SweepersWould a civilization that is heard be wiped out?

Should we shout into the universe? Parallax, leaked signals, waste heat and the dark forest, with the real science behind the game.

If the universe holds civilizations that destroy others, the safest strategy may be silence. David Brin discussed this idea systematically in 1983 in "The Great Silence". The "dark forest" is Liu Cixin's literary version of it in his novels, not a scientific conclusion. The game's "Sweepers" channel turns the idea into a concrete scene: 27 light-years away, a picket machine waits for someone to give themselves away.

This page covers the real parts: how humans measure distance, how strong the signals leaking from Earth are, why waste heat cannot be hidden, how destructive a fast projectile would be, and the scientists' debate over whether to shout. Then it outlines the game's scenario. The Sweeper is fictional.

How it finds you

Parallax: look at the same star from two ends of Earth's orbit and its position shifts slightly, which gives its distance. The Gaia satellite measured the positions of nearly 2 billion stars and ended observations in 2025.
Leakage: a 1978 study concluded that the easiest things on Earth to hear from far away are television transmitters and ballistic-missile early-warning radars. A 2023 simulation estimated that the peak radio power leaking into space from today's cell towers is about 4 billion watts, but a receiver about as sensitive as the Green Bank Telescope still could not hear it from 10 light-years away.
Waste heat: in 1960 Freeman Dyson pointed out that a civilization enclosing its star must radiate the energy it has used as infrared. The second law of thermodynamics decides it: doing work produces heat, and it cannot be hidden.

To shout or not

In October 2017, "Sonar Calling GJ 273b" sent a piece of music and a message to Luyten's Star, 12.36 light-years away, due to arrive in 2030. Scientists are still arguing about whether to send messages to the stars deliberately.

How frightening a projectile is

Kinetic energy: a 1-ton object moving at 90% of the speed of light has kinetic energy of about 1.2×10²⁰ joules, equal to about 28 billion tons of TNT.
Warning: it arrives almost together with its own light. Fired from 27 light-years away, it gives the main civilization only about 3 years of warning.
Interstellar gas: at 90% of the speed of light, every hydrogen atom it hits head-on carries about 1.2 billion electron volts of energy, so thin interstellar gas itself becomes a beam of radiation.

Whale song

Humpback whale songs spread between populations. A 2011 study found that new songs started on Australia's east coast and reached French Polynesia in about two years, moving almost only eastward. The Sweepers' subtitle in the game is "Singing while working".

The idea in the game

What follows is game lore. The channel appears only after the main civilization has exposed itself: broadcasting for a long time, firing a kinetic weapon, actively sending a message, or building a Dyson sphere. In the scenario, a machine 27 light-years away points toward the Sun. It is not a civilization's home, only a picket, guarding a patch of sky and waiting for some place to leak a little light it should not.

The mechanic is certainty. Every beam of light the main civilization leaks takes 27 hours to reach it (27 years in the real universe). When light arrives, its certainty about your coordinates rises, and when you go quiet, the certainty slowly falls. At 100% certainty it fires a projectile at 90% of the speed of light, which arrives after 30 hours, and the main civilization sees the launch flash only 3 hours ahead. If certainty falls all the way back to 0, it crosses you off its list. The channel has four endings: Crossed out, Swept away, Coordinates and The wall. All of this is fiction.

Death's End (Liu Cixin, 2010; The Three-Body Problem III): gravitational waves were directly detected in 2015, and broadcasting with them would need stellar-scale masses. The dimensional strike is fiction.
The Dark Forest (Liu Cixin, 2008; The Three-Body Problem II): the Fermi paradox is a real scientific question. The "dark forest" is one literary explanation and can be neither confirmed nor refuted.

The studies below concern gravitational waves, interstellar communication and the Fermi question. They are related real science, not evidence that a Sweeper exists.

