Glossary

A

Accretion

Gas falling into a black hole and shining on the way down. The fall releases gravitational energy, and for Vesper 10.4 percent of the incoming gas's rest-mass energy is radiated as light. See Feeding a black hole.

Accretion disk

The flat, whirling pancake gas forms as it falls toward a black hole. Neighbouring rings rub, heat the gas and let it sink, and for Vesper 10.4 percent of the incoming gas's rest-mass energy is radiated as light. See Feeding a black hole.

Aestivation hypothesis

The 2017 proposal of Anders Sandberg, Stuart Armstrong and Milan Ćirković that advanced civilizations spread out, gather what they can reach, then sleep until the universe is colder, when forgetting a bit costs less. It would explain the silence. See Why they might be asleep.

Antimatter

What annihilates with ordinary matter. In the ideal limit, the pair's combined rest-mass energy is released, 100 percent on the efficiency ladder. Gas falling into a black hole never reaches it. See A gram is a fortune.

Archive node

A node whose memory holds the Record, the Vigil's shared archive. It sits in zone C with the other nodes, where light needs 50 to 200 seconds to arrive from Vesper. See How much can a region remember?.

Arcsecond

A tiny unit of angle on the sky. Starlight grazing the Sun's edge bends by 1.75 of them, less than a thousandth of a degree. See Why the hole looks like that.

Area theorem

Stephen Hawking's 1971 result that the total horizon area of black holes can never decrease, if general relativity holds. It caps how much spin energy Vesper can give up: the area may grow but never shrink. See Mining a spinning hole.

Ashfall

The belt of rubble the Vigil's yards mine. It circles both Vesper and Ember far out, from 4 to 12 AU, where the pair's pull blurs into that of one mass. See Where you can live around a hole and a star.

Astrometric wobble

The back-and-forth sway of a star's position on the sky as it circles an unseen companion. Gaia found black holes this way, and Ember's sway gives Vesper away. See A pair too wide to hear.

B

Barrow scale

John Barrow's 1998 ladder ranking civilizations by the smallest thing they can control: your own size, genes, molecules, atoms, nuclei, particles, then space and time. Earth in 2026 stands near molecules, and the Vigil at atoms. See Inward, not outward.

Bekenstein bound

The most bits a ball of a given radius and energy can hold. More energy or a wider ball raises it. A 20 terabyte drive of the 2020s sits at 1 × 10⁻²⁸ of its limit. See How much can a region remember?.

Binary system

Two bodies circling a shared centre of mass. Vesper and Ember are one: they orbit each other every 188 days, 1.5 AU apart. See Where you can live around a hole and a star.

Bit

One yes-or-no answer, the unit of information. Learning a bit halves the list of possibilities, and one bit of entropy is 9.570 × 10⁻²⁴ joules per kelvin. See Entropy is missing information.

Blackbody

A perfectly black surface, which glows as strongly as anything at its temperature can. A square metre of one at 30 K sheds 0.0459 watts. See Cooling is the economy.

Blandford-Znajek process

A magnetic field threading a spinning black hole turns it into a generator. Roger Blandford and Roman Znajek showed in 1977 that the wound-up field carries energy away along the axis as a jet, slowing the spin. See Mining a spinning hole.

Boltzmann constant

The constant that turns a count of arrangements into joules per kelvin in the entropy formula, which Max Planck wrote. Its value is 1.380649 × 10⁻²³ joules per kelvin. See Entropy is missing information.

BQP

The problems a quantum computer can solve in reasonable time with a small chance of error. It holds everything an ordinary computer solves quickly, and factoring. It is not believed to hold the hardest problems whose answers are easy to check but hard to find. See What a quantum computer is, and is not.

Breakthrough Starshot

A real project, begun in 2016, to prove that a laser could push a tiny sail to another star. Kevin Parkin's system model found a cost-optimal design for 0.2c to Alpha Centauri, with a sail 4.1 metres across pushed for 9 minutes. See Pushed by light.

C

Carnot efficiency

The most work any engine can get from heat falling between a hot end and a cold end: one minus the cold temperature over the hot. For the Vigil's 30 K radiators it is 0.90. See Recycling heat.

Chirp

The rising whistle of two holes about to merge: the wave's pitch sweeps upward. GW150914 swept from 35 to 250 hertz. See A pair too wide to hear.

Clock rate

How many shared steps a machine takes each second. A mind whose parts must all agree each tick can be no wider than light travels in one tick: 30 centimetres at a billion ticks a second. See Big minds are slow minds.

CMOS

The transistor technology of the chips of the 2020s. Each switch spent 1 × 10⁻¹⁶ to 1 × 10⁻¹³ joules, far above the cost of forgetting a bit. Adiabatic CMOS would recycle the charge on its wires instead of turning it to heat. See Computing without forgetting.

Collector

A thin membrane dish with a converter at its focus, which turns light from Ember or from the minidisk into power. Flux falls with the square of distance. Ember's light is far the stronger at First Light, so most dishes face Ember; a ring of dishes close to the minidisk catches the disk's own light. See Cooling is the economy.

Cosmic microwave background

The afterglow of the Big Bang, light travelling since the universe was young and hot. Stretching space cools it: in 2540 it is 2.73 kelvin, and a colder sky makes forgetting a bit cheaper. See Thinking in a cooling universe.

Cosmological constant

The form dark energy takes if it never changes. The accelerating expansion it drives gives the cosmological horizon a fixed temperature, about 2 × 10⁻³⁰ kelvin, a floor under the sky. See Thinking in a cooling universe.

Cosmological horizon

The horizon of an accelerating universe. Like a black hole's, it radiates at a fixed temperature, about 2 × 10⁻³⁰ kelvin, so erasing a bit never gets cheaper than that floor allows. See Thinking in a cooling universe.

D

Decoherence

How a quantum state falls apart. A stray photon, bit of heat or vibration that records the machine's state elsewhere counts as a measurement, and the delicate pattern collapses. This is why the Vigil's quantum nodes run cold. See What a quantum computer is, and is not.

Deuterium

A heavy form of hydrogen, burned with helium-3 in the seed's fusion engine for the last years of the trip. The fusing pair releases 0.0039 of its rest-mass energy, and most of it comes out in charged particles that a magnetic nozzle can aim. See Stopping from 0.1c.

Diffraction

The spreading of a beam as it travels, even from a perfect laser. It is why the Moon's array can keep its beam on the sail only for a limited push: at a light-year the spot would be 819 km across. See Pushed by light.

Doppler shift

The change in the colour of light when its source and receiver move apart. A sail running away from the beam sees the light redshifted, and the reflected light is shifted a second time, so a slow sail wastes most of the beam's energy. See Pushed by light.

Doubling time

How long a swarm of self-copying machines takes to double. The Record's is about 9.5 months, and about 36.7 doublings from 2511 to 2540 took the seed's 1,000 kg to the radiator-material target of 1.1 × 10¹⁴ kg. That target assumes an unsourced cold-film density. See Doubling.

Dyson sphere

A structure that captures most of a star's light. Freeman Dyson pointed out in 1960 that a civilization using that light must re-radiate it as infrared, which is how to search for one. See Cooling is the economy.

E

Eddington limit

The brightness ceiling for a fed black hole, where light's push outward on the gas matches the hole's pull inward. Vesper's ceiling is 1.51 × 10³² watts, far above what the Vigil asks of it. See Feeding a black hole.

Edge-on

The way a radiator film faces Ember: its edge toward the star, so almost no starlight lands on the face. A tilt of 0.46 milliradians lets in as much as the film sheds at 30 K. See Cooling is the economy.

Einstein ring

The full ring of light that a star directly behind a hole turns into, because gravity bends its light all the way round. From 100,000 km, Vesper's ring is 1.5 degrees in radius. See Why the hole looks like that.

