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Glossary of black holes and quantum gravity
The vocabulary of black holes and quantum gravity, defined in a few lines. Each term has its own anchor, so you can link straight to a definition.
Updated 8 October 2026
- Event horizon
- The boundary of a black hole beyond which nothing, not even light, can escape. It is not a material surface: a freely falling observer crosses it without noticing anything locally.
- Singularity
- A region where general relativity predicts infinite curvature and stops applying. Penrose (1965) showed that, under general assumptions, gravitational collapse inevitably leads to one. A quantum theory of gravity should replace it.
- Photon sphere
- A region at 1.5 times the horizon radius (for a non-rotating black hole) where light can orbit. It outlines the black hole "shadow" observed by the Event Horizon Telescope.
- Accretion disk
- A disk of gas spiralling into the black hole. Heated to millions of degrees by friction, it emits X-rays and makes visible an object that emits nothing itself.
- Tidal forces
- The difference in gravitational pull between two points of the same body. Near a stellar-mass black hole, it stretches objects lengthwise: "spaghettification".
- Hawking radiation
- Thermal radiation emitted by a black hole because of quantum mechanics (Hawking, 1974). Its temperature is inversely proportional to the mass: a stellar black hole is colder than the cosmic microwave background.
- Bekenstein-Hawking entropy
- S = A/4 in Planck units: a black hole's entropy is proportional to its horizon area, not its volume. It is the first hint of the holographic principle.
- Information paradox
- If a black hole evaporates into purely thermal radiation, the information about what fell in seems destroyed, which quantum mechanics forbids. This conflict has driven research since 1976.
- Page curve
- The expected evolution of the entropy of Hawking radiation if information is preserved: it rises, then falls after the "Page time", once the black hole has lost about half its entropy. 2019 calculations using "islands" reproduced it.
- Firewall
- A proposal (Almheiri, Marolf, Polchinski, Sully, 2012) that preserving information would require a wall of energy at the horizon, contradicting the equivalence principle. ER = EPR is one of the proposed answers.
- Entanglement
- Quantum correlation between two systems so strong they can no longer be described independently, even at a distance.
- Decoherence
- Loss of quantum superposition through interaction with the environment: quantum information becomes classical.
- AdS/CFT
- Holographic correspondence: a gravitational theory in Anti-de Sitter space is equivalent to a non-gravitational quantum theory on its boundary.
- ER = EPR
- Conjecture linking Einstein-Rosen wormholes (ER) to EPR entangled pairs: entanglement would be a geometric shortcut.
- Holographic principle
- Information in a volume of space can be encoded on its boundary, like a 2D hologram encoding a 3D scene.
- De Sitter / Anti-de Sitter
- Two cosmological curvatures: de Sitter (positive, our accelerating universe) and Anti-de Sitter (negative, where holography is best understood).
Keep exploring
- Black holes →How black holes form, which types exist, from Gaia BH1 to M87*, and how we photographed and heard them.
- Hawking radiation →The 1974 discovery that gave black holes a temperature, Hawking's formula and the evaporation time, explained simply.
- Information paradox →Do black holes destroy information? Hawking's paradox, an interactive Page curve and the 2019 "islands".
- Sources →The foundational papers cited on this site with their DOI and arXiv links, image credits and how facts are checked.