If Post Filter, We Are Alone

Me four years ago:

Imagine that over the entire past and future history of our galaxy, human-level life would be expected to arise spontaneously on about one hundred planets. At least it would if those planets were not disturbed by outsiders. Imagine also that, once life on a planet reaches a human level, it is likely to quickly (e.g., within a million years) expand to permanently colonize the galaxy. And imagine life rarely crosses between galaxies. In this case we should expect Earth to be one of the first few habitable planets created, since otherwise Earth would likely have already been colonized by outsiders. In fact, we should expect Earth to sit near the one percentile rank in the galactic time distribution of habitable planets – only ~1% of such planets would form earlier. …

If we can calculate the actual time distribution of habitable planets in our galaxy, we can then use Earth’s percentile rank in that time distribution to estimate the number of would-produce-human-level-life planets in our galaxy! Or at least the number of such planets times the chance that such a planet quickly expands to colonize the galaxy. (more)

New results:

The Solar System formed after 80% of existing Earth-like planets (in both the Universe and the Milky Way), after 50% of existing giant planets in the Milky Way, and after 70% of existing giant planets in the Universe. Assuming that gas cooling and star formation continues, the Earth formed before 92% of similar planets that the Universe will form. This implies a < 8% chance that we are the only civilisation the Universe will ever have. (more; HT Brian Wang)

Bottom line: these new results offer little support for the scenario where we have a good chance of growing out into the universe and meeting other aliens before a billion of years have passed. Either we are very likely to die and not grow, or we are the only ones who could grow. While it is possible that adding more filters like gamma ray bursts could greatly change this analysis, that seems to require a remarkable coincidence of contrary effects to bring Earth back to being near the middle of the filtered distribution of planets. The simplest story seems right: if we have a chance to fill the universe, we are the only ones for a billion light years with that chance.

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  • Dave Lindbergh

    Just what I need. Responsibility for all life in the universe.

    • Anon

      If there were already evil aliens, we would have to clash with them to prevent their evil.

      But since there are only evil humans, the good guys really just need a coalition against the bad guys here on earth. And since most humans should be roughly in the same ballpark in their degree of evil (compared with alien possibilities), we should converge on some vaguely inefficient outcome no matter what any one individual wants.

      Also don’t forget that the “If we grow” part is a huge if.

  • Lord

    One has to assume we haven’t been colonized but what if the first multicellular organism, first flowering plant, or first land animal was just a first terraforming step? It would seem colonization would require a fairly developed local life if a lengthy terraforming process is not involved.

    • Dave Lindbergh

      The universe doesn’t seem old enough for aliens to have that much patience.

  • Advael

    The accidents of history that lead to the evolution of humans (Or rather, cognitive-niche life of an intelligence level that resembles humans, which I’d say most importantly involves the ability to communicate or store information permanently, forming a basis for understanding the natural world and accumulating advances in technology over multiple generations) may differ even on earth-like planets for a variety of reasons without becoming impossible on those planets.

    It could be that on many such planets, atmospheres formed in a way that’s amenable to cognitive life much later in their history. Or that some large extinction event needed to disrupt an evolutionary equilibrium that makes it unlikely for the the cognitive niche animal to fully flourish to the extent of humans. There are pretty well-respected theories that say events of both of these kinds happened based on extremely low-frequency events on a cosmic timescale, leading to an expected variation in the speed at which human-like life forms on the order of possibly hundreds of millions of years.

    It’ll also be interesting to see strong results as to exactly what’s needed for life-like replicators to arise spontaneously on an earth-like planet in the first place. This might be another event that requires a rare causal factor that might push a variation on its timescale relative to the formation of the planet to a similar level.

    And that’s leaving aside quirks of human history that allowed us to get to where we are now. After all, agriculture didn’t arise until extremely recently in the history of humans (Less than 10,000 years ago, with humans being around for, conservatively, at least a million).

