Why No Signals?
Thus far, after 50 years of searching, we have received no intelligent signals from outer space and see no evidence of extraterrestrial visitors. The lack of signals can readily be explained by the vast amount of physical space and electromagnetic spectrum that has to be searched. The analogy has been drawn that our situation at present is like trying to described the contents of Earth’s oceans from examination of a single cup of water.
There may be other reasons for lack of communications, of course. Life may have only evolved once under highly improbable circumstance on Earth alone. That would certainly account for absence of extraterrestrial communications. Given the likely abundance of planets capable of supporting life, however, this view of Earth as unique in the universe seems to be not strongly supported by the evidence we have. Perhaps the answer is that intelligent life does not evolve very often. However, since evolving species survive by out-competing others in their environment, intelligence is the ultimate destination of evolution because it is a multiplier factor for survivability. Since we can see the evolution of intelligence on Earth in both mammals and birds it appears realizable in different brain structures. So the evolution of intelligence is likely, once life starts evolving.
That leaves civilization. Perhaps intelligent civilizations are improbable. Well, perhaps. But civilizations are the end point of evolution of social cooperation, which is also a multiplier for survivability. Intelligence and civilization are likely to come together at some stage to promote the survivability of a species. The problem is we know from our own experience that civilizations rarely last more than a few hundred years. If this is characteristic of extraterrestrial civilizations with evolving intelligence, then there will be few or none at this time, even though millions may have emerged on different planets in the past four billion years. If there are a few of these civilizations remaining, then they are likely to be very sparsely scattered in the Galaxy, thousands of light years away from us, poorly situated for communication.
If this is true, then even if we search for a few more centuries we may still come up empty handed, because the average life of planetary civilizations is shorter than the time it takes light or radio waves to travel between them. A civilization that recognizes itself to be in this situation could well decided that the mounting of a major effort of interstellar communication would be pointless. It would be communicating with nothing but empty space.
If extraterrestrial civilizations are uniformly distributed throughout the Galaxy, we can calculate their separation by estimating the number of civilizations that are present at any one time. It appears that the number of planets capable of sustaining life may be in the hundreds of millions or even billions. However, the number with civilizations is critically dependent on the probability that simple life will successfully evolve into highly complex social organisms. It is also strongly dependent on the probable lifetime of planetary civilizations. These controversial probabilities are discussed under ET Civilizations. Under different assumptions the estimates of the probable number of civilizations vary from one (us) to millions. A low probability of for evolution of civilizations from simple life, or a high probability that a civilization will destroy itself or its environment within a few centuries, can account for the absence of extraterrestrial communications.
Another explanation is that we receive no communications because we are at a stage in our evolution where we can potentially destroy our environment, ourselves, and other species, either by war or by disease, waste and pollution. Significantly, the physical constants of the universe, notably the strength of gravity, prevent us from damaging extra-solar planets in a similar fashion, because they are way beyond our technological reach. For this reach to be extended requires vast advances in our technology or discovery of new principles of physical science whose presence is merely hinted at. The time it takes to develop a suitable technology or to exploit new physical principles is likely to be very much longer than the time it takes to either annihilate ourselves or to evolve the necessary social qualities that will enable us to preserve and enhance our planet.
To put it another way, we cannot become a technically advanced civilization capable of extended space travel and communication unless we can evolve socially to live at peace and preserve our planet. From their own experience, any advanced extraterrestrial civilizations that exist will be aware of this, and will treat other civilizations exhibiting our present level of technology as something to be quarantined—no visits, no communications to be sent, restricted access to communications received. Our own experience with world wars, suicide bombing, bioweapons, and malicious software would indicate that this is a prudent strategy.
Thus we may have not heard from extraterrestrial intelligent civilizations because their typical lifetime is short. If their lifetime is long, they may simply not be interested in us. We tend to think of advanced civilizations as populated by gung ho engineers and scientists forever probing the mysteries of the universe. But an advanced civilization may resolve the mysteries of the universe within a few thousand years and, no longer pursuing advances in the physical and biological sciences, go on to other things. Our level of intelligence, science, and social accomplishments may be of no interest to them. They've seen it all before.
There have been suggestions that our planetary neighborhood may be like a nature park where the inhabitants are left undisturbed for the sake of longitudinal studies by more intelligent beings. But this suggests a uniqueness that we do not have if millions of advanced civilizations have evolved in the universe. An alternative explanation points to the fact that sampling by the Kepler Space Telescope indicates there may be 50 billion possible planetary systems in the Galaxy. There is a strong chance, then, that the solar system as a possible location of intelligent life has not yet been investigated by another civilization, and consequently no communication has been attempted.
We are actually in a poor position to speculate on how planetary civilizations evolve beyond the point we are at and what their future motivations may be. We have barely started human space travel but we have found that after six months in space there are serious questions whether organisms evolving in a one-g environment can survive for periods of years in zero-g. Even if we become capable in future decades of assembling the enormous rotating structures needed to restore a one-g environment in space, there are still questions of the effect of the Coriolis force and the mental stability of crews subject to the social stresses of isolation and sensory deprivation. If these were the only considerations, we would perhaps take an optimistic view that such problems will be overcome and expansion into the solar system and Galaxy would follow.
However, this would be to ignore the exponential increases that are surging forward in computation, artifical intelligence, and genetics. We have no idea how these will change our lives in the next three or four decades. Our slow, linear advances in propulsion and tin-bending pale in the face of the speed-multiplying effects of the feedback of advances in these new areas and their potential interactions. We do not know who we will be in mid-century, or how we will think of our role in the universe. There is already talk of immortality being achieved by then, by transferring the total contents of a individual's brain into a an advanced computer running a total brain simulation. An initial simulation of this type is already under consideration by the European Union as a billion-Euro project to stimulate the region's technical advances.
It is difficult to speculate on the nature of civilizations thousands of years older than ours that have gone well beyond such advances. Assuming, of course, that some civilizations are able overcome the many threats to their continuance.