Are we alone in the universe or is there intelligent life on other worlds? And if there was, but it’s no longer there, perhaps destroyed, which is why we haven’t made contact with anyone? And if that’s the case, how many civilizations were there and why did they all disappear? The subsequent questions would be who destroyed them and how, plus a third one that’s a bit unsettling: could we be next? The answer would probably be affirmative if we were to accept the so-called Berserker hypothesis, which suggests that self-replicating probes could be roaming space in search of life to interact with, either positively or negatively.

Berserker is a word that etymologically some believe derives from ancient Norse language and others from Germanic, but in any case, it was used in antiquity to refer to a warrior who entered battle naked, at most clad in wolf or bear skins, and, above all, plunged into a kind of psychotic trance that gave him blind fury and strength beyond normal, rendering him insensitive to pain. Julius Caesar noted their existence in his work De bello Gallico (“The Gallic War”), referring to the “furor teutonicus” displayed in battle by some Germanic enemies; Lucan also attested to this in his Pharsalia.

The hypotheses to explain the homicidal behavior of these peculiar warriors often focus today on the consumption of hallucinogens, whether in the form of mushrooms, or through the infection of bread or beer by ergot fungus, or even through the spicing of beer, which was often done with black henbane. This would explain why, under these conditions, they would attack anyone in their path, including their own comrades.

The term “berserker” was utilized in 1967 by Fred Saberhagen, an American science fiction writer, to title a series of stories and novels in the space opera genre, a type of stellar fantasy of adventure and action, akin to Star Wars. It consists of several anthologies with diverse stories but with a common thematic background, somewhat akin to Lovecraft’s Cthulhu myths. In them, there are robotic mechanical contraptions, emerged from an interstellar war between two extraterrestrial races fifty thousand years ago, and they continue the mission for which they were created—they have the capacity to self-replicate—destroying any intelligent life they encounter on their travels.

Thus, they come across a Humanity that has begun its expansion from Earth and already has populations on some planets in the galaxy, and even maintains an alliance with an alien species whose main characteristic is the ability to communicate telepathically. Both will be threatened by the arrival of the aforementioned robotic machines, the berserkers, with which they engage in a struggle to defend their survival against their objective of extermination (and they also receive the treacherous collaboration of some humans).

Saberhagen passed away in 2007, at the age of seventy-seven. The curious thing is that his saga, apart from entertaining many readers, has served to name a more or less scientific hypothesis about why we have not yet detected intelligent extraterrestrial life in the universe. The so-called Berserker hypothesis is based on the idea that these possible alien civilizations have been eliminated by what is known as von Neumann probes, that is, unmanned spacecraft that would travel through the cosmos seeking primitive life (or insufficiently developed cultures) to observe and, if deemed necessary, intervene, either positively or negatively.

The name refers to Neumann János Lajos, a Hungarian mathematician who emigrated to the US—where he was renamed John von Neumann—and participated in the Manhattan Project (the development of the first atomic bomb), also making significant contributions to science in various areas: quantum physics and mechanics, weaponry, statistics, economics, etc. Also in computational cybernetics, a field in which he created the concept of self-replicating automata: autonomous robotic machines capable of fabricating replicas of themselves and, for example, carrying out work in distant places that do not allow human presence.

The idea, embodied in his work “Theory of Self-Reproducing Automata”, was intended for mineral extraction on moons or asteroids; mechanical labor would grow exponentially in situ, similar to a virus, thus increasing the pace of work. He called them universal assemblers, although the name von Neumann machines caught on more. Other scientists used this concept to apply it to space exploration; for example, the English physicist and mathematician Freeman Dyson, who suggested the use of such an artifact—which he dubbed Astrochicken—to investigate the Solar System.

The Swedish philosopher Nick Bostrom theorized the possibility that in the future, our intelligence will reach such a level that we can manufacture probes based on the Von Neumann principle, whose self-replication will allow them to spread in many directions and go further. There are also those, like the British physicist and popularizer Paul Davies, who suggest the possibility that some alien probe may have reached the Moon in prehistoric times and remains there, vigilant, interacting with us when deemed necessary for our development; this would be what is known as a Bracewell probe (because the first to propose such a thing was the Australian mathematician and physicist Ronald Newbold Bracewell).

