String Theory: AdS/CFT Correspondence
Maldacena’s AdS/CFT correspondence brought the holographic principle to center stage in string theory. Though presented in 1993, even Leonard Susskind says he thought it would be decades before there would be any way to confirm the holographic principle.
Then, in 1997, Argentinean physicist Juan Maldacena published a paper, inspired by the holographic principle, that proposed something called the anti-de Sitter/conformal field theory correspondence, or AdS/CFT correspondence.
He proposed a new duality between a gauge theory defined on a 4-dimensional boundary (three space dimensions and one time dimension) and a 5-dimensional region (four space dimensions and one time dimension). In essence, he showed that there are circumstances in which the holographic principle scenario 2 is possible.
As usual in string theory, one of those conditions is unbroken supersymmetry. In fact, the theoretical world he studied had the most amount of supersymmetry possible — it was maximally supersymmetrical.
Another condition was that the 5-dimensional region was something called an anti-de Sitter space, which means it had negative curvature. Our universe (at least at present) is more similar to a de Sitter space. As such, it hasn’t yet been proved that the AdS/CFT correspondence (or something similar) specifically applies to our own universe (though thousands of papers have been written on the subject).
Even if the duality turns out not to be completely true, a growing body of theoretical work supports the idea that there is some sort of correspondence between string theory and gauge theory, even if only at some low levels of approximation.
Calculations that are hard in one version of the theory may actually be easy in the other one, meaning that it may be crucial in figuring out how to complete the theory. This has helped support the idea that the holographic principle may ultimately prove to be one of the fundamental principles of M-theory.
The holographic principle, and specifically the AdS/CFT correspondence, may also help scientists further understand the nature of black holes. The entropy (or disorder) of a black hole is proportional to the surface area of the black hole, not its volume. This is one of the arguments in support of the holographic principle, because it’s believed that it would offer further physical explanation of black holes.