Extraterrestrial Intelligence: A Skeptical View

I wrote this in the 1990s. I have verified and updated the web links, and added some new material. It really should get a much more thorough update, but I think it is still useful.

Do extraterrestrial civilizations exist? So far no others have been found, but this has not stopped scientific speculation. While the other planets of our solar system appear to be quite inhospitable, the past 25 years have seen many discussions about the possibilities for alien civilizations on planets around other stars. These discussions have frequently concluded that we are not unique; that there are many, perhaps millions, of other civilizations in our galaxy alone. However, in the last few years there has been a reaction against this belief. There are two independent lines of argument suggesting that there are very few technological civilizations in the galaxy, perhaps only one: our own. An excellent summary of the state of the debate c. 1990 was published by Casti.


The possibility of many civilizations in the galaxy has suggested the Search for ExtraTerrestrial Intelligence (SETI). SETI studies have concluded that the probability of detecting an alien civilization depends on the answers to several questions:

  1. How many suitable stars are there in the galaxy?
  2. How many of those stars are likely to have habitable planets?
  3. How many habitable planets will each such star have?
  4. How many of those planets will develop life?
  5. Given life, how likely is intelligent life?
  6. Will that intelligent life develop some kind of communication technology?
  7. Will they want to communicate?
  8. What is the lifetime of a communicating technological civilization?

From the above questions it is clear that SETI is actually a search for extraterrestrial technology, rather than intelligence, for an extraterrestial race without significant technology would be undetectable from our solar system. Furthermore, SETI is a search for technology that we can recognize. It may well be that extraterrestrials are in some sense so that their science and technology is incomprehensible to us. In this case we could never detect them even if they do exist and so SETI is futile. Rescher and Casti have both taken this position, while the noted artificial intelligence researcher Marvin Minsky has argued against it.

Assuming that we can detect aliens, the above questions have been formulated in terms of numbers and probabilities, giving an equation which can in principle be used to estimate the number of communicating technological civilizations in the galaxy. This equation is known variously as the Drake equation, the Sagan-Drake equation, or the Green Bank equation The result of this equation, the number of such civilizations, is conventionally denoted N. It depends, of course, on how the above questions are answered. Unfortunately most of the questions are very difficult to deal with, especially toward the end of the list. However, Carl Sagan and some other SETI researchers have proposed answers that imply that there are many communicating technological civilizations in the galaxy, perhaps as many as 1,000,000. This high number suggests that it should be possible to detect at least one of these civilizations, which the SETI Institute is trying to do.

The Pessimistic Response

However, the Drake equation has been analysed by other scientists with different results. Are we alone? by Rood and Trefil is a good popularization of the work of these revisionists. Their approach is similar to that of Sagan and Shklovskii, but their answers are far more pessimistic. By applying more recent research to the Green Bank equation, such as that of Hart on planetary atmospheres, they conclude that N is very small, and that quite possibly the only technological civilization in the galaxy is our own. A similar argument has also been proposed by Barrow and Tipler using the anthropic principle.

As Tipler, and Barrow have pointed out, the advocates of SETI are primarily astronomers and physicists. Many biologists, including Ernst Mayr (both on-line and in print) and G.G. Simpson have been very skeptical of the arguments for extraterrestial civilizations, and are similarly inclined toward the view that N = 1. Leonard Ornstein presented this view in Physics Today during the controversy that followed Frank Tipler’s 1981 paper as well as in other papers. See also Avoiding “Sagan Syndrome.” Why Astronomers and Journalists should pay heed to Biologists about ET.

Rood and Trefil also presented a completely independent argument for a small value of N. This is an extended reductio ad absurdum that begins with the work on space colonization by Gerard K. O’Neill and others.

Like O’Neill, they argued that space colonies offer an alternative to the Malthusian catastrophe that an expanding population on a single planet must inevitably face. Access to the moon and asteroids would vastly increase the mineral resources of the race. Solar energy is far easier to obtain and exploit in space, and so could be the primary form of energy for a civilization of space colonies. O’Neill has show that space colonies are feasible at the present level of human technology. Therefore, if there are many advanced technological civilizations in the galaxy, then a large percentage of them should have colonized space to get around the limitations of a single planet.

Given a well developed system of space colonies, a civilization could easily proceed to interstellar travel. A starship can be considered as a space colony provided with a supply of raw materials, an energy source for when it is far from any star, and a propulsion system. It may not be a very fast starship, but it would still be able to make the voyage.

Therefore a civilization with space colonies has the means for interstellar travel. If there are really many such civilizations, at least some of them eventually will travel over interstellar distances. Some may do so to find new resources, or to avoid overcrowding or persecution in the home system. Others may be interested in pure exploration. Whatever the motives, if there are many technological civilizations in the galaxy, it seems reasonable to expect that some of them will be interstellar travellers.

