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    More on the Fermi paradox (from David Wallace).

    In a comment to the previous post on the Fermi Paradox, David Wallace wrote this terrific exposition of it.  I agree with almost everything he says, so I will save my own quibbles for a comment.  David writes:

    Just to clarify the force of the Fermi paradox (which is intended to rest on quantitative factors, not just a general "where are they?"), here are the main basically-uncontroversial premises that get it going:

    1. The Galaxy contains an awful lot of stars, and plenty of them have planets. There are 100+ billion stars in the Galaxy, and in our neighbourhood, around 5% of them are G-type stars like the Sun in non-binary systems. Our best theories of planet formation make planets look generic, and the current era of exoplanetology backs that up. I think it would be very hard now to argue for fewer than several hundred million Earth-type planets, and that's almost certainly a significant underestimate.
    2. The Galaxy is much older than it is large. The Galaxy is something like 100,000 light years across. But it's nearly ten billion years old, which is to say that light could have travelled across the Galactic disk and back some 50,000 times.
    3. Slow interstellar travel (c.0-1%-1% of light speed) is technologically possible. Pioneer 10 is already a starship, travelling at (1/30,000) of the speed of light, and that was by accident. There are a quite large number of designs and plans for spacecraft capable of reaching between 0.1% and 10% of light speed. That we are unlikely to build any of them any time soon – the mission time is too long and the cost is currently rather painful – isn't my point; the point is that this kind of speed is borderline possible with *current technology*, let alone the extrapolated capabilities of much-more-advanced civilisations. (Contrast relativistic starflight, which doesn't violate any known physical principle but isn't remotely possible with any feasible technology we can think of now; contrast faster-than-light starflight, which actually looks *physically* impossible.)
    4.  Self-replicating intelligent machinery is technologically possible. We have an existence proof for this: humans. (I guess I can't absolutely rule out some strange principle of emergent engineering which makes it physically impossible to make a slow-moving space probe capable of carrying machines (humans or not) that can reproduce the probe even though self-replicating intelligence and probes are separately possible, but it seems ad hoc.) Given 2-4, it is technologically possible for a civilisation to send physical vehicles to every solar system in the Galaxy in a timescale *much* less than the age of the Galaxy. (Err on the pessimistic side: assume 0.1% light-speed propulsion and a 1000-year turnaround time; you still get an expansion wave moving at 0.05% of lightspeed, enough to span the galaxy in 200 million years, or 2% of its current age. Why might a civilisation do so? Several plausible reasons: (i) colonisation. As Allen Olley points out, this isn't a plausible way to move your existing population (I don't really agree with Eric's resource-based argument for colonisation) but it is a plausible way to spread your civilisation, or your species, across the Galaxy. (ii) exploration. Robot probes is a really great way to explore space, as we've found ourselves. (See the Wiley paper cited above.) (iii) malevolence or prudence. People very often react by noting that each of (i)-(iii) is speculative, and maybe intelligent civilisations don't tend to do that kind of thing. Fair enough. But the crucial issue in the Fermi paradox is that *it only has to happen once*. And unless life *basically never* evolves, or intelligence *basically never* evolves, or civilisations *basically always* die off before they get to the stage of exploring the galaxy, there ought to have been so many civilisations that it seems implausible that not even one would do this. (Note that I'm pretty sure *we* would do it, if we got to the point at which it was reasonably inexpensive.) An illustration: suppose life only turns up on 1% of earth-type planets, and that intelligence only evolves in 1% of biospheres, and that 99% of civilisations wipe themselves out through war or resource exhaustion before reaching the stage when it's feasible (indeed relatively cheap) to spread across the Galaxy. Then, on a pretty conservative estimate of 1 billion habitable planets, there should have been 1000 suitable civilisations so far in Galactic history. Yet *not even one* decided to try the explore-the-galaxy strategy? Things get several orders of magnitude worse when you consider that the strategy works perfectly well over intergalactic distances too, and that there are hundreds or thousands of galaxies in realistic exploration range. Could they be here but not be noticed? Well, it's quite hard to hide a decelerating starship, but maybe none happen to have turned up recently. There could be all manner of things hiding in the Solar System. But again: does *every single* civilisation behave that way? It seems unmotivated to suppose so. (Unless there is a single hegemonic species or coalition that requires it coercively.) I think the force of the Paradox is this: yes, for any given civilisation we can imagine reasons why we could be completely oblivious of it despite it being millions of years older than us and quite capable of having spread across the Galaxy. But unless technological civilisations are fantastically unlikely to arise on a per-planet basis, there should have been such an enormous number of them by now that it becomes really implausible that *none of them* have done so.
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    16 responses to “More on the Fermi paradox (from David Wallace).”

