AestivatorsWhy a civilization might wait for a colder universe

The minimum energy needed to erase one bit of information is proportional to temperature: the colder, the cheaper. In 2017 Anders Sandberg, Stuart Armstrong and Milan Ćirković used this to propose the "aestivation hypothesis". A civilization that saves its energy for the very cold far future might do about 10³⁰ times more computation, so we see no aliens because they are asleep. The "Aestivators" channel in the game turns this hypothesis into a story.

This page first covers the physics behind the hypothesis, then the argument against it, then outlines the game's scenario and lists related real research. The hypothesis is disputed. It is not an accepted conclusion.

Temperature and computation

• 2.7 K: the cosmic microwave background is about 2.725 K today. In a future dominated by dark energy, the scale of the universe roughly doubles every 12 billion years, and the background temperature halves with it.
• Landauer's principle: in 1961 Rolf Landauer showed that erasing 1 bit must release at least kT·ln2 of heat, so a lower temperature is cheaper. A 2012 experiment trapped a small glass bead with a laser and measured this limit for the first time.
• Boomerang Nebula: the coldest known natural place is the Boomerang Nebula, at about 1 K, colder than the cosmic background. Gas blown out by a dying star expanded so fast that it cooled itself.
• Temperature floor: the universe will not keep cooling forever. An accelerating universe has a horizon, and the horizon itself has a temperature of about 10⁻³⁰ K, as Gibbons and Hawking calculated in 1977.

Why "sleep"

"Aestivation" originally means summer dormancy. African lungfish burrow into mud in the dry season, wrap themselves in mucus and drop their metabolism very low. They can sleep like this for several years until the rains return. The hypothesis borrows the word.

To wait for a colder universe you first have to hold on to your resources. When a star fuses hydrogen into helium, only about 0.7% of the mass becomes energy, and most of it scatters into space as starlight. That is the motive for hoarding. A black hole of one solar mass has a Hawking temperature of only about 6×10⁻⁸ K, far colder than today's cosmic background. Until the background falls below that, it will not evaporate. It slowly absorbs background radiation instead, so it could serve as a warehouse.

Sleeping does not mean computing stops entirely. As long as no information is erased, computation can in principle use almost no energy. This is called reversible computing, and Charles Bennett proved it in 1973.

The rebuttal: no need to wait

In 2019 Charles Bennett, Robin Hanson and Cameron Riedel argued back. The universe is full of "cold reservoirs" far from equilibrium into which entropy can be dumped right now, with no need to wait for the universe to cool. Waiting only pays off after a civilization has used up all the matter.

The idea in the game

What follows is game lore. The channel is found by asking "where are they?" in the Black Cloud channel, or by unlocking "Degenerate era" in the Tree of the Beyond. In the scenario, the Observer finds an unusually cold, unusually orderly structure at the edge of the Local Group. A great deal of matter has been gathered together. It barely glows and gives off no heat. It is asleep.

The mechanic is waiting for the universe to cool. This channel runs on a cosmic clock: every 2 hours of real time, the background temperature halves (in the real universe, about 12 billion years). While asleep, they only store energy. When you wake them, the stored energy is converted into "bits" at the current temperature, and they keep thinking at that temperature while awake. The later you wake them, the more you get. To use a colder universe, you let them keep sleeping. The scenario also has a night watch: a few watchers stay awake and look after the universe for the others. The channel has two endings: they wake at the coldest moment, or some of the aestivators wake early and think together with the main civilization. All of this is fiction.

Two related novels

• The Last Question (Isaac Asimov, 1956): the second law of thermodynamics has no known exception, and the heat death of the universe is one of the mainstream predictions. In the story, people over generations ask an ever more powerful computer whether the increase of entropy can be reversed. Each time it answers "insufficient data for a meaningful answer", until the last moment of the universe.
• The Black Cloud (Fred Hoyle, 1957): Hoyle was a real astrophysicist. Interstellar clouds really do contain complex organic molecules, and several have been found in Sagittarius B2.

Related real research

The studies below concern cosmic expansion, background temperature, black hole evaporation and the far future. They are related real science, not evidence that civilizations are asleep.

• Distant supernovae revealed that the expansion of the universe is accelerating, and dark energy entered physics. The work won the 2011 Nobel Prize (Riess et al., The Astronomical Journal, 1998).
• A second team independently reached the same conclusion (Perlmutter et al., The Astrophysical Journal, 1999).
• Noise that could not be removed from an antenna turned out to be the afterglow of the Big Bang, which won the 1978 Nobel Prize (Penzias and Wilson, The Astrophysical Journal, 1965).
• Quantum effects make black holes radiate energy slowly, so black holes will eventually evaporate (Hawking, Nature, 1974).
• A calculation of the fate of the universe over trillions of years and beyond: stars go out, protons may decay, black holes finally evaporate (Adams and Laughlin, Reviews of Modern Physics, 1997).
• Ordinary matter makes up about 5% of the total energy of the universe, dark matter about 27%, and the rest is dark energy (Planck Collaboration, Astronomy & Astrophysics, 2020).