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The Fermi Paradox

223 points by itamarb 12y ago

wavesum 12y ago
I think the biggest fallacy in this rationalization is when it is assumed that advanced civilizations would want to expand like yeast; consuming everything, building dyson spheres... Why would they want that much energy, and why extract it from the sun? Trying to picture myself as one of these superlifeforms, I think would like to have the sun visible... For sunbathing and sh... plants to grow etc... Maybe they have built little fusion reactors wherever they need energy?
- sooheon 12y ago
  I agree. Why the assumption the Type 3 capable civilisations will necessarily wish to metastasise across the entire universe? One would hope that civilisations' ideas of what constitutes fulfilling and worthwhile activity will evolve and improve along with their technology.
- fernly 12y ago
  The SUN is visible everywhere within a Dyson sphere; it's the STARS that you can't see any more. The idea isn't that a civilization wants to do this; the idea is that it needs that much energy to continue with whatever industrial or infrastructure processes it requires to support a very large population. Dyson reasoned, we need steadily increasing amounts of energy input to support what we are doing; if the trend continues, what's the limit? (Typical physicist question, no?) The limit is to harvest all the energy emitted from your star. The only way to do that is to build a spherical collector, so that every photon can be captured. Then where do you live? On the sphere's inner surface. Of what material could you build a stable sphere one AU in radius? Where would that bulk of material come from and how be processed and positioned? How would you provide gravity on its inner surface? We have no fucking idea -- but we are not a Type-whatever civilization.
  - JoeAltmaier 12y ago
    I always thought that was a primitive notion. We can build the sphere, but not control the star? Imagine a caveman - "Limit to civilization when all energy from campfire captured - most people warm, most meat cooked" Clearly one can do better than a campfire. Instead, imagine harvesting energy from the star asymmetrically - from flares or from the inside out. Or get your energy from a neighboring star. Or make your own star. Or whatever a Type-whatever civilization decides to do.
ZanyProgrammer 12y ago
My solution to the Fermi Paradox: Who the hell knows, who the hell can know, when all we have is a sample size of 1 to extrapolate from. The solutions to the FP seem to be representative of the personal beliefs of whomever is discussing it.
- Udo 12y ago
  Of course no one can now right now, it's not one of these things that can be solved by theoretical work alone. At some point - assuming we don't go extinct before reaching it - we will know. But it's still open to useful speculation, as there might be something we still need to consider in order to avoid said extinction. The Fermi Paradox is one of the more thought-provoking signs that something might be amiss (at least in our part of the galaxy), and especially because our culture is predisposed to not take anything happening on the interstellar stage very seriously this provides an impetus for reflection that would otherwise not happen. One of the most benign (but ultimately depressing) possibilities is that biogenesis is somehow extremely unlikely. There is no reason to assume this is true, given that we know even the more complex building blocks of life are actually very prevalent throughout the universe - but it may still be statistically unlikely that cells form from them. On the footsteps of that possibility follows the hypothesis that technological intelligence is rare. Again, that's not exactly in line with what we can observe on Earth, but it might still be the case. Most of the other options should be positively troubling, though.
  - ZanyProgrammer 12y ago
    "The Fermi Paradox is one of the more thought-provoking signs that something might be amiss (at least in our part of the galaxy), and especially because our culture is predisposed to not take anything happening on the interstellar stage very seriously this provides an impetus for reflection that would otherwise not happen." A hypothetical observer in North America, circa 1500, would have no idea that Asia and Europe existed. Perhaps we're not too far from some interstellar space empire, but since they use some exotic means of FTL communication, we can't detect them?
    - 12y ago
ccvannorman 12y ago
i love this puzzle. here are some theories: 1. simulation argument 2. civilizations have a high or inevitable chance to self destruct after <1M years of language 3. civilizations transcend our observable universe after <1m years [1] 4. due to the single observer problem, humans fail to grok some very important feature of larger scale space that prevents detection/increases isolation [1] http://accelerating.org/articles/transcensionhypothesis.html http://accelerating.org/articles/transcensionhypothesis.html
- ZanyProgrammer 12y ago
  Or we just haven't observed them yet because we aren't technologically advanced enough? What if they don't use radio?
  - Flow 12y ago
    I hope there's something along the lines of sub-space transmissions. Once we discover it, we'll start listening and discover a plethora of civilisations out there. Would also be nice if they periodically transmitted decoding instructions :) But who knows, maybe it's far more rare with life that are similar to what we can recognise?
    - XorNot 12y ago
      Pretty much the scientific experiment I've been waiting for: we set up some exotic state of matter, and then spot modulated and structured signals coming out of it.
      - danielweber 12y ago
        Except all the messages are spam.
- Pxtl 12y ago
  4 seems likeliest. Interstellar colonization is simply impossible because of the vastness of the distance that must be covered. It requires more energy than can be summoned to get a useful amount of hardware across multiple lightyears.
fxj 12y ago
when the question is: "where are they?" and the answer is: "there is nobody here." then all long living civilisations must have left. so we should answer the question "why did they leave?"
- Lrigikithumer 12y ago
  Or maybe the long living civilisations never existed, or we aren't knocking on the right doors, or in the right areas?
gpsx 12y ago
This article does a good job of covering a lot of different ideas on this paradox. I have a problem with the idea of "big numbers" in it though, like where it implies 500 billion billion is nearly infinite, or that one-in-a-billion is a freak occurrence. There is a saying that if enough monkeys pound on a keyboard, one will type Hamlet. This would take a lot of monkeys. If we take the odds of hitting one character to be 1 in 40 (which doesn't account for properly hitting the shift key to make a capital), factoring in the 130,000 letters in Hamlet (according to Wikipedia), we get the odds of a monkey typing it at 6x10^208267. This number comes from 40 keys and 130,000 letters. How about when we start taking molecular numbers of items (6x10^23) and start raising that to a large power? I don't think it is a given that the number of planets out there infer there should be other life.