    Basically, I think there are a lot of potential factors that could lead to a huge variation in how quickly life appears and then gets its shit together and starts storing information and building tech that can reasonably be seen as orthogonal to the factors leading to earth-like planet formation in the first place. We may not be one of the first amenable planets to form, but for a lot of reasons we could be the first technological life-form, or at least one of the first. My point isn’t that I think we are, but that you’re drawing your conclusion too early.

    • truth_machine

      On top of that, human industrial civilization will likely end within the next two hundred years, and a reoccurrence won’t be possible due to resource depletion.

  • lump1

    Here is my summary of the argument, which I think is rather persuasive:

    If there was another us-like civilization in the Milky Way and we are both post-filter, then with a very high probability, they would already have visible colonies around here. Since they don’t, we are either pre-filter, or we are the only us-like civilization in the Milky Way.

    I’m working on a draft of an article which considers an argument much like this, though the topic is SETI. According to my thesis in that article, SETI is basically over and it found nothing. The way I motivate this conclusion is by trying to first establish some sort of a priori metric of ways in which aliens are likely to be detected.

    Imagine if we had our present understanding of science, including cosmology, biology, economics, chemistry, etc. And yet, for some reason, we never began looking around in the world, though we are just about to start. Once we begin examining the world, we are alert for evidence of aliens. We begin at home, first examining easily accessible parts of Europe. But before we do, we are supposed to estimate ahead of the search the odds that we find evidence of aliens in the search, conditional on the assumption that intelligent aliens exist in our galaxy. We make that estimate based on all the science we know.

    This sort of thinking is parallel to the familiar procedure where we estimate the probability that my keys are in the coat pocket, conditional on the the assumption that they’re in the apartment. Once we checked the coat pocket and found no keys, our subjective probability that the keys are not in the apartment should increase by a certain percentage. As we check other possible hiding places and find no keys, that probability will increase in some disciplined way that we can outline even before we start looking.

    I think it’s a reasonable guess that if there are intelligent aliens in the Milky Way, the a priori odds of them having left easily visible evidence somewhere in Europe is something like 30%. This is because of what we know about life: It tends to spread in such a way that it doesn’t leave out accessible habitats. Why would it ignore a perfectly fine continent of a perfectly usable planet? So basically, simply by opening our eyes, about 30% of SETI’s search space has been explored.

    Then of course we widen our search beyond Europe, and beyond the immediately visible. We dig underground, we examine microscopic scales, look under the sea, check remote places like Antarctica, see if there are alien structures visible by telescopes. This covers another substantial portion of the a priori SETI search space. By the time we start doing the sort of stuff that we now call SETI, the majority of the SETI search space had already been exhausted. But of course, good telescopes that cover a wide range of the electromagnetic spectrum will knock out another large part of the search space, provided they find nothing.

    Given this way of setting up the issue, I’m curious how much of the total a priori search space we have covered by now. I hold that we are well above 90% – that is, on the assumption that intelligent life exists in the galaxy, we should have estimated before started looking that once we have examined all the stuff we’ve actually looked at, we should have estimated a 90%+ probability that we would have spotted them by now. It’s just a priori improbable that intelligent life would be so hard to see that it would have evaded all our searches.

    What we are doing now is the equivalent of tearing up the linoleum in the kitchen as we search for our keys. Sure, it’s not impossible that we’d find them there, but why the heck would they be under the kitchen linoleum? That’s just very a priori improbable, and if all the more probable places have already been explored, we should just accept the idea that the keys are not in the apartment. That’s basically the argument. I’d appreciate comments.

    • Sigivald

      It sure seems like the apartment metaphor for “the entire galaxy”, where “everywhere but the surface of the earth” is 90+% of “the apartment” is fundamentally flawed.

      (All it takes is the example of the Earth “hippie” to realize that it’s perfectly plausible to have intelligent aliens all over, if their dominant government says “hands off the fledgling planets, don’t mess with them”.

      [This is not meant as an ‘obviously aliens would be deep ecology environmentalists because shut up!’, but to point out that we already have, in our own cultures, a pattern that would explain such a possibility.]

      The rest of the galaxy is more like 99.99% of the search surface, not 5%.