    1. Eric Winsberg Avatar
      Eric Winsberg

      I have only two small amendments I would make to this, and they both have to do with with the finite resources of a planet (where I am including the capacity of a planet to absorb pollution and waste as a “resource.”)
      1. I agree that interstellar travel is not a plausible way to move a planet’s population. But I don’t think it follows from that that running out of planetary resources cant be a motivation for interstellar travel. You might want to simply save the species, and send away a small colonization group to do this.
      2. Though David say so explicitly, he doesn’t seem to worry as much as I do about the possibility that its a practically exception-less generalization that “civilisations basically always die off before they get to the stage of exploring the galaxy.”
      First, I think that’s got to be the weakest link in all the above. I might not be inclined to believe this without the Fermi paradox as evidence, but if I have to break the chain somewhere, this is where I probably do it.
      Second, I am sometimes struck by how incredibly short the history of human civilization is even compared only to the history of human life let alone to the history of animals, of of life, or the earth, or the galaxy. We have shockingly little evidence that planets can sustain industrial civilization for very long at all, and some non-trivial evidence that they cant.

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    2. Alan White Avatar
      Alan White

      Eric (if I may)–
      As you can see from my comment on the original thread, I concur that David (if I may be so familiar) covers a lot of good ground. But there’s a lot of uncovered ground as well.
      In our own instance of an evolved space-capable species, the almost inconceivable contingencies of evolution have had a lot to do with our being here. It’s well-known that a global extinction-event probably made room for us. That is incredible (and by “incredible” I really mean incredible!) serendipity against a happily (for us) and powerfully biodiverse history of species that posed our possible evolution but itself might well have not occurred, and thus by failure of one conjunct of history or asteroid not us. Not to mention the possibilities of utter extinction events that might have occurred, or still might. I’d wager that that extra extra-terrestrial factor–the possibilities of ET evolution being crucially interrupted in its course or simply destroyed as opposed to being enabled as in our own case by an extinction event–is one that might well change the estimates of the possibility of tech-savvy life evolving in the universe. I’ve nothing but a slightly informed gut-feeling about this–but evolution that parses out to beings aspiring to their local planets–much less the stars–must be very rare indeed.

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    3. Alan White Avatar
      Alan White

      Eric (if I may)–
      Just lost an hour of writing to trusting that the post would work, so I’ll be briefer. Better for the blog anyway.
      I agree that David’s (if I may be so familiar) post was excellent, as I said in the previous thread. But there is a lot of uncovered ground that affects this whole line of argument.
      We are here most plausibly because of an extinction event. That was a form of serendipity cast against the very contingent fact that evolution up to that event cast a happily (for us) rich biodiversity of species that could give rise to us. We owe our being here to that conjunction of factors, neither of which was likely separately but in combination is even more unlikely. The strong possibility for extinction events for influencing ongoing forms of ET evolution (overwhelmingly as potentially eliminative of the development of intelligent life, and only extremely rarely as a necessary condition for that as with us) must be an extra extra-terrestrial consideration that we must consider to evaluate how frequently such processes result in tech-savvy intelligence in the universe, and my slightly-informed gut tells me that this kind of contingency greatly reduces those chances.

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    4. Alan White Avatar
      Alan White

      Eric (if I may)–
      Just lost an hour of writing to trusting that the post would work, so I’ll be briefer. Better for the blog anyway.
      I agree that David’s (if I may be so familiar) post was excellent, as I said in the previous thread. But there is a lot of uncovered ground that affects this whole line of argument.
      We are here most plausibly because of an extinction event. That was a form of serendipity cast against the very contingent fact that evolution up to that event cast a happily (for us) rich biodiversity of species that could give rise to us. We owe our being here to that conjunction of factors, neither of which was likely separately but in combination is even more unlikely. The strong possibility for extinction events for influencing ongoing forms of ET evolution (overwhelmingly as potentially eliminative of the development of intelligent life, and only extremely rarely as a necessary condition for that as with us) must be an extra extra-terrestrial consideration that we must consider to evaluate how frequently such processes result in tech-savvy intelligence in the universe, and my slightly-informed gut tells me that this kind of contingency greatly reduces those chances.