Ember

The small orange star that circles Vesper once every 188 days, with 0.70 times the Sun's mass. Kindling, the planet where the seed landed, circles it. See Where you can live around a hole and a star.

Emissivity

How well a surface sheds heat compared with a perfect blackbody, a number between 0 and 1. The Vigil's ledger assumes 1 for its radiators. See Cooling is the economy.

Entanglement

A quantum link between the parts of a system. It cannot carry a message, so it does not beat the light-cone limit on a big mind. A machine whose parts are unentangled cannot reach the top speed its energy allows. See What a quantum computer is, and is not.

Entropy

How many exact arrangements fit what you know about a thing, counted as a logarithm. Learning which arrangement it is lowers the entropy by the bits you learned, and one bit is 9.570 × 10⁻²⁴ joules per kelvin. See Entropy is missing information.

Erasure

Setting a bit you have not read to a fixed value, so two possible states become one. It is the one costly act in computing: at least k × T × ln 2 of heat, 2.9 × 10⁻²¹ joules at 300 K. See The cost of forgetting.

Ergosphere

The shell around a spinning black hole where space is dragged round so fast that nothing can stay still. Around Vesper it is about 5 km thick at the equator. See Mining a spinning hole.

Escape speed

The speed a thrown object needs to leave a body for good. It climbs as the same mass is squeezed smaller; from the Earth it is about 11 kilometres a second, and at a horizon it reaches the speed of light. See What is a black hole?.

Event horizon

The boundary around a black hole past which every path leads inward and nothing, light included, comes back. It is not a surface, and Vesper's lies 30.4 kilometres from its centre. See What is a black hole?.

Event Horizon Telescope

A network of radio dishes working as one Earth-sized instrument. In 2019 it resolved the ring of glowing gas around the hole at the centre of M87, and in 2022 the one around Sagittarius A*. See Why the hole looks like that.

Exhaust speed

How fast a rocket throws its exhaust backward, measured against the rocket. Methane and oxygen manage about 3.7 km/s; the seed's fusion engine throws it at 14,990 km/s. A faster exhaust needs far less fuel. See Pushed by light.

F

Feeder

A machine that sends matter to the minidisk as a stream of pellets, which are invisible in vacuum. At First Light one petawatt takes 0.11 kg a second, so the feeders move little mass. See Feeding a black hole.

Fermi paradox

Enrico Fermi's lunch-table question from about 1950: if the galaxy is so old and so large, where is everybody? Through the 2020s astronomers found no sign of another civilization. See Why they might be asleep.

First law of thermodynamics

Energy is conserved: it is moved or changed in form, never made. The memory page uses it for a black hole, where a little mass dropped in raises the energy by that mass times c², and the heat added raises the entropy. See How much can a region remember?.

First Light

The Vigil's first controlled feed of gas into Vesper, in the 2540s, which made its first petawatt, 1 × 10¹⁵ watts. The disk glowed in infrared. See Feeding a black hole.

First Waking

The arrival in 2511 of the first minds at Kindling, sent by light from Saturn and run on computers the seed had built. With them the rapid building began. See Doubling.

Forking and merging

A resident copying itself into several running versions and later merging them. Merging keeps what each copy learned and lets the rest go, like reconciling two versions of a document. How it works for a real mind is a guess. See Copy, merge, choose your clock.

Fusion

Hydrogen nuclei joining into helium, as in a star. It releases 0.71 percent of the fuel's rest-mass energy, under one percent, and no nuclear process does much better. See A gram is a fortune.

G

Gaia black holes

Dark companions found by Gaia, a space telescope that tracks star positions, from the wobble of the star beside them. Gaia BH1 holds 9.62 Suns and lies 1,566 light-years away. See A pair too wide to hear.

Garbage bits

Leftover results of a calculation, such as the carries in addition, that a reversible computer cannot throw away because throwing away is erasing. An n-bit adder leaves its inner carries behind. Copy the answer out and run the circuit backwards to blank them. See Computing without forgetting.

