Fermi’s paradox solved?


We have little to guide us on the question of the existence intelligent life elsewhere in the universe. But the physicist Enrico Fermi came up with the most obvious question: if the universe is teeming with advanced civilizations, where are they?

The so-called Fermi Paradox has haunted SETI researchers ever since. Not least because the famous Drake equation, which attempts put a figure on the number intelligent civilisations out there now, implies that if the number of intelligent civilisations capable of communication in our galaxy is greater than 1, then we should eventually hear from them.

That overlooks one small factor, says Reginald Smith from the Bouchet-Franklin Institute in Rochester, New York state. He says that there is a limit to how far a signal from ET can travel before it becomes too faint to hear. And when you factor that in, everything changes.

Smith uses this idea to derive a minimum density of civilizations below which contact is improbable within a given volume of space. The calculation depends on factors such as the lifetime of a civilization and the distance that it might be possible to communicate over and it produces some interesting scenarios:

Assuming the average communicating civilization has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 communicating civilization in the galactic neighborhood to reach a minimum density.

So if there are only 200 advanced civilizations in our galaxy, the chances are that they’ll never notice each other.

Of course, we’ve no way of knowing how many advanced civilizations are out there. But this kind of thinking could, for the first time, put a limit on the number that could be out there: less than 200 perhaps?

It also has significant implications for Fermi’s line of thinking.

Would it be too early to say the paradox has been solved?

Ref: arxiv.org/abs/0901.3863: Broadcasting But Not Receiving: Density Dependence Considerations for SETI Signals

59 Responses to “Fermi’s paradox solved?”

  1. Jared Lessl says:

    This doesn’t take into account the fact that a space-faring species could colonize the entire galaxy fairly quickly; 20 million years or so. Pretty speedy compared to geological or evolutionary timescales.

    The hardest part of space exploration is getting off the planet. Once that’s done, getting anywhere else is a cinch. Heck, we could build a ship to get to Alpha Centauri today. It’d be hideously expensive, but we could do it.

    So we’re back to Fermi’s paradox. In the Milky Way’s 13 billion year history, not one single species rose to sapience, civilization, and interstellar travel? Because once it spans more than a few light years, there’s no possible event that can wipe them out nor stop them from continuing. So… where are they?

  2. Seth Zenz says:

    Far too early. Jared has the critical point in his comment. From the paper (page 4): “It is also assumed that no other factors such as colonization or interstellar travel are present.” That seems a bit unlikely, doesn’t it?

    This is a much more interesting paper on the same question: http://arxiv.org/abs/astro-ph/9901322 I have no idea if the ideas are still valid given the research on gamma ray bursts in the last decade, but it’s a neat idea.

  3. shan says:

    Jared: You are saying we can build a ship to Alpha Centauri today. Dude – the challenge is not building the ship, it is how long it will take to get there… at the current speads we can achieve. And that is very long, maybe thousands of years

  4. […] than just make things up — there’s possibly big possible news. The physics ArXiv blog reports: We have little to guide us on the question of the existence intelligent life elsewhere in the […]

  5. zushiba says:

    shan, the problem people have with space travel is that they expect it to happen in their time. If it’s not then it’s too slow.
    Jared’s supposition that we could build a ship capable of reaching Alpha Centauri while suspect. Doesn’t say anything about making it in our time. A generational ship could in theory make it. So while those that initially set out towards Alpha Centauri wouldn’t be those that arrive, humans would. Of course we’ve no idea what kind of form humans would take by the time they got there.

    Assume for the moment that interstellar travel is not only possible but other species have already achieved it. The likelihood of being able to detect it is pretty unlikely.
    Any sufficient advance in technology is indistinguishable from magic from our point of view.
    We listen for radio waves because that’s what we use to communicate but any civilization that has made the jump to interstellar colonization won’t be using something so slow. Even communicating with a directed laser device would be too slow.
    In fact by the time we heard any radio waves that could reach us the civilization that produced them would either be extinct or have moved on and is using a technology that we simply wouldn’t understand to get about in a universe that we’re only starting to comprehend.