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    5. Daniel Greco Avatar
      Daniel Greco

      We’re asked to suppose that intelligence turns up in “only” 1% of biospheres, and given the context, it looks like that is supposed to sound like a conservative estimate. I don’t really have any idea, but I wouldn’t be surprised if it were orders of magnitude too high. After all, intelligence has evolved only once, while lots of other traits (eyes, flight) have independently evolved in many different lineages. In what percentage of biospheres do you suppose elephant-style trunks evolve? Would 1% seem conservative, too high, or would you have no idea? I’m inclined towards either “too high” or “no idea”, and it’s not clear to me why intelligence ought to be thought of as all that different. For all we know, the evolution of intelligence in humans was a highly contingent event that depended, e.g., on our already having the rare trait of bipedalism (perhaps, without bipedalism, which may have evolved for independent reasons, intelligence wouldn’t be much use, since we wouldn’t have free hands to use for building/using tools). I don’t offer these hypotheses as an argument for thinking that the true frequency with which intelligence evolves on life-supporting planets is much less than 1%–just as considerations in favor of the idea that it might be much less than 1%, consistent with what (little) we know about how and why intelligence evolved in our lineage.

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    6. CJ Avatar
      CJ

      Doesn’t showing that it is probable that some intelligent species spreads across the galaxy only show that most habitable planets will at some point have been colonised? If the death rate of planetary societies is high enough, it needn’t be a surprise that there’s no-one else here right now. Given that planets that become vacant will be re-colonised eventually, a planet should expect to be colonised every Y years where Y is a number I can’t be bothered to estimate. Perhaps Y is small enough, though, that we should expect paleontologists to find evidence of alien visitors in the recent past, let alone the 3 billion years or so of Earth’s history that we can study.
      Say that P is the proportion of the number of planets in the galaxy that are inhabited. If, as you argue, it is probable that a civilisation has already arisen that had means and motive to colonise the galaxy, then P probably rose quickly at that point and has been relatively stable ever since. I wonder what the dynamics of P are? I imagine that, if it neared 1, the rate of colonisation would decrease as it became harder to find unoccupied planets. If P neared 0, the rate of colonisation would again drop as fewer societies capable of seeding colonies remained, though it would still be positive. The death rate of planetary societies might increase or decrease with P, depending on whether cooperation or competition dominates. There is also the rate of increase of P due to evolution of new intelligent species to consider; it probably doesn’t depend much on P, though (the rate of evolution of intelligent species will surely increase with P as existing intelligent species diversify, but the resulting new species will be on already inhabited planets, so they won’t contribute to increasing P).

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    7. Charles R Avatar
      Charles R

      Has intelligence only evolved once on this planet?
      It’s interesting reading this while also reading Montaigne’s Apology for Raymond Sebond, where Montaigne spends quite a bit of time working out, with some success and some failure, the various ways animals demonstrate intelligence that our particular form of pride denies to them. So, that stuck out to me.
      What’s meant by intelligence? Strictly the kind that builds the infrastructure necessary to leave the gravity well of a planet?

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    8. Avital Lamarck Avatar
      Avital Lamarck

      This ‘intelligence’ may be thought in too teleological a manner. Was it necessary for the people of say, what is now called Myanmar, to come to Western European thought?
      I find a read of some so-called ‘existentialism’ (read as neo-historisist thought) ought to shake our certainty about these preoccupations, that orbit around Western European notions of progress, as scientific notions of science, as the meaning of all things ‘intelligence.’
      That could be a very rare form of intelligence and by no means a necessary mutation of thought.
      On the other hand, why assume that such cultures would have the same curiosity about zipping about here and there, that may be totally unimpressive to most beings. Just as many intelligent people find theorizing about such twaddle totally fruitless.
      just as Kant never imagined people on other planets would differ from us in anyway but morally, we hardly bring ourselves beyond the Western European concept, morphological and mutational statistics.

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    9. David Wallace Avatar
      David Wallace

