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The ‘Great Filter’ Tackles Fermi

Suppose for a moment that life really is rare in the universe. That when we are able to investigate the nearby stars in detail, we not only discover no civilizations but few living things of any kind. If all the elements for producing life are there, is there some kind of filter that prevents it from proceeding into advanced and intelligent stages that use artifacts, write poetry and build von Neumann probes to explore the stars? Nick Bostrom discusses the question in an article in Technology Review, with implications for our understanding of the past and future of civilization.

Choke Points in the Past

Maybe intelligent beings bring about their own downfall, a premise that takes in more than the collapse of a single society. Alaric’s Goths took Rome in 410, hastening the decline of a once great empire, but the devastated period that followed saw Europe gradually re-build into the Renaissance. And as Bostrom notes, while a thousand years may seem like a long time to an individual, it’s not terribly significant in the overall scheme of a civilization, which theoretically might last millions of years. No, a true filter must be something larger, a potential civilization-killer.

A galaxy like our own

Bostrom’s idea of a ‘Great Filter’ comes from Robin Hanson (George Mason University), and consists of the kind of transition that a civilization has to endure to emerge as a space-faring culture. The key question: Is the filter ahead of us or behind? If behind, wonderful — we have already passed the test and can look with some confidence to the future. Recent work, for example, indicates that human beings were reduced to a band of as little as 2000 individuals some 70,000 years ago, near extinction. Yet somehow migrations out of Africa began 60,000 years ago, and all the tools of civilization would emerge in their wake.

Image: The galaxy NGC 6744, a barred spiral about thirty million light years from Earth. Is it possible that such vast congregations of stars may be utterly devoid of life? Credit: Southern African Large Telescope (SALT).

But that’s a filter that still gets intelligent life well on its way, and surely with the number of stars in our galaxy, that would imply at least a few civilizations should have made it through besides ourselves, their presence obvious by now. No, to explain the Fermi paradox, we would like to go further back, making the emergence of complex life of any kind problematic. Making it, in fact, so rare that a galaxy devoid of it (other than here on Earth) is an explicable outcome. That kind of filter gives us hope, because we’ve survived it even though no one else has. The galaxy may be empty of life, but it is also a vast frontier awaiting our expansion.

The Shape of Future Menace

But maybe the filter is still ahead of us. If so, we may be able to see its outline in fairly familiar terms, such as nuclear war, asteroid impact, genetically engineeered disease used as weaponry, and so on. Or maybe, and more likely, it’s something we cannot foresee:

The study of existential risks is an extremely important, albeit rather neglected, field of inquiry. But in order for an existential risk to constitute a plausible Great Filter, it must be of a kind that could destroy virtually any sufficiently advanced civilization. For instance, random natural disasters such as asteroid hits and supervolcanic eruptions are poor Great Filter candidates, because even if they destroyed a significant number of civilizations, we would expect some civilizations to get lucky; and some of these civilizations could then go on to colonize the universe. Perhaps the existential risks that are most likely to constitute a Great Filter are those that arise from technological discovery. It is not far-fetched to imagine some possible technology such that, first, virtually all sufficiently advanced civilizations eventually discover it, and second, its discovery leads almost universally to existential disaster.

Better to have the Great Filter behind us. Then, at least, we know that we are here and that the experience was survivable. And the parameters of the filter have implications for our search for life. Bostrom hopes we find no sign of life elsewhere because such a find would imply that life is commonplace, that the Great Filter kicked in after the point in evolution that that life represents. Well and good if the discovered lifeforms were simple — we could still assume the filter operated early in evolutionary history and that we are past it. But if we found complex life, this would eliminate a larger set of early evolutionary transitions as the filter, and would imply that it is ahead rather than behind us.

Explaining the Great Silence

Remember, we are trying to explain why we are not finding signs of intelligence elsewhere, no von Neumann probes, no artifacts from civilizations that should have had plenty of time to expand through the galaxy. In Bostrom’s view, no news from the stars may actually be good news. It could imply that life itself is improbable, that the Great Filter happened well in our past and we somehow survived it, and that therefore we may be able to make the transition to a higher and better civilization. We are the one species lucky enough to make it this far, and while we cannot rule out the possibilities of other Great Filters lying ahead, we can at least hope we have weathered the worst.

All of which seems to put Earth back into the center of the universe again, a bizarre exception to the overwhelming norm. Bostrom thus has no choice but to explain the observation selection effect, a way to make sense out of our good fortune in being the lucky exception to the rule:

Consider two different hypotheses. One says that the evolution of intelligent life is a fairly straightforward process that happens on a significant fraction of all suitable planets. The other hypothesis says that the evolution of intelligent life is extremely complicated and happens perhaps on only one out of a million billion planets. To evaluate their plausibility in light of your evidence, you must ask yourself, “What do these hypotheses predict I should observe?” If you think about it, both hypotheses clearly predict that you should observe that your civilization originated in places where intelligent life evolved. All observers will share that observation, whether the evolution of intelligent life happened on a large or a small fraction of all planets. An observation-selection effect guarantees that whatever planet we call “ours” was a success story.

