The Holographic Principle

A hologram is a laser-generated photograph that appears three-dimensional. It contains all the information about a 3-D object in a flat, 2-D surface. If a hologram is cut in half, each half still shows the entire image of the 3-D subject. Divide the halves again and the image remains intact.

The holographic principle is proposed, in 1993, by Gerard 't Hooft and Leonard Susskind, claiming that all of the information contained in a volume of space can be represented by a theory that lives in the boundary of that region. They based their observation on the entropy of a black hole.

In 1970s, Bekenstein and Hawking showed that the entropy of a black hole is proportional to the area of its event horizon (roughly its surface area.) Therefore, the maximal limit of entropy for any ordinary region of space is proportional to the surface area , not its volume. Their result showed that the black hole entropy equals the number of Planck squares that can fit on its surface. We may speculate that perhaps the true fundamental degrees of freedom may reside on the region�s surface. This leads to the consideration that the universe we are in may be described as a hologram, whose property is isomorphic to the information represented on its boundaries.

The Holographic Principle states that all of the information contained in some region of space can be represented as a hologram on the boundary of that region. What we experience within the �volume� of the universe (the bulk) may be determined by what can be represented on the bounding surface. One can choose a description in which the fundamental laws operate in the bulk or a description in which the fundamental physics takes place on the boundary of the universe. Each viewpoint will be equally valid.

For further reading, SeeBrian Greene's "The Fabric of the Cosmos," Random House, 2004
Lee Smolin, "Quantum Gravity," Basic Book, New York, 2001