Practical arguments [about exterrestrials]
If extraterrestrials exist at all an obvious question—notably asked by Enrico Fermi in the 1940s—is why we have not encountered them. For there seems no fundamental reason that even spacecraft could not colonize our entire galaxy within just a few million years.
Explanations suggested for apparent absence include:
• Extraterrestrials are visiting, but we do not detect them;
• Extraterrestrials have visited, but not in recorded history;
• Extraterrestrials choose to exist in other dimensions;
• Interstellar travel is somehow infeasible;
• Colonization is somehow ecologically limited;
• Physical travel is not worth it; only signals are ever sent.
Explanations for apparent lack of radio signals include:
• Broadcasting is avoided for fear of conquest;
• There are active efforts to prevent us being contaminated;
• Extraterrestrials have no interest in communicating;
• Radio is the wrong medium;
• There are signals, but we do not understand them.
The so-called Drake equation gives a straightforward upper bound on the number of cases of extraterrestrial intelligence that could have arisen in our galaxy through the same basic chain of circumstances as humans. The result is a product of: rate of formation of suitable stars; fraction with planetary systems; number of Earth-like planets per system; fraction where life develops; fraction where intelligence develops; fraction where technology develops; time communicating civilizations survive. It now seems fairly certain that there are at least hundreds of millions of Earth-like planets in our galaxy. Biologists sometimes argue that intelligence is a rare development—though in the Darwinian approach it certainly has clear benefit. In addition, particularly in the Cold War period, it was often said that technological civilizations would quickly tend to destroy themselves, but now it seems more likely that intelligence—once developed—will tend to survive indefinitely, at least in machine form.
It is obviously difficult to guess the possible motivations of extraterrestrials, but one might expect that—just as with humans—different extraterrestrials would tend to do different things, so that at least some would choose to send out signals if not spacecraft. Out of about 6 billion humans, however, it is notable that only extremely few choose, say, to explore life in the depths of the oceans—though perhaps this is just because technology has not yet made it easy to do. In human history a key motivator for exploration has been trade. But trade requires that there be things of value to exchange; yet it is not clear that with sufficiently advanced technology there would be. For if the fundamental theory of physics is known, then everything about what is possible in our universe can in principle be worked out purely by a computation. Often irreducible work will be required, which one might imagine it would be worthwhile to trade. But as a practical matter, it seems likely that there will be vastly more room to do more extensive computations by using smaller components than by trading and collaborating with even millions of other civilizations. (It is notable that just a couple of decades ago, it was usually assumed that extraterrestrials would inevitably want to use large amounts of energy, and so would eventually for example tap all the output of a star. But seeing the increasing emphasis on information rather than mechanical work in human affairs this now seems much less clear.)
Extrapolating from our development, one might expect that most extraterrestrials would be something like immortal disembodied minds. And what such entities might do has to some extent been considered in the context of the notion of heaven in theology and art. And it is perhaps notable that while such activities as music and thought are often discussed, exploration essentially never is.