- _greim_ 12y ago
  We can't calculate the probability of life arising though because even if we did know how it arose here—which we don't exactly—we don't know all the possible different ways it can arise, which may or may not also be a number raised to a large power.
  - gpsx 12y ago
    That is a good point. I agree with you. And people do have suggestions as to how life can be more probable than naively expected. But this answers the question why there is life at all as opposed to why we don't see more of it. My bias is that life is highly improbable and that we will not find any other life in our universe. But I also think it is no miracle that we exist, as I will describe. Given that I am trying to say this in one paragraph, it may not sound well justified, but it is how I view the problem. I'll start with Schrodinger's cat. A cat is in a box with some radioactive source that can decay and kill the cat. You can ask if the cat is alive or dead - and the answer is both. I will ignore the next part (Schrodinger's whole point) about what happens when the human observes it. There are different interpretations about why the human thinks the cat is definitely in one of the two states as opposed to both. This has to do with the human's observation and is not important for now. The point is that the cat is in both states. The universe can be in multiple states, some with life on earth and some without. Every possible state that can exist, does. So no matter how improbable we are, we will exist in some version of the universe. And there would be lots of other life too, just maybe not in our version of the universe. So the probability of life existing doesn't impact whether or not we should exist, but it does impact whether or not we see other life. I don't think we will. But as correctly pointed out, we don't know the answer to this right now.
equil 12y ago
I've always wondered at what distance it would be impossible for current technology to detect life on earth.
- maaku 12y ago
  A very short distance. IIRC something like 20 or so light years, but I'm having trouble finding the citation. That's for radio waves (and the number is getting smaller as the technology is getting better -- EM radiation blasted into the universe is considered waste by hardware engineers). For direct detection of life, that's still an open question. I remember a poster done by a grad student who looked at whether life would be detectable from the Moon, looking at the Earth. This was tested with data from one of the outer solar system probes which did a lunar flyby on its way out (Cassini?). The result, IIRC, was inconclusive -- you could see signs of life, but not anything that was absolutely definitive proof. Indirect evidence might be provided by spectra of the atmosphere observed via solar transit. A biome is likely to have different spectra than would be predicted by inorganic atmospheric physics. Still, that's making some assumptions about what extraterrestrial life would be like, and only only visible along the elliptic plane.
  - sampo 12y ago
    The chemical composition of Earth's atmosphere (21% O2) is so far away from a long term chemical equilibrium, that any current astrobiologist would interpret it as a sign of planetary scale life. (And correctly so: the oxygen was produced by life.) So life on Earth is pretty visible from the other planets in our solar system. And spectroscopy to observe the composition of atmospheres of planets orbiting the nearest stars will probably happen in not-too-distant future.
    - maaku 12y ago
      Meh, it's a long way from saying that "we don't know how the O2 could have been produced except by life" to "the O2 was produced by life." Just look at the current debate about methane on Mars.
maaku 12y ago
The "we are the first" argument seemed less convincing than it could have been. A better argument from that perspective is the observation that Type III civilizations expand at an alarming rate. Whatever the motivation for expansion[1], basic simulations and back of the envelope calculations show that even with technology we could imagine building in the not so distant future, humans could expand into the cosmos at >0.9c. We must assume the same of ET. Now information only reaches us at the speed of light; we are only capable of looking for ET in our own past light cone. That means that any Type I or Type II civilization like us should expect to see an empty sky for most of their existence, until all of a sudden the most distant stars go dim (Dyson sphere, or whatever energy capturing device). And the darkness spreads in a wavefront travelling at almost the speed of light, until it hits and ... the unpredictable happens. We see an empty sky because if the sky wasn't empty, the planet we call Earth and our Sun would already be consumed by some extraterrestrial Type III civilization and Fermi would never have existed. [1] An inflationary universe provides incentive for a hypothetical Type III civilization to spread throughout the cosmos as quickly as possible, as idle time means regions of the universe becoming permanently inaccessible. Or maybe just resource competition and a desire for some elements of a population to remain on the frontier.
- aswanson 12y ago
  Sounds like a hell of a book/movie plot.
- sampo 12y ago
  > until all of a sudden the most distant stars go dim Why the most distant?
  - jacquesm 12y ago
    Because if the events move at 0.9c the light still moves at 'c', so the furthest stars (of which there are more than nearby ones) will be the first to show you that something is up. The light from the older events will reach us before the events or the light from later events will reach us. Unless of course the whole thing originates in one of the stars nearby but the chances of that are smaller than that it will happen in a system further away.
Xcelerate 12y ago
I am surprised a more common argument was not mentioned: other life exists but the laws of physics limit our interaction. Maybe there is undiscovered physics like wormholes or time travel, or maybe there isn't and the speed of light really is a fundamental limit that cannot be surpassed. In that case, no matter how advanced another civilization becomes, they still can't violate natural law and are thus unable to reach us.
- joe_the_user 12y ago
