Holographic Principle
Introduction
The Holographic Principle is a principle of string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region—preferably a light-like boundary like a gravitational horizon. First proposed by Gerard 't Hooft, it was given a precise string-theory interpretation by Leonard Susskind.
Theoretical Background
The holographic principle resolves the black hole information paradox within the framework of string theory. However, there exist classical solutions to the Einstein equations that allow values of the entropy larger than those allowed by an area law, hence in conflict with the covariant entropy bound. The resolution of this conflict is the holographic principle.
Principle
The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information inscribed on the surface of its boundary.
Black Hole Entropy
In a larger and more speculative sense, the theory suggests that the entire universe can be seen as a two-dimensional information structure "painted" on the cosmological horizon, such that the three dimensions we observe are an effective description only at macroscopic scales and at low energies.
Holography and Quantum Gravity
The holographic principle is a property of quantum gravity and string theories which states that the description of a volume of space can be thought of as encoded on a boundary to the region.
Holography and String Theory
In string theory, the holographic principle is an integral part of the AdS/CFT correspondence, an equivalence between string theory on Anti-de Sitter (AdS) space and a conformal field theory defined on the boundary of the AdS space.
Criticisms and Controversies
Despite the success of the holographic principle in string theory, it remains a highly speculative idea in more general settings. It is not yet clear how the principle would be implemented in a full theory of quantum gravity.