Because of the energy requirements, as well as the relativistic speed limit of c, an interstellar journey will take many decades, perhaps centuries. Allowing FTL travel would only make the argument stronger, in that it would reduce the time needed to colonize the galaxy. However, even subluminal interstellar transit times could be very short compared to the lifetime of the galaxy. Suppose there is just one civilization that is seriously interested in interstellar colonization. One it has established itself in several solar systems no single catastrophe, such as nuclear war, ecological disaster, or asteroid impact could destroy it. From that time on this civilization, or civilizations descended from it would continue to expand throughout the galaxy. In about 30,000,000 years, quite a short time in astronomical terms, it could colonize the entire galaxy. Finney and Jones have suggested such a possibility for the future of the human race.

If there are many technological civilizations in the galaxy, it is unreasonable to suppose that none of them have taken the path outlined above. Such an expansion would eventually include our own solar system. If so, we should be able to see the travellers, or some trace of their works. We have seen no indication of their existence. The conclusion is that in fact there are very few technological civilizations in the galaxy. Quite possibly we are in fact alone.

The above argument does not depend specifically on O’Neill colonies. All it requires is that interstellar travel be possible. In its modern form it was first proposed by Hart, but it has a much longer history. Enrico Fermi posed the question quite simply: Where is everybody? and so the problem is generally known as the Fermi paradox. The significance of the question has grown as the possibility of interstellar travel has become more accepted.

There have been many ingenious attempts to answer this question and avoid the conclusion that technological civilizations are very rare. Notable among these is the “zoo hypothesis”: Our solar system is off limits to interstellar travellers to permit the uncontaminated development of our own civilization. These and similar arguments share a common flaw: While they may explain the behavior of one group of extraterrestrials at one time, they cannot explain the behavior of all extraterrestrials at all times without assuming that human beings are different from every other race in the galaxy, i.e., that we are unique. This is the problem with Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture.

Tipler’s Argument

A variant of the Fermi paradox argument has been proposed by Tipler and Barrow. Instead of physical colonization, Tipler suggests that an advanced civilization would use self-reproducing robot spaceships to explore the galaxy and attempt to contact other civilizations. Whenever such a spaceship reached a solar system, it would use a radio beacon or some other mechanism to announce its presence to any natives. It would also use locally available materials, such as asteroid fragments, to reproduce itself. It would leave the beacon or other some other signal behind for any intelligent life that might evolve later. It and its clones would then proceed to other stars. This technique would also cover the galaxy in a relatively short time. Since self-reproducing spacecraft would be used, the cost of such an exploration program would be quite low, eliminating a possible weakness in the above argument.

It may be that an alien civilization would not choose to build such spacecraft. Sagan and Newman have argued that no civilization would dare build such spacecraft for fear that they would “mutate” into monsters that would consume the entire galaxy. Tipler has argued that this could be prevented and that a civilization might not in fact be too concerned about this possibility. It has also been suggested that if every civilization followed Tipler’s reasoning, nobody would send out probes and every civilization would conclude that it is unique. Tipler’s reply is that the self-reproducing probes would be useful for any kind of interstellar exploration, not just SETI, and hence would be developed even by a civilization that believed in its own uniqueness.

Like Sagan, Hart and Tipler have made some assumptions about the motivations of the hypothetical aliens, and to that extent their arguments are equally subject to Rescher’s criticism that extraterrestrials would be so alien as to be unknowable. However, the SETI advocates cannot use this argument against them. The whole SETI enterprise of listening for extraterrestrial radio signals implicitly assumes that we can understand alien motivations to some extent. Tipler argues that any civilization that wished to make contact with us would not use the radio beacons that Sagan and Drake are looking for, but would instead send out robot spacecraft, simply because that is a vastly more efficient method (see his responses to Physics Today reader letters in 1982 and 1988). His position might be more cautiously phrased as comprehensible extraterrestrial intelligent beings do not exist.

Sagan and Newman also argued that the spread of a civilization throughout the galaxy would be much slower than that suggested above, as does Aliens have already visited Earth? Scientists’ new theory.. They do not explain why this should apply to all civilizations. Yet another response is to ignore the issue; Sagan and Drake’s contribution to a recent (1990) Scientific American publication does not mention the arguments of Hart and Tipler. The most recent reference is from 1973. was apparently just a reprint of a much earlier article, even though they apparently were given the opportunity to update it.

There is another possible resolution: There may have been many extraterrestial civilizations, but they are all dead now. They were all destroyed by nuclear war, some environmental catastrophe of their own making, or any of the other perils facing humanity on Earth now. See, for example, Maybe We Haven’t Seen Any Aliens Because They’re All Dead. The conclusions of Hart and Tipler are consistent with this: We are alone.

Alternatively, we are the outliers, and intelligent life is generally much slower to evolve elsewhere. See A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy. Again, this means that right now we are alone.

I have not considered UFO stories in this article. Here is why.


Some more recent references: