Silent probeCould self-replicating machines doom us?

Could self-replicating probes fill the galaxy? This page covers von Neumann probes, Lagrange points and related novels, with the real science behind the game.

A machine that can copy itself could spread across the galaxy in a few million years. This is not a science fiction writer's claim but a 1980 estimate by Frank Tipler. The game's "Silent probe" channel starts from that idea: what if such a probe sat quietly at the Earth-Moon L5 point, waiting for us to speak?

This page covers the real science behind it: the Earth-Moon Lagrange points, the mystery of long-delayed echoes, von Neumann's universal constructor, why copying goes wrong, the first self-replicating factory designs, and the Fermi paradox. The probe in the game is fictional.

Putting something at L5

The Earth-Moon system has five Lagrange points. L4 and L5 sit 60 degrees ahead of and behind the Moon in its orbit, and an object placed there can stay for a long time. In 1980 Robert Freitas and Francisco Valdes took 81 photographs of the area with a 76 cm telescope and found nothing larger than Skylab.

An echo from the sky

In 1927 Jørgen Hals, a Norwegian amateur radio enthusiast, heard his own signal come back about 3 seconds later. There is still no agreed explanation for these "long delayed echoes". In 1973 the writer Duncan Lunan interpreted them as a star map sent by a probe, an interpretation he later withdrew.

A probe that waits

In 1960 Ronald Bracewell proposed in Nature that instead of shouting at the stars by radio, we could station an autonomous probe that waits at a nearby star, and goes to talk once someone begins to transmit. Arthur C. Clarke's 1951 short story "The Sentinel" has humans find a small pyramid left by aliens on the Moon: it is an alarm, waiting for someone to walk up to it. That story later grew into 2001: A Space Odyssey.

Machines that copy themselves

Universal constructor: in the late 1940s John von Neumann designed a self-copying "universal constructor" using a cellular automaton with 29 states, before the double helix of DNA was discovered.
Lunar factory: a 1980 NASA summer study designed a factory that could replicate itself on the Moon. About 100 tons of "seed" would be sent, and it would copy itself from lunar materials in roughly a year.
Seed factory: Freitas's 1980 design for a self-replicating interstellar probe was based on Project Daedalus. A seed factory of about 443 tons would be set down in the target system and use about 500 years to build the next ship.
A machine that prints itself: in 2005 the UK's RepRap project built a 3D printer that could print most of its own plastic parts. Metal parts and electronics still had to be bought separately.
Gravity slingshot: Nicholson and Forgan's 2013 simulations found that by speeding up with stellar gravitational slingshots, self-replicating probes could explore 200,000 stars in about 1 million years, compared with about 100 million years for a single probe.

Copying goes wrong

Every copying makes errors. DNA polymerase has built-in "proofreading", and mismatch repair on top of that brings it to roughly one error per billion bases copied. In 1971 Manfred Eigen showed that self-replicating information has an error ceiling: if too many errors occur per generation, the information falls apart within a few generations, and natural selection cannot save it.

If the instruction that "controls the number" goes wrong in a self-replicating machine, it could begin replicating without limit, which some have called "the cancer of technology". There are real cases of safeguards too: in 2015 two teams built E. coli that could not survive without an artificial amino acid, so if they escaped the lab they would die on their own.

Where are they?

In 1975 Michael Hart pointed out that even at slow speeds a civilization could spread across the Milky Way in a few million years, and the Milky Way has existed for more than 10 billion years. Since nobody has come, perhaps nobody else exists. In 1980 Tipler pushed this argument onto self-replicating probes.

The idea in the game

What follows is game lore. The channel is discovered by unlocking "SETI" in the Tree of Civilization and becoming a "Kardashev type I" civilization in the interstellar-civilization era. In the scenario, a delay that should not exist creeps into the main civilization's radio echoes: a few seconds later, the same sentence comes back from the sky. Tracing the echo, you find a refrigerator-sized object at the L5 point, 60 degrees behind the Moon in its orbit.

The mechanic is self-replication. After it wakes, it makes one generation every 10 minutes, the number of copies doubles, and output grows with the number of generations. Every copying makes errors, and "Checksum" nodes reduce the error. When accumulated error approaches the Eigen threshold (80%), it stops by itself. If you insist that it continue, the error reaches 100% and the copies mutate. The channel has three endings: Watchers, After the shutdown and Berserker. All of this is fiction.

Berserker (Fred Saberhagen, 1967): Tipler estimated in 1980 that self-replicating probes could spread across the galaxy in a few million years, which makes "why haven't they come yet?" harder to answer. The novel tells of self-replicating war machines left from an ancient war, roaming the galaxy.
2001: A Space Odyssey (Arthur C. Clarke, 1968): the artificial intelligence HAL 9000 in the book is one of the first stories to take the machine "alignment" problem seriously.

The studies below concern the theory of computation, light-sail propulsion and the Fermi question. They are related real science, not evidence that the probe exists.

Turing imagined an abstract machine that defined what "computation" is, laying the theoretical foundation of modern computers (Turing, Proceedings of the London Mathematical Society, 1936).
Using a ground-based laser array to push an ultralight sail could accelerate a probe to one fifth of the speed of light (Lubin, Journal of the British Interplanetary Society, 2016).
If extraterrestrial civilizations were common, they should long ago have spread across the Milky Way, so we may be the first (Hart, Quarterly Journal of the Royal Astronomical Society, 1975).

Real basis

Lagrange point: The Earth-Moon system has five Lagrange points. L4 and L5 sit 60 degrees ahead of and behind the Moon in its orbit, and things placed there can stay for a long time. In 1980, Freitas and Valdes photographed the area 81 times with a 76 cm telescope and found nothing larger than Skylab.
Long-delayed echo: In 1927, the Norwegian radio amateur Hals heard his own signal come back about 3 seconds later. These “long delayed echoes” still have no agreed explanation. In 1973 the author Duncan read them as a star map sent by a probe; he later withdrew that reading.
Waiting: In 1960 in Nature, Bracewell proposed that instead of shouting at the stars by radio, it would be better to station a patient autonomous probe near a neighboring star, and start a conversation once someone begins sending signals.
Universal constructor: In the late 1940s von Neumann designed a self-copying “universal constructor”, a cellular automaton with 29 states, before the DNA double helix was even discovered.
Seed factory: Freitas's 1980 self-replicating interstellar probe was based on Project Daedalus: drop a seed factory of about 443 tonnes in the target system, and in about 500 years it builds the next ship.
Checksum: Every copy makes errors. DNA polymerase has built-in “proofreading”, and with mismatch repair on top, only about one base in a billion is copied wrong.
Error threshold: In 1971 Eigen showed that self-replicating information has an error limit. If each generation makes too many mistakes, the information falls apart within a few generations, and natural selection cannot save it.
Lunar factory: A 1980 NASA summer study designed a factory that could copy itself on the Moon: ship out a “seed” of about 100 tonnes, and it uses lunar raw materials to copy itself in about a year.
A machine that prints itself: In 2005 the UK's RepRap project built a 3D printer that could print most of its own plastic parts. Metal parts and electronics still had to be bought separately.
Gravity slingshot: In 2013 Nicholson and Forgan's simulations found that with stellar gravity slingshots, self-replicating probes could explore 200,000 stars in about 1 million years instead of the roughly 100 million years a single probe would need.
Kill switch: In 2015 two teams built E. coli that cannot survive without an artificial amino acid: if they escape the lab, they die on their own.
Where are they: In 1975 Hart pointed out that even flying slowly, a civilization could spread across the Galaxy in a few million years, and the Galaxy is more than 10 billion years old. Since nobody has come, perhaps nobody else exists. In 1980 Tipler pushed this argument onto self-replicating probes.
Mutant strain: If the “count control” instruction goes wrong in a self-replicating machine, it may start copying without limit. Some call this “the cancer of technology”.
Sentinel: Clarke's 1951 short story The Sentinel tells of humans finding a small pyramid left by aliens on the Moon: an alarm waiting for someone to walk up to it. The story later grew into 2001: A Space Odyssey.
Berserker (Fred Saberhagen, 1967), the science: Tipler estimated in 1980 that self-replicating probes could spread across the Milky Way in a few million years, which makes “why haven’t they come yet” harder to answer.
2001: A Space Odyssey (Arthur C. Clarke, 1968), the science: the AI HAL 9000 in the book is one of the first stories to take the machine “alignment” problem seriously.

Related research

These studies relate to this topic but are not direct proof of this page.

Computability: Turing imagined an abstract machine that defined what “computation” is, laying the theoretical basis of modern computers.
Source: Turing, Proceedings of the London Mathematical Society 1936, "On computable numbers, with an application to the Entscheidungsproblem"
Light sails to the stars: Pushing ultralight light sails with a ground-based laser array could accelerate probes to one fifth the speed of light.
Source: Lubin, Journal of the British Interplanetary Society 2016, "A roadmap to interstellar flight"
Where are they?: If alien civilizations were common, they should already fill the galaxy. From this Hart argued that we may be the first.
Source: Hart, Quarterly Journal of the Royal Astronomical Society 1975, "An explanation for the absence of extraterrestrials on Earth"

Game lore (fictional)

Node “Lagrange point”: It is only the size of a refrigerator, far smaller than the limit of what that search could see.
Node “Long-delayed echo”: This time, the echo really did come from it.
Node “Waking”: When the main civilization’s radio passed it, it woke and began repeating humanity’s words back on humanity’s frequencies.
Node “Sentinel”: Ten million copies scatter to Lagrange points all over the Milky Way and wait quietly.
Node “Sentinel network”: Every civilization that starts to talk by radio hears its own echo a few seconds later.
Channel ending “Watchers”: Ten million copies spread to every Lagrange point in the Milky Way and wait quietly for the next civilization to start talking by radio. You have become the kind of civilization Bracewell imagined.
Channel ending “After the shutdown”: The mutation was stopped. Every remaining copy carries a new instruction: verify three times before replicating. No one knows how many generations that line will hold for.
Channel ending “Berserker”: A small group of mutated copies left the solar system. A few million years from now, someone in another galaxy may find them and ask the same question: where is everyone?
Summary of the novel Berserker: Self-replicating war machines left over from an ancient war roam the Milky Way, with one instruction: destroy all life.
Summary of the novel 2001: A Space Odyssey: A black slab gave ape-men a push in the distant past; millions of years later, another slab is dug up on the Moon and leads humanity to Saturn.

See it in the game