  6. Chris K says:

    Detection is a problem, thus wide-angle sending works poorly. An interstellar civilization would talk to itself with narrow-angle transmissions. Latency is years to 1000’s of years, so continuous transmission is not important.

    One should expect powerful but rare and narrow-angle interstellar transmissions.

    Why colonize the whole galaxy densely? Once you are wide spread there is no pressing need to expend the effort to reach every star — unless you are consuming the solar systems at a fantastic (wasteful?) rate.

    As for reverse SETI (they look for us): If they have been civilized for X years, they only need to bother to search for us no more often then a reasonable fraction of X, such as every X/2 years. If X is 10^5 or more years then they may not search for us for a very long time. And our radio sphere (~100 year radius) is just under 2 parts per million of the galaxy. It will be 4600 AD before our radio sphere is arguably 1% of the galaxy.

  7. […] voor Fermi’s paradox ArXiv blog en voor Helfand over Drake’s formule Visual […]

  8. R.. Mirman says:

    The chance of ET is ext

  9. R.. Mirman says:

    The chance of ET even existing is extraordinarily minute. For full discussion see the book
    Our Almost Impossible Universe:
    Why the laws of nature make the existence of humans extraordinarily unlikely

    Drake’s equ. is probably the only case in which wishful thinking has been expressed in a mathematical equation. It is nothing more than that.

  10. Todd Grigsby says:

    Fermi’s Paradox fails to include a very likely scenario: technology as the end of the civilization.

    Look at us. We’ve critically altered the climate on our planet such that large numbers of species will likely be wiped out. We’re building scientific experiments that theoretically wipe out the earth. We have built weapons that could certainly destroy all life here. And then there’s the singularity, the point at which man either merges with his machines or is subsumed by them.

    I propose that the very ability to reach a given stage of technology opens the door to the inevitable and complete disappearance of the species, either through self-destruction or evolution to a non-biological stage. The span of time between discovering radio technology and creating intelligent machines would seem to be, cosmically speaking, a blink of an eye. For all we know, the only intelligence in the galaxy that is space-faring is machine based and has no desire whatsoever to attract our attention.

    Perhaps there are dozens of intelligent species out there, all waiting until our machines rise as the dominant life form before approaching to make contact? What are the odds that UFOs are merely probes all reporting back, “Nope, not yet.” Or more thought-provoking, what are the odds that intelligent beings capable of making it to another star system planet would still be organic? I think it would be startlingly small.

  11. Alistair Morley says:

    Another nitpick; The 1,000 year lifespan for civiliation looks…poorly phrased. It should be expressed 1,000 year lifepan of radio-technological species, not civilisation.

    Any civilisaton-ending catastrophe that doesn’t completely destroy the intelligent species will see a rebuild in short (<10,000yr?) periods. If that happens, every subsequent civilistion effectively counts as a continuation…

    Now, my point is its difficult to think of a catastrophe that will completely destroy a species that has spread throughout its own system, let alone gone interstellar. No, a nuclear war would NOT be sufficient, nor would your favourite ecological disaster. 99.9999% dead is not enough. You need to get everyone.

  12. Todd Grigsby says:

    And I would suggest that, as technology becomes more powerful, so do the potential accidents. You’re right, climate change and nuclear war by themselves are not enough, but black holes? Yeah, probably. Or something we can’t even foresee. Some particle smashing, space tearing, time ripping experiment that, unfortunately, won’t be completely thought through and will result in a massive gamma ray burst that, a million years later, some other civilization will report in their news as “the most energetic cosmological event yet detected.”

  13. Alistair Morley says:

    There also seems to be a statistics issue in the 1,000 year lifetime. Let us assume that this is the mean, but consider the impact of the variance about that mean. These kind of means are probably lognormal distributed, but that’s not as important as the effects of varying lifetimes.