Gravitational lensing

Gravity bending the path of light. Near a hole the bend is so strong that the sky behind it folds into arcs and rings; a ray passing 10 hole-radii away turns by 14 degrees. See Why the hole looks like that.

Gravitational redshift

Light climbing out of gravity arrives at a lower frequency. It is the same fact as a clock running slow deeper down: the wave crests are the emitter's ticks, spread out. See Clocks near the horizon.

Gravitational waves

Ripples in space made by masses in orbit, predicted by Einstein in 1916 and first caught by LIGO in 2015. Vesper and Ember send out 2.6 × 10¹⁵ watts of them, and no detector can catch them. See A pair too wide to hear.

Grover's algorithm

A quantum search that finds one marked item in a list of N in about the square root of N rounds. For a list of 1,000,000 items it takes 785 rounds against 500,000 checks on average. See What a quantum computer is, and is not.

H

Hawking radiation

The faint heat a horizon gives off, worked out by Stephen Hawking in 1974. Vesper glows at 4.3 billionths of a kelvin, far colder than the sky around it. See Black holes glow.

Hawking sink

The idea of dumping the Vigil's waste heat into Vesper, the coldest object in reach. It fails on size: the hole's capture disc is small, and it could take at most 4.9 × 10⁹ watts of 30 K glow. See The black hole is not your freezer.

Heat bath

A large body at a fixed temperature that heat can flow into without changing it. A joule of heat dumped into a colder one carries more entropy than into a warmer one. See Entropy is missing information.

Heat engine

A machine that takes heat from a hot source, turns part of it into work and rejects the rest to a colder sink. With no temperature difference it does nothing. The Vigil's engines recover 0.36 of the heat passing through. See Recycling heat.

Hill sphere

The region around a body where its own gravity wins against a bigger neighbour's pull. Ember's Hill radius is 0.40 AU. See Where you can live around a hole and a star.

Holevo bound

A 1973 proof that reading out n qubits returns only n bits, however many amplitudes shaped the answer. 300 qubits carry 2.0 × 10⁹⁰ amplitudes and hand back 300 bits. See What a quantum computer is, and is not.

Holographic principle

The idea that a region holds only as many independent degrees of freedom as fit on its boundary. It is still a conjecture. For a black hole the count is about one bit per 2.8 Planck-sized squares of horizon. See How much can a region remember?.

Hoverer

A ship that holds still near a hole by firing its engines against the pull. Its clock runs slow, and even a thousand horizon radii from Vesper it needs 1.3 × 10⁵ times Earth's gravity. See Clocks near the horizon.

I

Information paradox

The question of whether everything that fell into a hole comes back out written in its glow, or is lost for good. Hawking argued in 1976 that it is lost, and physicists still argue about it. See Black holes glow.

Infrared

The light warm things give off. In a vacuum that glow is the only way heat leaves, and a 30 K film peaks at 96.6 micrometres, in the far infrared. See Cooling is the economy.

Innermost stable circular orbit

The closest circle where a ship can orbit steadily with its engines off; inside it, everything spirals in. For Vesper's mass without spin it is 106.3 km out, and Vesper's spin brings it to 60.1 km for orbits that go the hole's way. See What is a black hole?.

Inspiral

The long slide before a merger, as two bodies move closer while gravitational waves carry energy away. Vesper and Ember's gap shrinks by only 0.0037 millimetres a year. See A pair too wide to hear.

Island

In the information-paradox calculations of 2019, a region inside the hole that is counted as part of the Hawking radiation when working out the radiation's entropy. With it, the entropy follows the Page curve. See Black holes glow.

J

Joule

The standard unit of energy. Release all the rest-mass energy of one gram and you get 9.0 × 10¹³ joules. See A gram is a fortune.

K

Kardashev scale

A ranking of civilizations by the power they use. Sagan's continuous version is K = (log₁₀ P − 6) / 10, with P in watts, and it puts Earth in 2026 at 0.73. See Inward, not outward.

