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Published May 6, 1977 | public
Journal Article

Searching for Extraterrestrial Civilizations

Abstract

We have argued that planning for a search for extraterrestrial intelligence should involve a minimum number of assumptions. In view of the feasibility (at our present level of understanding) of using nuclear fusion to effect interstellar travel at a speed of 0.1c, it appears unwarranted (at this time) to assume that it would not occur for at least some technologically advanced civilizations. One cannot even conclude that humans would not attempt this within the next few centuries. On the contrary, the most likely future situation, given the maintenance of technological growth and the absence of extraterrestrial interference, is that our civilization will explore and colonize our galactic neighborhood. A comparison of the time scales of galactic evolution and interstellar travel leads to the conclusion that the galaxy is either essentially empty with respect to technological civilizations or extensively colonized. In the former instance, a SETI would be unproductive. In the latter, a SETI could be fruitful if a signal has been deliberately directed at the earth or at an alien outpost, probe, or communication relay station in our solar system. In the former case, an existing antenna would probably be sufficient to detect the signal. In the latter case, success would depend on the way in which the communications were coded. Failure to detect a signal could permit any of the following conclusions: (i) the galaxy is devoid of technological civilizations, advanced beyond our own, (ii) such civilizations exist, but cannot (for some reason which is presently beyond our ken) engage in interstellar colonization, or (iii) such civilizations are not attempting overt contact with terrestrial civilizations and their intercommunications, if present, are not coded in a simple way. To plan at this time for a high-cost, large-array SETI based on the last two possibilities appears to be rather premature.

Additional Information

© 1977 American Association for the Advancement of Science. We gratefully acknowledge helpful criticisms from R. Carpenter, R. Edelson, B. Gary, S. Gulkis, M. Janssen, M. Jura, G. and S. Knapp, D. Kunth, R. McEliece, B. C. Murray, J. Rather, E. Rodriguez Kuiper, P. Swanson, and B. Zuckerman. This work has been carried out at the Jet Propulsion Laboratory (JPL) and the Owens Valley Radio Observatory, partially supported by a grant from the JPL Director's Discretionary Fund. Jet Propulsion Laboratory is operated by the California Institute of Technology under NASA contract NAS 7-100. Research at Owens Valley Radio Observatory is sponsored by the National Science Foundation under grant AST 73-04677AO3.

Additional details

Created:
August 19, 2023
Modified:
October 25, 2023