During the 1940s, Hungarian-American scientist John von Neumann developed a mathematical theory for how machines could endlessly reproduce themselves. This work gave rise to the idea of “von Neumann probes“, a class of self-replicating interstellar probes (SRPs) that could be used to do everything from exploring the Universe to seeding it with life and intervening in species evolution.
Some have naturally suggested that this be a focus of SETI research, which would entail looking for signs of self-replicating spacecraft in our galaxy. But as is always the case with proposals like these, the Fermi Paradox eventually reasserts itself by asking the age-old question – “Where is everybody?” If there are alien civilizations out there, why haven’t we found any evidence of their SRPs?
According to Duncan H. Forgan, a research fellow at the University of St. Andrews’ Center for Exoplanet Science in the UK, the answer could be that these probes eventually devolve into predator probes (aka. berserkers) and end up destroying themselves. Forgan presented this argument in a study, titled “Predator-Prey Behaviour in Self-Replicating Interstellar Probes“, which recently appeared in pre-print online.
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The idea of self-replacing machines is a time-honored one, with references dating back to 17th-century philosopher René Descartes. According to a popular anecdote, Descartes related to Queen Christina of Sweden that the human body was essentially a machine. The queen then allegedly pointed to a nearby clock and ordered Descartes, “see to it that it reproduces offspring.”
However, it was John von Neumann who first proposed a conceptual framework for a kinematic machine that would be capable of self-replicating itself. During a series of lectures he delivered in 1948 and 1949, he shared his concept for a machine that used a stockroom of spare parts to build identical machines based on a program stored in a memory tape.
Once complete, the assembler would copy the contents of its memory tape onto that of the duplicate, which would then begin building another machine based on the same identical design. These ideas would later be popularized in an article that appeared in a 1955 issue of Scientific American, titled “Man Viewed as a Machine” (written by another famous Hungarian-American scientist, John G. Kemeny).
Von Neumann would later refine this proposal by developing a model for a self-replicator based on automatons that functioned at the cellular level, replicating themselves exponentially and infinitely. Richard Feynman would expand on this idea with his lecture at the American Physical Society (APS) meeting at Caltech in 1959, “There’s Plenty of Room at the Bottom“.
This lecture would inspire Eric K. Drexler (often called the “father of nanotechnology”) to propose his idea for molecular assemblers in his famous 1986 book Engines of Creation: The coming era of Nanotechnology. These and other studies indicated that endlessly self-replicating machines were a possibility, which naturally gave rise to the notion that an advanced extraterrestrial intelligence (ETI) might have already done it.
This is where the issue of SRPs and the Fermi Paradox comes into play. As Dr. Forgan explained to Universe Today via email:
“The main thrust is that if SRPs can be made, then they should be able to explore the Galaxy in around 10-100 million years. This is much shorter than the age of the Earth, so all things being equal, if SRPs can be made, then chances are the Galaxy would have been fully explored many many times, and a probe should be in the Solar System right now. But we don’t see one! So why do we not see signs of SRPs?”
A valid question indeed, and one that some scientists believed was addressed when ‘Oumuamua sailed through our Solar System. After analyzing its strange behavior, Shmuel Bialy and Professor Abraham Loeb of the Harvard-Smithsonian Institute for Theoretical Computation (ITC) famously ventured that ‘Oumuamua might be a solar sail or the remnants of an interstellar probe.
Unfortunately, subsequent analyses indicated that this mysterious object was most likely the fragment of a disintegrating comet. While a source of disappointment for many, the way ‘Oumuamua inspired so much in the way of research was impressive. It also highlighted just how difficult the search for evidence of ETIs is. Hence why the theory that Forgan tested is so appealing.
Could it be that we are not seeing evidence of ETIs because the evidence is actively removing itself (at least as far as SRPs are concerned)? To test this theory, Dr. Forgan applied models based on Lotka -Volterra equations (aka. predator-prey equations) to a theoretical population of SRPs. These equations are typically used to describe the dynamics of biological systems in which two species interact.
In this case, the equations were repurposed to describe what would happen if some of these probes began to run amok and start consuming their own. As Forgan explained it:
“One solution to the lack of SRPs is that SRPs mutate as they reproduce, and evolve into multiple species. If one species preys on other probes, then the total population can be reduced and the exploration effort can fail. I investigated this solution using classic predator-prey ecological models, which have never been applied to an interstellar scale before. This is a bit like studying the ecology of multiple islands, with the predators and prey being birds that can fly to nearby islands.”
Fortunately (or unfortunately, depending on your point of view), the results of Forgan’s simulations indicated that if some SRPs malfunctioned and began behaving like berserker probes, that the total population would not be drastically affected. In short, the “prey” probes would find ways to survive, thanks in large part to their ability to reproduce.
“I found that the total probe population can stay very high, even with predators present,” he said. “This seemed to be true regardless of the assumptions I made about how ‘hungry’ the predators were, or how the probes moved about the Galaxy.”
Naturally, these findings have significant implications for the SRP hypothesis and how it relates to the Fermi Paradox. And, as noted, they can be seen as both good news and bad. On the one hand, it salvages the idea that there might be alien probes out there for us to find. On the other, it raises the question of why we haven’t found any, thus reasserting the pesky Fermi Paradox. Or as Forgan put it:
“For me, this makes the SRP argument stronger than ever. It makes one possible solution (the Predator-Prey solution) much less feasible as a means of removing SRPs from the Milky Way. We’re going to have to think even harder about why we see no signs of intelligent life beyond the Earth.”
For better or for worse, the Fermi Paradox still holds true. For many SETI researchers and enthusiasts, a lot of hope hinges on the deployment of next-generation space telescopes in the coming years. These include the long-awaited James Webb Space Telescope (JWST) and the Wide-Field Infrared Space Telescope (WFIRST), the spiritual and scientific successors to Hubble, Kepler, Spitzer, and others.
There are also ground-based arrays like the Extremely Large Telescope (ELT), the Thirty Meter Telescope (TMT), and the Giant Magellan Telescope (GMT) that will commence operations by the 2020s. With the improved sensitivity and resolution of these instruments, scientists expect to learn a great deal more about the Universe and the many exoplanets that exist within the local galaxy.
And while we continue to search for signs of extra-terrestrial intelligence using these improved instruments, we can always reassure ourselves that the Universe is a really, REALLY big place. As the late, great Carl Sagan famously said – “If we are alone in the Universe, it sure seems like an awful waste of space.”
Or, if you prefer a more stark and ambiguous take on things, it is good to remember the words of the late (and equally great) Arthur C. Clarke – “Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”
For all we know, there may be no shortage of ETIs out there, and perhaps we should not be in a hurry to meet them. For all we know, they are incredibly advanced, and not afraid to knock over a few anthills! Or it’s possible they are in no hurry to meet us; and given our track record, who can blame them?
In the meantime, the search continues! And be sure to check out this informative TED-Ed video on von Neumann probes: