What Does Solution to Fermi Paradox Implies?
December 4, 2009 5 Comments
No present observations suggest a technologically advanced extraterrestrial intelligence
(ETI) has spread through the galaxy. However, under commonplace assumptions about galactic
civilization formation and expansion, this absence of observation is highly unlikely. This
improbability is the heart of the Fermi Paradox. The Fermi Paradox leads some to conclude that
humans have the only advanced civilization in this galaxy, either because civilization formation
is very rare or because intelligent civilizations inevitably destroy themselves. I find this case more implicitely appropriate. However, there are many cases which may be plausible.
The classic Fermi Paradox can lead to the conclusion that humans have formed the first
advanced civilization in the galaxy because extraterrestrial intelligence (ETI) has not yet been
observed. Numerous resolutions to this paradox have been proposed , spanning the range
of cosmological limits to sociological assumptions. A popular class of solutions assumes that the
evolution of life is rare in the Universe: Earth may not be wholly unique, but other inhabited
planets in the Universe could be too far away for any interaction or detection. But if life is a
common phenomenon in the galaxy, then it seems reasonable to expect observable evidence.
Furthermore, if the evolution of intelligence is commonplace, then there is hope for projects such
as the search for extraterrestrial intelligence (SETI), even though no present observations suggest
a technologically advanced ETI has spread throughout the galaxy.
The conclusion that other ETI do not exist contains implicit assumptions about the nature
and pattern of ETI. Specifically, this argument requires that ETI expand exponentially from their
home location throughout the entire galaxy , an assumption that is based on observations of
the expansion of human civilization on Earth. The assumption of exponential or other fastergrowth
is crucial to the conclusion that extraterrestrial civilizations should have colonized the
galaxy by now.
However, a closer look at human civilization suggests two problems with this
assumption. First, where human populations are exponentially expansive, they often—perhaps
always—do so unsustainably,
expansion. Second, not all human populations are exponentially expansive, such as the !Kung
San of the Kalahari Desert. These slower-growth human populations are without question
intelligent. Indeed, global human population growth is currently slowing, and humanity as a
whole may be transitioning towards a slower-growth, sustainable development pattern. A slowergrowth
humanity would even remain capable of space colonization.
It is possible that extraterrestrial civilizations face similar sustainability constraints. This
possibility suggests a resolution to the Fermi Paradox, which we name the “Sustainability
If the Sustainability Solution is true,
i.e. if intelligent civilizations cannot sustainexponential growth, then no exponentially expansive civilizations should likely be observed.
However, the Sustainability Solution does not rule out the possibility of civilizations following
slower-growth patterns. Such slower-growth civilizations expand sufficiently slowly that they
would not necessarily have colonized the entire galaxy by now. The Sustainability Solution also
does not rule out the possibility of faster-growth civilizations colonizing the galaxy and then
collapsing. The existence of slower-growth or collapsed civilizations is thus consistent with the
lack of human observations of extraterrestrial civilization.
The Fermi Paradox ultimately concerns the spatial expansion of civilizations, but spatial
expansion is closely linked with expansion in population, environmental impact, and resource
consumption. For example, migration is often driven by resource shortages, which in turn may
result from large population and/or environmental degradation. Likewise, migration to
uninhabited regions can lead to resource surpluses, which can in turn drive population growth.
Finally, broadly expansionist policy can cause expansion in each of space, population,
environmental impact, and resource consumption.
The Fermi Paradox posits that if intelligent life were common in the Universe, then in all
likelihood there would exist some extraterrestrial intelligence (ETI) capable of interstellar travel.
This ETI would then explore and colonize the galaxy, just as humans have explored and
colonized Earth and have begun exploring the Solar System. The magnitude of time required for
a technological ETI to spread throughout the galaxy is on the order of 1-100 Myr [4, 15],
significantly less than the ~10 Gyr age of the galactic thin disk, so the question arises:
If they exist, advanced ETI could have colonized the galaxy several times over by now, sothe lack of evidence for their presence implies their non-existence.
i.e. in a way that leads to an eventual end to the exponential.