Into a Barren Universe

Bostrom is director of the Future of Humanity Institute at Oxford, a transhumanist philosopher (this is George Dvorsky’s description) who notes that even if the Great Filter were in our past, this would not absolve us from future danger. But this is a man who would like to see all that interesting technology, from nanotech to life extension, kicked in to provide us with a ‘posthuman’ existence whose outline we cannot presently imagine. He’s actively pulling against finding life anywhere else because he’s convinced that life’s rarity implies most organisms run into a buzzsaw before they can colonize space. We survivors, then, may find no one else to talk to, but we should have a fighting chance to use our technologies in a transformative way.

And here is where I truly disagree with Bostrom:

…surely it would be the height of naïveté to think that with the transformative technologies already in sight–genetics, nano­technology, and so on–and with thousands of millennia still ahead of us in which to perfect and apply these technologies and others of which we haven’t yet conceived, human nature and the human condition will remain unchanged. Instead, if we survive and prosper, we will presumably develop some kind of posthuman existence.

I see no evidence in history that the basics of human nature are amenable to change, whether or not such change would be a positive or negative thing. Nor can I go along with those who think we will be able to control our own evolution into some kind of higher lifeform, but long-time readers know my doubts that a genuine ‘transhumanism’ is possible to us. That would be another discussion, though, and I leave this one with the thought that if complex life of any kind is rare, we may have survived only to move outwards into an unexpectedly bleak universe.

Comments on this entry are closed.

  • Adam May 14, 2008, 15:30

    Hi Larry

    The Church has believed in the possibility of ETs since at least the 15th Century with Nicholas of Cusa’s writings. It’s an unappreciated fact that the Medieval thinkers broke the Aristotlean belief in “one world” and actively discussed the possibility of a “multiverse”.

    On the issue of evolution of life the Church has sided with science, but things weren’t always so clear cut. James Blish’s “A Case of Conscience” is a dramatisation of the conflict between science, ETs and religion

  • James M. Essig May 14, 2008, 23:03

    Hi Adam and other Folks;

    It is interesting to note that the Theory of Chaotic Inflation proposes the existence of a fractal verse or multiverse wherein, in virtually each Planck scale differential volumetric element of space time within our universe, a new universe forms and then immediately becomes effectively casually decoupled from our universe. The well known principle of this theory, which some of the new readership may not be aware of is that the proposed existence of so-called scalar fields which produce ultramicroscopic quantum fluctuations are normally self canceling in their effects over larger scales because of the net cancellation of the randomly oriented ultramicroscopic short duration gradients within these fields. Due to statistical probabilities on a somewhat larger level, occasionally, the fields line up or otherwise reinforce each other in such a way that a universe forms and branches off. In some cases, the universe immediately collapses and in other cases, the universe inflates much as our universe supposedly did.

    Accordingly, each universe including ours is constantly giving birth to ensembles, perhaps infinite numbers of baby universes and thus an infinite number of universe family trees. Our universe was accordingly one such baby universe that formed from a previous universe. The theory suggests that their might be an infinite number of such effectively independent fractal-verses or eternal family trees. It is even suggested that there might exist an independent number of different vacuum fields from which each potentially infinite set of fractal verses subsists. Moreover, it has even been suggested that there are what I will term an infinite number of meta fields wherein each meta-field underwrites the existence of an infinite set of vacuum fields. My thinking is that if the Theory of Chaotic Inflation has any merit to it, maybe there are an infinite number of meta-meta-quantum fields, an infinite number of meta-meta-meta-quantum fields and perhaps the nested series goes on forever.



  • ljk May 15, 2008, 9:24

    How “official” were Nicholas of Cusa’s thoughts on the subject
    of exterrestrial life with the Church? How many others sided
    with him, if they considered the subject at all?

    While the Church mainly persecuted Giordano Bruno for his
    religious views, which they naturally considered heretical, it
    did not help that he also believed in an infinite Universe with
    planets around every star and life on each one.


    Galileo had some positive views on life elsewhere, using the
    analogy of a field of grapes where it would be absurd to
    assume only one grape would grow, mirroring Metrodorus
    of Chios’ quote:

    “A single ear of corn in a large field is as strange as a single
    world in infinite space.”

    But Galileo kept most of his views on alien life to himself,
    figuring he was already in enough trouble just for promoting
    that Earth went around the Sun.

  • Administrator May 15, 2008, 10:34

    Nicholas of Cusa was in fact a cardinal of the Roman Catholic Church; his opinions carried great weight within it. Copernicus, Galileo and Kepler were all acquainted with his work, and his denial of the finiteness of the universe was an influence on Bruno.

    Medieval philosophers were indeed interested in the prospects of life elsewhere, and many did not see this as violating Church teachings. In 1277, Pope John XXI turned to the bishop of Paris, Stephen Tempier, to produce a series of writings against theories currently spreading at the university there. Tempier’s ‘Sentence 34’ is explicit in saying that whoever believes that God cannot create numerous worlds is a heretic. The statement did not say that many worlds existed, but that to deny the possibility was heresy. John XXI was the author of numerous scientific studies, incidentally.