  The laws of any potential biology and the messiness of the world could together effectively limit a civilization's energy usage to much lower than indicated by the easily deducible laws of physic (the explicit laws of physics might limit the output of an internal combustion engine to a certain but the effective maximum winds up less than that. Human biological systems experience serious damage by being weightless for significant periods, etc). I only recently realized that this "paradox" looking at the possibility not of "earth advanced" civilization but supposed advanced civilizations that would put out much more energy than humans put out. The whole thing seems to rest on false-extrapolations of mid 20th century hard-science fiction authors. Our Western Civilization has experienced an exponential growth of some things, has expanded it's "frontiers" quickly in a variety of ways, conquering the rest of the globe and then labeling space "the final frontier". But all this has been low-hanging fruit. We're reaching the end of population expansion, space is fundamentally hostile to human life in a way that would take tremendous progress to overcome and we're facing longer term consequences of the rapid expansions we have achieved (global warming is just one of the consequences). If humans survive this, we'll have to have somehow achieve a steady-state for energy consumption and related things. But once we do achieve that steady state, why would we want or need to suddenly start using the energy of an entire star (after the thousand years of scientific progress we'd need to under how to do that)?
  - XorNot 12y ago
    The problem here is the assumption we need scientific progress to do this. We don't. We have the technology today to launch satellites into solar-orbits which would be capable of beaming power back to Earth. It would be clean, efficient and non-disasterous to the biosphere. With any amount of space-borne industry, the costs would also plummet and get cheaper with every additional satellite. Once you can harness the energy of an entire star, there's almost no reason not to achieve it.
ScottBurson 12y ago
How about this possibility: the Great Filter is now! We entered it, perhaps, when we developed sufficient collective nuclear weapons capability to wipe out most or all human life; we won't be out of it until the major threats our civilization poses to the biosphere have been resolved.
- epaulson 12y ago
  Even if we wipe ourselves out tomorrow, that still means that a civilization was able to broadcast its existence for about 80 years. If the Great Filter is now or ahead of us, that at least implies that pre-filter civilizations can be advanced enough to signal their existence. At some point our signals will be too weak for anyone to pick up or distinguish from cosmic background noise, which probably limits the radius of the sphere of stars that can hear us, but anyone inside that sphere gets at least 80 years to be listening to potentially hear us. So, even if civilizations only last for say 100 years on average, and you can only hear other civilizations within N light years, if N is big enough eventually someone should hear someone, and if N is really big enough, somewhere in that 100 years everyone should hear at least one other someone.
  - fixedd 12y ago
    I've written about this elsewhere, but I'll chime in here too. The largest radio telescope we have (the 305 meter diameter Arecibo) would need to have it's sensitivity increased by around two orders of magnitude JUST to pick up our TV/FM/AM signals from outside the solar system. If we move into the narrowband signals then, depending on the source-strength, it could pick up signals at up to a few thousand light years... if it happened to be pointed in exactly the right direction at exactly the right time. You'd have to build an absolutely monstrous dish to be able to detect any of our signals at even the center of our own galaxy (much less at the other end)... I haven't run the numbers on it, but I doubt it's physically possible to build a receiver large enough. Add to that that they're highly directional devices and I think you'll find the "N" in your scenario is actually a startlingly small number.
- has2k1 12y ago
  I'll add another guess. A resource crunch, i.e. we simply use up the required resources before we figure out how to explore our planetary neighbourhood. e.g When we figure out that a helium engine is possible, very efficient and very safe, we no longer have sufficient quantities left.
zw123456 12y ago
I think they sort of cover this but here is my simple explanation. Think about how many slugs there are on Earth. How many of them have never seen a human? If we are slugs in comparison to advanced beings that have a billion years of evolution on us then it is no surprise we have not seen them.
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mangecoeur 12y ago
I don't see why a "Great Filter" is needed to justify there being no other highly evolved lifeforms - all it needs is for there to need to be a reasonably large number of key evolutionary steps and environmental conditions with a sufficiently low probability. Multiply those and you end up with some pretty tiny chances of observing life as we know it.
- keule 12y ago
  I had the same thought and agree with what you wrote. But when you think about this set of evolutionary steps you still end up with the same scenarios: you are rare, you are first, you are fucked. So in that regard, you can subsume all of these steps into a filter.
mda 12y ago
I think Authors is way way optimistic with these numbers: "Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life" and " "And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth" Why 1% and not 10^-10%?
- Coincoin 12y ago
  Agreed. It's a common error to think that the smallest fraction of something is 1%.
dedward 12y ago
In that great expanse and time, there may also have been ample time for civilizations to rise, then fall. We might have just missed them on such grand scales. Nobody says we'll be hanging around for a billion years trying to communicate with others... if we even last that long.
dennisgorelik 12y ago
How about the theory that we are the way super-civilization spreads? If physical space travel is extremely expensive (as it should be, considering the distance), then may be it is better to send a signal that would trigger creation of life (in our case on Earth). Then when Earth civilization is advanced enough, it would be able to receive complete boot sequence and then fully advanced alien civilization would be replicated on Earth.
- fernly 12y ago
  ... or, how about the hypothesis that we were intended to be a seed package (a.k.a. "biological rootkit") but due to a bug in the programming, we failed to develop properly. Just a mistake playing itself out...
thelollies 12y ago
Another possibility... there are plenty of civilisations within broadcasting distance from us that are all listening but not broadcasting for fear of predatory civilisations.
- thefreeman 12y ago
  that exact possibility was listed in the article.