As the cinephile reader may have noticed, the concept of the Bracewell probe was adopted by Arthur C. Clarke for his story “The Sentinel”, which Stanley Kubrick adapted into the film “2001: A Space Odyssey”. But, although similar, a Bracewell probe does not necessarily equate to a von Neumann machine, as they both have superintelligence but the former does not self-replicate. In fact, neither the latter would necessarily be benign, if they were to come from outer space. And it is then, if their intentions turn malicious, that we refer to them as berserkers.

How did the berserker hypothesis arise? Knowing whether we are alone or accompanied in the Universe is a question that has fascinated scientists for a long time. Programs have even been created to search for intelligent life, such as SETI (Search for Extra-Terrestrial Intelligence), which tracks and analyzes electromagnetic signals and sends similar messages in hopes of being detected. But, although SETI began operating in the 1970s, there has still been no luck, leading some experts to deduce that there is no one out there. There is great controversy over this assertion.

Paleontologist and geobiologist Peter Douglas Ward and astronomer and astrobiologist Donald Brownlee followed the line of Fermi’s paradox (“The common belief that the universe harbors numerous technologically advanced civilizations, combined with our observations suggesting otherwise, is paradoxical, suggesting that our knowledge or observations are defective or incomplete”) and formulated the Rare Earth hypothesis, according to which our planet has so many and such anomalous special conditions that the probability of finding life is practically nil, as it would exist too far away to be seen.

On the other hand, scientists like Carl Sagan believed that the cosmos teems with life; it’s just too far away to contact because, to use his own words, we are just a tiny point in the vastness of the stellar ocean. In that sense, radio astronomer Frank Drake created an equation – bearing his name – to calculate how many civilizations there might be in the Milky Way, later refined and improved by others using many more variables than he did.

Only five percent of the planets in our galaxy are in what is called the habitable zone (close enough to a star to receive its warmth and far enough to have a bearable temperature), but there are about two trillion galaxies in the observable universe, each with an average of 7×1022 stars. So, the question arises again: where are those aliens? And that’s where Michael H. Hart comes in, a New York astrophysicist specializing in the study of the possibility of extraterrestrial civilizations, author of the chapter “An Explanation for the Absence of Extraterrestrials on Earth” from the book “Extraterrestrials. Where are they?”

A proponent of Fermi’s paradox and the Rare Earth hypothesis, he picked up on a proposal from an Alabama colleague, Frank Jennings Tipler, who in 1980 had published an article entitled “Extraterrestrial Intelligent Beings Do Not Exist” and together they named the Hart-Tipler conjecture, according to which the non-detection of von Neumann probes is counter-evidence that there is no intelligent alien life outside the Solar System. It should be noted that Hart is a white supremacist and Tipler a proponent of intelligent design, both often falling into pseudoscientific fantasies.

Against them, the Quarterly Journal of the Royal Astronomical Society published in 1983 an intriguing article titled “The Great Silence. The Controversy Concerning Extraterrestrial Intelligent Life”, signed by Glen David Brin. Perhaps some readers may be familiar with him, as he is a Californian writer who has won the most important awards in science fiction literature, such as the Hugo and the Nebula, among others. Brin’s most important work, who is also a graduate in astronomy and philosophy, is titled “Uplift’s Elevation”, a series of novels whose plot has to do with all of this.

In them, he tells how there is an extraterrestrial civilization that, for eons, has been dedicated to stimulating the advancement of other less advanced civilizations throughout the universe, so that they, in turn, do the same with third parties in a stellar chain. Earth is an exception, lacking a tutor and convinced that its level of evolution is due to its own effort, something that others consider absurd. Literature aside, in the aforementioned article Brin talked about the berserker theory – which he presented as perfectly compatible with Fermi’s paradox – and suggested that the lack of contact with aliens could be due to their destruction.

In other words, von Neumann probes would have annihilated all living beings outside the Solar System, and then perhaps they ended up self-destructing themselves, perhaps when the probes of one civilization competed with those of another or faced contradictory instructions (which again brings us back to “2001: A Space Odyssey”, but now to Hal’s behavior). This is explained by Brin in his story “Lungfish”, and in a way corroborated by Carl Sagan—always in the hypothetical realm, obviously—with the response he gave to Tipler’s thesis in “The Solipsist Approach to Extraterrestrial Intelligence”.

Although he wrote it jointly with astronomer William I. Newman, it has become popular as Sagan’s Response and it says that Tipler underestimated the rate of replication of von Neumann probes, which should already be present in most of the galaxy’s mass… but an intelligent race would never create Neumann machines that destroy, and it would be paradoxical that they would end up destroying each other precisely because of that. Of course, the debate would be biased from the outset in the opinion of other scientists, who do not see it as clear that the probes could have so many capabilities.