      The weakness of SETI might just be “assuming that aliens even bother with radio emissions of any power”…)

      • lump1

        Yeah, I mean, if intelligent life could sit around quietly like a set of keys, I’d agree. But what are the a priori odds of that? Very low, I think, based on the most reasonable inferences about how intelligent life works.

        Searching for intelligent life in the galaxy is much more likely to be like a search for a ringing cell phone. If you visited all the rooms in the apartment and still haven’t found it, it’s probably not in the apartment at all. Sure, the speaker might have busted today, or it might be under a pile of pillows, so I’m not saying the chance is zero. But the chance is low as long as the a priori probability of such states is very low.

        A lot also depends on your estimate of the prior probability of it being in the apartment. If you can be almost certain that it’s there somewhere, then yes, it’s still worth checking under all the pillows, in the toilet, etc.

        Here’s the rub, though: The probability of the conditional “If intelligent alien life exists in the Milky Way, we would have probably detected it by now” is a priori high. The proposition “There is intelligent alien life in the Milky Way” also has a high a priori probability, but I think the probability of the conditional is higher, ergo the “we’re alone” conclusion. This is not a knock-down argument, but I think it’s the right way to frame the problem.

    • Ilya1981

      Fairly recently, there was an discussion/argument between Hanson, Brin, etc on http://www.cato-unbound.org/2014/12/01/david-brin/seti-meti-paradox-extraterrestrial-life-there-libertarian-perspective about similar topic, but mostly from the perspective of “Should we scream out into Space about our existence?”

      If I understood everything properly, Hanson and Brin broadly agreed on us needing to shut up. Their differences were mostly on the topic of what enabled (and allows) civilization to flourish. Brin (whom I respect) is very much mesmerized with the idea of “diamond-shaped” distribution of power, whereas Hansom (with whom I agree, in this case), said that yes, competition is needed, but it doesn’t need to be between individuals within a global framework: competition between societies themselves (think group selection/”multiple-pyramid-shaped” power distribution) is quite sufficient, and that eventually for the advancement. Moreover, eventually (long-term) most important criteria will become how cohesive said societies are, especially, with the emergence of genetic engineering and stoppage of “natural” selection. Hanson is correct, IMO.

      Anyway, the point that’s applicable here is that aliens may decide fairly early on in their evolution to not broadcast into space. They may also mask their activities within Dyson spheres or even by figuring out how to use Hawking radiation of their own/artificially produced black holes (latter would probably be residents of different galaxy). So, I don’t think that your “key” is an apt analogy.

      We might be the only civilization for the next 10**9 years, but that’s still not a certainty. What is close to a certainty, is that *any* possibly existing civilization within the Milky Way Galaxy (100,000 light-years in diameter) is either too primitive or is very advanced. If the latter exist, depending on their philosophy, it’s very possible we’re going to be detected within the next 50K-250K or so, and possibly annihilated. It’s a virtual certainty (though *even this* isn’t guaranteed) that we are the only civilization with current level of development.

      I also don’t disagree with Sigivalid’s response.

      • lump1

        Yeah, but Hanson and many others also believe that the future of our society will be dominated by emulated minds, or some other sort of non-biological cognizers. And it would be puzzling why non-biological agents could respect arbitrary expansion boundaries, given that basically, any concentrated source of energy and matter make a fertile territory in which to sew duplicates/descendents/allies. The argument works almost as well even if you assume that we’ll remain biological to the end. We don’t need to find habitable planets in order for us to comfortably colonize extrasolar space, just like past a certain level of technology, we don’t need to find caves in order to move in to new lands.

        Alien technological societies, if they exist, are probably extremely old. I’m not saying that it’s impossible that they all live under the thumb of a dictatorship that is so stable and powerful that it thwarts every individual effort to escape its containment and spread into the galaxy, for hundreds of millions of years. I just have to assign that a very low probability. Same with the non-interfering groupthink hippie aliens who, for the sake of civilizations like ours, successfully scrub out all traces of their activities. It’s a possibility, but you can’t tell me that you would assign it any significant a priori likelihood.