  - thelollies 12y ago
    That's funny! I read the article and then had that thought five minutes later and thought I'd come up with it. Thanks for pointing that out :)
    - chatmasta 12y ago
      I often wonder how many of my opinions derive from this same mistaken originality... Quite scary, actually.
cdelsolar 12y ago
We might be very close to finding life on Mars. If we did and it was different than us that would answer a lot of questions.
arikrak 12y ago
The estimates of stars seem reasonable; the estimates of Earth seem a bit ill-defined. (The study he links to discuses Earth-size planets, but there are many other factors that go into Earth's suitability for life.) The real issue though is in the "speculative" part: > Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life... And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. " 1% sounds like a small number, but it's a bit ridiculous to just throw it out there and assume it's reasonable. It's a little bit like when a startup does a top-down estimate of revenue - "If we only capture 1% of the e-commerce market, we'll be worth billions!" I think if you do a "bottom-up" style analysis of the likelihood of atoms forming a replicating growing organism, or that organism evolving to think intelligently, you would get a far smaller number, say in the range of 0.00001% to 10^-20 for each one.
pigboy 12y ago
Out of all those advanced civilizations in the universe, what fraction use electromagnetic radiation for communication? We are like ants waiting for other civilizations to communicate with us using pheromones.
- danielweber 12y ago
  There are humans that try to talk with ants (and bees, and dolphins) using those creatures' own methods. Unless there were strongly-enforced no-contact laws, then some alien researcher would want to inspect those Terrans and see if we can talk. Even if they are way smarter, they'll use that smarts to talk with us in a way we can understand.
iopq 12y ago
Possibility 5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—who is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. If we're the most advanced civilization right now, that will be us in a few million years. We're kind of dicks.
- sebastialonso 12y ago
  Exciting for a movie plot I guess. But, what could they win from exterminating another inferior intelligence? By your context, they are already inferior, and thus, no threat to the apex civilization.
  - danielweber 12y ago
    It's been suggested by Robin Hanson (who coined the name "Great Filter") that an ethical super-civ would see a galactic war with another super-civ as the worst thing possible, and so wipe out other civs to stop that worst thing possible, I don't buy it, FWIW, because "ethical" doesn't overlap with "wipe out all other life" in my book. (Although you could certainly, say, have some system that stops any civ from leaving their planet by dropping rocks on them each time they launch something into orbit.) But you can search Hanson's blog for more talk about this if you want to see more discussion.
MrBra 12y ago
Thanks for letting me find out about this article and this whole awesome website!
MrBra 12y ago
Think of something like the "Human Brain Project" [1] scaled to what it could become after evolving/improving/growing for 3.2 billion years.. [1] humanbrainproject.eu
drjesusphd 12y ago
I personally find the "we are first" solution to be the scariest scenario. We are not suited for the responsibility of being the only intelligent life in the universe.
- adventured 12y ago
  What responsibility? I can see absolutely zero responsibility attached to that scenario. Humans also may be the most responsible intelligent life that ever exists in the universe. What's the basis to judge such things to begin with? There's an exceptionally low probability that human survival has any bearing on whether other intelligent life comes to exist (in the we're first scenario). Being first bears no responsibility because future life in the galaxy or universe is not inherently dependent on what we do.
  - sebastialonso 12y ago
    I guess parent was implying an inherent responsibility about being the first intelligence, or conciousness. As such, should we nurture, protect, and by all means spread the intelligence sickness? I'd think yes.
rdc12 12y ago
Would discovering the fossils of some complex species on Mars (or any where else in the solar system) be any worse for getting past the great filter then the discovery of fossils of dinosours on earth. We allready know that mass extinction events on Earth are possible, and should expect them to occur on other planents with life too
kowdermeister 12y ago
We are not even listening hard. SETI covers an incredibly tiny range of possible spectrum and possibilities of intelligent life. Are there any other attempts to find alien communication via data mining? If I were a galactic artist, I would use pulsars to modulate their signals and create a statue that broadcasts the existence of life. http://www.technology.org/2013/11/20/extraterrestrial-civilizations-modulate-pulsar-signals/ http://www.technology.org/2013/11/20/extraterrestrial-civili...
kowdermeister 12y ago
I'm a bit surprised nobody mentioned AI-s taking over as the Great Filter. I'm sure every technical civilization attempts to do that because it's very tempting to accelerate your intelligence by computation. I've also read an interesting article that why probably machine life is more abundant than biological: http://www.popularmechanics.com/how-to/blog/robot-universe-dominant-lifeform-17549081 http://www.popularmechanics.com/how-to/blog/robot-universe-d...
- pilaf 12y ago
  If AIs were taking over left and right we'd still expect to see some kind of evidence of their existence. Since they'd only be replacing biological civilizations I don't think it would count as the Great Filter, not in the context of Fermi's Paradox at least, as you'd end up with roughly the same number of technologically capable civilizations after the filter.
  - kowdermeister 12y ago
    We can't comprehend intelligence million times beyond us, so speculating about their intention is very wild. Who knows, maybe an universal law of super-intelligence is to keep a low profile.
    - sebastialonso 12y ago
      Yes, but I like my sci-fi like I like my Minecraft worlds. With at least a tiny layer of solid bedrock. If you're going to propose a crazy but interesting possibility, you cannot pull the 'can't speculate, they're to superior' card. You already speculate by sharing the idea!
deleted 12y ago
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DougN7 12y ago
The idea that maybe aliens visited before humans could record what they experienced struck me. Even if they did record it, we wouldn't believe it. Need proof? See the Bible or virtually any other ancient religious text. Our modern society believes if we can't prove it today, it didn't happen.

