In 1948-49, mathematician, physicist, computer scientist, and engineer John von Neumann introduced the world to his idea of “Universal Assemblers,” a species of self-replicating robots. Von Neumann’s ideas and notes were later compiled in a book titled “Theory of self-reproducing automata,” published in 1966 (after his death). In time, this theory would have implications for the Search for Extraterrestrial Intelligence (SETI), with theorists stating that advanced intelligence must have deployed such probes already.
The reasons and technical challenges of taking the self-replicating probe route are explored in a recent paper by Gregory L. Matloff, an associate professor at the New York City College of Technology (NYCCT). In addition to exploring why an advanced species would opt to explore the galaxy using Von Neumann probes (which could include us someday), he explored possible methods for interstellar travel, strategies for exploration, and where these probes might be found.
His paper, “Von Neumann probes: rational propulsion interstellar transfer timing,” was recently published in the International Journal of Astrobiology, a Cambridge University publication. In addition to being an Adjunct and Emeritus professor of physics at NYCCT, Matloff is a Fellow of the British interplanetary Society (BIS), a Member of the International Academy of Astronautics (IAA), and has been a consultant for the NASA Marshall Space Flight Center.
His pioneering research in solar-sail technology has been utilized by NASA to develop concepts for interstellar probes and diverting potentially-hazardous objects (PHOs) – in other words, asteroids. His writings have helped establish interstellar-propulsion studies as a sub-division of applied physics in academia. He also co-authored books with fellow luminaries like MIT science-writer Dr. Eugene Mallove, noted physicist, author, and NASA technologist Les Johnson, and Italian researcher Dr. Giovanni Vulpetti.
In April 2016, Matloff was appointed an advisor to Yuri Milner’s Breakthrough Starshot alongside fellow astrophysicists like Prof. Abraham Loeb (Harvard Smithsonian Center for Astrophysics) and Dr. Philip Lubin – leader of the Experimental Cosmology Group at UC Santa Barbara. In January 2017, he presented a Frontiers Lecture on interstellar travel at the American Museum of Natural History in Manhattan, where he is also a Hayden Associate.
Anybody Out There?
It is essential to address questions about Von Neumann probes, considering their implications for SETI and the Fermi Paradox. For decades, theoretical physicists and researchers have used the possible existence of Von Neumann probes to constrain the search for intelligence beyond Earth. As Matloff told Universe Today via Zoom, the road that brought us to this point was long and winding and went beyond any single person.
As he explained, the connection between Von Neumann’s idea of “Universal Assemblers” and space exploration emerged sometime in the 1970s. This was largely due to interstellar studies like Project Daedalus, a fusion rocket concept developed by the British Interplanetary Society (BIS) between 1973 and 1977. Amid the debate over whether or such missions should be crewed or robotic, the idea of the Von Neumann probe was revived and applied.
In no time at all, the old SETI saw came up, where humanity’s ability to conceive an idea is seen as a possible indication that an older, more advanced species might have done it already! As Michael Hart and Frank Tipler noted in their respective studies, the fact that we see no evidence for extraterrestrial interstellar probes is the most compelling evidence that humanity is alone in the Universe. This is the basis of the Hart-Tipler Conjecture, the earliest-known proposed resolution to Fermi’s Paradox.
According to Tipler, if ETIs did exist, they would have developed the capacity for interstellar travel and explored the Milky Way within ~300 million years:
“What one needs is a self-reproducing universal constructor, which is a machine capable of making any device, given the construction materials and a construction program… In particular, it is capable of making a copy of itself. Von Neumann has shown that such a machine is theoretically possible… As the copies of the space probe were made, they would be launched at the stars nearest the target star. When these probes reached these stars, the process would be repeated, and so on until the probes had covered all the stars of the Galaxy.”
Comparison of the Daedalus spacecraft and Saturn V Moon rocket. Credit/copyright: Adrian Mann.
Famed astronomer and science communicator Carl Sagan rebutted their conclusions a few years later in an essay titled “The Solipsist Approach to Extraterrestrial Intelligence.” In this famous paper (nicknamed “Sagan’s Response”), he and co-author William Newman declared that while there was an apparent absence of probes and other technological marvels, this was by no means conclusive. As they poetically summarized: “the absence of evidence is not the evidence of absence.”