Two black holes merged 1.3 billion years ago, and humans finally heard the ripples in spacetime (Abbott et al., LIGO and Virgo, Physical Review Letters, 2016).
A proposal to use radio telescopes to listen for alien signals near the 21 cm frequency of hydrogen, which started SETI (Cocconi and Morrison, Nature, 1959).
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

Parallax: Look at the same star from opposite ends of Earth's orbit and its position shifts a little. This is parallax, and it gives the distance. The Gaia satellite measured the positions of nearly 2 billion stars and ended observations in 2025.
Leakage: A 1978 study concluded that the things on Earth easiest to hear from afar are TV transmitters and ballistic-missile early-warning radars. A 2023 simulation estimated that cell towers today leak a peak of about 4 billion watts into space, but a receiver as sensitive as the Green Bank Telescope within 10 light-years still couldn't pick it up.
Waste heat: In 1960 Dyson pointed out that a civilization wrapping a star must radiate the used energy as infrared. The second law of thermodynamics decides it: doing work gives off heat, and it can't be hidden.
Song: Humpback whale songs spread between populations: a 2011 study found new songs travelling from Australia's east coast to French Polynesia in about two years, almost always eastward.
Rules: In 1983 Brin discussed this systematically in The Great Silence: if civilizations that destroy others exist in the universe, the safest strategy may be to keep quiet. The “dark forest” is a literary version of this by Liu Cixin in his novel, not a scientific conclusion.
To shout or not: In October 2017, “Sonar Calling GJ 273b” sent music and messages to Luyten's Star, 12.36 light-years away, due to arrive in 2030. Scientists still argue over whether to send signals to the stars on purpose.
Projectile: A 1-tonne object at 90% of light speed carries about 1.2×10²⁰ J, roughly 28 billion tonnes of TNT. It arrives almost together with its own light: launched from 27 light-years away, it gives the main civilization only about 3 years of warning.
Interstellar gas: At 90% of light speed, every hydrogen atom hit head-on carries about 1.2 billion electron-volts, so even thin interstellar gas becomes a beam of radiation.
Death’s End (Liu Cixin, 2010), the science: gravitational waves were directly detected in 2015; broadcasting with them would take the mass of an astronomical body. Dimensional reduction attacks are fiction.
The Dark Forest (Liu Cixin, 2008), the science: the Fermi paradox is a real scientific question; the “dark forest” is one literary explanation, and can be neither confirmed nor refuted.

Related research

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

Gravitational waves: Two black holes merged 1.3 billion years ago, and ripples in spacetime were finally heard by humans, confirming Einstein's prediction from a century earlier.
Source: Abbott et al. (LIGO & Virgo), Physical Review Letters 2016, "Observation of gravitational waves from a binary black hole merger"
Searching for interstellar signals: Two physicists suggested listening for alien signals with radio telescopes near hydrogen's 21 cm frequency, launching SETI.
Source: Cocconi & Morrison, Nature 1959, "Searching for interstellar communications"
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 “Picket”: It is not a civilization’s home, only a picket: one machine, one keeper, guarding a patch of sky.
Node “Parallax”: It measures distance the same way, only far more precisely.
Node “Song”: It sings whenever it works, a song older than its own civilization.
Node “Rules”: It has only one rule: once certain, sweep.
Node “Projectile”: In the game it is about 3 hours.
Node “Patience”: It is in no hurry. If it isn’t certain enough, it just keeps waiting; once the other side goes quiet on its own, it crosses the line out.
Node “List”: It keeps a list, and every line was once a leak of light. Once you can read the list, you can see how certain it really is about the main civilization.
Node “The oldest song”: The song’s author was swept away long ago. It is still singing.
Node “It is watched too”: It knows very well that the flash of its own launch can be seen by others too.
Node “The last line”: The last line on the list is its own coordinates.
Channel ending “Crossed out”: Its certainty fell back to zero bit by bit. The machine crosses the line off the list, turns to the next patch of sky, and keeps singing. The main civilization never knew how close it came to being swept.
Channel ending “Swept away”: The projectile arrived. Those who had dispersed survived and started over on other planets. From then on they knew one thing: someone in the universe is listening.
Channel ending “Coordinates”: The projectile arrived. But its coordinates had already spread outward at the speed of light. 27 light-years away, the machine keeps singing, knowing that one day it will be its turn.
Channel ending “The wall”: The projectile was deflected far from the Sun and vanished into the dark. No one saw what stopped it. The Keepers had used their one and only intervention.
Summary of the novel Death’s End: The story runs from the age of deterrence to the end of the universe: gravitational-wave broadcasts, light-speed ships, dimensional reduction attacks, and a small universe with only two people in it.
Summary of the novel The Dark Forest: Facing an enemy that is bound to arrive, humanity stakes its hopes on four “Wallfacers”. One of them derives a cold cosmic sociology: every civilization is an armed hunter.

See it in the game