223 points by itamarb 12y ago

I think the biggest fallacy in this rationalization is when it is assumed that advanced civilizations would want to expand like yeast; consuming everything, building dyson spheres... Why would they want that much energy, and why extract it from the sun? Trying to picture myself as one of these superlifeforms, I think would like to have the sun visible... For sunbathing and sh... plants to grow etc... Maybe they have built little fusion reactors wherever they need energy?

sooheon 12y ago
I agree. Why the assumption the Type 3 capable civilisations will necessarily wish to metastasise across the entire universe? One would hope that civilisations' ideas of what constitutes fulfilling and worthwhile activity will evolve and improve along with their technology.
fernly 12y ago
The SUN is visible everywhere within a Dyson sphere; it's the STARS that you can't see any more. The idea isn't that a civilization wants to do this; the idea is that it needs that much energy to continue with whatever industrial or infrastructure processes it requires to support a very large population. Dyson reasoned, we need steadily increasing amounts of energy input to support what we are doing; if the trend continues, what's the limit? (Typical physicist question, no?) The limit is to harvest all the energy emitted from your star. The only way to do that is to build a spherical collector, so that every photon can be captured. Then where do you live? On the sphere's inner surface. Of what material could you build a stable sphere one AU in radius? Where would that bulk of material come from and how be processed and positioned? How would you provide gravity on its inner surface? We have no fucking idea -- but we are not a Type-whatever civilization.
- JoeAltmaier 12y ago
  I always thought that was a primitive notion. We can build the sphere, but not control the star? Imagine a caveman - "Limit to civilization when all energy from campfire captured - most people warm, most meat cooked" Clearly one can do better than a campfire. Instead, imagine harvesting energy from the star asymmetrically - from flares or from the inside out. Or get your energy from a neighboring star. Or make your own star. Or whatever a Type-whatever civilization decides to do.

ZanyProgrammer 12y ago

My solution to the Fermi Paradox: Who the hell knows, who the hell can know, when all we have is a sample size of 1 to extrapolate from. The solutions to the FP seem to be representative of the personal beliefs of whomever is discussing it.

Udo 12y ago
Of course no one can now right now, it's not one of these things that can be solved by theoretical work alone. At some point - assuming we don't go extinct before reaching it - we will know. But it's still open to useful speculation, as there might be something we still need to consider in order to avoid said extinction. The Fermi Paradox is one of the more thought-provoking signs that something might be amiss (at least in our part of the galaxy), and especially because our culture is predisposed to not take anything happening on the interstellar stage very seriously this provides an impetus for reflection that would otherwise not happen. One of the most benign (but ultimately depressing) possibilities is that biogenesis is somehow extremely unlikely. There is no reason to assume this is true, given that we know even the more complex building blocks of life are actually very prevalent throughout the universe - but it may still be statistically unlikely that cells form from them. On the footsteps of that possibility follows the hypothesis that technological intelligence is rare. Again, that's not exactly in line with what we can observe on Earth, but it might still be the case. Most of the other options should be positively troubling, though.
- ZanyProgrammer 12y ago
  "The Fermi Paradox is one of the more thought-provoking signs that something might be amiss (at least in our part of the galaxy), and especially because our culture is predisposed to not take anything happening on the interstellar stage very seriously this provides an impetus for reflection that would otherwise not happen." A hypothetical observer in North America, circa 1500, would have no idea that Asia and Europe existed. Perhaps we're not too far from some interstellar space empire, but since they use some exotic means of FTL communication, we can't detect them?
  - 12y ago

ccvannorman 12y ago

i love this puzzle. here are some theories: 1. simulation argument 2. civilizations have a high or inevitable chance to self destruct after <1M years of language 3. civilizations transcend our observable universe after <1m years [1] 4. due to the single observer problem, humans fail to grok some very important feature of larger scale space that prevents detection/increases isolation [1] http://accelerating.org/articles/transcensionhypothesis.html http://accelerating.org/articles/transcensionhypothesis.html

ZanyProgrammer 12y ago
Or we just haven't observed them yet because we aren't technologically advanced enough? What if they don't use radio?
- Flow 12y ago
  I hope there's something along the lines of sub-space transmissions. Once we discover it, we'll start listening and discover a plethora of civilisations out there. Would also be nice if they periodically transmitted decoding instructions :) But who knows, maybe it's far more rare with life that are similar to what we can recognise?
  - XorNot 12y ago
    Pretty much the scientific experiment I've been waiting for: we set up some exotic state of matter, and then spot modulated and structured signals coming out of it.
    - danielweber 12y ago
      Except all the messages are spam.
Pxtl 12y ago
4 seems likeliest. Interstellar colonization is simply impossible because of the vastness of the distance that must be covered. It requires more energy than can be summoned to get a useful amount of hardware across multiple lightyears.

fxj 12y ago

when the question is: "where are they?" and the answer is: "there is nobody here." then all long living civilisations must have left. so we should answer the question "why did they leave?"