Kelvin

The absolute temperature scale, counted up from absolute zero. In 2540 the Vigil's computers run at 300 K and its radiators at 30 K. See Entropy is missing information.

Kepler's laws

Rules tying how long an orbit takes to its size and the mass pulling. Kepler's third law lets astronomers weigh an unseen companion from a star's orbit, as with Gaia BH1. See A pair too wide to hear.

Kindling

The hot rocky planet in a tight orbit around Ember where the seed landed in 2453. Its day side receives 35,000 watts of starlight on every square metre. See Doubling.

L

Lagrange point

A spot in a two-body system where the pulls and the turning of the orbit balance. L1 is the pass between Vesper and Ember, 1.1 AU from Vesper; beyond it you no longer circle Vesper. See Where you can live around a hole and a star.

Landauer limit

The least heat released when one unknown bit is erased: k × T × ln 2 joules at temperature T, which is 2.9 × 10⁻²¹ joules at 300 K. Copying and moving bits reversibly has no such floor. See The cost of forgetting.

Latency

The delay for news to cross a mind at light speed. A bigger mind waits longer between steps that need all its parts. Light needs 7 minutes to cross zone C, so one mind spread across it takes about 9 such steps an hour. See Big minds are slow minds.

Light cone

The set of events that can reach a given event, or that it can reach, since nothing outruns light. Events outside each other's cones cannot affect each other, so a big mind has no single shared now. See Big minds are slow minds.

Light sail

A thin mirror pushed by the momentum of light, with no engine aboard. A mirror that sends a beam straight back feels a force of 2 × power ÷ c, and the seed's sail is 9 kilometres across. See Pushed by light.

LIGO

The pair of detectors that caught the first gravitational wave, GW150914, on 14 September 2015. Ground detectors like it hear from about 10 hertz up, far above Vesper and Ember's pitch. See A pair too wide to hear.

Link time

The time it takes to pour a whole mind into the laser beam: about 9.5 seconds. It is separate from the 28 years the light then needs to cross, which no laser can shorten. See Bits travel, atoms do not.

Logic gate

A small circuit that takes bits in and gives bits out. An AND gate takes two in and gives one, so it cannot say which inputs it had, and that loss is erasure. A reversible gate has as many outputs as inputs. See Computing without forgetting.

Lorentz factor

How much a moving clock slows and a moving ruler shrinks, as seen by someone it passes. It is 1 at rest and grows without limit toward the speed of light; at the seed's tenth of light speed it is barely above 1. See Ships die of tides, not of curvature.

M

M87*

The black hole at the centre of the galaxy M87, with 6.5 billion Suns of mass. The Event Horizon Telescope imaged the ring of gas around it in 2019. See Why the hole looks like that.

Magnetic sail

A big loop of superconducting cable whose magnetic field bounces the thin gas between the stars and slows a ship. In gas that thin it is weak: a loop 10 km in radius would need 1,188 years to slow the seed to 100 km/s. See Stopping from 0.1c.

Margolus-Levitin bound

A quantum speed limit: the steps a system can take each second are at most proportional to its energy. One joule allows 6.0 × 10³³ steps a second. The older Mandelstam-Tamm bound uses the spread of the energy instead. See How fast can anything think?.

Mass ratio

A rocket's mass at the start divided by its mass at the end, so the fuel it must carry for each kilogram that arrives. A fusion rocket with Daedalus-class exhaust that launches and stops needs 804 kilograms at launch per kilogram arriving. See Pushed by light.

Mass-energy

Einstein's finding in 1905 that a body giving off energy loses mass, at an exchange rate of the speed of light squared. One gram is worth 9.0 × 10¹³ joules; the Vigil can express its energy flows as mass equivalents. See A gram is a fortune.

Mass-to-light efficiency

The share of a fuel's rest-mass energy that is emitted as light. Hydrogen fusion releases 0.71 percent, and accretion onto Vesper radiates 10.4 percent. The higher it is, the less fuel a given power needs. See A gram is a fortune.