    Take, for example 2 civilisations with a communications history of 1,900 and 100 years, a signal horizon equal to their lifetime, and randomly placed in the galaxy. Detection volume is proportional to cube of lifespan. They are far more likely (~8 times?) to make contact that 2 civilisations with a 1,000 year communications history each. The odds also favour the shorter-lived civilisation being the first detector.

  14. Alistair Morley says:


    Interesting point; I agree about capacity for accidents advancing with scale of technology. It’s always possible… (But our own civilisation shows a long-term decreasing accidental death rate per capita with higher technology/energy use…).

    And without wanting to pour too much cold water on the idea, the scale looks a bit equivocal. The accident has to be just big enough to get everyone without being big enough to leave supsicious remnants spread across the sky for others to see.

    It has to be powerful-but-discreet and brief. Spectacular accidents might well attract post-humous attention; making other civilisations belatedly aware of their existence. “That’s weird. Yellow dwarfs don’t just explode. Play back everything we heard from that direction earlier…”

    I agree, it’s possible. But it feels like an unsatisfactory fix to me.

  15. Todd Grigsby says:


    How often have we viewed the remnants of gamma ray bursts? I’m just saying…


  16. Brian Mingus says:

    Wouldn’t a sufficiently advanced species be able to alter the natural properties of their Sun in a manner that could eventually be detected by a distant civilization?

    Suppose there were two advanced civilizations in our galaxy – Earth, and some other planet that was far away but could still resolve our sun. Suppose they were advanced enough to have a very accurate simulation of what Earth’s solar system should look like, and what the properties of its sun should be. Suppose that we then modified the properties of our sun using some unspecified means.

    Any ideas of how we might pull this off?

  17. Ryan Davis says:


    When pulsars were first discovered that was one of the more wild speculations going around, that the stars pulsed due to macro-level modifications or wholesale creations by distant species.

    Why not find a pulsar, throw some shades around it and use a single rotation as a base value. Shades = 0. Light = 1. Interstellar Morse code. Still, seems like a crude fudge since it would have terribly low bandwidth.

  18. Seerak says:

    Fermi’s Paradox fails to include a very likely scenario: technology as the end of the civilization.

    It also does not include *willful* cultural suicide, such as a technophobic-induced return to primitivism.

    The potential dangers involved in ever more powerful technologies means we need to advance our *morality* in the same fashion that we advance technology.

    Our technology only advanced slowly, if at all, until we abandoned primitive religion in favor of reason as a means of comprehending the universe.

    However, thanks to such philosophical abominations as the “is-ought” dichotomy, we have not done so for morality. Our culture is dominated by the view that either imaginary friends in the sky are the source of morality.. or that there is no such thing as morality.

    With such primitive moral concepts, it’s a wonder we haven’t blown ourselves up.

  19. rchoetzlein says:

    There is a much simpler path.. Direct observation. Its not necessary for alien civilizations to send us a narrow-band message via radio/etc..

    With powerful enough telescopes, we should be able to directly observe structures on the surface. This is not inconceivable. In 100 yrs we went from imaging our own planets to first images of extra-solar planets.

    Also, with direct observation, it doesn’t matter when the message was sent. The structures/lifeforms will be there.

  20. darksolon says:

    I saw no mention of special relativistic effects. I don’t have any numbers, so maybe its irrelevant.

    If one assumes that these civilizations communicate with light, then what does “200 advanced civilizations” mean? Does this mean “right now?” If so, then one should define what simultaneous means within this framework of communication.

    One should be careful about claiming two events are simultaneous, since this has different meanings when observers travel at two different velocities with respect to each other.

    Also, who says that civilization lifetimes scale linearly? In my opinion, as time goes on, the likelihood of destruction increases. Maybe one should talk about some sort of “half-life” of a civilization instead?

  21. Colin says:


    Perhaps if we dumped our fission product waste in to the sun’s atmosphere, the artificial elements might show up in the solar spectrum. Not a very elegent solution, but simpler than re-engineering the properties of the sun itself.

