Holographic principle

The holographic principle proposes that the information describing a region of space can be represented by degrees of freedom on its boundary. In quantum gravity, it suggests that a lower-dimensional description can encode the physics of a higher-dimensional volume.

Bekenstein–Hawking entropy: The entropy of a black hole, proportional to the area of its event horizon in Planck units. Its area scaling inspired the proposal that gravitational information is stored on boundaries.
Jacob Bekenstein: The physicist who proposed that black holes have entropy proportional to horizon area. His black-hole entropy bound supplied an early clue that information scales with area.
Event horizon: A boundary beyond which events cannot influence a distant observer. A black hole’s horizon is the surface whose area measures its entropy.
Local quantum field theory: A framework describing quantum fields whose interactions are local in spacetime. Holography can encode a gravitational bulk using a boundary theory with different dimensionality.
Quantum gravity in de Sitter space: The unresolved problem of formulating a complete quantum theory of gravity in positively curved spacetime. Our expanding universe is closer to de Sitter than anti-de Sitter, where holography is less understood.
AdS/CFT correspondence: A duality equating gravity in anti-de Sitter spacetime with a conformal field theory on its boundary. It provides a concrete example of a boundary theory encoding a gravitational bulk.
Stephen Hawking: The physicist who derived black-hole radiation from quantum effects near an event horizon. His calculation fixed the black-hole entropy formula that underlies holographic arguments.
Planck length: The length scale formed from the gravitational constant, the speed of light, and Planck’s constant. Black-hole entropy counts area in units set by this quantum-gravity scale.
De Sitter space: A spacetime with constant positive curvature, often used to model cosmic expansion. Unlike anti-de Sitter space, it lacks the same well-understood timelike boundary for holographic duality.
Black hole firewall: A proposed high-energy horizon that could arise if black-hole information constraints cannot all be reconciled. Firewall arguments test whether holographic information recovery is compatible with a smooth horizon.

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