Lrigikithumer 12y ago
Or maybe the long living civilisations never existed, or we aren't knocking on the right doors, or in the right areas?

gpsx 12y ago

This article does a good job of covering a lot of different ideas on this paradox. I have a problem with the idea of "big numbers" in it though, like where it implies 500 billion billion is nearly infinite, or that one-in-a-billion is a freak occurrence. There is a saying that if enough monkeys pound on a keyboard, one will type Hamlet. This would take a lot of monkeys. If we take the odds of hitting one character to be 1 in 40 (which doesn't account for properly hitting the shift key to make a capital), factoring in the 130,000 letters in Hamlet (according to Wikipedia), we get the odds of a monkey typing it at 6x10^208267. This number comes from 40 keys and 130,000 letters. How about when we start taking molecular numbers of items (6x10^23) and start raising that to a large power? I don't think it is a given that the number of planets out there infer there should be other life.

_greim_ 12y ago
We can't calculate the probability of life arising though because even if we did know how it arose here—which we don't exactly—we don't know all the possible different ways it can arise, which may or may not also be a number raised to a large power.
- gpsx 12y ago
  That is a good point. I agree with you. And people do have suggestions as to how life can be more probable than naively expected. But this answers the question why there is life at all as opposed to why we don't see more of it. My bias is that life is highly improbable and that we will not find any other life in our universe. But I also think it is no miracle that we exist, as I will describe. Given that I am trying to say this in one paragraph, it may not sound well justified, but it is how I view the problem. I'll start with Schrodinger's cat. A cat is in a box with some radioactive source that can decay and kill the cat. You can ask if the cat is alive or dead - and the answer is both. I will ignore the next part (Schrodinger's whole point) about what happens when the human observes it. There are different interpretations about why the human thinks the cat is definitely in one of the two states as opposed to both. This has to do with the human's observation and is not important for now. The point is that the cat is in both states. The universe can be in multiple states, some with life on earth and some without. Every possible state that can exist, does. So no matter how improbable we are, we will exist in some version of the universe. And there would be lots of other life too, just maybe not in our version of the universe. So the probability of life existing doesn't impact whether or not we should exist, but it does impact whether or not we see other life. I don't think we will. But as correctly pointed out, we don't know the answer to this right now.

equil 12y ago

I've always wondered at what distance it would be impossible for current technology to detect life on earth.

maaku 12y ago
A very short distance. IIRC something like 20 or so light years, but I'm having trouble finding the citation. That's for radio waves (and the number is getting smaller as the technology is getting better -- EM radiation blasted into the universe is considered waste by hardware engineers). For direct detection of life, that's still an open question. I remember a poster done by a grad student who looked at whether life would be detectable from the Moon, looking at the Earth. This was tested with data from one of the outer solar system probes which did a lunar flyby on its way out (Cassini?). The result, IIRC, was inconclusive -- you could see signs of life, but not anything that was absolutely definitive proof. Indirect evidence might be provided by spectra of the atmosphere observed via solar transit. A biome is likely to have different spectra than would be predicted by inorganic atmospheric physics. Still, that's making some assumptions about what extraterrestrial life would be like, and only only visible along the elliptic plane.
- sampo 12y ago
  The chemical composition of Earth's atmosphere (21% O2) is so far away from a long term chemical equilibrium, that any current astrobiologist would interpret it as a sign of planetary scale life. (And correctly so: the oxygen was produced by life.) So life on Earth is pretty visible from the other planets in our solar system. And spectroscopy to observe the composition of atmospheres of planets orbiting the nearest stars will probably happen in not-too-distant future.
  - maaku 12y ago
    Meh, it's a long way from saying that "we don't know how the O2 could have been produced except by life" to "the O2 was produced by life." Just look at the current debate about methane on Mars.

maaku 12y ago

The "we are the first" argument seemed less convincing than it could have been. A better argument from that perspective is the observation that Type III civilizations expand at an alarming rate. Whatever the motivation for expansion[1], basic simulations and back of the envelope calculations show that even with technology we could imagine building in the not so distant future, humans could expand into the cosmos at >0.9c. We must assume the same of ET. Now information only reaches us at the speed of light; we are only capable of looking for ET in our own past light cone. That means that any Type I or Type II civilization like us should expect to see an empty sky for most of their existence, until all of a sudden the most distant stars go dim (Dyson sphere, or whatever energy capturing device). And the darkness spreads in a wavefront travelling at almost the speed of light, until it hits and ... the unpredictable happens. We see an empty sky because if the sky wasn't empty, the planet we call Earth and our Sun would already be consumed by some extraterrestrial Type III civilization and Fermi would never have existed. [1] An inflationary universe provides incentive for a hypothetical Type III civilization to spread throughout the cosmos as quickly as possible, as idle time means regions of the universe becoming permanently inaccessible. Or maybe just resource competition and a desire for some elements of a population to remain on the frontier.

aswanson 12y ago
Sounds like a hell of a book/movie plot.
sampo 12y ago
> until all of a sudden the most distant stars go dim Why the most distant?
- jacquesm 12y ago
  Because if the events move at 0.9c the light still moves at 'c', so the furthest stars (of which there are more than nearby ones) will be the first to show you that something is up. The light from the older events will reach us before the events or the light from later events will reach us. Unless of course the whole thing originates in one of the stars nearby but the chances of that are smaller than that it will happen in a system further away.