  22. […] this thinking fails to take into account the limit to how far a signal from ET can travel before it becomes too faint to hear. Factor that in and everything changes. Assuming the average communicating civilization has a […]

  23. Assume one civilization that was stable for a long time and that had benefited from interstellar contact. Then that civilization should be grateful and make a serious effort, perhaps sending out a large radio pulse every year or two. Primitive as we are, we already have that capability. Considering how long the dinosaurs lasted, “stable” should be much longer than it would take to saturate our galaxy.

    I don’t buy the proposed analysis. I think the most likely reason is that experience has taught them not to talk to animals–and we’re still regarded as animals. They are most probably being entertained and slighted informed by our antics, and waiting to see if we stop being animals before we destroy ourselves.

  24. Mister Scott says:

    Well spoke, all of you. Did any one consider the “Prime Directive”, like in TV’s Star Trek? The rule to not interfear in a civilization’s development ando/or not to make first contact until the species is light speed capable. Just a thought from a life-long Trekkie and beleiver that there is too much evidence here on Earth proveing life exists out there, you also gotta consider what our governments already know…something to do with reptile like life… I dont know google or youtube it, see for yourself..

  25. Zephir says:

    This estimation is relevant for light speed, but more advanced civilization could communicate via gravity waves, which are much faster (but more dispersed by environment as well). Anyway, an advanced civilization can always make a larger radiolocator to handle such problem.

    But by my opionion, we aren’t supposed to see more advanced civilization at all, simply because it’s not advantageous for such civilization. Only silly, less advanced civilizations could attempt to communicate with more advanced civilizations, which could destroy them anytime.

  26. Brian Mingus says:

    > Perhaps if we dumped our fission product waste in to the sun’s atmosphere, the artificial elements might show up in the solar spectrum.

    Great idea!

  27. Brian Mingus says:

    > terribly low bandwidth

    How much bandwidth does it take to say “You are not alone.” Sheesh!

  28. Dan says:

    > How much bandwidth does it take to say “You are not alone.” Sheesh!

    It’s information impossible without any common signal/symbolic system.

    > Gravity waves, relativity

    Gravity doesn’t have waves, it’s differential drag.

    There also isn’t a centrifical force, that’s really just momentum.

    Defining an infinite set of possible futures also isn’t a multiverse, it’s a dimension. An unreal dimension at that.

    Oh, and with that double-slit, particles don’t know you’re watching, you’re interfering. You can’t watch without doing so. Waves quantize to interact.

    Not entirely sure if the universe is expanding or light is slowing down universally.

    Life… christ. It’s a self replicating pattern. Chances are a sufficiently evolved form would self replicate over us rather than communicate with us.

  29. […] physics arXiv blog comes the possibility that maybe Fermi’s paradox is solved? The so-called Fermi Paradox has haunted SETI researchers …because the famous Drake equation, […]

  30. Daniel says:

    I’m working on a partial solution to the Fermi Paradox myself. Actually, this author’s approach fits mine nicely. His assumption of 1000 years lifespans sounds deeply flawed. (If a civilization manages to leave its home planet, extinction becomes quite improbable.) Let me attempt to patch this flaw in his argument:

    My first assumption is this: for a spacefaring civilization, it takes a relatively short amount of time to acquire the ability to expand with the speed of light. (Here “relatively short” may mean a million years of technological advance.) At this point, the civilization becomes unobservable from the outside.

    My second assumption is this: Meeting with such a quickly expanding civilization is fatal for other, slower civilizations. They are “burning the cosmic commons”, to quote Robin Hanson.

    Based on this two assumptions, we can deduce that a civilization can observe another one only if BOTH of them are in their short interim phase: already emitting signals, but not yet becoming signals themselves.

  31. PasiK says:

    There is a very bad assumption in most Fermi theorems: It is assumed that some, or most, or even all of the other civilizations would use radio or other ranges of electromagnetic spectrum for communications. What if this isn’t the case? What if all of their stellar communications happens on the most efficient way, in beam-like transmissions, like laser? Not in broadcast like radio? We wouldn’t know if they were sitting right next to us.

