Introduction

One of the most controversial aspects of the problem of life in the universe is the value of N, the number of technological civilizations that exist in an average spiral galaxy such as the Milky Way. N has been debated at various meetings (e.g. Papagiannis, 1980) and extreme values between 1010 and 10–24 have been suggested. One of the strongest arguments in favor of small N is the so-called ‘Fermi paradox’: If N is a large number, then why are extraterrestrials not physically present in our solar system (see, e.g., Hart & Zuckerman, 1982, hereafter HZ)? Various arguments have been advanced to explain this paradox and yet allow a large value of N. For example, Drake (see Papagiannis, 1980, p. 27) has contended that it is not cost-effective to travel between the stars using rocket ships and, therefore, even if N is a large number, the extraterrestrials will choose to stay home. The question of cost-effectiveness is a debatable one, in any event (e.g. Singer in HZ, p. 46).

The purpose of this chapter is to point out that, if N is large, then, for a wide class of reasonable scenarios, extensive rocket travel between the stars seems not only likely but inevitable, quite independent of considerations of cost-effectiveness, speed of colonization waves, etc. The basic reason is that large N implies that L, the lifetime of an average technological civilization, must be very long, at least millions of years.