      to Eric:
      Fair points. But consider:
      (i) There are no fundamental depletable resources on Earth other than solar energy (which is depleted on a timescale much longer than relevant here). Basically everything else is technologically reversible (even radioisotopes in principle, but certainly anything lost to a chemical process). Building a civilisation that’s long-term sustainable in this fashion doesn’t seem technologically unreasonably difficult. In fact, it looks about as hard as slow starflight, i.e. demanding given our current technology but just-about-in-reach even now.
      (ii) It’s legitimate to have extreme doubts about whether our species will avoid resource-led extinction in the next century or two. But the drivers here are mostly sociological, not technological. It is, in particular, not really controversial that we could decarbonise the economy without really dramatic hits in our level of technological sophistication. The blocks to doing so are political and social. But I find it really implausible that there are iron laws of sociology and politics, across the whole range of possible technologically advanced civilisations, that rule out technologically-possible resource management with the reliability required to block the Fermi conclusion.
      (iii) Saving the species from resource depletion by extrasolar colonisation assumes that terraforming extrasolar worlds is easier than working out how to live sustainably on this one. I think there are serious reasons to doubt this. We’re very adapted to Earth, and if we had a sufficiently good grasp of its biosphere to replicate it elsewhere, I’d have thought we’d have a sufficiently good grasp to fix it here.
      To Daniel Greco: you misunderstand my argument. I don’t claim that a 1% intelligence chance is a sensible or conservative estimate; I claim only that it’s a reasonable proxy for “not really unlikely”. Name notwithstanding, the Fermi paradox isn’t a literal paradox: it’s a attempted demonstration that a bunch of premises – that the evolution of life on Earthlike planets isn’t really unlikely, that the evolution of intelligence in biospheres isn’t really unlikely, that the survival of intelligent species long enough to build a reasonably-advanced technological species isn’t really unlikely – are jointly inconsistent with the data. (And those premises aren’t a straw man: they’re basically the underlying premises of SETI.) If you read the thread from which this one emerged, you’ll see that I’m sympathetic to exactly the point you make (and indeed partly as a result of reading exactly the Steven Pinker discussion of the point from which I assume you drew your trunk example).
      To Alan White’s worries (on the previous thread) about Klaatu and Gort: well, I said it was a science-fictional comment. But in seriousness, it’s pretty standard in SETI to assume that advanced intelligent life is guaranteed to be benevolent, and I do find that curious. The only example of such life that we know has a very questionable track record of handling first-contact situations with more primitive societies. It’s difficult to be confident of a hypothesis when there’s only one data point; it’s doubly difficult when that datapoint is a counter-example to the hypothesis! If we ever do discover an intelligent extraterrestrial civilisation, I’m in favour of keeping very quiet and hoping it doesn’t notice us.

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    10. Allan Olley Avatar
      Allan Olley

      Surely one possible conclusion of the Fermi paradox is the lack of evidence of what we would recognize as alien technological civilizations is evidence for their absence
      Obviously worrying about that particular kind of alien life is very anthropocentric (since it is looking for life like us), but I don’t think that negates it as a valid observational category we can theorize or speculate about, it may even be a valid pragmatic category since we might well get a lot more use out of aliens like us in the use of technology than other forms of intelligence.
      I would say the lack of evidence fails to tell us anything because it can be explained by any number of links in the chain, and I don’t see a reason for finding one scenario more probable than another, although we may yet forge more links in the chain. Before the discovery of exoplanets there was still at least some reason to question our ability to consider the link in the chain of whether Earth-like planets exist in abundance in this galaxy. If we discover that life independently emerged on multiple planets in this solar system that would presumably also make us able to answer with some confidence what we expect the number of planets with life on them spread over the galaxy to be and so on.
      A point about self-replicating machines, sure all living creatures are self-replicating machines, but living creatures are dependent on a great many support structures (mostly other living things, but also a nice planet to live on and shield them from deadly radiation etc.) and very narrow conditions to do their self-replication. Indeed decades of human space flight has demonstrated how ill adapted humans are for space flight, if your proof that self-replicating machines can exist are life, you still need proof that self-replicating machines that are capable of long distance space travel can exist and flourish in all the conditions other solar systems offer. All our machines tend to depend on particular support strucutres also.
      I think the kind of technological optimism suggested by the view that we could have some interstellar spanning technology in a few decades or centuries, calls into question the idea that resource collapse or war would be a big threat for technological aliens, since the ability to construct technology usable for interstellar travel suggests the ability to construct technology to weather such a global catastrophe (it should be easier to create life support systems for beings on a desolated planet than for ones traversing the vast desolation of space).
      Note, if we remove mass colonization and Malthusian scenarios as the reason for colonizing and make it about small colonization to insure species survival/continuity/legacy then I don’t see that there is a rational for exponential colonization which Eric originally thought was likely by analogy to biological exponential expansion.
      Sure perhaps we might imagine that civilizations often destroy their origin world and need to take a Mulligan on a new one with a small group of survivors (perhaps augmented by a giant bank of genetic material from the old world) or some appropriate legacy (some artificial or genetically engineered successor beings), but having screwed up the first time, do we really expect them to decided to go whole hog on replicating the problems of the prime world in secondary world? I would guess such survivors would favour a much smaller footprint on world 2 with a much longer mean time to civilization collapse/resource depletion thus removing the need to colonize further to avoid resource depletion. An infinity of other motives remain including not wanting all ones eggs in one basket.