Okey just take following implications
A = ETI exist, B = ETI are here, and
C = ETI are observed:
A, then (probably B)
B), then (probably C)
This inference can be criticized because it is only correct if
not-(probably C) is true. If (probablyC)
For example, ETI exploration of the galaxy could take the form of messenger probes that may have
already reached the Solar System, residing in the asteroid belt, Lagrange points, or other stable
orbits. Such probes with a limiting size of only ~1-10 meters may have so far eluded
observation. If ETI exploration takes such a remote form, then artifacts in the Solar System may
yet be observed, but ETI colonization of the Solar System, so far as we know, has not occurred.
Technological ETI are typically assumed to explore and colonize the galaxy just as
humans have explored and colonized Earth. This expansion implicitly assumes an exponential
growth pattern, leading to the colonization of the entire galaxy:
Assume that we eventually send expeditions to each of the 100 nearest stars. (These
are all within 20 light-years of the Sun.) Each of these colonies has the potential of
eventually sending out their own expeditions, and their colonies in turn can colonize,
and so forth. If there were no pause between trips, the frontier of space exploration
would then lie on the surface of a sphere whose radius was increasing at a speed of
0.10 c. At that rate, most of our Galaxy would be traversed within 650 000 years.
The assumption of exponential growth is in turn based on observations of the expansion of
human civilization on Earth:
If, the argument goes, there were intelligent beings elsewhere in our Galaxy, then
they would eventually have achieved space travel, and would have explored and
colonized the Galaxy, as we have explored and colonized the Earth.
However, as discussed above, exponential human population growth and colonization of the
planet may not be a sustainable development pattern. This fact calls into question a core
justification for the assumption of exponential expansion of ETI civilizations. If ETI civilizations
share similar development issues as human civilization, as is assumed in the Fermi Paradox, then
ETI civilizations would not be able to sustain exponential expansion. Likewise, if
exponential expansion could not be sustained, then ETI civilizations would either have switched.
to a slower-growth development pattern or collapsed. Collectively, these possibilities suggest the“Sustainability Solution” to the Fermi Paradox: The absence of ETI observation can be explainedby the possibility that exponential growth is not a sustainable development pattern for intelligentcivilizations.The Sustainability Solution implies that the existence of slower-growth ETI civilizationscannot be ruled out by the lack of observed ETI because these civilizations would grow tooslowly to have reached Earth by now. These civilizations may have always followed a slowergrowthdevelopment pattern, or they may have started with an exponential or other faster-growthgrowth pattern only to transition towards slower-growth as faster-growth became unsustainable. Both of these development patterns can be observed in human populations , suggestingthat both could be possible among ETI civilizations. Furthermore, just as slower-growth humanpopulations (including the global human civilization if it transitions successfully towardssustainable development) are highly intelligent and technologically capable, slower-growth ETImay still be as well. Indeed, slower-growth ETI may even possess space colonization capacity,just without having expanded so rapidly as to colonize the entire galaxy.The Sustainability Solution also implies that ETI civilizations may have previouslyfollowed an exponential or other faster-growth development pattern but eventually collapsed.This collapse could occur at the planetary scale, as is suspected may happen to human civilization , at the solar system scale, or even at the galactic scale. If the entire galaxy wereonce colonized by an ETI civilization, then the colonizing civilization must have collapsed insuch a way that no evidence of the colonization has been detected. Evidence of such a graveyardcivilization may still exist and may eventually be detectable by humans using search effortsdifferent from those already attempted. Furthermore, just as human populations sometimespersist in diminished numbers after undergoing collapse, a collapsed ETI civilization may stillexist at a smaller scale.Having considered the sustainability of ETI civilizations, we can now revisit the FermiParadox. If exponential or other faster-growth is unsustainable at the sub-galactic scale, then thesupposition by Hart and others that advanced ETI civilization could easily colonize thegalaxy is false. Alternatively, this supposition could be true if ETI civilizations that colonize thegalaxy eventually collapse, but we are unlikely to observe a galactic colony because fastergrowthcivilizations collapse quickly relative to astronomical timescales. In principle acivilization could colonize the galaxy through faster-growth and then avoid collapse bytransitioning towards sustainable slower-growth; however, the absence of observation of galactic7civilization suggests that this has not occurred. In either case, the Fermi Paradox cannot rule outthe possibility that slower-growth or post-collapse ETI civilizations currently exist.