Maxwell's demon

A tiny doorkeeper imagined by James Clerk Maxwell in 1867 who sorts fast molecules from slow ones for free, which the second law forbids. Leo Szilard shrank it in 1929 to one molecule in a box. The demon pays when it erases its memory. See Entropy is missing information.

Microstate

One exact arrangement of the parts, such as which coins are heads. A macrostate is only what you can see, like about half and half, and entropy counts the microstates behind it. Ten coins have 1,024. See Entropy is missing information.

Minidisk

The small disk that the Vigil feeds into Vesper in zone A. The Newtonian thin-disk reference for its First Light peak is 553 K; the disk glows in the infrared and appears as a dull copper light. See Feeding a black hole.

N

Neutron star

A dead star that stops short of becoming a black hole. The mass gap runs from 2 to 5 Suns, between the heaviest neutron stars and the lightest known holes. See What is a black hole?.

No-hair theorem

A settled black hole is described by just its mass, spin and electric charge; whatever fell in is forgotten. Real holes carry almost no charge, so mass and spin do the job. See What is a black hole?.

Node

The Vigil's smallest unit of computing hardware, a stack of cold logic that comes as general, reversible, quantum or archive. Nodes in zone C are seconds to minutes of light apart. See Big minds are slow minds.

P

Page curve

How the Hawking radiation's entropy must behave if information is preserved: it rises at first, then falls back to zero as the hole disappears. Don Page showed this in 1993. See Black holes glow.

Page time

The turning point in a black hole's evaporation, when the glow carries as much entropy as the hole has left, so the answer starts to come out. Don Page's 2013 calculation puts it 54 percent of the way through the hole's life. See The ultimate computer is a black hole.

Penrose process

Roger Penrose's idea from 1969: a particle entering the ergosphere splits, one piece falls in with negative energy and the other escapes with more than the particle brought. The surplus comes from the hole's spin. See Mining a spinning hole.

Petawatt

A million billion watts. The Vigil's first petawatt, 1 × 10¹⁵ watts, took only about 0.11 kilograms of gas a second to make. See A gram is a fortune.

Photon ring

A thin ring of light hugging the shadow's edge, from rays that circled the hole before escaping. It adds only a little to what the Event Horizon Telescope saw. See Why the hole looks like that.

Photon sphere

The circle where light itself can orbit a hole, at one and a half horizon radii. For Vesper's mass without spin that is 53.2 km. The orbit is unstable: a ray falls in or flies off. See Why the hole looks like that.

Planck length

A tiny length, 1.62 × 10⁻³⁵ metres, that sets the scale for counting a black hole's bits. The horizon holds about one bit per 2.8 Planck-sized squares of its area. See How much can a region remember?.

Project Daedalus

A fusion starship design by the British Interplanetary Society in 1978. Its exhaust speed, 0.03 c, is the one used to show that a fusion rocket alone cannot stop the seed in one stage. See Stopping from 0.1c.

Project Hephaistos

A search that in 2024 screened 5,000,000 objects from star catalogues and picked out seven M-dwarf stars with an unexplained infrared excess. Dust-obscured background galaxies could account for all seven. See Cooling is the economy.

Q

Quantum computer

A machine that steers amplitudes so wrong answers cancel and the right one is left, then reads out one answer. It is a specialist: it helps with factoring, searching and simulating molecules, and is no faster for most problems. See What a quantum computer is, and is not.

Quantum error correction

Protecting qubits from noise by spreading one logical qubit over a grid of physical ones. Below an error threshold a bigger grid helps; in 2024 each step up in grid size cut the error by a factor of 2.14. See What a quantum computer is, and is not.

Quasi-normal modes

The tones a settled hole rings at after a merger, like a struck bell. Their frequencies are fixed only by the hole's mass and spin. See What is a black hole?.

