The Great Filter, a possible explanation for the Fermi Paradox – interview with Robin Hanson

I grew up wondering about the nature of alien life, what it might look like, what they might do, and whether we will discover any time soon. Though aside from a number of conspiracy theories, and conjecture on Tabby’s Star, so far we have not discovered any signs of life out there in the cosmos. Why is it so?
Given the Drake Equation (which attempts to quantify the likelihood and detectability of extraterrestrial civilizations), it seems as though the universe should be teaming with life.  So where are all those alien civilizations?

The ‘L’ in the Drake equation (length of time civilizations emit detectable signs out into space) for a technologically advanced civilization could be a very long time – why haven’t we detected any?

There are alternative many explanations for reasons why we have not yet detected evidence of an advanced alien civilization, such as:
– Rare earth hypothesis – Astrophysicist ‘Michael H. Hart’ argues for a very narrow habitable zone based on climate studies.
– John Smart’s STEM theory
– Some form of transcendence

The universe is a pretty big place. If it’s just us, seems like an awful waste of space.Carl Sagan - 'Contact'


Our observable universe being seemingly dead implies that expansionist civilizations are extremely rare; a vast majority of stuff that starts on the path of life never makes it, therefore there must be at least one ‘great filter’ that stops the majority of life from evolving towards an expansionist civilization.

Peering into the history of biological evolution on earth, we have seen various convergences in evolution – these ‘good tricks’ include things like transitions from single cellular to multi-cellular (at least 14 times), eyes, wings etc. If we can see convergences in both evolution, and in the types of tools various human colonies created after being geographically dispersed, Deducing something about the directions complex life could take, especially ones that become technologically adept could inform us about our future.

The ‘Great Filter’ – should we worry?

The theory is, given estimates (including the likes of the Drake Equation), it’s not an unreasonable to argue that there should have been more than enough time and space for cosmic expansionist civilizations (Kardashev type I, II, III and beyond) to arise that are at least a billion years old – and that at least one of their light cones should have intersected with ours.  Somehow, they have been filtered out.  Somehow, planets with life on them make some distance towards spacefaring expansionist civs, but get’s stopped along the way. While we don’t specifically know what that great filter is, there have been many theories – though if the filter is real, seems that it has been very effective.

The argument in Robin’s piece ‘The Great Filter – Are We Almost Past It?’ is somewhat complex, here are some points I found interesting:

  • Life Will Colonize – taking hints from evolution and the behavoir of our human ancestors, it feasible that our ancestors will colonize the cosmos.
    • Looking at earth’s ecosystem, we see that life has consistently evolved to fill almost every ecological niche in the seas, on land and below. Humans as a single species has migrated from the African Savannah to colonize most of the planet filling new geographic and economic niches as requisite technological reach is achieved to take advantage of reproductively useful resources.
    • We should expect humanity to expand to other parts of the solar system, then out into the galaxy in so far as there exists motivation and freedom to do so.  Even if most of society become wireheads or VR addicted ‘navel gazers’, they will want more and more resources to fuel more and more powerful computers, and may also want to distribute civilization to avoid local disasters.
    • This indicates that alien life will attempt to do the same, and eventually, absent great filters, expand their civilization through the cosmos.
  • The Data Point – future technological advances will likely enable civilization to expand ‘explosively’ fast (relative to cosmological timescales) throughout the cosmos – however we a yet have no evidence of this happening, and if there was available evidence, we would have likely detected it by now – much of the argument for the great filter follows from this.
    • within at most the next million years (absent filters) it is foreseeable that our civilization may reach an “explosive point”; rapidly expanding outwards to utilize more and more available mass and energy resources.
    • Civilization will ‘scatter & adapt’ to expand well beyond the reach of any one large catastrophy (i.e. a supernova) to avoid total annihilation.
    • Civilization will recognisably disturb the places it colonizes, adapting the environment into ideal structures (i.e. create orbiting solar collectors, dyson spheres or matrioshka brains thereby substantially changing the star’s spectral output and appearance.  Really really advanced civs may even attempt wholesale reconstruction of galaxies)
    • But we haven’t detected an alien takeover on our planet, or seen anything in the sky to reflect expansionalist civs – even if earth or our solar system was kept in a ‘nature preserve’ (look up the “Zoo Hypothesis”) we should be able to see evidence in the sky of aggressive colonization of other star systems.  Despite great success stories in explaining how natural phenomenon in the cosmos works (mostly “dead” physical processes), we see no convincing evidence of alien life.
  • The Great Filter – ‘The Great Silence’ implies that at least one of the 9 steps to achieving an advanced expansionist civilization (outlined below) is very improbable; somewhere between dead matter and explosive growth lies The Great Filter.
    1. The right star system (including organics)
    2. Reproductive something (e.g. RNA)
    3. Simple (prokaryotic) single-cell life
    4. Complex (archaeatic & eukaryotic) single-cell life
    5. Sexual reproduction
    6. Multi-cell life
    7. Tool-using animals with big brains
    8. Where we are now
    9. Colonization explosion
  • Someone’s Story is Wrong / It Matters Who’s Wrong –  the great silence, as mentioned above seems to indicate that more or more of plausible sounding stories we have about the transitions through each of the 9 steps above is less probable than they look or just plain wrong. To the extent that the evolutionary steps to achieve our civilization were easy, our future success to achieve a technologically advanced / superintelligent / explosively expansionist civilization is highly improbable.  Realising this helps may help inform how we approach how we strategize our future.
    • Some scientists think that transitioning from prokaryotic (single-celled) life and archaeatic or eukaryotic life is rare – though it seems it has happened at least 42 times
    • Even if most of society wants to stagnate or slow down to stable speeds of expansion, it’s not infeasible that some part of our civ will escape and rapidly expand
    • Optimism about our future opposes optimisim about the ease at which life can evolve to where we are now.
    • Being aware of the Great Filter may at least help us improve our chances
  • Reconsidering Biology – Several potentially hard trial-and-error steps between dead matter and modern humans (lifecomplexitysexsocietycradle and language etc) – the harder they were, the more likely they can account for the great silence
  • Reconsidering AstroPhysics – physical phenomena which might reduce the likelihood we would see evidence of an expansionist civ
    • fast space travel may be more difficult even for superintelligence, the lower the maximum speed, the more it could account for the great silence.
    • The relative size of the universe could be smaller than we think, containing less constellations
    • There could be natural ‘baby universes’ which erupt with huge amounts of matter/energy which keep expansionist civs occupied, or effectively trapped
    • Harvesting energy on a large scale may be impossible, or the way in which it is done always preserves natural spectra
    • Advanced life may consistently colonize dark matter
  • Rethinking Social Theories – in order for advanced civs to be achieved, they must first loose ‘predispositions to territoriality and aggression’ making them ‘less likely to engage in galactic emperialism’

We can’t detect expansionist civs, and our default assumption is that there was plenty of time and hospitable space for advanced enough life to arise – especially if you agree with panspermia – that life could be seeded by precursors on roaming cosmic bodies (i.e. comets) – resulting in more life-bearing planets on them.  We can assume plausible reasons for a series of filters which slow down or halt evolutionary progress which would otherwise finally arrive at technologically savvy life capable of expansionist civs – but why all of them?

It seems like we as a technologically capable species are on the verge of having our civilizaiton escape earths gravity well and go spacefaring – so how far along the great filter are we?

Though it’s been thought to be less accurate than some of its predecessors, and more of a rallying point – let us revisit the Drake Equation anyway because its a good tool for helping understand the apparent contradiction between high probability estimates for the existence of extraterrestrial civilizations, and the complete lack of evidence that such civilizations exist.

The number of active, communicative extraterrestrial civilizations in the Milky Way galaxy N is assumed to be equal to the mathematical product of:

  1. R, the average rate of star formations, in our galaxy,
  2. fp, the fraction of formed stars that have planets,
  3. ne for stars that have planets, the average number of planets that can potentially support life,
  4. fl, the fraction of those planets that actually develop life,
  5. fi, the fraction of planets bearing life on which intelligent, civilized life, has developed,
  6. fc, the fraction of these civilizations that have developed communications, i.e., technologies that release detectable signs into space, and
  7. L, the length of time over which such civilizations release detectable signals,


Which of the values on the right side of the equation (1 to 7 above) are the biggest reasons (or most significant filters) for the ‘N’ value  (the estimated number of alien civilizations in our galaxy capable of communication) being so small?  if a substantial amount of the great filter is explained by ‘L’, then we are in trouble because the length of time expansionist civs emit signals likely correlates with how long they survive before disappearing (which we can assume likely means going extinct, though there are other possible explanations for going silent).  If other civs don’t seem to last long, then we can infer statistically that our civ might not either.  The larger the remaining filter we have ahead of us, the more cautious and careful we ought to be to avoid potential show stoppers.

