Holography

Holograms are two-dimensional images that convey the illusion of three dimensions. Holography, more broadly, deals with systems that possess one fewer dimension of freedom than they appear to. What does this have to do with black holes? Well, we said that the entropy of a black hole is related to its surface area rather than to its volume. So in this case we appear to be missing one dimension, as if all the information inside the black hole is secretly encoded on its surface (or event horizon)–just like a hologram. See Fig. 77 . In this way, a three-dimensional problem has suddenly been reduced to a two-dimensional problem. And this line of thinking has become an important source of inspiration to physicists trying to relate gravity in one setting to a physical system of one lower dimension. The principle at play here, known as holography, lies at the heart of some of the most exciting work going on in theoretical physics today. And it is fair to say that much of the impetus behind this work came from thinking about black holes–the impossible objects, which first emerged from calculations in 1915, that now seem more important than could ever have been imagined.