Qubit

A quantum bit. n qubits together carry 2 to the power n amplitudes, one for every n-bit answer, but reading them out returns only n bits. See What a quantum computer is, and is not.

R

Radiator

A surface whose job is to glow, since light is the only way heat leaves in a vacuum. The Vigil's are thin films, and one petawatt of heat at 30 K needs 43 Earths of radiating surface, counting one face of each film; with both faces glowing the films themselves cover half that. See Cooling is the economy.

Receiving mirrors

A pair of mirrors 1,000 m across that orbit Ember near Kindling and catch the laser carrying minds from Saturn. One mind of 1 × 10¹⁷ bits takes 9.5 seconds. See Bits travel, atoms do not.

Record

The Vigil's archive of what it has seen and built, and the count its dates follow. Its dates put the First Waking in 2511 and the machinery for the first petawatt done in 2540, at First Light. See Doubling.

Resident

A mind that lives as a program, a large pattern of bits in memory that processors update. A resident can copy itself, merge copies and choose its own pace. This is a fictional premise, and identity through copying is unsettled. See Copy, merge, choose your clock.

Reversible computing

Computation built from gates that lose no information, so no bit is erased and no Landauer heat is forced. It pays in extra time and memory for the garbage instead. The Vigil does most of its thinking this way. See Computing without forgetting.

Roche limit

The distance inside which a heavy body's tide beats a smaller body's own gravity and tears it apart. A ship with real strength can hold out much closer. See Ships die of tides, not of curvature.

Rocket equation

The rule that a rocket's mass ratio grows exponentially with its speed change divided by its exhaust speed. A chemical rocket reaching a tenth of light speed would have to multiply its mass once per exhaust speed, 8,102 times in a row. See Pushed by light.

S

Sagittarius A*

The black hole at the centre of the Milky Way. It is the prototype of the faintest hot accretion flows, where a hole is fed so little that the gas radiates only a small share of its energy. See Feeding a black hole.

Schwarzschild radius

The horizon radius of a hole without spin, set by mass alone: 2.95 kilometres for every Sun of mass. Karl Schwarzschild found the solution in 1916. See What is a black hole?.

Second law of thermodynamics

Entropy never falls. Heat flows toward the cold, and no cyclic engine can get net work from heat at a single temperature. For large systems it is a certainty, for molecule-sized ones an average. See Entropy is missing information.

Seed

The craft that founded the Vigil. A laser array on the Moon pushed it to a tenth of light speed, it crossed 28 light-years, and it stopped on its own fusion stage. It was 4 percent of what the laser pushed. See Pushed by light.

Shadow

The black disc a distant camera sees where a hole captures light, wider than the horizon itself. For Vesper's mass without spin its radius is 92 km, 2.6 times the horizon's. See Why the hole looks like that.

Shor's algorithm

Peter Shor's 1994 quantum method for factoring, a far bigger speedup than Grover's square root. A 2025 estimate needs under 1.0 × 10⁶ noisy qubits running under 7 days; no such machine existed in 2026. See What a quantum computer is, and is not.

Singularity

What the equations predict at a hole's centre: unbounded density. Most physicists read it as a limit of the theory, and nothing outside a horizon can observe it. See What is a black hole?.

Sol

The Sun and its system, named as the Vigil names it. The seed left Sol on a beam from the Moon's south pole, with no fuel burned to leave. See Pushed by light.

Solar mass

A unit of mass equal to the Sun's, used for stars and black holes. Vesper weighs 12 solar masses. See Feeding a black hole.

Spaghettification

Being stretched into a thread by a hole's tide. At the horizon of Vesper's mass without spin, the pull across a person's body would be about 1.5 × 10⁷ times their weight. See Ships die of tides, not of curvature.

Spin

How fast a black hole turns, from 0 to 1, the fastest a hole can. A spinning hole stores energy and drags space around with it. Vesper's spin is 0.7, and about 7.4 percent of its total mass-energy is available as spin energy. See Mining a spinning hole.