A popular class of explanations for this absence of observation involves speculation into
the behavior or sociology of ETI. For example, a solution known as the zoo hypothesis predicts
that ETI civilization has set aside Earth as an undisturbed wildlife preserve , stealthily
observing Earth (perhaps using a virtual planetarium ) and waiting for its inhabitants to cross
a technological threshold before making themselves known . A recent hypothesis involving
common economic assumptions proposed a solution derived from resource issues,
concluding that ETI, like humans, will necessarily lack the patience required to conserve
resources for space colonization. Testing such hypotheses may require future technology; for
example, the zoo hypothesis might not be falsified (or vindicated) until humans begin interstellar
exploration. Nevertheless, most solutions of this class are falsifiable and thus legitimate avenues
of scientific inquiry.
Other possible explanations invoke the non-linearity of migration. If colonization through
the galaxy proceeds as a percolation problem, then expansion should halt after a finite number of
colonies, resulting in sub-galactic scale clusters around the parent star. Under this scenario,
colonized regions of the galaxy would remain isolated from each other, even in a galaxy teeming
with intelligent life. Alternatively, a relatively young civilization that engages in economic
interstellar travel may find its rapid expansion self-limited by the speed of light.
Civilizations that pursue aggressive growth may quickly collapse because growth outpaces
migration, while ETI that grow with the limits of the carrying capacity may expand too slow to
have colonized the galaxy yet. The persistence hypothesis suggests ETI civilization remains
undetected because the solar vicinity is persistently unvisited by ETI civilization—just as regions
of Earth such as the Amazon Basin, Siberia, and Indonesian islands are largely untouched by the
global human civilization. Persistent sites may remain persistent for a long time, explaining the
lack of ETI civilization in the neighborhood of the Sun. Many factors including these may limit
the expansion of ETI civilization at the sub-galactic scale. If any ETI civilization overcomes
such barriers, then the Sustainability Solution predicts an upper limit to faster-growth galactic
The classic Fermi Paradox can now be rephrased to account for its implicit assumptions.
If faster-growth development is unsustainable, then a faster-growth ETI civilization could
expand throughout the galaxy, only to collapse shortly thereafter. As a result, we would likely
not observe such a short-lived ETI civilization. This leads us to the inference that exponentially
expansive ETI civilization does not exist—contrary to the classic conclusion that ETI do not exist
at all. However, the non-existence of exponentially expansive ETI civilization does not preclude
the existence of ETI. Just as there are human populations maintaining sustainable, slower-growth
development, it is entirely possible that ETI exist with slower-growth development patterns.
Likewise, just as human populations sometimes persist in diminished numbers after a collapse, it
is possible that there exist post-collapse ETI.
Implications For SETI
The Sustainability Solution suggests a recalibration of the human search for ETI,
focusing on slower-growth and post-collapse ETI. Each of these forms of ETI would likely yield
different signs of their existence, which in turn could be detectable through different strategies.
Traditional SETI projects search for electromagnetic signals broadcast from ETI
civilizations . Electromagnetic signals could be broadcast by slower-growth ETI
civilizations, just as human civilization would retain the capacity to broadcast signals if it
transitions to slower-growth sustainable development. Electromagnetic signals could also be
broadcast by post-collapse graveyard civilizations: if part of the population survives the collapse,
then the survivors could make graveyard broadcasts. Alternatively, if the collapse leaves no
survivors, then the signal could, at least in principle, be broadcast by an automatic system
deployed before the collapse.
