
We keep finding earlier and earlier fossil evidence for life on earth, which has finally shrunk the time window for abiogenesis on earth down to near zero.
The late heavy bombardment sterilized earth repeatedly until about 4.1 billion years ago, and our earliest fossil evidence for life is also now (probably) 4.1 billion years old. Thus life probably either evolved from inorganics near instantly, or more likely, it was already present in the comet/dust cloud from the earth's formation. (panspermia)
With panspermia, abiogenesis may be rare, but the effect is similar to abiogenesis being common.
I think Robin Hanson has a mathematical model kicking around that shows that, given anthropic selection bias, early life on earth is not evidence that life is an easy step.
I think the argument is that if you need (say) five hard steps in sequence to happen for technological civilization to arise, and each step succeeds very rarely, then if you look at the set of all planets where the first step succeeded, you will see that it is unlikely to happen early.
However, if you look at the set of planets where ALL five steps happened, you always tend to find that the first step happened early! Why? Well, because those were the only ones where there was even a chance for the other four steps to happen.
Anthopics then comes in and says that we are guaranteed to find ourselves on a planet where all five steps happened, so seeing the first step happen quickly isn't really evidence of anything in particular.
~5 billion years out of an expected ~10 billion year lifespan for a star like the sun - mediocrity all the way down!
Sun luminosity is increasing and oceans will boil down in 1 billion years or sooner.
Correct. We are somewhere between 250 megayears and 2 gigayears away from the Earth becoming another Venus depending on whose models you look at (the runaway greenhouse being one of 2 or 3 endpoint outcomes for a terrestrial planet given enough time).
This being said, the whole of earth's history might not be relevant to look at for complex life. Eukaryotes are OLD, gigayears old, but there's a set of paleontologists who think that the Cambrian diversification of macroscopic animals 550+ megayears ago might have been CAUSED by increasing oxygen concentrations which might have something to do with the running down of Earth's geology. More on this in another post; for now I recommend the book "Oxygen: A 4 Billion Year History." If one uses this notion of a windowed subset of time when complex life is possible we could be roughly in the middle of THAT.
Thanks for the link on the book.
I think that intelligent life tends to find itself not in the middle but near the end of any "a windowed subset of time when complex life is possible". One reason for it is observation selection and some considration about distribution of peroids of stability. The longer periods of stability should be more rare. The large catastrophe may be overdue and could be easily triggered by our actions.
Another reason is more speculative (if anything could be more speculative here). It suggest that large changes in climate helps human evolution as they favour more universal ways of fitness, that is human intelligence. But such unstable environment may be near the end of stability period needed for evolution of complex life. (Think about recent Ice ages in last several millions years that coincide with development of Homo.)
My earlier article on the topic is here (need to be retranslated and rewriten):https://www.scribd.com/doc/8729933/Why-anthropic-principle-stopped-to-defend-us-Observation-selection-and-fragility-of-our-environment
Wouldn't mediocrity imply intelligence evolving many times in the Earth's past, which would imply a Great Filter in our near future? See also my other comment.
Depends on what you mean by 'intelligence'.
If you mean tech/culture/language capable, well it isn't surprising that has only happened once, because it is so recent, and the first tech species tends to takeover the planet and preclude others.
If you mean something more like "near human problem solving capability", then that has evolved robustly in multiple separate vertebrate lineages: - corvids, primates, cetaceans, proboscids. It also evolved in an invertebrate lineage (octopi) with a very different brain plan. I think that qualifies as extremely robust, and it suggests that evolution of culturual intelligence is probably inevitable, given enough time/energy/etc.