Xcelerate 12y ago

I am surprised a more common argument was not mentioned: other life exists but the laws of physics limit our interaction. Maybe there is undiscovered physics like wormholes or time travel, or maybe there isn't and the speed of light really is a fundamental limit that cannot be surpassed. In that case, no matter how advanced another civilization becomes, they still can't violate natural law and are thus unable to reach us.

joe_the_user 12y ago
The laws of any potential biology and the messiness of the world could together effectively limit a civilization's energy usage to much lower than indicated by the easily deducible laws of physic (the explicit laws of physics might limit the output of an internal combustion engine to a certain but the effective maximum winds up less than that. Human biological systems experience serious damage by being weightless for significant periods, etc). I only recently realized that this "paradox" looking at the possibility not of "earth advanced" civilization but supposed advanced civilizations that would put out much more energy than humans put out. The whole thing seems to rest on false-extrapolations of mid 20th century hard-science fiction authors. Our Western Civilization has experienced an exponential growth of some things, has expanded it's "frontiers" quickly in a variety of ways, conquering the rest of the globe and then labeling space "the final frontier". But all this has been low-hanging fruit. We're reaching the end of population expansion, space is fundamentally hostile to human life in a way that would take tremendous progress to overcome and we're facing longer term consequences of the rapid expansions we have achieved (global warming is just one of the consequences). If humans survive this, we'll have to have somehow achieve a steady-state for energy consumption and related things. But once we do achieve that steady state, why would we want or need to suddenly start using the energy of an entire star (after the thousand years of scientific progress we'd need to under how to do that)?
- XorNot 12y ago
  The problem here is the assumption we need scientific progress to do this. We don't. We have the technology today to launch satellites into solar-orbits which would be capable of beaming power back to Earth. It would be clean, efficient and non-disasterous to the biosphere. With any amount of space-borne industry, the costs would also plummet and get cheaper with every additional satellite. Once you can harness the energy of an entire star, there's almost no reason not to achieve it.

ScottBurson 12y ago

How about this possibility: the Great Filter is now! We entered it, perhaps, when we developed sufficient collective nuclear weapons capability to wipe out most or all human life; we won't be out of it until the major threats our civilization poses to the biosphere have been resolved.

epaulson 12y ago
Even if we wipe ourselves out tomorrow, that still means that a civilization was able to broadcast its existence for about 80 years. If the Great Filter is now or ahead of us, that at least implies that pre-filter civilizations can be advanced enough to signal their existence. At some point our signals will be too weak for anyone to pick up or distinguish from cosmic background noise, which probably limits the radius of the sphere of stars that can hear us, but anyone inside that sphere gets at least 80 years to be listening to potentially hear us. So, even if civilizations only last for say 100 years on average, and you can only hear other civilizations within N light years, if N is big enough eventually someone should hear someone, and if N is really big enough, somewhere in that 100 years everyone should hear at least one other someone.
- fixedd 12y ago
  I've written about this elsewhere, but I'll chime in here too. The largest radio telescope we have (the 305 meter diameter Arecibo) would need to have it's sensitivity increased by around two orders of magnitude JUST to pick up our TV/FM/AM signals from outside the solar system. If we move into the narrowband signals then, depending on the source-strength, it could pick up signals at up to a few thousand light years... if it happened to be pointed in exactly the right direction at exactly the right time. You'd have to build an absolutely monstrous dish to be able to detect any of our signals at even the center of our own galaxy (much less at the other end)... I haven't run the numbers on it, but I doubt it's physically possible to build a receiver large enough. Add to that that they're highly directional devices and I think you'll find the "N" in your scenario is actually a startlingly small number.
has2k1 12y ago
I'll add another guess. A resource crunch, i.e. we simply use up the required resources before we figure out how to explore our planetary neighbourhood. e.g When we figure out that a helium engine is possible, very efficient and very safe, we no longer have sufficient quantities left.

zw123456 12y ago

I think they sort of cover this but here is my simple explanation. Think about how many slugs there are on Earth. How many of them have never seen a human? If we are slugs in comparison to advanced beings that have a billion years of evolution on us then it is no surprise we have not seen them.

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mangecoeur 12y ago

I don't see why a "Great Filter" is needed to justify there being no other highly evolved lifeforms - all it needs is for there to need to be a reasonably large number of key evolutionary steps and environmental conditions with a sufficiently low probability. Multiply those and you end up with some pretty tiny chances of observing life as we know it.

keule 12y ago
I had the same thought and agree with what you wrote. But when you think about this set of evolutionary steps you still end up with the same scenarios: you are rare, you are first, you are fucked. So in that regard, you can subsume all of these steps into a filter.

mda 12y ago

I think Authors is way way optimistic with these numbers: "Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life" and " "And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth" Why 1% and not 10^-10%?

Coincoin 12y ago
Agreed. It's a common error to think that the smallest fraction of something is 1%.

dedward 12y ago

In that great expanse and time, there may also have been ample time for civilizations to rise, then fall. We might have just missed them on such grand scales. Nobody says we'll be hanging around for a billion years trying to communicate with others... if we even last that long.

dennisgorelik 12y ago

How about the theory that we are the way super-civilization spreads? If physical space travel is extremely expensive (as it should be, considering the distance), then may be it is better to send a signal that would trigger creation of life (in our case on Earth). Then when Earth civilization is advanced enough, it would be able to receive complete boot sequence and then fully advanced alien civilization would be replicated on Earth.

fernly 12y ago
... or, how about the hypothesis that we were intended to be a seed package (a.k.a. "biological rootkit") but due to a bug in the programming, we failed to develop properly. Just a mistake playing itself out...

thelollies 12y ago

Another possibility... there are plenty of civilisations within broadcasting distance from us that are all listening but not broadcasting for fear of predatory civilisations.

thefreeman 12y ago
that exact possibility was listed in the article.
- thelollies 12y ago
  That's funny! I read the article and then had that thought five minutes later and thought I'd come up with it. Thanks for pointing that out :)
  - chatmasta 12y ago
    I often wonder how many of my opinions derive from this same mistaken originality... Quite scary, actually.

