The ‘Great Filter’ Tackles Fermi

by Paul Gilster on April 29, 2008

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.

Administrator May 15, 2008 at 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 at 10:12

A Critique of Shortsighted Anthropic Principles 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 at 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 at 14:53
ljk June 12, 2008 at 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 at 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: Broadcasting But Not Receiving: Density Dependence Considerations for SETI Signals

ljk March 3, 2009 at 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.

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