Physicist Enrico Fermi asked: "If there are extraterrestrials,
where are they?" The fact that no convincing evidence had been found of
extraterrestrial activity in or near the solar system suggested to him that
there were no intelligent extraterrestrial societies in the Galaxy. "If
they existed," he said, "they would be here."
| Why has no one visited us? asked
The same point, also known as the Space Travel Argument, was subsequently
raised more forcefully and visibly, in 1966 by Freeman Dyson,1
in 1975 in two articles by Michael Hart2 in the United States
and David Viewing3 in Britain, and by others. Straightforward
calculations showed that a technological race capable of interstellar
travel at (a modest) one tenth the speed
of light ought to be able to colonize the entire Galaxy within a period
of one to 10 million years. Given that credible strategies for achieving
human space colonies, interstellar
colonization, and interstellar travel were already being discussed,
the Galaxy-wide spread of advanced space-faring civilizations seemed readily
achievable. Therefore, if such civilizations existed, why had they not come
to the solar system? To advocates of the extraterrestrial
hypothesis of unidentified flying objects,
or the paleocontact hypothesis, the
answer was simple: intelligent beings from the stars had arrived.
Yet the continuing lack of hard evidence to support these claims, together
with the availability, in most cases, of more mundane explanations, led
the scientific community to consider other possibilities. Two camps emerged.
In one were those who accepted Fermi's conclusion that the absence of extraterrestrial
artifacts in the solar system implied an absence of extraterrestrial intelligence
throughout the Galaxy. According to this view, SETI
was a pointless exercise – "a waste of time and money" as Hart put
it. A particularly extreme anti-SETI stance was taken by Frank Tipler
(see Tipler's Argument). In the
opposing camp, were those who favored the existence of advanced civilizations
elsewhere in the Galaxy and who sought, therefore, to circumvent the Fermi
Various explanations have been put forward, including that extraterrestrials
- Interested in us but do not want us (yet) to be aware of their presence
(see sentinel hypothesis; zoo
- Not interested in us because they are by nature xenophobic or not
curious (see extraterrestrial intelligence,
- Not interested in us because they are so much further ahead of us
(see extraterrestrial intelligence, more
advanced than us).
- Prone to annihilation before they achieve a significant level of interstellar
(a) They self-destruct.
(b) They are destroyed by external effects,
(i) The collision
of an asteroid or comet with their home world.
(ii) A galaxy-wide
sterilization phenomenon, e.g. a gamma-ray
(iii) Cultural or
(See also extraterrestrial civilizations,
- Capable of only interplanetary or limited interstellar travel because
of fundamental physical, biological, or economic restraints (see
- Dyson, Freeman J. "The Search for Extraterrestrial Technology." In
R. E. Marshak, ed., Perspectives in Modern Physics: Essays in Honor
of Hans Bethe. New York: John Wiley & Sons (1966).
- Hart, M. H. "An Explanation for the Absence of Extraterrestrial Life
on Earth," Quarterly Journal of the Royal Astronomical Society,
16, 128-135 (1975).
Abstract: Four categories of facts are explored for explaining
the lack of observable extraterrestrial beings (ETs) on earth. The
physical reasons are suggested to be the long travel times required
by sublight-velocity spaceships, a problem that may be overcome by
beings that live several millennia or that can be stored and reproduced
from zygotes on arrival. Also, the energy requirements for interstellar
travel, though large, are not an insurmountable difficulty. Sociologically,
it is suggested that ETs have no interest in space travel, or they
may have destroyed themselves with atomic wars, or the earth is being
used as a wildlife preserve. No procedures exist to test these hypotheses,
however. The consideration that ETs have not yet had time to find
earth is discounted by calculations that show that another intelligent
species in the Galaxy would have found earth if their space exploration
efforts began at least 2,000,000 yr ago. It is concluded that if the
earth has not yet been visited, then colonization of the Galaxy will
most probably be done by humans, who may have the first advanced civilization
in the Galaxy.
- Viewing, D. "Directly Interacting Extraterrestrial Technological Communities,"
Journal of the British Interplanetary Society, 28, 735
- Freitas, R. A. "Extraterrestrial Intelligence in the Solar System:
Resolving the Fermi Paradox," Journal of the British Interplanetary
Society, 36, 496-500 (1983).
Abstract: The assertion that extraterrestrial intelligences
(ETI) do not exist based on the apparent contradictions inherent in
the Fermi Paradox, rests upon an unproven and untenable presumption:
That ETI are not now present in the Solar System. Most advanced civilisations
also would be either invisible or unrecognisable using current human
observational methods, so millions of advanced societies may exist
and still not be directly detectable by us. Thus the Fermi Paradox
cannot logically be raised as an objection to the existence of ETI
until these major observational deficiencies have been corrected.
- Freitas, R. A, Jr. "There Is No Fermi Paradox," Icarus, 62,
Abstract: The "Fermi Paradox," an argument that extraterrestrial
intelligence cannot exist because it has not yet been observed, is
a logical fallacy. This "paradox" is a formally invalid inference,
both because it requires modal operators lying outside the first-order
propositional calculus and because it is unsupported by the observational
- Annis, James. "An Astrophysical Explanation for the Great Silence,"
Journal of the British Interplanetary Society, 52, 19
nightmare: the Fermi paradox (Oct 26, 2001)
AND NON-HUMAN INTELLIGENCE