Matloff similarly takes the Hart-Tipler conjecture to task in his paper for its simplistic and presumptuous nature. As he explained to Universe Today via email:
“The Solar System is huge and mostly unexplored, and the probes could be very small. There could be probes everywhere: in craters on the Moon, or lurkers in the Asteroid Belt and Kuiper Belt. There are 100 million objects in the Kuiper Belt alone and we have examined only two, one of which was very anomalous in its shape.”
The object he refers to is MU69 (aka. Arrokoth), a “contact binary” that New Horizons studied during its historic flyby on January 1st, 2019. As the images acquired showed, the object appeared to be two icy bodies that “pancake”-like in shape (rounded by flattened) and connected by a “neck.” This strange appearance led New Horizons Principal Investigator Alan Stern to nickname the object “Snowman.”
In short, humanity has barely scratched the surface when it comes to cosmic exploration, including our backyard. For all we know, there could be countless probes lurking in our Solar System actively watching us, or which became inoperable long ago and have since settled into orbit around the Sun. The only way to resolve questions related to Von Neumann probes (and the Fermi Paradox) is to refine our search methods and keep searching!
Propulsion Methods
As we addressed in a previous article, traveling through interstellar space is incredibly time-consuming! Using conventional technology, it would take anywhere from 19,000 to 81,000 years to reach even the nearest star system (Alpha Centauri). This includes chemical propellants, Hall-effect thrusters (ion engines), gravity assists, and solar sails. Hence, more advanced propulsion methods need to be considered when addressing interstellar travel.
Many concepts are currently being investigated by researchers here on Earth. These include nuclear-thermal and nuclear-electric propulsion (NTP/NEP), fusion propulsion, photon and electric sails, matter/antimatter annihilation, and even some truly exotic concepts (like the Alcubierre Warp Drive). In keeping with the idea that humanity is a recent arrival to the Universe, SETI researchers assume that more advanced civilizations are likely to have researched these concepts already.
First, Matloff considers unpowered gravity assists, where spacecraft use the gravitational force of giant planets to achieve higher velocities. To date, five space probes have been launched from Earth that used a gravity-assist maneuver to achieve escape velocity from the Solar System. These include the Pioneer 10/11, the Voyager 1/2, and the New Horizons mission. The fastest of these missions (Voyager 1) will reach the Alpha Centauri star system in about 70,000 years based on its current velocity.
Powered gravity assists, otherwise known as an “Oberth Maneuver,” consist of a spacecraft making a powered maneuver while deep within a massive planet’s gravity well. According to Matloff, such a maneuver could allow a spacecraft to achieve twice the velocity of the Voyager 1 mission (41 km/s; 25.5 mi/s) and make the journey to Alpha Centauri in roughly 30,570 years.
When adjusted for nuclear fission and fusions concepts (using NASA research as a template), Matloff concludes that a nuclear-electric spacecraft could traverse one light in 1500 years while a fusion spacecraft could do the same in 3000 years. That works out to a one-way transit time of 6,550 and 13,100 years to Alpha Centauri, respectively.
Based on several factors, like sail material and whether the probe is “nano-miniaturized,” Matloff estimates that photon and electric sails could achieve relativistic speeds (a fraction of the speed of light) and make the transit in 1000 years. This is considerably longer than the Breakthrough Starshot concept, which calls for velocities of 0.2 c and a transit time of just 20 years. However, this is based on an estimated velocity of 300 km/s (186 mi/s) and not Starshot’s ambitious goal of 60,000 km/s (37,280 mi/s).
Matloff’s study provides no estimates for antimatter propulsion because the technology is simply not feasible yet. According to a report prepared by NASA scientist Robert Frisbee for the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (2003), a two-stage rocket could make it to Alpha Centauri in about 40 years. However, Frisbee indicated that the spacecraft would need over 815,000 metric tons (900,000 US tons) of fuel.
Artist concept of lightsail craft approaching the potentially habitable exoplanet Proxima b. Credit: PHL @ UPR Arecibo
No FTL concepts are considered for precisely the same reason (i.e., the technology is not verifiable and may never be). Meanwhile, the estimate for photon probes is based on several factors, predominantly the types of materials used for the sail. Said Matloff:
“Conservative values for sails were assumed. For instance, the industrial infrastructure necessary to produce a slower aluminum sail is a lot simpler than the infrastructure required to produce a faster graphene sail. A graphene sail could do this in ~1,000 years at an interstellar cruise velocity in excess of 1,000 km/s. My estimate of multi-millennia travel by solar photon sails at ~300 km/s is for the much more conservative aluminum sail. Less industrial infrastructure would be necessary for Al than for graphene.
