Fermi paradox

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This article is about Enrico Fermi's paradox. For the music album, see
Fermi Paradox
(album).

"The Great Filter" redirects here. For the Tub Ring
album, see The Great
Filter (album).

A graphical representation of the Arecibo message -
Humanity's first attempt to use radio waves to actively communicate its
existence to alien civilizations

The Fermi paradox is the apparent contradiction between high estimates
of the probability of the existence of extraterrestrial
civilizations and the lack of evidence for, or contact with, such
civilizations.

The extreme age of the universe and its vast number of stars suggest that if
the Earth is typical, extraterrestrial life should be common.[1]
Discussing this proposition with colleagues over lunch in 1950, the physicist Enrico
Fermi asked: "Where are they?" (alternatively, "Where is
everybody?")[2][3] Fermi
questioned why, if a multitude of advanced extraterrestrial civilizations exist
in the Milky
Way galaxy,
evidence such as spacecraft or probes are not seen. A
more detailed examination of the implications of the topic began with a paper by
Michael
Hart in 1975, and it is sometimes referred to as the Fermi-Hart
paradox.[4] Another
closely related question is the Great Silence[5] -
even if travel is hard, if life is common, why don't we detect their radio
transmissions?

There have been attempts to resolve the Fermi Paradox by locating evidence of
extraterrestrial civilizations, along with proposals that such life could exist
without human knowledge. Counterarguments suggest that intelligent extraterrestrial
life does not exist or occurs so rarely that humans will never make contact
with it.

Starting with Hart, great deal of effort has gone into developing scientific
theories and possible models of extraterrestrial life and the Fermi paradox has
become a theoretical reference point in much of this work. The problem has
spawned numerous scholarly works addressing it directly, while various questions
that relate to it have been addressed in fields as diverse as astronomy, biology, ecology and philosophy. The emerging
field of astrobiology has brought an
interdisciplinary approach to the Fermi paradox and the question of
extraterrestrial life.

Contents

1 Basis of the
paradox

2 Related concepts

2.1 Drake equation and the
anthropic principle

3 Resolving the paradox
empirically

3.1 Radio
emissions

3.2 Direct planetary
observation

3.3 Alien
constructs

3.3.1 Probes, colonies,
and other artifacts

3.3.2 Advanced stellar
scale artifacts

4 Explaining the paradox
theoretically

4.1 No other civilizations
currently exist

4.1.1 No other
civilizations have arisen

4.1.2 It is the nature of
intelligent life to destroy itself

4.1.3 It is the nature of
intelligent life to destroy others

4.1.4 Human beings were
created alone

4.2 They do exist, but we
see no evidence

4.2.1 Communication is
impossible due to problems of scale

4.2.2 Communication is
impossible for technical reasons

4.2.3 They choose not to
interact with us

4.2.4 They are here
unobserved

5 See also

6 References

7 Suggested
reading

8 External links

[edit]
Basis of the paradox

The Fermi paradox is a conflict between an argument of scale and probability and a lack of evidence. A more
complete definition could be stated
thus:

The size and age of the universe suggest that many technologically advanced
extraterrestrial civilizations ought to exist. However, this hypothesis seems
inconsistent with the lack of observational evidence to support
it.

The first aspect of the paradox, "the argument by scale", is a function of
the raw numbers involved: there are an estimated 250 billion (2.5 x 1011) stars
in the Milky
Way and 70 sextillion (7 x 1022) in the visible
universe.[6] Even if
intelligent life occurs on only a minuscule percentage of planets around these
stars, there should still be a great number of civilizations extant in the Milky
Way galaxy alone.
This argument also assumes the mediocrity
principle, which states that Earth is not special, but merely a
typical planet,
subject to the same laws, effects, and likely outcomes as any other world. Some
estimates using the Drake equation support
this argument, although the assumptions behind those calculations have
themselves been challenged.

The second cornerstone of the Fermi paradox is a rejoinder to the argument by
scale: given intelligent life's ability to overcome scarcity, and its tendency to
colonize new habitats, it seems
likely that any advanced civilization would seek out new resources and colonize
first their star system, and then surrounding star systems. As there is no
conclusive or certifiable evidence on Earth or elsewhere in the known universe
of other intelligent life after 13 billion years of the universe's history, it
may be assumed that intelligent life is rare or our assumptions about the
general behavior of intelligent species are flawed.

The Fermi paradox can be asked in two ways. The first is "Why are no aliens
or their artifacts physically here?". If interstellar travel is possible, even
the "slow" kind nearly within the reach of Earth technology, then it would only
take from 5 million to 50 million years to colonize the galaxy.[7] This is
a relatively small amount of time on a geological scale, let
alone a cosmological
one. Since there are many stars older than the sun, or since intelligent
life might have evolved earlier elsewhere, the question then becomes why the
galaxy is not colonized already. Even if colonization is impractical or
undesirable to all alien civilizations, large scale exploration of the galaxy is
still possible; the means of exploration and theoretical probes involved are
discussed extensively below.
However, no signs of colonization or exploration have been found.

The argument above may not hold for the universe as a whole, since travel
times may well explain the lack of physical presence on Earth for far away
galaxies. However, the question then becomes "Why do we see no signs of
intelligent life?" as a sufficiently advanced civilization[8] could
potentially be seen over a significant fraction of the size of the
observable universe[9] Even if
such civilizations are rare, since they could be detected from far away, many
more potential sites for their origin are within our view. However, no signs of
such civilizations have been detected.

It is currently unclear which version of the paradox is stronger.[10]

[edit]
Related concepts

[edit]
Drake equation and the anthropic principle

Main articles: Drake equation and anthropic
principle

While numerous theories and principles attend to the Fermi paradox, the two
most closely related are the Drake equation and the anthropic
principle.

