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Friday, August 19, 2011




Are we alone in the universe? Is it really possible that no sentient being on a faraway planet ever contemplated the stars and felt awe? That only humans ever wondered about the nature of the universe, or pondered whether similar beings might be asking similar questions? In the view of some people it is extremely parochial to suppose that we are alone – one more instance of the syndrome that once made us believe that the Earth was the center of the universe. According to those people, we have no more reason now to believe that we must be the pinnacle of creation than we had once upon a time to believe that the Earth was so special.

Thus begins the reasoning that takes them to the conclusion that extraterrestrial intelligences (ETI) are likely to exist, a presupposition without which the search for them (SETI) would make little sense. This does not mean, however, that the proponents of SETI advocate the building of starships at all. Indeed, many of its practitioners believe that star travel is not very likely, at least not for a very long time. They urge instead that we scan the skies for the radio signals of other advanced species.

Success in their mission is seen by SETI proponents as of such extraordinary importance that at some point they proposed Project Cyclops, a very elaborate, and expensive, arrays of radio telescopes to carry it out. Their proposals were not received with much sympathy by those who control the purse strings, and thus over the years they had to content themselves with ever meager levels of support (from tens of billions for the proposed Cyclops[1] to less than two million per year in actual funding, and then to nothing). But what seemed like a deplorable situation to them appeared far too exorbitant to opponents of SETI. For in the view of such opponents, the very foundation of SETI, that extraterrestrial intelligence probably exists, was not only unwarranted but preposterous. U.S. Senator William Proxmire gave the program his Golden Fleece Award, for the most inane waste of taxpayers’ money. Eventually NASA cut SETI of its budget altogether. But the program lives on, bolstered by the privately-funded SETI Institute and by the ingenuity and good will of many contributing scientists.

Ironically, the opposition to SETI is buttressed by the key assumption of the SETI proponents themselves: Carl Sagan’s so-called "Principle of Mediocrity."[2] The Principle of Mediocrity asserts that the sun is a typical star in having a planet like the Earth in which life could arise, that terrestrial life is typical in having produced intelligence, and that human intelligence is typical in giving rise to a technological civilization.

Presumably Copernicus taught us humility when he argued that the Earth was not privileged but average, and later astronomy reinforced the lesson by discovering that the sun itself was merely an average star in an average galaxy. By extending the Copernican lesson, the reasoning goes, we should learn to be humble about our own position in the scheme of life. The principle of mediocrity thus purports to recognize that humanity and the conditions that have brought it about are pretty much average. In their arguments, the opponents of SETI stretch this principle slightly to add that a technological civilization is typically expansionist. As a result they are able to produce a variety of "impossibility proofs" against the existence of extraterrestrial intelligence.

In the pages that follow I will examine the justification of SETI in light of this controversy. This examination, I trust, will lead naturally to a discussion of some of the important philosophical assumptions made by SETI in estimating our ability to communicate with ETIs if they exist, a very interesting issue in its own right. Let me begin with a brief account of the reasons for optimism with respect to SETI. I will then proceed with an explanation of how such optimism actually sets up the impossibility proofs.


First there is the incredibly large number of stars. This galaxy alone contains over 100 billion, and there may be at least 100 billion galaxies. We do not know how many of those stars have planetary systems, but most theories of star formation would encourage us to believe that planets are rather common, at least in the average stars of what is called the Main Sequence (of star evolution) such as our own Sun. As we saw in Chapter 5, this optimism has been born out by the recent discovery of over 200 Jupiter-planets and a few rocky (“terrestrial”) planets around other stars, as well as by the very credible evidence for forming planetary systems around young stars.

All this has some very convenient aspects for SETI. One is that the average stars may live longer than ten billion years. Since it has taken about four and half billion years to produce a technological civilization on this planet, it is encouraging to know that the stars that live long enough are also the ones most likely to have planets in the first place.

From here on matters generally become far more speculative. Those who are in the business of making probability estimates for SETI often use the so-called "Drake Equation" (named after Frank Drake, the contemporary astronomer who first proposed it). According to this equation, the number of intelligent civilizations in this galaxy is equal to the product of the rate of star formation, the percentage of favorable stars, the number of planets around such stars, the fraction of Earth-like planets among those, the fraction of such planets in which life begins, the fraction of planets with life in which intelligence develops, and then the number of planets with intelligence in which technological civilizations arise. This product is then multiplied by the average longevity of a technological civilization.

