In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?“
This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which includes the Berserker Hypothesis. This theory suggests we haven’t heard from any alien civilizations because they’ve been wiped out by killer robots!
Also known as the “deadly probes scenario,” this hypothesis may sound like something science fiction (the name itself is actually taken from an SF franchise, in fact), but it’s actually rooted in scientific research. It also touches on other proposed resolutions to the Fermi Paradox, such as the Hart-Tipler Conjecture (i.e. aliens don’t exist) and that it’s the nature of intelligent life to destroy itself or others.
Fermi and the Great Filter
Central to Fermi’s famous question was a discrepancy between the assumed likelihood that extraterrestrial intelligence (ETI) and the lack of evidence to support this assumption. But given the number of stars in our galaxy (200 to 400 billion), the number of Earth-like planets in our galaxy (an estimated 6 billion), the number of galaxies in the Universe (as many as 2 trillion), it’s not farfetched to assume intelligent life must exist elsewhere.
In 1961, American physicist and SETI researcher Dr. Frank Drake illustrated this conundrum during a meeting at the Green Bank Observatory. In preparation for the meeting, Drake created an equation that summed up the probability of finding ETIs in our galaxy. Thereafter known as the Drake Equation, this probabilistic argument is expressed mathematically as:
N = R* x fp x ne x fl x fi x fc x L
- N is the number of civilizations we could communicate with;
- R* is the average rate of star formation in our galaxy;
- fp is the fraction of those stars which have planets;
- neis the number of planets that can support life;
- fl is the number of planets that will develop life;
- fiis the number of planets that will develop intelligent life;
- fc is the number of civilizations that would develop transmission technologies;
- L is the length of time that these civilizations would have to transmit their signals to space.
And yet, after an additional 70 years of searching, Fermi’s Paradox and the “Great Silence” persist as no compelling evidence has been found. This has led to multiple proposed resolutions from astrophysicists, astrobiologists, and other scientists and researchers. One of the most notable is the Great Filter Hypothesis, which was coined by economist Robin Hanson from Oxford University’s Future of Humanity Institute (FHI).
The term first appeared in a 1996 essay by Hanson titled “The Great Filter – Are We Almost Past It?“, where he proposed that there must be something that prevents non-living matter from coming together to form living organisms (abiogenesis) and reaching a high level of development. Using humanity as a template, Hanson created a nine-step process for the evolution of life. This consisted of:
- Habitable star system (organics and habitable planets)
- Reproductive molecules (e.g. RNA)
- Prokaryotic single-cell life
- Eukaryotic single-cell life
- Sexual reproduction
- Multi-cell life
- Animals capable of using tools
- Industrial civilization
- Wide-scale colonization
Essentially, Hanson’s “Filter” would have to be located somewhere within this process, possibly indicating life has a hard time emerging (if its at an early step) or becoming advanced (if its at a later step). Philosopher Nick Bostrom (also from the FHI) summarized the hypothesis beautifully in his 2008 essay, “Where Are They? Why I Hope the Search for Extraterrestrial Intelligence Finds Nothing“:
“The Great Filter can be thought of as a probability barrier. It consists of [one or] more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth-like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology.”
In the case of the Berserker Hypothesis, the Filter would be placed between step 8 and 9 with the assumption that life doesn’t have a hard time taking root or evolving, but ends up creating the machines that will destroy it. In this sense, intelligent life dooms itself to extinction at the very moment that it is poised on becoming an interstellar civilization.
The Berserker Hypothesis traces its origins to the mid-20th century, where mathematician John von Neumann, who began lecturing about his idea for self-replicating automata in 1948-1949. He called these machines “Universal Assemblers,” which would come to be referred to as “von Neumann machines.” Essentially, these machines would harvest local resources in order to produce copies of themselves.
In 1966, almost a decade after his death, von Neumann’s ideas and notes concerning Universal Assemblers were compiled and published in a book titled, “Theory of self-reproducing automata.” In time, his theories about robots that could endlessly self-replicate had implications for the Search for Extraterrestrial Intelligence and the Fermi Paradox, since the presence of such machines in our galaxy would be easily detectable.
