John von Neumann, a mathematician, physicist, computer scientist, and engineer, introduced the world to his idea of Universal Assemblers in 1948. After his death, Von Neumann's ideas and notes were published in a book titled " Theory of self-reproduction automata". This theory would have implications for the Search for Extraterrestrial Intelligence, with theorists saying that advanced intelligence must have deployed such probes already.
Gregory L. Matloff is an associate professor at the New York City College of Technology. In addition to exploring why an advanced species would use Von Neumann probes to explore the galaxy, he explored other ways to travel and where these probes might be found.
His paper was published in the International Journal of Astrobiology. Matloff is a Fellow of the British interplanetary Society, a Member of the International Academy of Astronautics, and a consultant for the NASA Marshall Space Flight Center.
NASA has used his research in solar-sail technology to develop concepts for probes and asteroids. His writings helped establish the sub-division of applied physics. He co-authored books with other notables like Dr. Eugene Mallove, Les Johnson, and Giovanni Vulpetti.
In April 2016 Matloff was appointed an advisor to the author of the book. In January of last year, he presented a lecture on the topic of interstellar travel at the American Museum of Natural History in Manhattan.
Von Neumann probes have implications for SETI and the Fermi Paradox. Physicists have used Von Neumann probes to search for intelligence beyond Earth for decades. Matloff told Universe Today that the road that brought us to this point was long and winding and went beyond any single person.
The connection between Von Neumann's idea of universal assemblers and space exploration emerged in the 1970s. The British Interplanetary Society (BIS) developed a fusion rocket concept called Project Daedalus. The Von Neumann probe was revived as a result of the debate over whether or not such missions should be crewed or robotic.
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! The most compelling evidence that humanity is alone in the Universe is the fact that we see no evidence for extraterrestrial probes. This is the basis of the Hart-Tipler Conjecture.
According to Tipler, if aliens existed, they would have explored the world 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.”
In an essay titled "The Solipsist Approach to Extraterrestrial Intelligence", Carl Sagan rebutted their conclusions a few years later. The absence of evidence is not the evidence of absence.
Matloff takes the Hart-Tipler conjecture to task for its simplistic nature. He told 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.”
He refers to an object. During its historic flyby on January 1st, 2019, New Horizons studied a contactbinary. The object appeared to be two icy bodies that were similar in shape to a pancake and connected by a neck.
We have barely scratched the surface when it comes to exploration of the universe. We don't know if there are many probes in our Solar System that are actively watching us or if there are only a few probes that have settled into the Sun's vicinity. The only way to resolve questions related to Von Neumann probes is to keep searching.
Traveling through space is very time-Consuming. It would take between 19,000 and 81,000 years to reach the nearest star system. Chemical propellants, Hall-effect thrusters, gravity assists, and solar sails are included. There are more advanced methods that need to be considered.
Many concepts are being investigated by researchers. Nuclear-thermal and nuclear-electric propulsion, fusion propulsion, photon and electric sails, matter/antimatter, and even some truly exotic concepts are included. SETI researchers assume that more advanced civilizations are likely to have researched these concepts already, in keeping with the idea that humanity is a recent arrival to the Universe.
Matloff considers unpowered gravity assists, where spaceships use the force of giant planets to get higher speeds. Five space probes have been launched from Earth and used a gravity assist maneuver to escape the Solar System. The Pioneer 10/11, the Voyager 1/2, and the New Horizons mission are included. Voyager 1 will reach the Alpha Centauri star system in about 70,000 years.
A powered gravity assist, also known as an Oberth Maneuver, is a maneuver made by a spaceship deep within a massive planet's gravity well. Matloff said that a maneuver could allow a craft to travel twice the speed of the first mission and reach Alpha Centauri in roughly 30,000 years.
Matloff found that a fusion spacecraft could do the same thing as a nuclear-electric one in 3000 years. The transit time to Alpha Centauri is 6,550 and 13,100 years.
Matloff estimates that photon and electric sails can achieve a fraction of the speed of light and make the transit in 1000. This is much longer than the concept of the Breakthrough Starshot, which calls for a transit time of 20 years. This is based on an estimated speed of 300 km/s, not Starshot's goal of 60,000 km/s.