Launch Strategies
In terms of rationale, Matloff explores many possibilities as to why a civilization would launch a fleet of Von Neumann probes. In this section, many of the arguments put forth by theorists who have explored questions related to “alien probes.” These include the Hart-Tipler Conjecture, the Berserker Hypothesis, and other research that attempted to place constraints on their reproduction and expansion rates.
Among the more popular rationales that have been explored include “life after death,” where an advanced civilization facing imminent demise would send out probes to broadcast messages. These could include stories of their accomplishments (“look upon our works and be impressed!”), instructions on how to avoid the same fate (“it’s not too late!”), or just advertisements of their existence (“This is who we were. Remember us!”).
There is also the possibility that probes would take the form of “benign lurkers” watching planet Earth from a distance. These probes could have been dispatched from a nearby star system as it made a close pass to our Solar System (Benford, 2021a, 2021b). A variant on this, “malignant lurkers,” suggests that extraterrestrials might dispatch armed probes (aka. “berserker probes”) to investigate Earth as a potential threat and destroy it.
It has also been ventured that some of these probes could still be here – likely on the Moon, Earth Trojans, and Earth co-orbital objects – and would make viable targets in the Search for Extraterrestrial Artifacts (SETA). Examples include recent studies by Jim Benford, Prof. Abraham Loeb, Konstantin Batygin, and the Initiative for Interstellar Studies (i4is) that show how interstellar objects (ISOs) like ‘Oumuamua and 2I/Borisov regularly enter our Solar System and are periodically captured.
Proposed Theoretical Propulsion Antimatter Rocket. Credit: Dr. Mike LaPointe/NASA Marshall Flight Center
Related research has also shown that the study of the captured ISOs (and new arrivals) will be possible in the near future thanks to the Vera C. Rubin Observatory and initiatives like Breakthrough Listen and the Galileo Project. Another rationale is directed panspermia, where an advanced civilization may choose to forgo sending crewed ships to distant stars (which could take thousands of years) and instead send spacecraft equipped with “gene banks” or fertilized ova.
Matloff cites Tipler’s 1994 book, The Physics of Immortality, where he elaborated on how humans could achieve interstellar colonization with probes someday. As Matloff summarizes it, “A Von Neumann probe could carry fertilized human ova to be raised robotically and populate in-space habitats circling nearby stars that would be constructed by the probe. A more advanced civilization might replace embryos with computer uploads of human ‘essences.'”
In recent years, a similar idea has been proposed by Claudius Gros, a researcher with Goethe University’s Institute for Theoretical Physics and the founder of the Project Genesis. The purpose of Genesis is to send spacecraft with gene factories or cryogenic pods to “transiently-habitable” planets that orbit M-type (red dwarf) stars. This refers to rocky planets with atmospheres rich in abiotic oxygen (not produced biologically) that would be uninhabited but still capable of supporting life.
By “seeding” these worlds with basic life, entire biomes could develop in places where life would not otherwise arise. “If life turns out to be a very rare phenomenon in the Universe, a space-faring civilization might deploy Von Neumann probes with a much happier purpose,” writes Matloff. “Simply lifeforms might be ‘planted’ within oceans on sterile, water-bearing worlds to spread life through the Universe.”
A final possibility Matloff considers has been explored extensively in science fiction: could advanced ETIs be sending out probes to direct galactic or universal evolution? A popular version of this scenario known as “paleocontact” argues that advanced life may have visited Earth in the past and deliberately directed humanity’s cultural (or even physical) evolution (2001: A Space Odyssey, Prometheus, Stargate, etc.).
Graphic representation of the relative distances between the nearest stars and the Sun. Credit: IEEC/Science-Wave/Guillem Ramisa
While some versions of this argument are pure pseudoarchaeology (i.e., “aliens built the pyramids”), Carl Sagan argued the paleocontact is something that scientists should not dismiss. As he and Iosif Shklovsky stated in their seminal book, Intelligent Life in the Universe, evidence of this contact may be preserved in the oral traditions of ancient cultures. As examples, they cite Romanian folklore and the Tlingit story of their encounter with the La Perouse expedition in 1786.