  32. Jeff says:

    SETI has always bothered me in that it seems to make assumptions about communication that I don’t think are necessarily apropo given the vast distances of interstellar space.

    As humans, we tend to look at things from a human perspective. At most, we live 120-130 years. We are accustomed to vocal communication, with words and information being exchanged at a certain speed. Because of this, realtime communication between stars seems impossibly tedious to us, given the speed of light. Even if there was life on the closest of stars, who has patience for an 8-year lag between the sending of messages?

    I think it is likely that any advanced civilizations, given enough time, will simply adapt their manner of living and communication to suit the distances involved in interstellar space.

    For example, let us assume that there is a lifeform with a lifespan of 30 million years. It does not seem unreasonable to me that this intelligence might experience time in a different way than humans do. To them, humans might seem like so many fruit flies or bacteria, being born, living and dying in a matter of days or hours or minutes.

    What seems to us to be a lifetime might only be a few short moments to an interstellar intelligence.

    Imagine that you are a creature that lives for hundreds of millions of years, and process information slowly enough that these hundreds of millions of years feel to you like a normal human lifetime feels to us. Suddenly communication between the stars is not unreasonable. Who cares if it takes 8 human years to say a single “word” to an interstellar neighbor? If they have a similar lifespan, perhaps decades pass as they “listen” to your transmission before replying at an equivalent pace.

    I think there is no reason to assume that any other lifeforms in the universe must have lifespans comparable to ours. I think there is no reason to assume that other lifeforms in the universe communicate at the same rate that we do. This, to me, is a laughably human-centric point of view.

  33. Roberto says:

    Perhaps we’re the first civilization with technology advanced enough? Someone has to be first.

  34. Faraday says:

    It doesn’t make any difference if you’re operating in the “radio” band or using a “laser” to communicate. They are all within the EM spectrum, and all propogate through space at the same speed – the speed of light.

  35. […] хорошая дискуссия @ /. и @ arxivblog парадоксе Ферми — не то, чтобы нечто революционное, […]

  36. SpottedMarley says:

    One day we are going to stumble upon some form of FTL communication that allows us to communicate over vast distances instantaneously, and when we do I believe we are going to find out pretty quickly that this communication channel is already filled with extra-terrestrial communication.

  37. jon says:

    “By Jeff on Feb 3, 2009”

    Interesting point. Humans will probably have suspended animation within decades, so some individuals could live out their lives over a much longer timespan. It would be an interesting way to pass along history from one generation to future generations.

    There will also be a singularity when AI reaches human levels. It’s impossible to predict what will happen after that, but it would allow intelligence to survive nearly any event short of a disruption of the solar system.

  38. Chris says:

    What would be the impact of quantum communications? Once a paired device reaches a solar system it monitors normal EM spectrum for signals and forwards them instantaneously. If such technology is available we are back to “who is watch us and why haven’t they called?”
    So how far away are we from such a device?

  39. dave says:

    So what if there are NO other civilisations out there? Wouldn’t what we are observing at the moment be consistent with that?

  40. On Request says:

    >The hardest part of space exploration is >getting off the planet. Once that’s done, >getting anywhere else is a cinch. Heck, we >could build a ship to get to Alpha Centauri >today. It’d be hideously expensive, but we >could do it.

    Unlikely. Its not vacuum out there. Its chock full of stuff that will destroy you if you hit it at high speed. There isn’t going to be any ramship traffic happening any time soon, if ever. It’s not as easy as you make it sound.

  41. B&Massa says:

    I’m with Chris (Feb 3) and his question on quantum communications. We’re arrogantly monitoring for radio signals, but as recent experiments have shown, quantum transmission holds the promise super-relativistic communications that can only be intercepted within the context of the link itself. We don’t even have a technology which could determine if a quantum signal is passing us, let alone know how we might decrypt such a signal.

    If a civilisation could use quantum “radio” and get near zero latency over vast distances, with near zero interference, EM transmission is going to be abandoned very quickly, just like telegraph was abandoned for telex, and telex for email.

  42. Matthew C. Tedder says:

    Nature evolves life. It if far less true to suggest that life evolves to survive in nature. Similar environmental changes to similar species cause similar changes in that species, regardless of whether or not they are completely isolated from each other.

    That said, don’t think the Earth was “just right” in so many ways for life to form here. It evolved life to its conditions through changes in those conditions.

    I think more sophisticated forms of intelligence must form in more sophisticated, dynamic environments that did not change too quickly, too often (as to wipe out life in its current stage).

    Planets are like eggs. Some are more fertile than others. Most never hatch. Most that hatch, don’t survive long after.

    And I also have to wonder, even if our species survives to become a space faring people before the destruction of our world, would we be accepted by others? We are a very war-like species. We are not a safe species to share technology with.

  43. […] You can’t hear them because you aren’t listening at the right time you silly Fermi dude […]

  44. Sean OBrien says:

    Let me get this straight: every single civilization in the Milky Way stops broadcasting EM after 1000 years? Every one? What is the basis of that assumption?

    Here’s the answer to Fermi’s paradox: the probability that a sentient race will evolve in any galaxy is less than 1 per galaxy per 13 billion years. We are the only sentient species in the Milky Way. There is almost zero chance that another will evolve before we colonize our galaxy.

  45. […] Smith from the Bouchet-Franklin Institute in Rochester, New York has written a paper which claims that every civilization in the Milky Way stops broadcasting EM after 1000 years.

  46. The only extraterrestrial life that mankind is likely to discover will be found right here in the Solar System, or one close to mankind’s level of evolution nearby in the Milky Way.

    If one can agree with the Ray Kurzweil’s singularity, as I do, then science will accelerate exponentially, and accomplish almost anything imaginable within a few hundred years (or sooner). Why would a highly advanced civilization desire to communicate with mankind at our level of evolution?

    It’s more likely that they already know that we’re here, or have visited us, than that they would wish to communicate with us. Why communicate with mankind when within a short time we will advance to an ultimate level of evolution on our own.

    It would be a fruitless effort to communicate with every civilization at our level of evolution when they will advance themselves in such a short period of time.

    I say that it’s more likely that Earth has already been discovered by an advanced civilization, than that we will discover them at this level of Earth scientific advancement.

    I’ve never seen a UFO, but using the exponential argument, it’s just as likely (or more likely) that UFO’s are real. It’s possible that they have developed technologies to discover the Earth and remain difficult, or impossible for us to prove they exist…

  47. Justin Mattarocchia says:

    the energy levels required to move something at super-luminal speeds are likely enormous. An advanced space fairing race would (in my opinion) require the power of singularities (and i mean those beasts of mass and gravitational distortion, not the coined phrase regarding the acceleration of technology by Moors law.) A technologically advanced, self manipulated, engineered or otherwise merged bio-organic species might be ultra integrated like the “Borg” of Star Trek (although not necessarily slavish and “evil”)And thus, as was mentioned earlier in other peoples comments, capable of not only out living us in our current states, but also through quantum communication (which correct me if i’m wrong, doesn’t propagate waves that are detectable like EM transmissions) Communicate instantaneously. The resources needed to feed this kind of vast and potent organism would perhaps require the whole output from stars and material substance of whole worlds. the manipulation of the environment by a species is usually detectable, and thus the manipulation of the galactic environment may too be noticed once we really know what to look for. Or then again, there may not be another sufficiently advanced race currently within our local area, or even galaxy to observe.

  48. […] Other factors could include the range of usable and probable frequencies, modalities, and modulations monitored and the range of signal-processing and pattern-recognition algorithms employed (in the case of Earth, these are negligible) – Ed. (Source: http://arxivblog.com/?p=1167) […]