This is the case of Robert Edward Freitas Ortega, a physicist who worked for NASA precisely in that field, the creation of self-replicating spacecraft, and who in 2004 published an article on the topic—along with engineer Ralph Merkle—entitled “Kinematic Self-Replicating Machines”, in which he doubted that they could be as perfect as claimed. This is also endorsed by others who assume that any small flaw in replication would have a cascade effect on all copies and would ruin the project; unless they also had the ability to detect it, of course, but that would bring the question back to the beginning.

David Brin said in his aforementioned article “The Great Silence” that if berserker probes existed, they would probably have already found and destroyed Humanity. In 2013, an analysis conducted for the Future of Humanity Institute (a research center affiliated with the University of Oxford) arrived at the same conclusion with the title “Eternity in Six Hours: Intergalactic Spreading of Intelligent Life and Sharpening the Fermi Paradox”. It is true that its authors, Swedish computational neuroscientist Anders Sandberg and AI expert Stuart Armstrong, are very skeptical about the existence of life not only in our galaxy but in the observable universe.

But Brin does believe that there might be someone out there. In what he baptized as the dark forest hypothesis (a term taken from the 2008 novel “The Dark Forest” by Chinese writer Liu Cixin), he suggests that there could be many extraterrestrial civilizations, but they remain hidden to avoid the risk of encountering any aggressive ones and being exterminated. This kind of stellar paranoia would lead berserker probes to only visit places emitting some kind of signal that could be interpreted as evidence of intelligent life. In other words, keeping silent and unnoticed would be the best recourse to ensure survival, in a curious version of game theory.

What is game theory? It is a model of applied mathematics in which players act sequentially, one after the other, without any of them knowing all the available information, and the only way to win is to continually survive each play. Applied to the dark forest hypothesis, the possible actions of each player, in this case, a civilization, would be to destroy another civilization known by the player, spread the word and alert other civilizations of their own existence, or do nothing. In his novel “The Forge of God”, science fiction writer Greg Bear compares Humanity to a child abandoned in the forest whose cries attract the wolves.

One of the characters in “The Dark Forest” says that any intelligent life in the universe will face all other forms of life in the struggle for survival, which is the primary need of any civilization. However, to become such, there is a long and arduous path that economist Robin Hanson outlined in nine stages in his essay “The Great Filter—Are We Almost Past It?” published in 1996. They would be, analogous to the Kardashev Scale: a suitable and potentially habitable planetary system; reproductive molecules (RNA); simple unicellular life (prokaryotes); complex unicellular life (eukaryotes); sexual reproduction; multicellular life; animals with large brains that use tools; current level; and expansion and colonization of space.

According to Hanson, at least one of these steps would be improbable and would constitute that great filter from the title, hence there being no known civilization other than ours, which managed to overcome them all. Or perhaps we are the first to do so. Between phases eight and nine (that is, the transition from our current situation to space expansion), the filter would be the berserker hypothesis, the destruction of civilization by a von Neumann probe created by a more advanced entity. Of course, there are more alternatives that explain why we have not yet encountered such an entity. After all, as Arthur C. Clarke himself said, truly advanced engineering would seem like magic to us or would be completely unrecognizable.

This article was first published on our Spanish Edition on February 14, 2024. Puedes leer la versión en español en Hipótesis del Berserker, la posible destrucción de toda vida inteligente universal por sondas robóticas autorreplicantes


John von Neumann, Theory of Self-Reproducing Automata | Matt Williams, Beyond “Fermi’s Paradox” VI: What is the Berserker Hypothesis? (en Universe Today. Space and Astronomy News) | Glen David Brin, The Great Silence: the controversy concerning extraterrestrial intelligence life | Robert A. Freitas Jr., A Self-Reproducing Interstellar Probe | Stuart Armstrong y Anders Sandberg, Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox | Peter D. Ward y Donald Brownlee, Rare Earth: why complex life is uncommon in the universe | Michael H. Hart, An explanation for the absence of Extraterrestrials on Earth (en Extraterrestrials. Where are they?) | Carl Sagan y William I. Newman, The solipsist approach to Extraterrestrial intellingence | Robin Hanson, The great filter. Are we almost past it? | Fred Saberhagen, Berserker. The early thales | Cixin Liu, The dark forest | Greg Bear, The forge of God | Wikipedia

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