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    11. Gordon Knight Avatar
      Gordon Knight

      A good case can be made that whales and dolphins are a form of intelligent life on the earth that we are only beginning to recognize as a form of intelligent life (in the last 30 years or so). Dolphins and such are unlikely to develop technology, but that does not make them less intelligent. Similar water life or other stranger life forms may evolve on other planets and yet not develop a “civilization” as we understand it. From our own case its a hasty generalization to suppose that cites, technology and the like are typical results of intelligence (though this is likely the case for life forms that are similar to us)

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    12. David Wallace Avatar
      David Wallace

      Gordon: I’m using “intelligence” as a shorthand for “technological intelligence”, just because that’s the sort that shows up in SETI and could plausibly be starfaring. Neither SETI nor the Fermi Paradox is concerned either way with possible non-technological species. If you like, break the Fermi paradox’s reductio into four assumptions, not three: life is not really unlikely; intelligence is not really unlikely given life; technology is not really unlikely given intelligence; interstellar colonisation/exploration is not really unlikely given technology.
      That having been said, last time I looked at the science the case for human-level intelligence in dolphins or whales looked very poor. But you might well know more about this – if so I’d be interested what the current state of play is. (It’s highly relevant for the topic at hand: if dolphins have human-level intelligence then intelligence would have evolved twice, independently, on Earth.)

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    13. Daniel Butt Avatar
      Daniel Butt

      David: “If we ever do discover an intelligent extraterrestrial civilisation, I’m in favour of keeping very quiet and hoping it doesn’t notice us.”
      The, er, renowned science-fiction writer Robert Nozick has a line on this: http://myweb.lmu.edu/tshanahan/Nozick-RSVP.pdf

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    14. TEDchris Avatar
      TEDchris

      Great discussion. One extension of Fermi’s Paradox puzzles me as much as the standard thought about colonization. Why don’t we see any signs of massive artwork or engineering? The analogy would be this. Suppose another species on earth became intelligent enough to start being puzzled by something like the Fermi Paradox reduced to planetary scale. “If we’re intelligent, surely we’re not the first creatures on earth to be so. Do we see any other evidence of intelligence?” And a quick look around would discover supertankers, cities, electric lights, etc. Humanity’s impact is highly visible, even from space. Hard to imagine that at least some of David Wallace’s intelligent species wouldn’t have grand ambitions to, say, make the night sky more beautiful, or turn a nearby star into a chemical factory with a completely different spectroscopic signature from other stars. My own instinct is that type of visibility is prima facie even more likely than that of a spaceship showing up. But certainly, when we look out there, so far we haven’t seen any cosmic artworks or engineering. Maybe we should steer a portion of SETI effort more in this direction? I haven’t seen much discussion around this.
      Two intriguing ‘solutions’ to the paradox overall. 1) Advanced intelligences realize that all the fun of existence is generated by complex information flow, and that biology is a deeply inefficient way of achieving this. So overtime they miniaturize into more sustainable, more efficient, faster forms. Is it just possible that the galaxy is teeming with life and we’re just looking at the wrong scale? 2) If you take the multiverse as a whole, it’s possible that the majority of technologically intelligent civilizations appear in universes in which they are incredibly rare. This would be so if the physics of fine-tuning for life were such that only a tiny number of universes had abundant life, and a much larger number had conditions for life being generated only incredibly rarely, perhaps once per galaxy (if they even had galaxies). Life (and technological intelligence) appears to have evolved only once on the so-called ‘life-friendly’ planet we call earth. That’s evidence that even here it’s a long-shot for life to get started (unless you assume that once DNA life forms exist they make it impossible for other life forms to get under way).
      I hope 2 is false, and personally think 1) may turn out to be the case.

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    15. Gordon Knight Avatar
      Gordon Knight

      David: the example of dolphins is intended to be illustrative. Whether they are as intelligent as humans or not quite so intelligent is besides the point though this article: http://www.spiegel.de/international/world/scientists-dolphin-intelligence-may-be-overrated-a-924614.html is full of logical flaws. Clearly creatures such as Cetaceans can be as intelligent as us or mores o and at the same time not develop a technological civilization.
      I think its really hard to judge how intelligent or “what it is life” to be a dolphin because, while they evolved on the same planet as we did. They show signs of intelligence, but its a lot harder for us to interpret them because, unlike apes, they are both smart and unlike us. The point is that even if life evolves relatively often, and likewise gives rise to intelligence, the le of civilization does not follow. So I accept your new version of the paradox, though now (to me anyway) it does not seem very paradoxical. Maybe intelligence often leads to technology, maybe not–we just don’t have the evidence.

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    16. Mark Cordell Avatar
      Mark Cordell

      Maybe other civilizations in the galaxy developed a communication network with each other before the physical exploration effort got underway, and then decided not to pursue physical exploration because they exchanged information so much more quickly than could done via physical travel.

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