        At one point I worried about berserker sterilizers who quietly monitor and destroy all potential emerging competition. I stopped worrying when I realized that the best way for them to kill competitors is not with quiet kill probes but simply by expanding as fast as possible, and paving over the upstarts along the way. The best way to expand quietly is to do it at the speed of light. Then literally, nobody will see you coming. So, yeah, I have to assign quiet lurking berserkers a very low initial probability. My point is that basically every kind of technological society that would be invisible to us has a vanishingly small a priori probability, and that’s a good reason for thinking that they don’t exist.

        If you’re not convinced, try to imagine knowing everything you know about scientific principles, but being entirely ignorant about what is in reality, as in: you haven’t yet investigated what exists beyond your nose. Then assume technological aliens exist in the Milky Way, and make a list of all the most probable sorts of societies that they would have, judging probability based on your current knowledge of the natural laws, not based your awareness of contingent facts about what’s where. I think if you do this right, over 90% of the probability space you sketch will be composed of alien societies which, if they existed, would have been detected by now. Honestly, I think it’s much higher than 90%, I’m just being conservative.

  • http://praxtime.com/ Nathan Taylor (praxtime)

    This is a great paper. Thanks for posting. One thing I’m confused about is you saying (at least I think you are) our chance of meeting aliens has appreciably changed. Meeting aliens depends most on time scale of expanding to fill the galaxy compared to the time scale of life to evolve. As long as that time scale is fast (1 million years) relative to time for life to evolve (billions of years), the chance of meeting aliens in our own galaxy remains miniscule. For example if the result was not Earth coming 80 percentile of time but instead was 1%, the chance for more human like intelligence to evolve over the age of the universe goes up. Sure. But in terms of meeting aliens the time to expand compared to time to evolve is still super tiny, albeit slightly better. On the other hand, if expansion were super slow, say 100 million years, and time to evolve after formation of earth-like planet was super fast (say 10 million years), then chance of meeting is high. I think the latter case was never even close to being on the table though. Time to expand is so super short that the chance of meeting aliens remains nil, even if mathematically it’s better if that 80% were much lower (since that lowers time to evolve somewhat).

    Suspect you’d agree. Just a framing question here on “chance to meet aliens” is lower. Was never really going to happen.

    Also, seems like under these circumstances most likely way to meet aliens is by expanding to other galaxies. I assume on time scale of billions of years, that could be invented and done. Hence seems like cross galaxy looking for life makes most sense, regardless of number/rate of formation of Earthlike planets.

    • http://overcomingbias.com RobinHanson

      Even going to other galaxies won’t let you meet aliens if they are billions of light years away.

      • http://praxtime.com/ Nathan Taylor (praxtime)

        Have you done the math? Billions of light years away seems possible. It’s a race between expansion wave of (very very few) ETIs to find each other against end of stelliferous era and ever expanding and growing apart universe. If expansion wave is, say .1c on average even crossing between galaxies, that would get you pretty far in 100B years. Though the earlier you start the better before galaxies spread too far apart. Certainly could get to local group or perhaps virgo supercluster or Laniakea. If you have worked it out would love to see a link. Maybe I’ll try to do some basic arithmetic for a post if I have time this weekend.

      • Curt Adams

        0.1 c is very fast both from an energetic PoV and in terms of travel hazards. Project Daedelus-style vehicles need a fuel:payload ratio of 20,000:1 to make 0.12C and then stop at the destination, and that’s with He-3, which is not easy to come by. Collision hazards are pretty extreme at those speeds too.

        Also, in many senses we may not have 100 billion years. Star formation is dropping quickly, on the order of halving every billion and a half years.

        If the speed is reduced to 0.001 c (still pretty fast for a generation ship) and the time limit reduced to 10 billion years, we’re not going to ever get further than the clusters adjacent to the Local Group, and even that would be quite a stretch. Probably never beyond the Local Group itself.