Another approach is to search for terrestrial planets whose atmospheric spectral
signatures suggest a higher likelihood of life on the planet . Atmospheric composition alone cannot conclusively demonstrate the presence of life on a distant planet, nor can they necessarily
distinguish between intelligent and non-intelligent life, but certain spectral signatures would be
unlikely in an abiotic world. For example, the presence of O3 and O2 could be a good biomarker,
especially if coupled with atmospheric CH4, and anoxic atmospheres analogous to the early Earth
may also be suitable candidates for life . Additionally, the red edge of chlorophyll is a unique
biosignature on Earth [33, 34], and inhabited extrasolar planets may exhibit their own distinctive
biosignatures. Such signatures would likely occur for slower-growth ETI civilizations because
the civilizations’ planets necessarily have life on them. Spectral biomarkers may also occur for
post-collapse civilizations; if the collapse has survivors, then, as with slower-growth ETI, the
survivors’ planets necessarily have life on them. Alternatively, if the collapse leaves no
survivors, then the planets may still retain a similar biosignature if non-intelligent or nontechnological
A third search strategy allows for the possibility of remote exploration by ETI
civilizations. Though colonization of the galaxy may be problematic, slower-growth ETI could
conceivably achieve interstellar exploration using small long-lived probes . Remote
interstellar exploration by future humans is at least plausible, foreshadowed by the entry of
Voyager into the heliosheath at the edge of the Solar System , suggesting that slower-growth
ETI with sufficient technology could embark on this form of galactic exploration. Searches for
ETI probes known Solar System SETI, also called SETA (Search for Extraterrestrial Artifacts) or
SETV (Search for Extraterrestrial Visitation) , has been proposed at visible and radio
wavelengths, capable of detecting probes as small as ~10 meters or less. Calls for a Solar
System SETI acknowledge that the possibility of remote ETI exploration is at least as likely as
interstellar ETI broadcasts, and a survey of the solar vicinity may be more pragmatic than an allsky
search for encoded messages .
The Sustainability Solution suggests that Solar System SETI may be the preferred option
in searching for technological ETI. Spectral signatures can be detected even if civilization on the
planet has not yet developed the capacity to perform electromagnetic broadcast, and a slowergrowth
civilization may persist for an extended period of time before gaining broadcast capacity.
Additionally, spectral signatures can be detected if a post-collapse civilization loses broadcast
capacity, and experience with human civilization suggests that collapse is much more likely to
cause loss of broadcast capacity than significant change in long-term atmospheric composition.
Nevertheless, remote spectral signatures only provide probable biosignatures at best—far from
the confirmation of intelligence or technology elsewhere. Solar System SETI, on the other hand,
would search for probes of extraterrestrial origin in our stellar vicinity. Artifacts may originate from an extant slower-growth ETI or an extinct galactic empire, but the discovery of either
would be near conclusive evidence of extraterrestrial technology.
Ultimately, assumptions about life in the Universe are heavily based on what we observe
on Earth. This is because Earth hosts our only known example of life. However, we cannot rule
out the possibility that ETI civilization may follow a development pattern sufficiently different
that we wouldn’t recognize it even if we detected its signal. Therefore, the implications for SETI
discussed here cannot be taken as conclusive.
Finally I can that the Fermi Paradox cannot logically conclude that humans are the only intelligent
civilization in the galaxy. This is due to the Sustainability Solution to the Fermi Paradox
presented here: the absence of ETI observation can be explained by the possibility that
exponential growth is not a sustainable development pattern for intelligent civilizations. Thus,
the Paradox can only conclude that other intelligent civilizations have not sustained exponential
growth patterns throughout the galaxy. It is still possible that slower-growth ETI civilizations
exist but have not expanded rapidly enough to be easily detectable by the searches humans have
yet made. It is also possible that faster-growth ETI civilizations previously expanded throughout
the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might
have left have also remained undetected. Both of these growth patterns can be observed in
human civilization, suggesting that they may be possible for ETI civilizations as well.
The Sustainability Solution to the Fermi Paradox has practical implications for both the
search for extraterrestrial life and human civilization management. In the search for
extraterrestrial life, the Sustainability Solution allows that slower-growth ETI civilizations may
still transmit radio or other signals. Furthermore, ambitions such as Solar System SETI may
eventually discover extraterrestrial messenger probes residing in the asteroid belt and other
stellar orbits. For human civilization management, the Sustainability Solution increases the
likelihood that human civilization needs to transition towards sustainable development in order
to avoid its own collapse.
Jacob D. Haqq-Misra, & Seth D. Baum (2009). The Sustainability Solution to the Fermi Paradox J.Br.Interplanet.Soc.62:47-51, 2009 arXiv: 0906.0568v1