The former was formulated by Dr. Frank Drake in 1960, a
decade after the objections raised by Enrico Fermi, in an attempt
to find a systematic means to evaluate the numerous probabilities involved in
alien life. The speculative equation factors: the rate of star
formation in the galaxy; the number of stars with planets and the number
that are habitable; the number of those planets which develop life and
subsequently intelligent communicating life; and finally the expected lifetimes
of such civilizations. The fundamental problem is that the last four terms
(fraction of planets with life, odds life becomes intelligent, odds intelligent
life become communicative, and lifetime of communicating civilizations) are
completely unknown. We have only one example, rendering statistical estimates
impossible, and even the example we have is subject to a strong anthropic bias.

The Drake equation has been used by both optimists and pessimists with wildly
differing results. Dr. Carl Sagan, using optimistic
numbers, suggested as many as one million communicating civilizations in the Milky Way in
1966, though he later suggested that the number could be far smaller. Skeptics,
such as Frank Tipler, have put
in pessimistic numbers and concluded that the average number of civilizations in
a galaxy is much less than one.[11] (Note
that, even though there is at least one civilization in our galaxy, the average
or "most likely" number of civilizations in our galaxy as described by this
equation may still be smaller than one. In other words, the fact that there is
at least one civilization in our galaxy doesn't mean that this was a likely
outcome. This is an excellent example of anthropic bias. No
civilization can use its own galaxy to estimate likelihood, since if there was
not at least one civilization the question could not arise.)

Frank Drake himself has commented that the Drake Equation is unlikely to
settle the Fermi paradox; instead it is just a way of organizing our ignorance
on the subject.

[edit]
Resolving the paradox empirically

One obvious way to resolve the Fermi paradox would be to find conclusive
evidence of extraterrestrial intelligence. Various efforts to find such evidence
have been made since 1960, and several are ongoing. As human beings do not have
interstellar travel
capability, such searches are being carried out at great distances and rely on
careful analysis of very subtle evidence. This limits possible discoveries to
civilizations which alter their environment in a detectable way, or produce
effects that are detectable at a distance, such as radio emissions.
Non-technological civilizations are very unlikely to be detectable from Earth in
the near future.

One difficulty in searching is avoiding an overly anthropocentric
viewpoint. Conjecture on the type of
evidence likely to be found often focuses on the types of activities that humans
have performed, or likely would perform given more advanced technology.
Intelligent aliens might avoid these "expected" activities, or perform
activities totally novel to humans.

[edit]
Radio emissions

Further information: SETI, Project
Ozma, Project Phoenix
(SETI), SERENDIP, and Allen Telescope
Array

Radio technology and the ability to construct a radio telescope are
presumed to be a natural advance for technological species[12]
theoretically creating effects that might be detected over interstellar
distances. Sensitive observers of the solar system, for example, would note
unusually intense radio waves for a G2 star due to
Earth's television and telecommunication broadcasts. In the absence of an
apparent natural cause, alien observers might infer the existence of terrestrial
civilization.

Therefore, the careful searching of radio emissions from space for
non-natural signals may lead to the detection of alien civilizations. Such
signals could be either "accidental" byproducts of a civilization, or deliberate
attempts to communicate, such as the Communication
with Extraterrestrial Intelligence's Arecibo message. A
number of astronomers and observatories have attempted and are attempting to
detect such evidence, mostly through the SETI organization, although other
approaches, such as optical
SETI also exist.

Several decades of SETI analysis have not revealed any main sequence stars
with unusually bright, or meaningfully repetitive radio emissions, although
there have been several candidate signals: on August 15, 1977 the "Wow! signal" was picked up
by The
Big Ear radio telescope. However, the Big Ear only looks at each point on
the sky for 72 seconds, and re-examinations of the same spot have found nothing.
In 2003, Radio source
SHGb02+14a was isolated by http://en.wikipedia.org/wiki/SETI%40home
analysis, although it has largely been discounted by further study. There are
numerous technical assumptions underlying SETI that may cause human beings to
miss radio emissions with present search techniques; these are discussed below.

[edit]
Direct planetary observation

A composite picture of Earth at night. Human civilization is
detectable from space.

Detection and classification of exoplanets has come out of
recent refinements in mainstream astronomical instruments and analysis. While
this is a new field in astronomy — the first published paper claiming to have
discovered an exoplanet was released in 1989 — it is possible that planets which
are likely to be able to support life will be found in the near future.

Direct evidence for the existence of life may eventually be observable, such
as the detection of biotic signature gases (such as methane and oxygen) — or even the industrial
air
pollution of a technologically advanced civilization — in an exoplanet's
atmosphere by means of spectrographic
analysis.[13] With
improvements in our observational capabilities, it may eventually even be
possible to detect direct evidence such as that which humanity produces (see
right).

However, exoplanets are rarely directly observed (the first claim to have
done so was made in 2004[14]);
rather, their existence is usually inferred from the effects they have on the
star(s) they orbit. This means that usually only the mass and orbit of an
exoplanet can be deduced. This information, along with the stellar
classification of its sun, and educated guesses as to its composition
(usually based on the mass of the planet, and its distance from its sun), allows
for rough approximations of the planetary environment.

The current methods
for exoplanet detection are not likely to detect life-bearing Earth-like
worlds. Methods such as gravitational
microlensing can detect the presence of "small" worlds, potentially even
smaller than the Earth, but can only detect such worlds for very brief moments
of time, and no follow-up is possible. Other methods such as radial
velocity and astrometry allow prolonged
observations of exoplanet effects, but only work with worlds that are several
times the mass of Jupiter. These seem unlikely
candidates to harbor Earth-like life. However, exoplanet detection and
classification is a very active sub-discipline in astronomy, with 241 such
planets being detected between 1988 and 2007,[15]
and the first possibly terrestrial planet
discovered within a star's habitable zone being
found in 2007.[16] It is
hoped that refinements
in exoplanet detection methods will lead to increased detection and
prolonged observation of terrestrial planets. Such observational refinements
would allow us to better gauge how common potentially inhabitable worlds are,
and thus allow us a much better idea of how common life in the universe might
be, which of course has profound influence over the expectations behind the
Fermi Paradox itself.