  • Robert Koslover

    If ALL humans were to instantly vanish from the Earth, right now, how long would it take for a new comparable technological society to arise? Who among the remaining animals on the land or in the sea would evolve/advance enough to actually form societies that can effectively record their acquired knowledge and heritage, and convey it from one generation to the next? Chimps/apes, whales/dolphins, elephants, etc, all have large brains, and have coexisted with us on this planet for a very long time. So where are their great cities? And where are the cities built by those enormous dinosaurs (who actually lived on Earth much longer than we have)? Beavers, at least, build dams, but what do those oh-so-smart dolphins and whales ever build? Is it so unreasonable to guess that even in many millions of years, if left entirely alone, these brilliant animals just might not advance technologically whatsoever? In what way is a modern whale technologically ahead of any whale that lived 50 million years ago? How many more years will it take for elephants to master the practical use of fire? I assert that for every alien world with human-level (or more) achievements, there exist countless other worlds absolutely teaming with life, even including intelligent life (like whales, elephants, etc), but yet no technology whatsoever. Even in a life-filled galaxy, we may be quite anomalous.

    • http://overcomingbias.com RobinHanson

      There is surely a filter in the step from smart animals to civilization. But the key question is: how big a filter?

      • truth_machine

        You don’t seem to have understood a word that Robert wrote.

        Dinosaurs lived for 150 MYA without producing civilization or even tool users. Why not? because evolution does not generally favor such traits. Humans are a fluke.

        The most widespread misunderstanding of evolution is that it is directed and that humans are at “the pinnacle” and that cognitive capability is generally an evolutionary advantage. This is completely wrongheaded, but widely believed … a sad statement about the state of biological education.

      • IMASBA

        The question is: were there ever such “smart” dinosaurs? Where smart means as smart as a dolphin, whale or great ape (all of which have existed for much less 150 million years), and if so, for how much of that 150 million years did they exist (the highly specialized theropod dinosaurs who were probably the most intelligent dinosaurs mostly existed near the end of those 150 million years)?

      • truth_machine

        No, that’s not the question. There weren’t ever any dinosaurs “smart” enough to produce civilization because, again, it is not the case that “cognitive capability is generally an evolutionary advantage”. And so it will be on most planets with life … ecologies lasting hundreds of millions of years without giving rise to tool users.

      • IMASBA

        Robert Koslover specifically mentioned a filter between smart and civilized animals. Smart animals are rare, but did evolve indepently several times and these animals do use tools (perhaps velocitaptors did too, we’ll never know).

      • truth_machine

        “Robert Koslover specifically mentioned a filter between smart and civilized animals.”

        No he didn’t, Robin Hanson did … but a) “a filter” is a ridiculous way to talk about such evolutionary paths and b) neither that nor anything else you wrote has any bearing on what I wrote.

      • zarzuelazen

        Actually, its even worse than that, because even *if* intelligence was a clear evolutionary advantage, evolution can’t pre-plan, so some minimum level of intelligence needs to appear by chance *before* it starts getting selected for.

        But even animals with minimal intelligence are already extremely complex! So what are the odds all this complexity would arise in the first place? Very very low I would say.

        For billions of years on Earth there was no intelligence at all, just single-celled microbes. Nothing but MICROBES for BILLIONS of years. Zero chance of intelligence being selected for, because intelligence was not even there yet.

      • truth_machine

        pre-planning has nothing to do with it. If cognitive capability were *generally* an advantage, then *any* increase would get selected for. “some minimum level” is there from the get go in any organism that discriminates, including microbes. It is because being a better discriminator is not *per se* an evolutionary advantage that the dominant organisms on this planet are unicellular.

        ” So what are the odds all this complexity would arise in the first place? Very very low I would say.”

        This a Creationist argument. The odds are very high, *given enough time*; that’s how probability works. Complexity arises inexorably in evolution, for the same reason that a bunch of computer cables inexorably become more entangled over time, and stirring cream into coffee mixes them and never unmixes them … because tangling happens more readily than untangling. But while more intelligence requires more complexity, more complexity alone does not imply more intelligence. And the word “more” there is important; you are treating it like a binary attribute. Of course, there are gates, like harnessing fire and learning to make ever more effective tools, but those are cultural, and culture evolves far more rapidly than biology.