In 1981, Frank Tipler wrote a study titled, “Extraterrestrial intelligent beings do not exist,” where he argued that the absence von Neumann probes was evidence that ETIs didn’t exist. Consistent with the Hart-Tipler Conjecture, he asserted that a sufficiently-advanced species would eventually develop universal assemblers that would explore and/or colonize the galaxy in less than 300 million years
In short, Tipler argued that based on the history of our galaxy (and even at a moderate-rate of production), a species of self-replicating probes would be widespread in our galaxy and humanity would have encountered them by now. In 1983, Carl Sagan and William Newman produced a study in response titled “The Solipsist Approach to Extraterrestrial Intelligence.”
In it, Sagan and Newman argued that Tipler had underestimated the rate of replication and that any von Neumann probes would have consumed most of the mass in the galaxy by now. Therefore, Sagan and Newman argued that an advanced race would avoid building self-replicating machines and would destroy any they encountered.
However, the suggestion that von Neumann probes would inevitably be destructive inspired another interpretation. Could it be that the reason we haven’t heard from any ETIs is that they have been wiped out by von Neumann probes that were designed to seek out life and destroy it? Or could it be that a species of benign probes went “berserk”, wiped out their creators, and have been destroying any life they encounter ever since?
Enter the Berserkers
Rather than having a single proponent, the Berseker Hypothesis is one that appears to have emerged over time, both as a popular science fiction trope and as a potential resolution to the Fermi Paradox. In the case of the former, science fiction writers like Fred Saberhagen explored this idea at length with his Berserker novels (1963-2005), the popular SF series from which the theory gets its name.
Astrophysicist and SF writer Gregory Benford explored the idea further in his Galactic Center Saga (1976-1995). Throughout these novels, Benford depicts a first contact scenario with alien machines (the Mechs) that are the creation of an extinct species that sees all biological life as inherently unstable and prone to self-destruction. The fact that the Mechs annihilate all advanced life they find is offered as the reason for the Great Silence.
Beyond science fiction, you have scientists like Glen David Brin, an astrophysicist, science fiction author, and NASA-consultant. In 1983, he published a study titled “The Great Silence – the Controversy Concerning Extraterrestrial Intelligent Life,” where he conducted a thorough review of the proposed resolutions to the Fermi Paradox – which included the Berserker interpretation of the classic von Neumann probe theory.
Addressing the arguments put forth by Tipler and others who supported the notion that humanity is unique in the Universe, Brin summarized the case for Berserker probes thusly:
“Let us say many advanced ETIS get the robot-emissary idea and ship out first-generation probes as Tipler suggests, to replicate and fill the void with messages of brotherhood. Then suppose that for every 100 or 1000 or 10,000 “sane” ETIS, there is one that is xenophobic, paranoid even. Such a race might program its self-replicating emissaries to add powerful bombs to their repertoire, and command them to home in on any unrecognized source of modulated electromagnetic radiation.”
The frightening thing about these “Deadly Probes”, argued Brin, is that it is consistent with all of the facts and philosophical principles that have been raised ever since Fermi asked his famous question. In response to the many decades in which SETI efforts have been conducted (and their failure to find evidence of ETIs) the Berserker Hypothesis offers a sensible explanation:
“There is no need to struggle to suppress the elements of the Drake equation in order to explain the Great Silence, nor need we suggest that no ETIS anywhere would bear the cost of interstellar travel. It need only happen once for the results of this scenario to become the equilibrium conditions in the Galaxy. We would not have detected extra-terrestrial radio traffic – nor would any ETIS have ever settled on Earth – because all were killed shortly after discovering radio.”
There have been a number of variations on this argument, such as cosmologist Edward Harrison. In 1981, he argued that an advanced species that has overcome its own self-destructive tendencies might be motivated to create Berserker probes out of a sense of self-preservation. This idea, where destruction is implemented out of some sense of “the greater good,” has come to be known as the “Cosmic Quarantine Hypothesis.”