[edit]
Alien constructs

[edit]
Probes, colonies, and other artifacts

Further information: Von Neumann
probe and Bracewell probe

As noted, given the size and age of the universe, and the relative rapidity
at which dispersion of intelligent life can occur, evidence of alien
colonization attempts might plausibly be discovered. Additionally, evidence of
"unbeinged" exploration in the form of probes and information gathering devices
may await discovery.

Some theoretical exploration techniques such as the Von Neumann probe
could exhaustively explore a galaxy the size of the Milky Way in
as little as half a million years, with relatively little investment in
materials and energy relative to the results. If even a single civilization in
the Milky Way attempted this, such probes could spread throughout the entire
galaxy. Evidence of such probes might be found in the solar system—perhaps in
the asteroid belt where raw
materials would be plentiful and easily accessed.[17]

Another possibility for contact with an alien probe—one that would be trying
to find human beings—is an alien Bracewell probe. Such a
device would be an autonomous space probe whose purpose is to seek out and
communicate with alien civilizations (as opposed to Von Neumann probes, which
are usually described as purely exploratory). These were proposed as an
alternative to carrying a slow speed-of-light dialogue
between vastly distant neighbors. Rather than contending with the long delays a
radio dialogue would suffer, a probe housing an artificial
intelligence would seek out an alien civilization to carry on a close range
communication with the discovered civilization. The findings of such a probe
would still have to be transmitted to the home civilization at light speed, but
an information-gathering dialogue could be conducted in real time.[18]

Since the 1950s direct exploration has been carried out on a small fraction
of the solar system and no evidence that it has ever been visited by alien
colonists, or probes, has been uncovered. Detailed exploration of areas of the
solar system where resources would be plentiful—such as the asteroids, the Kuiper
belt, the Oort cloud and the various
planetary ring systems—may yet produce evidence of alien exploration, though
these regions are vast and difficult to investigate. There have been preliminary
efforts in this direction in the form of the SETA and SETV projects to search
for extraterrestrial artifacts or other evidence of extraterrestrial visitation
within the solar system.[19] There
have also been attempts to signal, attract, or activate Bracewell probes in
Earth's local vicinity, including by scientists Robert Freitas and
Francisco Valdes.[20] Many
of the projects that fall under this umbrella are considered "fringe" science by
astronomers and none of the various projects have located any artifacts.

Should alien artifacts be discovered, even here on Earth, they may not be
recognizable as such. The products of an alien mind and an advanced alien
technology might not be perceptible or recognizable as artificial constructs.
Exploratory devices in the form of bio-engineered life forms created through synthetic biology
would presumably disintegrate after a point, leaving no evidence; an alien
information gathering system based on molecular
nanotechnology could be all around us at this very moment, completely
undetected. Clarke's third law
suggests that an alien civilization well in advance of humanity's might have
means of investigation that are not yet conceivable to human beings.

[edit]
Advanced stellar scale artifacts

A variant of the speculative Dyson sphere. Such large
scale artifacts would drastically alter the spectrum of a
star.

Further information: Dyson sphere, Kardashev scale, Alderson disk, Matrioshka brain, Stellar engine

In 1959, Dr. Freeman Dyson observed
that every developing human civilization constantly increases its energy
consumption, and theoretically, a civilization of sufficient age would require
all the energy produced by its sun. The Dyson Sphere was the thought experiment
that he derived as a solution: a shell or cloud of objects enclosing a star to
harness as much radiant energy as possible. Such a feat of astroengineering would
drastically alter the observed spectrum of the sun,
changing it at least partly from the normal emission lines of a
natural stellar atmosphere,
to that of a black body radiation, probably
with a peak in the infrared. Dyson himself
speculated that advanced alien civilizations might be detected by examining the
spectra of stars, searching for such an altered spectrum.[21]

Since then, several other theoretical stellar-scale megastructures have been
proposed, but the central idea remains that a highly advanced civilization —
Type II or greater on the Kardashev scale — could
alter its environment enough as to be detectable from interstellar
distances.

However, such constructs may be more difficult to detect than originally
thought. Dyson spheres might have different emission spectra depending on the
desired internal environment; life based on high-temperature reactions may
require a high temperature environment, with resulting "waste radiation" in the
visible spectrum, not the infrared.[22]
Additionally, a variant of the Dyson sphere has been proposed which would be
difficult to observe from any great distance; a Matrioshka Brain is a
series of concentric spheres, each radiating less energy per area than its inner
neighbour. The outermost sphere of such a structure could be close to the
temperature of the interstellar background radiation, and thus be all but
invisible.

There have been some preliminary attempts to find evidence of the existence
of Dyson spheres or other
large Type-II or Type-III Kardashev scale
artifacts that would alter the spectra of their core stars, but optical surveys
have not located anything. Fermilab has an ongoing program
to find Dyson spheres,[23] but
such searches are preliminary and incomplete as yet.

[edit]
Explaining the paradox theoretically

Certain theoreticians accept that the apparent absence of evidence proves the
absence of extraterrestrials and attempt to explain why. Others offer possible
frameworks in which the silence may be explained without ruling out the
possibility of such life, including assumptions about extraterrestrial behaviour
and technology.

[edit]
No other civilizations currently exist

The simplest explanation is that the human species is alone in the
galaxy. Several theories along these lines have been proposed, explaining why
intelligent life might be either very rare, or very short lived.

[edit]
No other civilizations have arisen

See also: Rare Earth
hypothesis

Those who believe that extraterrestrial intelligent life does not exist argue
that the conditions needed for life—or at least complex life—to
evolve are rare or even unique to Earth. This is known as the Rare Earth
hypothesis, which attempts to resolve the Fermi paradox by rejecting the mediocrity
principle, and asserting that Earth is not typical, but unusual or even
unique. While a unique Earth has had historical support on philosophical or
religious grounds, the Rare Earth Hypothesis uses quantifiable
and statistical arguments to
argue that multicellular life is exceedingly rare in the universe because
Earth-like planets are themselves exceedingly rare and/or many improbable
coincidences have converged to make complex life on Earth
possible.[24] While
some have pointed out that complex life may evolve through other mechanisms than
those found specifically here on Earth, the fact that in the extremely long
history of life on the Earth only one species has developed a civilization to
the point of being capable of space flight and radio technology seems to lend
more credence to the idea of technologically advanced civilization being a rare
commodity in the universe.