      • zarzuelazen

        I don’t think the transition from smart animals to civilization is that unlikely. It took a long time in human terms (millions of years), but that’s like the blink of an eye on a geological time-scale. My intuitive guess is that at least 20% of intelligent species go on to civilization. So no, I don’t think that step is a great filter.
        See my other post for 2 far more plausible candidates for past great filters.

    • Diadem

      How do you know there were no earlier civilizations here on earth? An very advanced civilization earlier civilization we would see traces off, but it’s perfectly possible that many species before us developed hunter-gatherer level civilizations before, and just vanished without a trace.

  • TheBrett

    I suppose we’ll just have to agree to disagree on the whole “if human level intelligence develops, it will expand quickly across the galaxy” thing. I don’t see humans leaving the solar system at all, and what we do send out (if we do) will be digital/electronic and operating under different assumptions – an upload/AI doesn’t have to “colonize” much of anywhere, because it can send itself to do periodic missions then transmit itself back to a base to be re-integrated into the memories and mind of another version of itself so they share the memories gathered.

    If they’re out there at all, I’d expect them to be clustered in extremely metal-rich solar systems while sending out probes/using telescopes to periodically check up solar systems every few hundred thousand years or so. Our solar system is not a metal-rich one – in fact, considering that a majority of the solar systems we’ve discovered are compact systems with few or no gas giants and tons of potentially terrestrial worlds, we got the short end on rocky material.

    • http://overcomingbias.com RobinHanson

      Evolutionary pressures to expand apply just as strong to ems as to humans or animals.

      • truth_machine

        There is no “evolutionary pressure to expand”. Evolution is simply the change in allele frequency in a population over time. The pressures on populations come from the environment, not evolution. Properly understood, evolutionary pressure refers to the increase in a population of genetic traits that mitigate environmental threats; it is pressure on allele distribution. “evolutionary pressure to expand” is a category mistake.

        Populations expand when there’re nothing to prevent them from doing so, but that’s rare. Most populations are contained, not expansive.

        “Imagine also that, once life on a planet reaches a human level, it is likely to quickly (e.g., within a million years) expand to permanently colonize the galaxy.”

        Imagine all you want, but there’s no rational basis for this expectation. On the contrary, physical limitations make this extremely implausible.

      • arch1

        I don’t get the “evolutionary” modifier either. But assuming steadily increasing population requiring steadily increasing resources, this seems to imply over the long term an inexorable pressure to expand (one can only milk so much from a given volume of space).

      • truth_machine

        This is circular, since you *assumed* a steadily increasing population. Of course increasing populations imply expanding in area, but it’s the claim that there is “pressure” to increase population that needs substantiation.

      • Stephen Wordsworth

        “Properly understood, evolutionary pressure refers to the increase in a population of genetic traits that mitigate environmental threats”

        That is a secondary effect, evolution only responds to “environmental threats” by how much it affects the ability of that organism to reproduce. Evolution is just the almost tautological observation that those that are better at reproducing become more numerous over time. There is a common myth that “removing environmental threats stops evolution”, which is pure nonsense, it only shifts selection from survival traits to raw fertility. Modern living seems to neutralize some reproductive instincts in the present breed of humans but this will not be permanent over time humans will evolve genetic adaptations to modern living just as they did to farming etc, and like all life forms reproduce exponentially until they have filled the available habitat if we get something like fusion rockets then that could be the resources of the entire solar system, turn all the minerals in asteroids into cartwheel habitats. Before the solar system is totally mined out it would make sense for habitats to migrate to other solar systems as they will all be descended from those who had the strongest drive to reproduce of each generation.

        If robots take over and wipe out organic life its just the same thing the robots that make the most copies of themselves become more numerous over time.