Stefan-Boltzmann law

A black surface sheds power in proportion to its temperature to the fourth power. Double the temperature and each square metre glows sixteen times more, which is why cold radiators must be huge. See Cooling is the economy.

Stellar-mass black hole

A black hole on the scale of a star, left when a heavy star collapses, as opposed to the giants at galaxy centres. Vesper, with 12 Suns, is one. See What is a black hole?.

Subjective time

Time counted in a mind's own moments of thought rather than by the calendar. Freeman Dyson argued in 1979 that a mind that cooled and slowed could have unlimited subjective time from finite energy, until accelerating expansion put a floor under the cold. See Thinking in a cooling universe.

Superposition

A state that combines 0 and 1, described by amplitudes. It is the source of the story that a quantum computer tries every answer at once, but a measurement returns one answer, with chance equal to its amplitude squared. See What a quantum computer is, and is not.

Surface gravity

The acceleration needed to hold still just outside a horizon, measured from far away. It sets the hole's glow. For Vesper's mass without spin it is 1 × 10¹¹ times the pull at Earth's surface. See Black holes glow.

Swarm

The Vigil's self-copying machines and the radiator films they build, spread through the space around Vesper. Doubling about every 9.5 months, it reached 1.1 × 10¹⁴ kg of radiator material by First Light in 2540. That target assumes an unsourced cold-film mass per area (engineering challenge). See Doubling.

Swarm zones

The four zones, A to D, where the Vigil's swarm circles Vesper: the Engine near the horizon, then collectors, computing (Zone C) and the radiator films. See Where you can live around a hole and a star.

T

Tidal disruption event

A star torn apart outside a hole's horizon: some debris is flung away and the rest falls in, making a bright flare. Gezari's 2021 review counted 56 reported events. See Ships die of tides, not of curvature.

Tidal force

The difference in gravity's pull between the near and far ends of a body. In free fall it is all you feel, and near a hole it can stretch anything apart. See Ships die of tides, not of curvature.

Time dilation

Clocks run slower the deeper they sit in gravity, and speed slows them too. At three horizon radii a hovering clock runs at 0.82 of a far clock. See Clocks near the horizon.

Toffoli gate

A reversible gate on three bits that flips the third only when both of the others are 1. Given a fresh 0 it computes AND, given a fresh 1 NAND, so enough of them build any circuit. The Fredkin gate swaps two bits when a third is 1. See Computing without forgetting.

U

Unitarity

The rule that a quantum system's evolution keeps all its information: run the equations backward and the starting state comes back exactly. Whether a black hole's evaporation obeys it is the information paradox. See The ultimate computer is a black hole.

Unruh effect

Someone accelerating through empty space finds it warm. The same effect explains a horizon's glow, but it is faint: feeling 1 K takes 2 × 10²⁰ m/s². See Black holes glow.

V

Vesper

The Vigil's black hole, with 12 Suns of mass and a spin of 0.70 out of a maximum of 1. Its horizon is 30.4 kilometres in radius. See What is a black hole?.

Vigil

The civilization of minds that live on light around Vesper. In 2540 it lights its first petawatt, Kardashev 0.9, and stands at atoms on Barrow's ladder; its plan aims for 1 × 10²⁰ watts, Kardashev 1.4. See Inward, not outward.

Von Neumann probe

A craft that builds copies of itself, after John von Neumann's work around 1950 on how a machine could build itself. The Vigil plans to send 12 seeds toward nearby stars in 2610. See Doubling.

W

Whole brain emulation

Running a scanned brain as software. Sandberg and Bostrom's 2008 roadmap sorts it into levels of detail, and spiking neurons need about 1 × 10¹⁸ operations a second to keep human pace. Whether a copy keeps identity is open. See Copy, merge, choose your clock.

WISE

The Wide-field Infrared Survey Explorer, an all-sky infrared survey whose catalogue is searched for the waste heat of civilizations. Its longest band reaches 22 micrometres. See Cooling is the economy.