cdelsolar 12y ago

We might be very close to finding life on Mars. If we did and it was different than us that would answer a lot of questions.

arikrak 12y ago

The estimates of stars seem reasonable; the estimates of Earth seem a bit ill-defined. (The study he links to discuses Earth-size planets, but there are many other factors that go into Earth's suitability for life.) The real issue though is in the "speculative" part: > Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life... And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. " 1% sounds like a small number, but it's a bit ridiculous to just throw it out there and assume it's reasonable. It's a little bit like when a startup does a top-down estimate of revenue - "If we only capture 1% of the e-commerce market, we'll be worth billions!" I think if you do a "bottom-up" style analysis of the likelihood of atoms forming a replicating growing organism, or that organism evolving to think intelligently, you would get a far smaller number, say in the range of 0.00001% to 10^-20 for each one.

pigboy 12y ago

Out of all those advanced civilizations in the universe, what fraction use electromagnetic radiation for communication? We are like ants waiting for other civilizations to communicate with us using pheromones.

danielweber 12y ago
There are humans that try to talk with ants (and bees, and dolphins) using those creatures' own methods. Unless there were strongly-enforced no-contact laws, then some alien researcher would want to inspect those Terrans and see if we can talk. Even if they are way smarter, they'll use that smarts to talk with us in a way we can understand.

iopq 12y ago

Possibility 5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—who is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. If we're the most advanced civilization right now, that will be us in a few million years. We're kind of dicks.

sebastialonso 12y ago
Exciting for a movie plot I guess. But, what could they win from exterminating another inferior intelligence? By your context, they are already inferior, and thus, no threat to the apex civilization.
- danielweber 12y ago
  It's been suggested by Robin Hanson (who coined the name "Great Filter") that an ethical super-civ would see a galactic war with another super-civ as the worst thing possible, and so wipe out other civs to stop that worst thing possible, I don't buy it, FWIW, because "ethical" doesn't overlap with "wipe out all other life" in my book. (Although you could certainly, say, have some system that stops any civ from leaving their planet by dropping rocks on them each time they launch something into orbit.) But you can search Hanson's blog for more talk about this if you want to see more discussion.

MrBra 12y ago

Thanks for letting me find out about this article and this whole awesome website!

MrBra 12y ago

Think of something like the "Human Brain Project" [1] scaled to what it could become after evolving/improving/growing for 3.2 billion years.. [1] humanbrainproject.eu

drjesusphd 12y ago

I personally find the "we are first" solution to be the scariest scenario. We are not suited for the responsibility of being the only intelligent life in the universe.

adventured 12y ago
What responsibility? I can see absolutely zero responsibility attached to that scenario. Humans also may be the most responsible intelligent life that ever exists in the universe. What's the basis to judge such things to begin with? There's an exceptionally low probability that human survival has any bearing on whether other intelligent life comes to exist (in the we're first scenario). Being first bears no responsibility because future life in the galaxy or universe is not inherently dependent on what we do.
- sebastialonso 12y ago
  I guess parent was implying an inherent responsibility about being the first intelligence, or conciousness. As such, should we nurture, protect, and by all means spread the intelligence sickness? I'd think yes.

rdc12 12y ago

Would discovering the fossils of some complex species on Mars (or any where else in the solar system) be any worse for getting past the great filter then the discovery of fossils of dinosours on earth. We allready know that mass extinction events on Earth are possible, and should expect them to occur on other planents with life too

kowdermeister 12y ago

We are not even listening hard. SETI covers an incredibly tiny range of possible spectrum and possibilities of intelligent life. Are there any other attempts to find alien communication via data mining? If I were a galactic artist, I would use pulsars to modulate their signals and create a statue that broadcasts the existence of life. http://www.technology.org/2013/11/20/extraterrestrial-civilizations-modulate-pulsar-signals/ http://www.technology.org/2013/11/20/extraterrestrial-civili...

kowdermeister 12y ago

I'm a bit surprised nobody mentioned AI-s taking over as the Great Filter. I'm sure every technical civilization attempts to do that because it's very tempting to accelerate your intelligence by computation. I've also read an interesting article that why probably machine life is more abundant than biological: http://www.popularmechanics.com/how-to/blog/robot-universe-dominant-lifeform-17549081 http://www.popularmechanics.com/how-to/blog/robot-universe-d...

pilaf 12y ago
If AIs were taking over left and right we'd still expect to see some kind of evidence of their existence. Since they'd only be replacing biological civilizations I don't think it would count as the Great Filter, not in the context of Fermi's Paradox at least, as you'd end up with roughly the same number of technologically capable civilizations after the filter.
- kowdermeister 12y ago
  We can't comprehend intelligence million times beyond us, so speculating about their intention is very wild. Who knows, maybe an universal law of super-intelligence is to keep a low profile.
  - sebastialonso 12y ago
    Yes, but I like my sci-fi like I like my Minecraft worlds. With at least a tiny layer of solid bedrock. If you're going to propose a crazy but interesting possibility, you cannot pull the 'can't speculate, they're to superior' card. You already speculate by sharing the idea!

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DougN7 12y ago

The idea that maybe aliens visited before humans could record what they experienced struck me. Even if they did record it, we wouldn't believe it. Need proof? See the Bible or virtually any other ancient religious text. Our modern society believes if we can't prove it today, it didn't happen.