For example, the emergence of intelligence may have been an evolutionary
accident. Geoffrey
Miller proposes that human intelligence is the result of runaway sexual selection, which
takes unpredictable directions. Steven Pinker, in his book
How the Mind
Works, cautions that the idea that evolution of life (once it has
reached a certain minimum complexity) is bound to produce intelligent beings,
relies on the fallacy of the "ladder of evolution": As evolution does not strive for
a goal but just happens, it uses the adaptation most useful for a
given ecological niche, and
the fact that, on Earth, this led to language-capable sentience only once so far
may suggest that this adaption is only rarely a good choice and hence by no
means a sure endpoint of the evolution of a tree of life. Another
theory along these lines is that even if the conditions needed for life might be common in the
universe, that the formation of life itself, a complex array of molecules that
are capable simultaneously of reproduction, the creation or extraction of all
base components that it uses to build itself, from the environment, and of
obtaining energy in a form that it can use to maintain the reaction (or the
initial abiogenesis on a potential
life-bearing planet), might ultimately be very rare even if worlds that might
have the proper initial conditions for life might be common.

Insofar as the Rare Earth Hypothesis privileges Earth-life and its process of
formation, it is a variant of the anthropic principle.
The variant of the Anthropic Principle states the universe seems uniquely suited
towards developing human intelligence. This philosophical stance opposes not
only mediocrity, but the
Copernican
principle more generally, which suggests there is no privileged location in
the universe.

Opponents dismiss both Rare Earth and the anthropic principle
as tautological — if
a condition must exist in the universe for human life to
arise, then the universe must already meet that condition, as human life exists
— and as an unimaginative
argument. According to this analysis, the Rare Earth hypothesis confuses a
description of how life on Earth arose with a uniform conclusion of how life
must arise.[25] While
the probability of the specific conditions on Earth being widely replicated is
low, we do not know what complex life may require in order to evolve.

[edit]
It is the nature of intelligent life to destroy
itself

See also: Doomsday
argument

Technological civilizations may usually or invariably destroy themselves
before or shortly after developing radio or space flight technology. Possible
means of annihilation include nuclear war, biological warfare or
accidental contamination, nanotechnological catastrophe,
ill-advised
physics experiments, or a Malthusian
catastrophe after the deterioration of a planet's ecosphere. This general theme
is explored both in fiction and in mainstream scientific theorizing. Indeed,
there are probabilistic arguments which suggest that humanity's end may occur
sooner rather than later. In 1966 Sagan and Shklovskii suggested
that technological civilizations will either tend to destroy themselves within a
century of developing interstellar communicative capability or master their
self-destructive tendencies and survive for billion-year timescales.[26]
Self-annihilation may also be viewed in terms of thermodynamics: insofar as life
is an ordered system that can sustain itself against the tendency to disorder,
the "external transmission" or interstellar communicative phase may be the point
at which the system becomes unstable and self-destructs.[27]

From a Darwinian perspective, self-destruction would be a paradoxical outcome
of evolutionary success. The evolutionary
psychology that developed during the competition for scarce resources over
the course of human evolution has left the species subject to aggressive,
instinctual drives to consume resources, increase longevity, and to reproduce —
in part, the very motives that led to the development of technological society.
It seems likely that intelligent extraterrestrial life would evolve subject to
similar conditions and thus face the same possibility of self-destruction, yet
for destruction to provide a good answer to Fermi's Question, it would have to
be very nearly universal, that is, have a probability of very nearly 1.0. It has
been suggested, for instance, that a successful alien species will be a superpredator, as is Homo
sapiens.[28]

[edit]
It is the nature of intelligent life to destroy
others

See also: technological
singularity and Von Neumann
probe

Another possibility is that intelligent species beyond a certain point of
technological capability will destroy other intelligence as it appears. The idea
that someone, or something, is destroying intelligent life in the universe has
been well explored in science fiction[29] and
the scientific literature.[5] A
species might undertake such extermination out of expansionist motives, paranoia
or simple aggression. In 1981, cosmologist Edward Harrison also
pointed out that such behavior would be an act of prudence: an intelligent
species that has overcome its own self-destructive tendencies might view any
other species bent on galactic expansion as a kind of virus.[30]

The extermination of other civilizations might be carried out with
self-replicating spacecraft. Under such a scenario, even if a civilization that
created such machines were to disappear, the probes could outlive their
creators, destroying civilizations far into the future. It is hard to imagine
any intelligent species actually wanting to develop such probes as they are
dangerous to the creators as well as to all other life. Furthermore if their aim
is to colonize the galaxy first they can fulfill their aim simply by being the
first to colonize as the colonization time for the galaxy is much less than the
age of the galaxy.[31]

[edit]
Human beings were created alone

Religious and philosophical speculation about extraterrestrial intelligent
life long predates the modern scientific inquiry into the subject. Some
religious thinkers, including the Jewish rationalist commentator Rabbi Hasdai
Crescas (c. 1340–1410/1411)[32] and
the Christian philosopher Nicholas of Cusa
(1401–1464), posited the possibility of such extraterrestrial intelligence. On
the other hand, at least some strains within the various Western religious
traditions suggest the uniqueness of human beings in the divine plan and would
counsel against belief in intelligent life on other worlds.[33]

Religious reasons for doubting the existence of extraterrestrial intelligent
life resemble some forms of the Rare Earth
Hypothesis. The argument here would be a teleological form of the
strong anthropic principle:
the universe was designed for the express purpose of creating human (and only
human) intelligence.[34]

[edit]
They do exist, but we see no evidence

It may be that technological extraterrestrial civilizations exist, but that
human beings cannot communicate with them because of various constraints:
problems of scale or of technology; because their nature is simply too alien for
meaningful communication; or because human society refuses to admit to evidence
of their presence.

