Interstellar Travel and Extraterrestrial Intelligence

The success of several proposals to search for extraterrestrial intelligence (ETI) in the Galaxy (Cocconi & Morrison, 1959; Oliver & Billingham, 1971; Michaud, 1979) requires the existence of a large number of technologically competent cultures over a long period of time. For example, to expect to find one ETI within 1000 light-years in a perfectly efficient search would require about a million ETI in the Galaxy, each signalling for a million years. (Or it would require 108 ETI signalling 104 years, or 104 ETI signalling 108 years, etc.) Many people have asked why some of these ETI should not have taken advantage of their prolonged technological capability to find a method for interstellar travel and settlement of nearby stellar systems (see, e.g., Hart, 1975; Jones, 1976; Winterberg, 1979). If the initial problem of interstellar travel and settlement were solved, then it should become progressively easier for daughter settlements to eventually continue the process until every available stellar system in the Galaxy (including possibly our own) were inhabited.

The chances of this happening have been discussed extensively, often with minimal thought given to the physical requirements for interstellar settlement. In particular, it has been argued that interstellar settlement is either impossible (see, e.g., Purcell, 1960; Marx, 1973) or absurdly expensive (e.g. requiring trillions of man-years of effort to amass the nuclear fuel needed).