Matloff's study doesn't provide estimates for antimatter propulsion because it's not feasible yet. A two-stage rocket could reach Alpha Centauri in about 40 years according to a report prepared by NASA scientist Robert Frisbee. The spacecraft would need over 815,000 metric tons of fuel, according to Frisbee.
No FTL concepts are considered for the same reason. The estimate for photon probes is based on the 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.
Matloff explores many possibilities as to why a civilization would launch a fleet of Von Neumann probes. The Hart-Tipler Conjecture, the Berserker Hypothesis, and other research that attempted to place constraints are some of the arguments put forth by theorists.
Life after death is one of the more popular rationales that has been explored. They could include stories of their accomplishments, as well as instructions on how to avoid the same fate. Remember us!
There is a chance that probes would look at planet Earth from a distance. The probes could have been dispatched from a nearby star system. A variant on this suggests that extraterrestrials might send armed probes to investigate Earth.
Some of the probes could still be here, and they would make good targets in the Search for Extraterrestrial Artifacts. Recent studies by Jim Benford, Prof. Abraham Loeb, and the Initiative for Interstellar Studies (i4is) show how the stars can be seen.
The study of captured ISOs and new arrivals will be possible in the near future thanks to the Vera C. Rubin Observatory and other initiatives. In directed panspermia, an advanced civilization may choose to forgo sending crewed ships to distant stars and instead send spaceships equipped withgene banks or fertilized ova.
Matloff cites a 1994 book by Tipler, The Physics of Immortality, where he explained how humans could colonize other planets. A Von Neumann probe could carry fertilized human ova to be raised robotically and populate in-space habitats around nearby stars that would be constructed by the probe. A more advanced civilization might replace embryos with computer uploads.
The founder of the Project Genesis, Claudius Gros, has proposed a similar idea in the past. The purpose of Genesis is to send spaceships with gene factories to planets that are in the vicinity of M-type stars. This refers to rocky planets with atmospheres that are not produced biologically but still capable of supporting life.
If life turns out to be a very rare phenomenon in the Universe, a space-faring civilization could be formed.
Matloff considers the possibility that advanced tesseracts could be sending probes to direct evolution. A popular version of this scenario claims that advanced life may have visited Earth in the past and directed the evolution of our culture.
Carl Sagan argued that scientists should not dismiss the paleocontact argument. Evidence of this contact can be found in the oral traditions of ancient cultures. They cite the Tlingit story of their encounter with the La Perouse expedition.
Matloff addresses SETI research in the final section of his study because all of the scenarios have implications as far as SETI research is concerned.
Matloff concludes that human astronomer may feel compelled to focus on Sun-like stars when looking for evidence of Von Neumann probes. We assume that G-type stars are most likely to support planets because we are familiar with them. The implications of this could be that advancedETIs suffer from the same bias and prefer to send probes to stars similar to their own.
Recent exoplanets studies have shown that M-type stars are good candidates for finding exoplanets that are rocky. Matloff stresses how recent research has shown that these planets could potentially live on. They are not likely to overlook these star systems if they are an advanced ETI like us.
“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.”
Matloff considers various proposals for where probes could be found in the Solar System, as well as searching based on stellar classifications. This raises the issue of proposed resolutions to the Fermi Paradox and their 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 co-orbital asteroids, and Earth Trojan asteroids. Matloff had previously suggested that searches for etha will have a better chance of success in the outer Solar System. There is a possibility that the Kuiper Belt is a large location.
“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.”
The limited frame of reference is one of the hardest parts of SETI. Only one planet supports life and one technologically-advanced civilization. We are confined to looking for signs of life under the heading of "biosignatures".
We are forced to stick to what we know and use the conclusions we come up with to help refine the search when it comes to getting inside the minds of aliens. This approach has many positives. We know the laws don't change from place to place, so we assume that the physics will be the same.
We are pretty confident that if intelligent life is found elsewhere in the Universe, evolution will favor similar characteristics like curiosity. It is safe to assume that aliens would be equally motivated to explore. They would be interested in whether there are intelligent species other than themselves.
We can refine the search by subjecting famous questions like Where Is Everybody? to serious scrutiny. By asking the questions, we can find places and signals that will work. We can only look until we see what's out there.
Further reading.
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