[edit]
Communication is impossible due to problems of
scale

See also: Relativity of
simultaneity

NASA's conception of the Terrestrial Planet
Finder. Is it possible alien civilizations are too far away for meaningful
communication?

Intelligent civilizations are too far apart in
space or time

It may be that non-colonizing technologically capable alien civilizations
exist, but that they are simply too far apart for meaningful two-way
communication.[35] If two
civilizations are separated by several thousand light years, it is very
possible that one or both cultures may become extinct before meaningful dialogue
can be established. Human searches may be able to detect their existence, but
communication will remain impossible because of distance. This problem might be
ameliorated somewhat if contact/communication is made through a Bracewell probe. In this
case at least one partner in the exchange may obtain meaningful information.
Alternatively, a civilization may simply broadcast its knowledge, and leave it
to the receiver to make what they may of it. This is similar to the transmission
of information from ancient civilizations to the present.[36]

The problem of distance is compounded by the fact that timescales affording a
"window of opportunity" for detection or contact might be quite small. Advanced
civilizations may periodically arise and fall throughout our galaxy, but this may be such a
rare event, relatively speaking, that the odds of two or more such civilizations
existing at the same time are low. There may have been intelligent civilizations
in the galaxy before the emergence of intelligence on Earth, and there may be
intelligent civilizations after its extinction, but it is possible that human
beings are the only intelligent civilization in existence now. The term
"now" is somewhat complicated by the finite speed of light and the
nature of spacetime under relativity.
Assuming that an extraterrestrial intelligence is not able to travel to our
vicinity at faster-than-light
speeds, in order to detect an intelligence 1,000 light-years distant, that
intelligence will need to have been active 1,000 years ago.

There is a possibility that archaeological evidence of past civilizations may
be detected through deep space observations — especially if they left behind
large artifacts such as Dyson spheres — but this
seems less likely than detecting the output of a thriving civilization.

It is too expensive to spread physically
throughout the galaxy

See also: Project Daedalus, Project
Orion (nuclear propulsion), and Project
Longshot

Many assumptions about the ability of an alien culture to colonize other
stars are based on the idea that interstellar travel is technologically
feasible. While the current understanding of physics rules out the possibility
of faster than light
travel, it appears that there are no major theoretical barriers to the
construction of "slow" interstellar ships. This idea underlies the concept of
the Von Neumann probe and
the Bracewell probe as
evidence of extraterrestrial intelligence.

It is possible, however, that present scientific knowledge cannot properly
gauge the feasibility and costs of such interstellar colonization. Theoretical
barriers may not yet be understood and the cost of materials and energy for such
ventures may be so high as to make it unlikely that any civilization could
afford to attempt it. Even if interstellar travel and colonization are possible,
they may be difficult, leading to a colonization model based on percolation
theory.[37]
Colonization efforts may not occur as an unstoppable rush, but rather as an
uneven tendency to "percolate" outwards, within an eventual slowing and
termination of the effort given the enormous costs involved and the fact that
colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur
in "clusters," with large areas remaining uncolonized at any one time.

A similar argument holds that interstellar physical travel may be possible,
but is much more expensive than interstellar communication. Furthermore, to an
advanced civilization, travel itself may be replaced by communication, through
mind uploading and
similar technologies.[38]
Therefore the first civilization may have physically explored or colonized the
galaxy, but subsequent civilizations find it cheaper, faster, and easier to
travel and get information through contacting existing civilizations rather than
physically exploring or traveling themselves. In this scenario, since there is
little or no physical travel, and directed communications are hard to see except
to the intended receiver, there could be many technical and interacting
civilizations with few signs visible across interstellar distances.

Human beings have not been searching long
enough

Humanity's ability to detect and comprehend intelligent extraterrestrial life
has existed for only a very brief period — from 1937 onwards, if the invention
of the radio telescope is taken
as the dividing line — and Homo sapiens is a
geologically recent species. The whole period of modern human existence to date
(about 200,000 years) is a very
brief period on a cosmological scale, while radio transmissions have only been
propagated from since 1895. Thus it remains possible that human beings have
neither been searching long enough to find other intelligences, nor been in
existence long enough to be found.

One million years ago there would have been no humans for any
extraterrestrial emissaries to meet. For each further step back in time, there
would have been increasingly fewer indications to such emissaries that
intelligent life would develop on Earth. In a large and already ancient
universe, a space-faring alien species may well have had many other more
promising worlds to visit and revisit. Even if alien emissaries visited in more
recent times, they may have been misinterpreted by early human cultures as supernatural entities. (As
proposed by Erich von
Däniken)

This hypothesis is more plausible if alien civilizations tend to stagnate or
die out, rather than expand. However, "the probability of a site never being
visited, even [with an] infinite time limit, is a non-zero value".[39] Thus,
even if intelligent life expands elsewhere, it remains statistically possible
that such terrestrial life could eventually be discovered.

[edit]
Communication is impossible for technical
reasons

Human beings are not listening properly

There are some assumptions that underlie the SETI search programs that may cause
searchers to miss signals that are present. For example, the radio searches to
date would completely miss highly compressed data streams (which would be almost
indistinguishable from "white noise" to anyone who
did not understand the compression algorithm). Extraterrestrials might also use
frequencies that scientists have decided are unlikely to carry signals, or do
not penetrate our atmosphere, or use modulation strategies that
are not being looked for. The signals might be at a datarate that is too fast
for our electronics to handle. "Simple" broadcast techniques might be employed,
but sent from non-main sequence stars which
are searched with lower priority; current programs assume that most alien life
will be orbiting Sun-like stars.[40]

The greatest problem is the sheer size of the radio search needed to look for
signals, the limited amount of resources committed to SETI, and the sensitivity
of modern instruments. SETI estimates, for instance, that with a radio telescope
as sensitive as the Arecibo Observatory,
Earth's television and radio broadcasts would only be detectable at distances up
to 0.3 light years.[41]
Clearly detecting an Earth type civilization at great distances is difficult. A
signal is much easier to detect if the signal energy is focused in either a
narrow range of frequencies (Narrowband transmissions),
and/or directed at a specific part of the sky. Such signals can be detected at
ranges of hundreds to tens of thousands of light-years distance.[42]
However this means that detectors must be listening to an appropriate range of
frequencies, and be in that region of space to which the beam is being sent.
Many SETI searches go so far as to assume that extraterrestrial civilizations
will be broadcasting a deliberate signal (like the Arecibo message), in
order to be found.

