So far, no extraterrestrial civilization has been unequivocally observed by humans. Nearly 50 years of listening for ETI transmissions has found no artificial signals in space s1 F.D. Drake, "Project Ozma", Physics Today 14 (1961) pp. 40-46. s2J. Tarter, "The search for extraterrestrial intelligence (SETI)", Annual Review of Astronomy and Astrophysics 39 (2001) 511-548. 28, and the search for ETI artifacts in the Solar System has also produced null results s3R.A. Freitas, F. Valdes: "A search for natural or artificial objects located at the EarthMoon libration points", Icarus 42 (1980) 442-447. s4 F. Valdes, R.A. Freitas: "A search for objects near the Earth-Moon lagrangian points", Icarus 53 (1983) 453-457. B. Cornet, S.L. Stride: "Solar system SETI using radio telescope arrays". Contact in Context 1 (2003) v1i2/s3eti-ata.pdf.. However, a simple back-of-the-envelope calculation initially performed by physicist Enrico Fermi suggests that ETI should be widespread throughout the galaxy s5M.H. Hart: "An explanation for the absence of extraterrestrials on Earth", Quarterly Journal of the Royal Astronomical Society 16 (1975) 128-135.. Indeed, an advanced ETI civilization could easily colonize the galaxy to form a Galactic Club among intelligent societies, a concept popular in science fiction (such as the "United Federation of Planets" of Star Trek fame) that in the nonfiction literature dates back at least to Ronald Bracewell s6R.N. Bracewell, The galactic club: Intelligent life in outer space. San Francisco, CA, WH Freeman and Co., 1975.. This conspicuous absence of extraterrestrials is often referred to as the Fermi paradox s7M.H. Hart: "An explanation for the absence of extraterrestrials on Earth", Quarterly Journal of the Royal Astronomical Society 16 (1975) 128-135. or the Great Silence s8G.D. Brin, The great silence - The controversy concerning extraterrestrial intelligent life, Quarterly Journal of the Royal Astronomical Society 24 (1983) 283-309. and raises the question: if ETI should be widespread, then where are they? A number of resolutions to the Fermi paradox have been proposed and explored s9S. Webb, If the Universe is Teeming with Aliens--where is Everybody?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Springer-Verlag, New York, 2002. M.M. Ćirković, Fermi's paradox-The last challenge for Copernicanism? Serbian Astronomical Journal 178 (2009) 1-20., and three paradox resolutions are worthy of consideration in our discussion.
One resolution to the Fermi paradox is that life, or at least intelligence, is rare and thus sparsely distributed throughout the galaxy. This rarity could be because few intelligent civilizations form s10P.D. Ward, D. Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe, Springer-Verlag, New York, 2000. or because intelligent civilizations tend to have short lifetimes, perhaps because they quickly destroy themselves [14s11C. Sagan, Communication with Extraterrestrial Intelligence (CETI), MIT Press, Cambridge, MA, 1973.. If intelligence is rare, then it is quite unlikely that humanity would have detected ETI. In the extreme case, humanity is the only intelligent civilization in the galaxy or even in the universe. Along the same lines, other intelligent civilizations may be beyond the physical limits of contact even if they do exist [15-s12S.A. Kaplan, Extraterrestrial Civilizations: Problems of Interstellar Communication, Israel Program for Scientific Translations, Jerusalem, 1971.-s13 P.S. Wesson, Cosmology, extraterrestrial intelligence, and a resolution of the Fermi-Hart paradox, Quarterly Journal of the Royal Astronomical Society 31 (1990) 161-170.. These scenarios are of limited value to this paper because they imply that contact with ETI is impossible.
A second possible resolution to the Fermi paradox derives from the challenges of expanding rapidly throughout the galaxy. Perhaps rapid expansion is unsustainable at the galactic scale, just as rapid expansion is often unsustainable here on Earth. This suggests that the absence of extraterrestrials might be explained by the fact that exponential growth is an unsustainable development pattern for intelligent civilizations s14S. von Hoerner, Population explosion and interstellar expansion, Journal of the British Interplanetary Society 28 (1975) 691-712., a response to the Fermi paradox known as the Sustainability Solution s15J.D. Haqq-Misra, S.D. Baum, The sustainability solution to the Fermi paradox, Journal of the British Interplanetary Society 62 (2009) 47-51.. According to the Sustainability Solution, rapidly expanding civilizations may face ecological collapse after colonizing the galaxy, analogous to the fate of Easter Island s16J.M. Diamond, Collapse: How Societies Choose to Fail Or Succeed, Viking Press, 2005.. On the other hand, the galaxy could be teeming with ETI that expand too slowly to have reached Earth yet s17W.I. Newman, C. Sagan, Galactic civilizations: Population dynamics and interstellar diffusion, Icarus 46 (1981) 293-327.. These slowly expanding ETI civilizations could still be detected by us or send us messages, and their nature as slow expanders has some implications for contact scenarios.
A third response to the Fermi paradox suggests that ETI are actually already widespread throughout the galaxy but are somehow invisible to us. The ETI could be unintentionally invisible, if it just happens to take some form that is undetectable to or otherwise undetected by humans. Alternatively, the ETI could be intentionally invisible. The intentional form of this solution is sometimes known as the Zoo Hypothesis s18J.A. Ball, The zoo hypothesis, Icarus 19 (1973) 347-349. because it implies that ETI are treating Earth like a wildlife preserve to be observed but not fully incorporated into the Galactic Club. This idea has been popularized through the Star Trek series as the “prime directive” for noninterference with a primitive culture. The Zoo Hypothesis thus implies that ETI could make contact with humans at any time. Perhaps such stealthy ETI will reveal themselves once Earth civilization has reached certain milestones. They may be waiting until we have reached a sufficient level of sophistication as a society such as the start of a METI program or the 4discovery of light speed travel s19J.A. Ball, The zoo hypothesis, Icarus 19 (1973) 347-349. s20 J.W. Deardorff, Examination of the embargo hypothesis as an explanation for the great silence, Journal of the British Interplanetary Society 40 (1987) 373-379., or they could be applying a societal benchmark such as sustainable development or international unity. The possibility that the Zoo Hypothesis explains the Fermi paradox has several important implications for contact scenarios.