  • arch1

    0) what a cool paper to have written – thanks to the authors and to you for the pointer!
    1) the 8% upper bound is very interesting, but that concerns the currently-visible Hubble volume; and as noted in the last paragraph of 4.1, most of *that* region’s planet formation (thus presumably most of its civilizations) will be unobservable to us by the time they occur. As a practical matter wouldn’t it be even *more* interesting to calculate the analogous upper bound for *that portion of spacetime potentially observable by us*?
    2) When I try to confirm Eq (2) I lose a factor of N. Can someone confirm or (more likely) debug my derivation? a) applying Bayes Theorem, P(N|f,E=1) = P(E=1|f,N) * P(N)/P(f,E=1); b) the RHS denominator=1 by assumption so we need only evaluate the 1st factor in the RHS numerator; c) if each of the other (N-1) civilization-spawning planets falls in that fraction f of planets which haven’t yet formed (this has probability f^(N-1)), then E=1 definitely; but d) E=1 could also occur if some of the N-1 planets *have* formed but *haven’t* yet spawned their respective civilizations; *but* e) if we assume civilization gestation time is small relative to the overall timespan of 100BY-1TY this doesn’t affect the answer much and we have P(N|f,E=1) ~ f^(N-1) P(N).
    3) As a bonus I saw the spiffy diagram in Wikipedia which makes graphical sense of Bayes Theorem – check it out.
    4) As a double bonus, thanks to that diagram remembering the formula is now as ez as 1-2-3 (actually, easier):
    1: P(A|B)P(B) = P(B|A)P(A) is both ez to remember and obviously true (since LHS and RHS each equal P(A AND B)), right?
    2: divide both sides by P(B) and you’re done!

  • zarzuelazen

    Agree with the conclusion. The simplest explanation for the Fermi paradox that fits the facts is the boring one: that technological (space-faring) civilization is exceedingly rare, and that the filters are probably in the past.
    In order to cut the probabilities down low enough, one filter is probably not enough: you need at least 2 unlikely steps (where I define ‘unlikely’ as odds of less than 1 in 10 000 ). With 2 filters , the probabilities when multiplied cut the numbers down to what is observed: less than 1 space-faring civilization per galaxy.
    The 2 best candidates for the filters are:
    (1) The transition from single- to multi-cellular life and
    (2) The transition from multi-cellular to intelligent life
    I think these are the great filters due to the length of time it took on Earth for these steps to occur (billions of years). I assign both of these steps odds of less than 1 in 10 000 each of occurring within the available time (where ‘available time’ is defined as ‘the period when a planet is potentially hospitable to life’).

    • Stephen Wordsworth

      As we have no idea how hard the filters are to cross we dont know how many filters are needed. Could be a single trillion to one filter. Maybe the creation of the first cell and genetic replicator, it involves several components coming together. There could be a lot of lifeless, sterile yet potentially fertile and Earth like worlds out there just waiting for us to seed them.

  • turchin

    If any ET kills all other possible ETs, we could be only the first one. (And we will kill all other civilizations). It also explains why we are relatively early from all possible planets. ( first 10 per cent). It is the same as we are the first and only life on Earth, and the only intelligent beings ( we killed Neanderthals).

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  • Rafal Smigrodzki

    We are the Firstborn! So exciting!

    The only alternative would be if it was possible for many aliens to exist without at least one of them becoming an exponential interstellar replicator. Even a single such organism would very quickly take over all available resources, just as a single bacterium is capable of quickly using up all available media in a ridiculously short time. We can already envision the technologies needed to become interstellar replicators, capable of spreading at a substantial fraction of c, without postulating any new physics. If there were aliens, they would have the physical ability to visibly settle the universe. Since they are not visible, either they don’t exist, or else there is some universal non-physical (i.e. social) stricture that prevents, over billions of years and millions of solar systems, the emergence of exponential replicators. I feel it is highly implausible that such anti-exponential social law exists, although it is not completely impossible.

    So, yes, most likely we are the Firstborn. Let’s build the laser launchers for our von Neumann probes soon, before something bad happens to us.

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