Exploration of the Galaxy by Living Beings
This, of course, assumes that interstellar flight is possible. As we already saw, one problem with interstellar flight is that it takes a very long time. Even traveling close to the speed of light, it takes four years to get to the nearest star and over 30,000 years to arrive at the center of the galaxy. As we discussed in Chapter 7, these are not the times for the travelers themselves, who might be able to make a round trip to the center of the galaxy in their lifetimes. Unfortunately the energy involved may be such as to make prohibitive any more than an occasional probe. We have also seen that some extremely fanciful ideas, including ramjets driven by nuclear catalytic engines, and even superluminal starships, are consistent with current physical theory. Nevertheless, we cannot base an impossibility proof on technologies that are at best problematic, for an impossibility proof with weak links is not much of a proof (the same reasoning would apply with even greater force to the development of hyper-space travel, or some of the other fanciful inventions of science fiction writers).
Still, a velocity 1/100 that of light is within the scope of the technology described in Chapter 7. At this velocity, it would take us about eight million years to arrive at the furthest confines of the galaxy. A more centrally located species could have spread throughout the galaxy in a little over five million years, and that is only a bit more than 1/1000 the age of the Earth. Since the galaxy is at least twice as old as the Earth, if technological civilizations are as prevalent as the proponents of SETI would have it, many such civilizations should have arisen before ours. But that presumably means that they should have been here already. Even at a much lower rate of expansion, the time it takes to cover the entire galaxy is not much compared with the age of the galaxy itself. Our ancestors who migrated from Africa to the rest of the world never completed the journey themselves, but, by moving a little in each generation, eventually they covered the entire planet. And as long as the journey was, it took but a moment in the life of the homo-sapiens family.
Of course, a journey of eight million years for a species that is not yet a million years old would not be a small undertaking, but it is a journey that we may begin one step, one star, at a time. And at any rate, if we realize that complex creatures such as the dinosaurs lived for about 140 million years, and that moving into the cosmos would probably enhance the long-term survival of the species, we can see that the travel time may be relatively short for some species. Presumably this would make a complete expansion by someone or other seem almost inevitable.
There is no question that interstellar travel at that velocity would pose a variety of social difficulties for us. Chief among them is that it would take 400 years to arrive at the nearest star, perhaps 100 years with improvements in technology. Since it would be a second or third generation that would finish the trip--and if the nearest stars are not suitable, it would take an even later generation--we are not sure that we can entrust the success of the exploration to people that would not have been born when the decision to explore the galaxy is made. It may also be frightfully expensive to keep alive and healthy the many humans that would be necessary to send in a mission of that sort.
Nevertheless, we can cook up several scenarios in which the social obstacles are overcome and a species begins to migrate to the stars: an authoritarian regime forces the issue, or there is forewarning of a cosmic catastrophe, or the migration is simply a natural consequence of a long and massive colonization of the species' own planetary system. In such a case, this version looks more like the impossibility proof it is purported to be.
Moreover, a proponent of SETI cannot reply by bringing up reasons why a civilization may not choose to travel throughout the galaxy. Given that we are average, we can easily imagine why at least one of the many technological civilizations would eventually venture out with the purpose of colonization. This is easy to imagine because we can imagine why we ourselves might begin such an adventure. Accepting low odds (in the style of SETI proponents), let us say that it is one of a thousand options we have. Under certain conditions it may become the most reasonable option. Out of 1,000 advanced civilizations 100 million years ago, then chances are that one would have built starships. But where are they? As for the assumption of expansionism, again from our own case we know that we have a tendency to move onto new niches. Even if the tendency is not overwhelming, the existence of many civilizations will make it likely that at least one will act on it. And all it takes is one, as long as we assume that star travel is indeed possible and that the tendency to expand will give such a civilization the required persistence. The mediocrity principle supports this impossibility proof.
Of course, as in the previous impossibility proof, we may be able to find excuses for why we have not detected an alien presence in our solar system. Imagine for example, the enormous difficulty that we would have ourselves in trying to spot even a large starship that came within a few astronomical units from Earth, a distance that may be quite suitable for an alien species to conduct a survey of our solar system. At that sort of distance it is not easy to detect asteroids smaller than a kilometer across, even when we are searching for them. A starship may come in no closer than Saturn but send much smaller probes into orbit around the other planets. Their advanced stealth systems may be beyond our technological ability to detect. Or the ship may have been here already and gone home (or gone silent). The excuses may be limited only by our imagination.
In addition, in this discussion we have to assume that a slower conquest of the galaxy will not be hampered by lack of resources. But once again the Principle of Mediocrity comes to the rescue of the objection. In our own solar system it seems that the Kuiper Belt and the Oort Cloud would offer the resources needed for the survival of a civilization not unlike that envisioned by O’Neill’s in his proposal of space colonies. And since we are pretty much average, we should expect such resources to be spread throughout the galaxy.
As we will see below, the Principle of Mediocrity makes for an interesting philosophical target. But let us consider first whether seeking contact with advanced alien civilizations is wise.
The Wisdom of Contact
To make matters worse for SETI, if by some quirk of fortune we have not been found yet, the principle of mediocrity should lead us to question the wisdom of trying to communicate. It is clear that in our complete expansion in our own planet we have done our best to eliminate all significant competition from other species. The last thing we wish to do is advertise our presence to more advanced species who may then wish to occupy our niche, and in the process may need to get rid of the local pests, or at least bring them under control. In some circles there is the feeling that advanced creatures must somehow be wise and benevolent, although under the guidance of the principle of mediocrity it would be difficult to see why. In the first place we have a history of ruthlessness toward species that become obstacles to our aims; we have been ruthless even to other human cultures. Consider for example, as Ron Bracewell has pointed out, what the response of suburbanites might be if raccoons became much smarter. They would be such pests that suburbanites would go to great lengths to wipe them out. And few of us would lose much sleep over that. Indeed when we try to poison cockroaches and rats, or hunt the coyotes that prey on our sheep, the issue of benevolence or malevolence seldom comes up.
In light of these considerations, some suggest that we should lay low until we are in a better position to do battle if need be. There are others who argue that the issue is moot since we have been radiating into space our radio and television signals for a long time. That may be so, but those signals would be very weak and well scrambled by the time they leave the solar system, and in view of the difficulties we have recognized in trying to look for alien transmissions, and the low powers of most of our own transmissions, it is not unreasonable to suppose that detecting life on Earth from, say, 100 light years away would involve a rather substantial amount of luck. Of course, some transmissions, radar beams for example, are very powerful. And in any event, the great activity in the radio range alone might indicate to another civilization that a relatively advanced technology exists here. At the present time the SETI program (not longer at NASA, as I mentioned earlier) does not transmit any messages. There does not seem to be much harm in listening, anyway, and so officially the wisdom of communication is not yet a problem we must face. Nevertheless, unofficially we do have to concern ourselves with it, since some radio astronomers have already sent messages on their own.
 R. Bracewell. The Galactic Club. San Francisco: W. H. Freeman 1975.