So let’s hope that the great filter is behind us, or a substantial proportion is – meaning that the seemingly rare occurrence of expansionist civs is likely because the emergence of intelligent life is rare, rather than it being because the time expansionist civs exist is short.

The more we develop our theories about the potential behaviours of expansionist civs the more we may expand upon or adapt the ‘L’ section of the drake equation.

Many of the paramaters in the Drake Equation are really hard to quantify – exoplanet data from the Keplar Telescope has been used to adapt the Drake equation already – based on this data it seems that there seems to be far more potentially earth like habitable planets within our galaxy, which both excites me because news about alien life is exciting, and frustrates me because it decreases the odds that the larger portion of the great filter is behind us.

Only by doing the best we can with the very best that an era offers, do we find the way to do better in the future.'Frank Drake' - A Reminiscence of Project Ozma, Cosmic Search Vol. 1, No. 1, January 1979


…we should remember that the Great Filter is so very large that it is not enough to just find some improbable steps; they must be improbable enough. Even if life only evolves once per galaxy, that still leaves the problem of explaining the rest of the filter: why we haven’t seen an explosion arriving here from any other galaxies in our past universe? And if we can’t find the Great Filter in our past, we’ll have to fear it in our future.Robin Hanson - The 'Great Filter' - should we worry?

As stated on the Overcoming Bias blog:

We have three main sources of info on existential risks (xrisks):

  1. Inside View Analysis – where we try to use our best theories to reason about particular causal processes.
  2. Earth Track Records – the empirical distribution of related events observed so far on Earth.
  3. The Great Filter – inferences from the fact that the universe looks dead everywhere but here.

These sources are roughly equally informative. #2 suggests xrisks are low, even if high enough to deserve much effort to prevent them. I’d say that most variations on #1 suggest the same. However, #3 suggests xrisks could be very high, which should encourage more xrisk-mitigation efforts.

Ironically most xrisk efforts (of which I’m aware) focus on AI-risk, which can’t explain the great filter. Most analysis efforts also focus on #1, less on #2, and almost none on #3.

“What’s the worst that could happen?” – in 1996 (revised in 1998) Robin Hanson wrote:

Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?The Great Filter - Are We Almost Past It? - 'Robin Hanson'
If the ‘Great Filter’ is ahead of us, we could fatalistically resign ourselves to the view that human priorities too skewed to coordinate towards avoiding being ‘filtered’, or we can try to do something to decrease the odds of being filtered. To coordinate what our way around a great filter we need to have some idea of plausible filters.
How may a future great filter manifest?
– Reapers (mass effect)?
– Bezerker probes sent out to destroy any up and coming civilization that reaches a certain point? (A malevolent alien teenager in their basement could have seeded self-replicating bezerker probes as a ‘practical joke’)
– A robot takeover? (If this has been the cause of great filters in the past then why don’t we see evidence of expansionist robot civilizations? see here.  Also if the two major end states of life are either dead or genocidal intelligence explosion, and we aren’t the first, then it is speculated that we should live in a young universe.)

Robin Hanson gave a TedX talk on the Great Filter:


Robin Hanson is an associate professor of economics at George Mason University, a research associate at the Future of Humanity Institute of Oxford University, and chief scientist at Consensus Point. After receiving his Ph.D. in social science from the California Institute of Technology in 1997, Robin was a Robert Wood Johnson Foundation health policy scholar at the University of California at Berkeley. In 1984, Robin received a masters in physics and a masters in the philosophy of science from the University of Chicago, and afterward spent nine years researching artificial intelligence, Bayesian statistics, and hypertext publishing at Lockheed, NASA, and independently.

Robin has over 70 publications, and has pioneered prediction markets since 1988, being a principal architect of the first internal corporate markets, at Xanadu in 1990, of the first web markets, the Foresight Exchange since 1994, of DARPA’s Policy Analysis Market, from 2001 to 2003, and of Daggre/Scicast, since 2010.


Robin Hanson’s 1998 revision on the paper he wrote on the Great Filter in 1996
– The Drake Equation at connormorency  (where I got the Drake equation image – thanks)|
Slate Star Codex – Don’t Fear the Filter
Ask Ethan: How Fast Could Life Have Arisen In The Universe?
Keith Wiley – The Fermi Paradox, Self-Replicating Probes, Interstellar Transport Bandwidth