There’s also the version of this hypothesis where the first lifeform (or lifeforms) to achieve interstellar space travel and colonization will prevent others from arising and achieving the same ends. This may be intentional, or simply the result of the “tragedy of the commons” and the “anthropic principle”, where one party working for their own self-interests invariably holds back others.
While the Berserker Hypothesis does provide a reasonably succinct resolution to the Fermi Paradox, it is itself subject to some of the same problems. For instance, if berserk hunter-killer robots are the reason we are not seeing any advanced civilizations, then why is it that we’re not seeing any evidence of hunter-killer robots?
After all, a species of robots that have become so pervasive that they would have eliminated any ETIs in our corner of the galaxy would have surely left undeniable signs of its existence. If planets (and/or stars) were being dismantled within thousands of light-years from Earth, observers here would certainly notice the massive energy signatures produced. In addition, the sudden disappearance of planets and suns would be an indication.
In 2013, Anders Sandberg and Stuart Armstrong of the FHI tested the Berserker Hypothesis in a study titled “Hunters in the dark: game theory analysis of the deadly probes scenario.” They analysis determine that even at a slow rate of self-replication, a population of von Neumann probes would have been able to spread to the point that it would have encoutered (and annihilated) humanity already.
It has also been suggested that Berserker robots would turn on their own, eliminating the evidence of themselves the same way they would eliminate ETIs. However, a recent study by Duncan Forgan of the University of St. Andrews Centre for Exoplanet Science concluded that a “berserk” population of probes couldn’t possibly eliminate their non-berserk brethren fast enough to stop them from spreading throughout the galaxy.
Alas, it seems that theories involving von Neumann probes and berserker robots run into the same difficulties as other potential resolutions to the Fermi Paradox. In the end, these difficulties all come down to one stark reality: humanity is the only intelligent life and the only civilization in the Universe that we know about. At least, for now.
Until we find some other examples of ETI and learn to recognize the signs of their biological and technological activity (i.e. biosignatures and technosignatures), we won’t know how to find more like them. At present, theoretical analyses and probabilistic arguments are about the best we can do. That, and to keep on looking! Because, after all, Fermi’s Paradox only needs to be solved once!
We have written many interesting articles about the Fermi Paradox, the Drake Equation, and the Search for Extraterrestrial Intelligence (SETI) here at Universe Today.
Here’s Where Are All the Aliens? The Fermi Paradox, Where Are The Aliens? How The ‘Great Filter’ Could Affect Tech Advances In Space, Why Finding Alien Life Would Be Bad. The Great Filter, Where Are All The Alien Robots?, How Could We Find Aliens? The Search for Extraterrestrial Intelligence (SETI), and Fraser and John Michael Godier Debate the Fermi Paradox.
Want to calculate the number of extraterrestrial species in our galaxy? Head on over to the Alien Civilization Calculator!
And be sure to check out the rest of our Beyond Fermi’s Paradox series:
Astronomy Cast has some interesting episodes on the subject. Here’s Episode 24: The Fermi Paradox: Where Are All the Aliens?, Episode 110: The Search for Extraterrestrial Intelligence, Episode 168: Enrico Fermi, Episode 273: Solutions to the Fermi Paradox.
- von Neumann, J. & Burks, A. (ed.) Theory of Self-Reproducing Automata. University of Illinois Press (1966)
- Freitas, R. Jr. “A Self-Reproducing Interstellar Probe” (1980)
- Sagan, C. & Newman, W. “The Solipsist Approach to Extraterrestrial Intelligence” (1983)
- Binr, G.D. “The Great Silence – the Controversy Concerning Extraterrestrial Intelligent Life” (1983)
- Sandberg, A. & Armstrong, S. “Hunters in the dark: game theory analysis of the deadly probes scenario” (2013)
- Sanders, A. & Armstrong, S. “Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox” (2013)
- Forgan, D. “Predator-Prey Behaviour in Self-Replicating Interstellar Probes” (2019)