The

**holographic principle** is a property of

quantum gravity and

string theories that states that the description of a volume of

space can be thought of as encoded on a

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^{[1]} who combined his ideas with previous ones of 't Hooft and

Charles Thorn.

^{[1]}^{[2]} As pointed out by

Raphael Bousso,

^{[3]} Thorn observed in 1978 that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way.

In a larger 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 only an effective description at

macroscopic scales and at

low energies. Cosmological holography has not been made mathematically precise, partly because the

cosmological horizon has a finite area and grows with time.

^{[4]}^{[5]}
The holographic principle was inspired by

black hole thermodynamics, which implies that the maximal

entropy in any region scales with the radius

*squared*, and not cubed as might be expected. In the case of a

black hole, the insight was that the informational content of all the objects that have fallen into the hole can be entirely contained in surface fluctuations of the event horizon. The holographic principle resolves the

black hole information paradox within the framework of string theory.

^{[6]}