Thus to detect alien civilizations through their radio emissions, Earth
observers either need more sensitive instruments or must hope for fortuitous
circumstances: that the broadband radio emissions of alien radio technology are
much stronger than our own; that one of SETI's programs is listening to the
correct frequencies from the right regions of space; or that aliens are sending
focused transmissions such as the Arecibo message in our
general direction.

Civilizations only broadcast detectable radio
signals for a brief period of time

It may be that alien civilizations are detectable through their radio
emissions for only a short time, reducing the likelihood of spotting them. There
are two possibilities in this regard: civilizations outgrow radio through
technological advance or, conversely, resource depletion cuts short the time in
which a species broadcasts.

The first idea, that civilizations advance beyond radio, is based in part on the "fiber
optic objection": the use of high power radio with low-to-medium gain (i.e.,
non-directional) antennas for long-distance transmission
is wasteful of spectrum, yet this "waste" is precisely what makes these systems
conspicuous at interstellar distances. Humans are moving to directional or
guided transmission channels such as electrical cables, optical fibers,
narrow-beam microwave and lasers, and conventional radio
with non-directional antennas is increasingly reserved for low-power,
short-range applications such as cell phones and Wi-Fi networks. These
signals are far less detectable from space. Analog television, developed in the
mid-twentieth century, contains strong carriers to aid reception
and demodulation. Carriers are spectral lines that are very easily detected yet
do not convey any information beyond their highly artificial nature. Nearly
every SETI project is looking for carriers for just this reason, and UHF TV
carriers are currently the most conspicuous and artificial signals from earth
that could be detected at interstellar distances. But advances in technology are
replacing analog TV with digital television
which uses spectrum more efficiently precisely by eliminating or reducing
components such as carriers that make them so conspicuous.

Hypothetically, advanced alien civilizations evolve beyond broadcasting at
all in the electromagnetic spectrum and communicate by principles of physics we
don't yet understand. Thus it seems plausible that other civilizations would
only be detectable for a relatively short period of time between the discovery
of radio and the switch to more efficient technologies.

A different argument is that resource depletion will soon result in a decline
in technological capability. Human civilization has been capable of interstellar
radio communication for only a few decades and is already rapidly depleting
fossil fuels and grappling with the problem of peak oil. It may only be a few
more decades before energy becomes too expensive, and the necessary electronics
and computers too difficult to manufacture, for societies to continue the
search. If the same conditions regarding energy supplies hold true for other
civilizations, then radio technology may be a short-lived phenomenon. Unless two
civilizations happen to be near each other and develop the ability to
communicate at the same time it would be virtually impossible for any one
civilization to "talk" to another.

Critics of the resource depletion argument point out that an energy-consuming
civilization is not dependent solely on fossil fuels. Alternate
energy sources exist, such as solar power which is
renewable and has enormous potential relative to technical barriers.[43] For
depletion of fossil fuels to end the "technological phase" of a civilization,
some form of technological
regression would have to invariably occur, preventing the exploitation of renewable energy
sources.

They tend to experience a technological
singularity

See also: Sentience
Quotient and Matrioshka
brain

Another possibility is that technological civilizations invariably experience
a technological
singularity and attain a posthuman
(or postalien) character. Theoretical civilizations of this sort may have
altered drastically enough to render communication impossible. The intelligences
of a post-singularity civilization might require more information exchange than
is possible through interstellar communication, for example. Or perhaps any
information humanity might provide would appear elementary, and thus they do not
try to communicate, any more than human beings attempt to talk to ants.

Even more extreme forms of post-singularity have been suggested, particularly
in fiction: beings that divest themselves of physical form, create massive
artificial virtual environments, transfer themselves into these environments
through mind transfer, and exist
totally within virtual worlds, ignoring the external physical universe.
Surprisingly early treatments, such as Lewis Padgett's short
story Mimsy were the
Borogoves (1943), suggest a migration of advanced beings out of the
presently known physical universe into a different and presumably more agreeable
alternate one.

One version of this perspective, which makes predictions for future SETI
findings of transcension "fossils" and includes a variation of the Zoo
hypothesis below, has been proposed by singularity scholar John
Smart.[44]

[edit]
They choose not to interact with us

Earth is purposely isolated (The zoo
hypothesis)

Main article: Zoo
hypothesis

It is possible that the belief that alien races would communicate with the
human species is a fallacy, and that alien
civilizations may not wish to communicate, even if they have the technical
ability. A particular reason that alien civilizations may choose not to
communicate is the so-called Zoo hypothesis: the idea that Earth is being
monitored by advanced civilizations for study, or is being preserved in an
isolated "zoo or wilderness area".[45]

Many other reasons that an alien race might avoid contact have been proposed.
Aliens might only choose to allow contact once the human race has passed certain
ethical, political or technological standards i.e. ending poverty/war or being
able to master interstellar travel. They may not want to interfere with our
natural independent progress.[46] The
Earth may have been set as an explicit experiment that contact would ruin, or it
may be felt that it is too dangerous to make contact (for us or them). Perhaps
advanced civilizations actively hide, not only from Earth but from everyone
else, since the
galaxy is a dangerous place.

These ideas are most plausible if there is a single alien civilization within
contact range, or there is a homogeneous culture or law amongst alien
civilizations which dictates that the Earth be shielded. If there is a plurality
of alien cultures, however, this theory may break down under the uniformity of
motive flaw: all it takes is a single culture or civilization to decide to
act contrary to the imperative within our range of detection for it to be
abrogated, and the probability of such a violation increases with the number of
civilizations.[47]

A related idea is that the perceived universe is a simulated reality. The
planetarium hypothesis[48] holds
that beings may have simulated a universe for us that appears to be empty of
other life, by design. The simulation argument[49] by
Bostrom holds that although such a simulation may contain other life, such life
cannot be much in advance of us since a far more advanced civilization may be
correspondingly hard to simulate.

