Abyssinia-iffat

Robleh Wais
10/27/03

Note: The Drake equation has been proposed in several forms, with different variables used by different astronomers. The version below is, in my estimation, the most familiar.

In Chapter 9 of their book The Anthropic Cosmological Principle, physicists John Barrow and Frank Tipler propose an argument that they claim shows that extraterrestrial intelligent life does not exist anywhere in our Galaxy.

In Sections 9.1 and 9.2, they spend some time setting up their refutation. Not much in these sections directly advances their negative thesis. Instead, the discussion seems designed to illustrate how human beings might travel through the cosmos and colonize alien worlds.

Finally, in Section 9.3, they reach the heart of their argument.

The equation Barrow and Tipler use in arguing that there cannot be more than one intelligent species in the Galaxy is a version of the Drake equation:

N = f_p · n_e · f_l · f_i · f_c · N_g

The Drake equation is a numerical estimate based on probabilities. When the component probabilities are multiplied by N_g, the estimated number of stars in the Galaxy, the result is intended to represent the number of worlds on which alien civilizations might exist.

To support the Barrow–Tipler thesis, if n_e, the number of habitable planets, is small, while N_g, the number of stars in the Galaxy, is large, then the probabilities represented by the remaining terms must be very small. At least, that is what is inferred. It is important to notice that this is an inference rather than an implication in the strict deductive sense. This distinction becomes important when evaluating what I regard as a weak anti-alien-life argument.

One estimate they give is one chance in 100 billion. If that were a fair estimate, I would agree that the likelihood of another intelligent species in the Galaxy is extremely small, and thus we are number one. The Drake equation can serve as an effective predictor of alien life only if its component probabilities are accurate. Yet, from our Earthbound position, we cannot make reliable estimates of most of them. We do not know where planets containing species like ours, or species unlike ours, exist. Nor do we know where such planets might eventually arise. The numbers of stars and planets can be estimated; most of the other terms remain conjectural.

Another important feature of the equation is that it places a limit on how long an intelligent species might take to begin searching for others. The universe has a finite age and, by deduction, so do galaxies. Intelligent life elsewhere would therefore have to evolve and begin searching within a restricted period. The argument hinges on this assumption. Barrow and Tipler suggest that if the normal distribution associated with f_c peaks at six billion years, with a standard deviation of one billion years, the result implies only one intelligent species. A broader distribution would increase the estimated likelihood, but I will not press that issue here.

The argument also bears upon the Principle of Mediocrity: the idea that humanity occupies no privileged position in the Galaxy and that similar conditions could have produced life in many other star systems. Estimates of the number of stars in the Milky Way vary widely. If the relevant probability turns out to be extremely small, the Principle of Mediocrity could be challenged, and humanity might indeed be something unusual.

The problem with the Drake equation is that terms such as f_p and n_e depend upon observation and estimation, and the estimates vary. The final value can change dramatically depending on the numbers assigned to these terms. Barrow and Tipler acknowledge this.

Let us now put the probabilistic argument aside. It contains variables that cannot yet be reliably determined and may therefore be inaccurate. The argument would work better if restricted to our Solar System. Barrow and Tipler are correct that, if extraterrestrial intelligent species exist, they have not openly revealed themselves to us. But this does not prove that they do not exist.

Dolphins provide a useful terrestrial analogy. They display considerable intelligence, yet their physical form prevents them from constructing radio telescopes, spacecraft, or other technological devices. Intelligent life can therefore exist while being incapable of communicating with species beyond its own world. That is a major flaw in the Barrow–Tipler argument. They assume that extraterrestrial intelligence will possess both the means and the environmental circumstances necessary for interstellar communication.

We cannot assume that self-aware life elsewhere would take a physical form capable of building instruments that could signal or reach us. There could be many impediments to extraterrestrial intelligent life contacting us, quite apart from the possibility of self-destruction. Barrow and Tipler answer the claim that an absence of evidence is not evidence of absence by asking: If aliens exist, why do we not see them? Should they not have reached our Solar System by now? They then describe possible incentives for exploration and colonization, such as a dying world or diminishing resources.

What they overlook is that the physical structure of an alien species could itself prevent technological communication. An extraterrestrial intelligence might exist without possessing the means to contact us, just as we might remain undetectable to it. In such a case, the species would exist even though neither civilization had detected the other.

Barrow and Tipler also do not appear to exclude non-intelligent extraterrestrial life. Since their own argument is framed probabilistically, one can easily ask whether planets containing single-celled or microbial organisms might exist. Nothing in their reasoning rules out that possibility. Simple organic life might remain at a primitive level for billions of years without ever attaining human-like intelligence. Nevertheless, it would still be alien life in our Galaxy.

A different but related astronomical argument appeared in the nineteenth century. It bears a striking similarity in method to the Barrow–Tipler conjecture and also appealed to scientific reasoning:

Olbers’ Paradox

Conclusions and Remarks

Barrow and Tipler do not present a sound inductive argument for the nonexistence of extraterrestrial intelligence in the Galaxy. They present a speculative argument. Their central error is to treat the absence of evidence for extraterrestrial intelligence in our Solar System as implying the absence of extraterrestrial intelligence throughout the Galaxy.

Using the probabilistic framework associated with Drake and Sagan, they believe they have reached a valid conclusion explaining why no extraterrestrial intelligence exists in the Galaxy. This is the mistake. They write:

It is important to note that the above argument uses the observed evidential fact that the ETI are not present in our Solar System; the situation is not the one implied by the epigram to this section, “absence of evidence is not evidence of absence.” Rather, the evidence is that ETI are absent from our Solar System, and from this observed fact, together with other astrophysical observations and theories, it is inferred as a logical consequence that ETI are absent from the Galaxy.

Put differently, we do not observe aliens in our Solar System, so a theory is proposed to explain that absence. A probabilistic model is then constructed to predict not merely whether life exists in our Solar System, but whether it exists anywhere in the Galaxy. The model yields a low probability, and the conclusion is extended from the Solar System to the entire Galaxy.

The error is that the model is speculative and contingent upon uncertain estimates. Barrow and Tipler add further constraints, but I do not believe those constraints are sufficient. Their conclusion cannot properly be applied to the entire Galaxy.

It can, however, be applied more readily to our Solar System. We know the relevant planetary population here, and the available evidence supports the conclusion that there is one technologically intelligent species in the Solar System: us. Even that conclusion is not universally accepted. Some people claim that extraterrestrials have visited Earth or helped create ancient monuments in Egypt and South America. The Drake framework gives us no reason to accept those claims. Yet Barrow and Tipler still go too far. Their conclusion should be restricted to the Solar System rather than extended to the Galaxy.

Both physicists appear confident in their conclusion because they believe they have used the scientific method to reach it. They and their critics agree that we possess little evidence of extraterrestrial intelligence on Earth. Unlike those who simply continue searching, Barrow and Tipler use a model to account for the apparent absence. If the model agrees with observation and predicts no alien life, they infer that there is no alien life. But if the model is wrong, or if it is applied too broadly, a false conclusion can still result.

Their argument for the scarcity of extraterrestrial intelligence is not entirely without merit. I believe they may be close to the truth, even though their method is weak. I speculate that there may be at most one or two other species comparable to ours in the entire Galaxy. A conservative estimate based on the Drake equation illustrates the point.

Let N_g represent the number of stars in our Galaxy, and assume:

If only one percent of these stars have planetary systems, then the number of such systems is:

If one percent of these planetary systems contain an Earth-like planet, then:

If one percent of those Earth-like planets contain life, then:

If one percent of those life-bearing planets develop intelligent, communicative life, then:

This estimate leaves 5,000 planets in the Galaxy that might contain life broadly comparable to ours and capable of communication. Although this may seem like a small number, I believe it is far too large. Earth is a plenum of life: billions of human beings, trillions of insects, vast numbers of other animals, and microbial organisms beyond ordinary counting. Considered historically, the total is larger still. Our civilization also produces detectable electromagnetic emissions that travel into space.

If 5,000 worlds produced comparable emissions, radio telescopes and other detectors might be expected to encounter many signals. Let us add one more estimate: the probability that such civilizations have used nuclear weapons. Based on human experience, assign this a value of five percent:

If N_nd equals 250 worlds, the original estimate appears even less plausible. The effects of so many nuclear catastrophes might conceivably produce detectable signatures. Thus, even under a conservative estimate, one is led to speculate that the actual number of extraterrestrial technological civilizations must be exceedingly small.

Since writing this essay, I have noticed that the Barrow–Tipler argument resembles a well-known logical fallacy: denying the antecedent. The fallacious form is commonly written:

The argument under discussion is better represented by a closely related invalid inference:

Strictly speaking, this second form is denying the consequent only if the second premise negates the consequent, but the inference is invalid because the first premise has not been established as a biconditional. The absence of detection does not entail absence of existence. Detection could fail for many reasons: distance, time, incompatible technology, physical limitations, deliberate silence, or the short duration of detectable technological activity.

A simple analogy makes the error clear. Suppose I say, “If it does not rain today, I will jog for three hours.” If I do not jog, it does not follow that it rained. Many other circumstances could explain why I failed to jog. In the same way, the failure to detect extraterrestrial intelligence does not establish that none exists.

Barrow and Tipler combine two different forms of inquiry. Hypothetical or scientific reasoning proposes an explanation and seeks evidence to support it. It is inductive and revisable. Categorical logic, by contrast, concerns valid deductive forms in which a conclusion follows necessarily from stated premises. Barrow and Tipler attempt to give an inductive model the force of a categorical conclusion. That move does not succeed.

For this reason, I cannot agree that there are no alien species in the Galaxy. Only a valid deductive argument could support such an absolute conclusion. A hypothetical argument can, however, justify a more modest general conclusion. I therefore assent to the claim that, if extraterrestrial intelligent species exist in the Galaxy, their number is probably small.