    As we move into the Renaissance, Giordano Bruno’s views were quite theological. He believed in the existence of many worlds not because of any scientific observations (in any case, nothing was possible to confirm or deny this with the instrumentation of his time), but because to deny the infinity of the universe and the numerous worlds it could sustain was to deny the power of God. Giancarlo Genta, in his book Lonely Minds in the Universe, points out that Bruno, although he is often cited by scientists today, was actually much less ‘scientific’ than Copernicus and other astronomers of his day.

    And you’re right, Larry, Bruno wasn’t burned because of his belief in many worlds, but because of his theological beliefs, including his denial of transubstantiation. It’s telling that none of the other philosophers who defended theories about the plurality of worlds received such a sentence.

  • Administrator May 15, 2008, 15:14

    Larry also passed along an excellent source for further studies of early thinking on life on other planets:

    The Extraterrestrial Life Debate, Antiquity to 1915

    A Source Book

    Edited with Commentary by Michael J. Crowe

    This book presents key documents from the pre-1915 history of the
    extraterrestrial life debate. Introductions and commentaries accompany
    each source document, some of which are published here for the first
    time or in a new translation. Authors included are Aristotle,
    Lucretius, Aquinas, Nicholas of Cusa, Galileo, Kepler, Pascal,
    Fontenelle, Huygens, Newton, Pope, Voltaire, Kant, Paine, Chalmers,
    Darwin, Wallace, Dostoevski, Lowell, and Antoniadi, among others.
    Michael J. Crowe has compiled an extensive bibliography not available
    in other sources.

    These materials reveal that the extraterrestrial life debate, rather
    than being a relatively modern phenomenon, has extended throughout
    nearly all Western history and has involved many of its leading
    intellectuals. The readings also demonstrate that belief in
    extraterrestrial life has had major effects on science and society,
    and that metaphysical and religious views have permeated the debate
    throughout much of its history.

    Michael J. Crowe is the Rev. John J. Cavanaugh Professor Emeritus in
    Humanities in the Program of Liberal Studies and Graduate Program in
    History and Philosophy of Science at the University of Notre Dame. He
    has published a number of books, including The Extraterrestrial Life
    Debate, 1750–1900: The Idea of a Plurality of Worlds from Kant to

    Details are here:


  • ljk May 19, 2008, 10:12

    A Critique of Shortsighted Anthropic Principles

    PhysOrg.com May 16, 2008


    a new paper in Physical Review
    Letters from a group of physicists
    at Case Western Reserve University
    argues that any connection between
    the laws of physics and the
    existence of life is likely to be an
    illusion stemming from our
    shortsighted definition of
    intelligent life. The paper implies
    that the anthropic claim that “we
    observe things the…


  • ljk May 29, 2008, 13:46

    Unclogging “The Great Filter” – (Or Why We’ve Never Seen Any Sign of Alien Life)

    By Luke McKinney

    The Great Filter is the idea that there is some single, almost insurmountably improbable barrier on the path to the stars that explains why we’ve never seen any sign of alien life. It combines aspects of astrology, biology and history to arrive at one inescapable conclusion: University professors dream of book deals.

    Robin Hanson of George Mason University posits a “Great Filter” that prevents the rise of intelligent, self-aware, technologically advanced, space-colonizing civilizations. The “filter” would be one or more improbable steps along the path that starts with the creation of a planet and ends with a race capable of colonizing the galaxy.

    Full article here:


  • ljk May 30, 2008, 14:53
  • ljk June 12, 2008, 0:11

    A note on space dimensionality constraints relied on Anthropic arguments: methane structure and the origin of life

    Authors: Francisco Caruso

    (Submitted on 4 Jun 2008)

    Abstract: A general sketch of how the problem of space dimensionality depends on Anthropic arguments is presented. A new argument in favor of a stable scenario for space dimensionality for a time scale longer than that required for the existence of human or another kind of highly-evolved life on Earth is proposed.

    Comments: 7 pages, contribution to J.M.F. Bassalo’s Festschrift

    Subjects: History of Physics (physics.hist-ph); Astrophysics (astro-ph)

    Cite as: arXiv:0806.0684v1 [physics.hist-ph]

    Submission history

    From: Francisco Caruso [view email]

    [v1] Wed, 4 Jun 2008 03:51:54 GMT (12kb)


  • ljk February 2, 2009, 10:53

    Fermi’s paradox solved?

    February 2, 2009 | by KFC |

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


  • ljk March 3, 2009, 13:20

    A geochronologist’s take on the Fermi Paradox:


    In summation: It is not only a very big galaxy with lots of star
    systems, the Milky Way is also a very OLD place. Two ETI species
    being at the right level of development and time to detect each
    other may be a very problematic thing, Star Trek notwithstanding.