While these scenarios are all plausible in their own way, all of them have implications as far as SETI research is concerned – which Matloff addresses in the final section of his study.
Anywhere Nearby?
In the end, Matloff concludes that human astronomers may feel compelled to focus on Sun-like stars when looking for evidence of Von Neumann probes. This is perhaps the result of a Sol-centric bias, where we assume that G-type (yellow dwarf) stars are most likely to support habitable planets because that’s what we are familiar with. The implications of this could be that advanced ETIs suffer from the same bias and prefer to send their probes to stars similar to their own.
However, recent exoplanets studies have demonstrated that M-type (red dwarf) stars are very good candidates for finding find “Earth-like” (aka. rocky) exoplanets that orbit within the Habitable Zone (HZ). In particular, Matloff stresses how recent research has shown that these planets could be potentially-habitable. If an advanced ETI is anything like us (evolved on a rocky planet), they are not likely to overlook these star systems.
“If the spacing is less with M-type stars, you have [orbital] resonances, where a planet wouldn’t be tidally-locked because other planets cause perturbations in its orbit. Even if they are tidally locked, that doesn’t rule out the possibility of life. Von Neumann probes wouldn’t rule them out. [Future surveys should] look for probes and life at all stable and mature F, G, K, M main-sequence stars. M stars in particular seem to have lots of planets in or near the habitable zone.”
Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech
In addition to searching based on stellar classifications, Matloff also considers various proposals for where probes could be found in our Solar System. This once again raises the issue of proposed resolutions to the Fermi Paradox and their possible implications for SETI:
“Unless humanity is the first space-faring civilization or we are under some form of quarantine [a la the Planetarium and Zoo Hypotheses], it is reasonable to wonder where such probes might be found in the Solar System. Due to dynamic geophysical and meteorological processes, space might be a better place to search than Earth’s surface.”
Possible locations include the Moon, Earth Trojan asteroids, and Earth co-orbital asteroids. However, as Matloff himself previously suggested, searches for ET will have a better chance of success in the outer Solar System. One possible (rather large) location is the Kuiper Belt:
“An advantage of the Kuiper Belt for the construction of a subsequent generation of Von Neumann probes is the availability of resources including volatile materials,” he said, adding: “if they wish to keep their activities hidden, an outer Solar System location for a probe or a probe base makes the most sense. I think the Kuiper Belt is the best place to start looking.”
One of the hardest parts of SETI is the limited frame of reference we have. We know of only one planet that supports life (Earth) and one technologically-advanced civilization (ourselves). As such, all of our efforts fall under the heading of “the low-hanging fruit” approach, where we are confined to looking for signs of life (aka. “biosignatures”) as we know it and evidence of technological activity (aka. “technosignatures”) that we are familiar with.
So when it comes to getting inside the minds of ETIs, we are forced to stick to what we know (and what we might do in their place) and use the conclusions we come up with to help refine the search. While somewhat limiting, this approach does have many upsides. We have to assume that ETIs will be bound by the same physics we are since we know the laws don’t change from one place and time to another.
We are also pretty confident that if intelligent life exists elsewhere in our Universe, evolution will favor certain similar characteristics – like curiosity. While nothing definitive can be said about alien physiology, psychology, communications, or technology, it’s a safe assumption that they would be equally motivated to explore. Besides the allure of learning more about the cosmos and “seeing what’s out there,” they would surely be interested in whether there are intelligent species other than themselves.
In that respect, theoretical studies like this one help us refine the search by subjecting Fermi’s famous questions (“Where Is Everybody?”) to serious scrutiny. By asking the questions, “what would work best?” and “why would we do it?” we select places and signals that we can look for. Beyond that, the only thing we can do is to keep looking until we see what’s out there!
Further Reading:
Matloff, G.L. “Von Neumann probes: rationale, propulsion, interstellar transfer timing.” International Journal of Astrobiology, vol. 21, no. 1 (2022) Sagan, C. & Newman, W.I. The Solipsist Approach to Extraterrestrial Intelligence.” QJRAS, vol. 24 (1983) Tipler, F.J. “Extraterrestrial intelligent beings do not exist.” QJRAS, vol. 21 (1980) Hart, M.H. “Explanation for the Absence of Extraterrestrials on Earth.” QJRAS, vol. 16 (1975)