We believe that in this galaxy the rate of star formation is about twenty per year. And the existence of other planets is now established, although not the rate of planet formation. But as we progress through Drake's equation, the estimates are not as well grounded. This situation does not prevent SETI enthusiasts from assigning optimistic probabilities to every factor. One often hears, for example, that once life begins on a planet, intelligence is very likely to result eventually. Such optimism surely deserves examination.

Impossibility Proofs: A Summary

The most interesting impossibility proof against the existence of ETIs is the famous question by Enrico Fermi, which assumed an early version of Sagan’s Principle of Mediocrity: “Where are they?” With such good omens for the existence of ETIs, they should be everywhere, including our own solar system, watching us, making contact with us, and so on. But we don’t see them, hear them, or in any other way detect them. This of course assumes also that all the talk about UFOs, alien abductions, and the like is a delusion, or at best an illusion. So, if aliens do exist, they should be all over the place, but we have no trace of them; therefore, they do not exist.

SETI proponents like Bernard Oliver, however, argued that the reason we don’t see them is because star trekking takes too long, since the distances between solar systems are so vast. So no one can really be expected to undertake such a trip. That is why we need to resort to electromagnetic signals as a means to search for ETIs, as well as a means for communicating with them if we ever find them.

We have seen in the previous two postings why Oliver’s case is not as good as he might have thought. We cannot rule out the possibility of travel between the stars, either by traveling space colonies, or by ships that approach the speed of light. It is also physically possible, as we have seen, that warp engines might actually allow us to go faster than light (cf. Alcubierre[3]). Nevertheless we cannot affirm it either. This result weakens the impossibility proof some, but not completely: It still manages to cast serious doubt upon the existence of very advanced civilizations.

Some opponents of SETI have brushed Oliver’s response aside for a different reason. Even if star trekking takes too long for living things, an advanced civilization could still send self-reproducing machines to report about every interesting solar system, including ours. All they need do is send one. Once it gets to the backyard of another star, it will make copies of itself, which will then move on to other stars and do likewise. This self-reproducing probes will reproduce and cover any galaxy, give or take a few million years, the way a bacteria culture ends up taking over a petri dish. Thus Fermi’s question arises all over again: “Where are they?”

I do not believe that this impossibility proof succeeds. It is based on John von Neumann’s “proof” for the possibility of self-reproducing automata. I make two main points against such proof as the basis for exploring the galaxy in the fashion considered here. The first is that the conditions that make von Neumann’s proof plausible are not likely to be met under the exigencies of exploration. The second is that von Neumann assumes that a genome is like a computer program, and I think that such an assumption is unwarranted. I then criticize some clever proposals to apply von Neumann’s ideas to interstellar exploration (e.g. with space probes based on collections of nanorobots). Unfortunately I will have to ask my readers’ forgiveness for not providing the details of my arguments. As it turns out, such details will appear in my contribution to a book on imaging outer space that will be published in December. I promised the editor, Prof. Alexander Geppert, that I would not post the article, since the publisher would be naturally upset were my chapter to appear in this blog right before the publication of the book.

I will, however, provide a little plug for the book, since it is likely to interest most of you (I do not share in the profits). The title of my chapter is “Self-Reproducing Automata and the Impossibility of SETI.” The title of the book is Imagining Outer Space, and the editor is Alexander C. T. Geppert, as I said. The publisher will be Palgrave MacMillan

You can find out more information about the volume by clicking on this link:

[1] For a descripton of Cyclops see Bernard Oliver’s description in Carl Sagan, ed., Communication with Extraterrestrial Intelligence, MIT Press, 1973, pp. 279-301. The report on the project was published by NASA: CR 11445.

[2] Carl Sagan, Pale Blue Dot, Random House, 1994, pp. 39, 372-73.

[3] It seems that I left out the reference to Alcubierre’s seminal paper in my previous posting. It is as follows: Miguel Alcubierre (1994): "The Warp Drive: Hyper-Fast Travel within General Relativity". Classical and Quantum Gravity, 11: L73-L77.

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