string theory still faces the daunting issues of the landscape and hierarchy problems. The more recent work of the 90’s is of greater pertinence for heavy ion collisions. It is during this time that string theory ideas were applied to black holes. 

## 2.3.2 AdS/CFT Background 

## Holography 

The most important idea to emerge from attempting to consistently treat quantum mechanics and black holes is that of holography (for a review of holography, and especially its connection with AdS/CFT, see [318]). A holo- gram in the vernacular is a 2D object that projects a 3D image of the recorded subject; in this way the full information of the 3D subject is encoded on a 2D surface. Holography in string theory has the same connotation: given some volume the physics of the bulk is determined by the physics of the boundary. A standard example of this principle is the derivation of the maximum entropy of a volume of space, V . Imagine V filled with mass and just enough energy that, should the system collapse into a black hole, its event horizon would be exactly the boundary of V , ∂V . We know that the black hole entropy asso- ciated with this horizon is A/ 4 G , where A is the surface area of ∂V . By the second law of thermodynamics this entropy must be greater than or equal to the entropy of the system before the collapse; therefore it gives a maximum entropy bound for V . (There is some subtlety skimmed over in this argument. 