They are too alien

See also: technological
singularity

Another possibility is that human theoreticians have underestimated how much
alien life might differ from that on Earth. Alien psychologies may simply be too
different to communicate with human beings, and they are unable or unwilling to
make the attempt. Human mathematics, language, tool use, and other
concepts and communicative capacity may be parochial to Earth and not shared by
other life.[50]

They are non-technological

It is not clear that a civilization of intelligent beings must be
technological. If an alien species does not develop technology, because it is
difficult in its environment, because it chooses not to, or for any other
reason, it will be very hard for human beings to detect.[51]
Intelligence alone, as opposed to life, is not necessarily visible across
interstellar distances. While there are various remote sensing techniques which
could perhaps detect life-bearing planets, none of them has any ability to
distinguish intelligent but non-technical life from non-intelligent life. Not
even any theoretical methods for doing so have been proposed, short of an actual
physical visit by an astronaut or probe. This is sometimes referred to as the
"algae vs. alumnae" problem.[51]

[edit]
They are here unobserved

It may be that intelligent alien life forms not only exist, but are already
present here on Earth. They are not detected because they do not wish it, human
beings are technically unable, or because societies refuse to admit to the
evidence.[52]

It is not unreasonable that a life form intelligent enough to travel to Earth
would also be sufficiently intelligent to exist here undetected. In this view,
the aliens have arrived on Earth, or in our solar system, and are observing the
planet, while concealing their presence. Observation could conceivably be
conducted in a number of ways that would be very difficult to detect. For
example, a complex system of microscopic monitoring devices constructed via molecular
nanotechnology could be deployed on Earth and remain undetected, or
sophisticated instruments could conduct passive monitoring from elsewhere.

Many UFO researchers
and watchers argue that society as a whole is unfairly biased against claims of
alien abduction, sightings,
and encounters, and as
a result may not be fully receptive to claims of proof that aliens are visiting
our planet. Others use complex conspiracy theories to
allege that evidence of alien visits is being concealed from the public by
political elites who seek to hide the true extent of contact between aliens and
humans. Scenarios such as these have been depicted in popular culture for
decades.

[edit]
See also

Astrosociobiology

Fermi problem

Anthropic
principle

[edit]
References

^
Sagan, Carl. Cosmos, Ballantine Books 1985

^ Eric Jones,
"Where is everybody?", An account of Fermi's question", Los
Alamos Technical report LA-10311-MS, March, 1985.

^ Shostak,
Seth (25
October 2001).
"Our Galaxy Should Be Teeming With Civilizations, But Where Are
They?". Space.com. Space.com. Retrieved on April 8, 2006.

^ Wesson, Paul (June 1992). "Cosmology, extraterrestrial intelligence, and a resolution of the
Fermi-Hart paradox". Royal Astronomical Society, Quarterly Journal
31: 161–170. ISSN
0035-8738.
Retrieved on 2007-05-06.

^ a
b
Brin, Gen David, 1982, The 'Great
Silence': The Controversy Concerning Extraterrestial Intelligent Life,
Quarterly Journal of Royal Astronomical Society, fall 1983, v.24, pp. 283-309.

^ Craig,
Andrew (2003). "Astronomers count the stars". BBC News. BBC.
Retrieved on April 8, 2006.

^ Crawford,
I.A., "Where are They? Maybe we are alone in the galaxy after all",
Scientific American, July 2000, 38-43, (2000).

^ The Soviet astronomer
Nikolai Kardashev
has stated that an alien civilization
on Kardashev scale of 3
could send signals up to 10 billion light years.

^ Intelligent
Life in the Universe by I. S. Shklovskii(Author), Carl Sagan (Author) p394

^ Let N(r) be the number of civilizations (per
unit volume) that can be seen out to radius r. Let Rg be the radius of the galaxy.
So the number of civilizations we see is:

where the first integral are those in the galaxy, and the second
those outside. Which integral is bigger depends on how fast N(r) decreases,
which is completely unknown. This observation is due to Kardashev.

^ Barrow J. D. and Tipler, F. J. (1986).
The Anthropic Cosmological Principle p588. Oxford Univ. Press. ISBN
0-19-282147-4.

^ Mullen,
Leslie (2002). "Alien Intelligence Depends on Time Needed to Grow Brains".
Astrobiology Magazine. Space.com. Retrieved on April 21, 2006.

^ Habitable Planet Signposts, Astrobiology magazine.

^ G. Chauvin; A.M. Lagrange; C. Dumas; B. Zuckerman;
D. Mouillet; I. Song; J.-L. Beuzit; P. Lowrance (2004). "A giant planet candidate near a young brown dwarf".
Astronomy & Astrophysics 425: L29 – L32.

^
Schneider, Jean (2007-04-25). Interactive Extra-solar Planets Catalog. The Extrasolar
Planets Encyclopedia. Retrieved on 2007-04-25.

^ Udry et
al. (2007). "The HARPS search for southern extra-solar planets, XI. An
habitable super-Earth (5 M⊕) in a 3-planet system". Astronomy and Astrophysics
preprint: preprint.

^
Papagiannis, M. D. "Are We Alone or Could They be in the Asteroid Belt?,"
Quarterly Journal of the Royal Astronomical Society, 19, 277–281 (1978)

^ Bracewell,
R. N. "Communications from Superior Galactic Communities," Nature, 186,
670–671 (1960). Reprinted in A.G. Cameron (ed.), Interstellar
Communication, W. A. Benjamin, Inc., New York, pp. 243–248, 1963.

^ SETV projects

^ Freitas
Jr., Robert A. and Valdes, Francisco. "The Search for Extraterrestrial
Artifacts," Acta Astronautica, 12, No. 12, 1027–1034 (1985).

^ Dyson,
Freeman, "Search for Artificial Stellar Sources of Infra-Red Radiation", Science,
June 1960.

^ Niven,
Larry, "Bigger than Worlds", Analog,
March 1974.

^ Fermilab Dyson Sphere search program. Fermi National Accelerator Laboratory. Retrieved on
May 12, 2006.

^ Peter
Ward and Donald
Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe.
Copernicus Books. January 2000. ISBN
0-387-98701-0.

^ Athena
Andreadis. "E. T.,
Call Springer-Verlag!" SETI League Publications,
2000.

^ Darling,
David. "Extraterrestrial intelligence, hazards to". 'The
Encyclopedia of Astrobiology, Astronomy, and Spaceflight'. Worlds of
David Darling. Retrieved on May 11, 2006.

^ Hawking,
Stephen. "Life in the
Universe". Public Lectures. University of Cambridge. Retrieved on
May 11, 2006.

^ Archer,
Michael. "Slime Monsters Will Be Human Too," Nature Australia, vol. 22,
1989.

^ See, for
example, Berserker
(Saberhagen)

^ Soter,
Steven (2005). "SETI and the Cosmic Quarantine Hypothesis". Astrobiology
Magazine. Space.com. Retrieved on May 3,
2006.

^ Webb,
Stephen. If the Universe Is Teeming With Aliens...Where Is Everybody?,
Springer, 2002, pp. 112

^ Or Hashem
4:2

^ Wiker,
Benjamin D.. Christianity and the Search for Extraterrestrial Life.
crisismagazine.com.

^ See, for
instance: Gonzalez, Guillermo and Richards, Jay W. The Privileged
Planet, Regnery, 2004.

^ Webb,
Stephen. If the Universe Is Teeming With Aliens...Where Is Everybody?,
Springer, 2002, pp. 62–71

^ Douglas
Vakoch, Decoding E.T.: Ancient Tongues Point Way To Learning Alien
Languages, SETI Institute, November 15, 2001

^ Landis, Geoffrey.
"The Fermi Paradox: An Approach Based on Percolation Theory",
Journal of the British Interplanetary Society, London, vol 51, page
163–166, 1998.

^ Scheffer,
L.K., 1994, Machine Intelligence, the Cost of Interstellar Travel and Fermi's
Paradox, Quarterly Journal of the Royal Astronomical Society, volume={35},
1994, p. 157.

^ Kinouchi,
Osame. "Persistence solves
Fermi Paradox but challenges SETI projects," Condensed Matter,
0112137 v1, December 2001.

^ Margaret
C. Turnbull and Jill C. Tarter. "Target selection for SETI: A catalog of
nearby habitable stellar systems," The Astrophysical Journal Supplement
Series, 145: 181–198, March 2003.

^ SETI's FAQ, Sec
1.2.3

^ SETI's FAQ, Sec 1.6

^ History of
Solar Energy, www.solarenergy.com

^ Smart,
John, "Answering the Fermi Paradox: Exploring the Mechanisms of
Universal Transcension", Journal of Evolution and Technology, June
2002.

^ John A.
Ball. "The Zoo Hypothesis," Icarus, vol 19, issue 3, pp 347–349, July
1973.

^ For an
example from popular culture, see the Prime Directive of Star Trek

^ Crawford,
July 2000.

^ Baxter,
Stephen, 2001, The Planetarium Hypothesis: A Resolution of the Fermi
Paradox, Journal of the British Interplanetary Society, Vol. 54, no. 5/6,
pp. 210-216.

^ Nick
Bostrom, 2003, Are You Living In a Computer Simulation? Philosophical
Quarterly, 2003, Vol. 53, No. 211, pp. 243-255. See also this web site about
the simulation argument.

^ Schombert,
James. "Fermi's paradox (i.e. Where are they?)" Lectures,
University of Oregon.

^ a
b
J. Tarter, (2001)What is SETI, Annals of the New York Academy of Sciences
950 , pp. 269–275 (2001)

^ Webb, pp.
27–60

[edit]
Suggested reading

Ben Zuckerman and Michael H. Hart, Extraterrestrials: Where Are They? ISBN
0521448034 Amazon

Savage, Marshall T. (1992). The Millennial
Project: Colonizing the Galaxy in 8 Easy Steps. Denver: Empyrean
Publishing. ISBN
0-9633914-8-8.

Webb,
Stephen (2002). If the Universe Is Teeming with Aliens... Where Is
Everybody?. Copernicus Books. ISBN
0-387-95501-1.

Michaud, Michael (2006). Contact with Alien
Civilizations: Our Hopes and Fears about Encountering Extraterrestrials.
Copernicus Books. ISBN
978-0387-28598-6.

Evolving the Alien: The Science of Extraterrestrial Life

[edit]
External links

They're Made Out Of
Meat

So much space, so little time: why aliens haven't found us
yet by Ian Sample,The Guardian January 18, 2007

The Possibilities of FTL: Or Fermi's Paradox Reconsidered by
F.E. Freiheit IV

Fermi's Paradox (i.e. Where are They?) by James Schombert

Life
in the Universe, by Eric Schulman, Mercury
Magazine (May/June to November/December 2000)

Answering the Fermi Paradox: Exploring the Mechanisms of
Universal Transcension by John Smart

The Great Filter —
Are We Almost Past It? by Robin Hanson

Extraterrestrial Intelligence in the Solar System: Resolving the
Fermi Paradox, which argues that our observations are incomplete, and
There Is No Fermi Paradox, arguing that the paradox is
based on a logical flaw, both by Robert Freitas

Beyond
Kardaschev: Possible Answer to Fermi's Paradox by Paul Hughes

Fermi Paradox
debate Astrobiology Magazine July 2002. Michael Meyer, Frank Drake,
Christopher McKay, Donald Brownlee, & David Grinspoon.

Introduction an Drake
equations for the Fermi's paradox

The Fermi Paradox: Back With a Vengeance by George Dvorsky.

Retrieved from "http://en.wikipedia.org/wiki/Fermi_paradox"

Categories: Extraterrestrial
life | Physical
paradoxes | Unsolved
problems

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