In 1995 Leonard Susskind a Professor of Theoretical Physics at Stanford University published an unusual paper. He suggested that the entire universe might be a hologram in which people are just seeing a projection of the real thing. His theory is supported by a Dutch Professor and Nobel prizewinner called Gerard t’Hooft who works at the university of Utrecht. Although their theory sounds arcane the professors have deep intellectual reasons for their view which may turn out to be one of the most profound insights to be gained in modern physics for decades.

To appreciate their view it is worthwhile considering the field in which they work. These scientists are among the elite who are searching for a connection between the laws of quantum mechanics and the fundamental laws of gravitation. The laws of quantum mechanics explain the world on very small scales of space and time, inside the atom for instance. The laws of gravitation describe the universe over very large scales of space and time. Physicists think that there should be an underlying theory that connects the two worlds and are working hard to find the connection.

Scientists hope to make the connection in one of two ways. The first is to work from the rules of quantum mechanics and gradually build up a picture that would embrace gravity. The other is to work from the rules of gravitation and see how the laws need to be changed to avoid contradictions on a very small scale. Many quantum theorists believe that the universe can be described as a multidimensional space or “brane”. Just as the vibration on a string describes the music of a violin these scientists believe that the rules of physics are described by vibrations on the “brane”. At the other scale scientists are working on the physics of black holes where the rules of gravity are obliged to operate over very small length scales. Leonard Susskind and Gerard t’ Hooft make the observation that if the world is a hologram the mathematics of black hole theory and that of the “brane” are similar. Their work is a very encouraging step towards the unification of quantum theory and gravitation.

Leonard Susskind and Gerard t’Hooft call their theory a holographic principle in direct analogy with the holograms that can be produced in experimental optics. An optical hologram records the details of light scattered by an object on a two dimensional plate in such a way that the three dimensional image can latter be constructed. Using the analogy Leonard Susskind and Gerard t’Hooft believe that the observed universe may have one (or more) dimension less than the true universe

The great advance has come from considering the thermodynamics of black holes. According to the theory of general relativity an object falling into a black hole should descend to a singularity which is a place where it collapses to a single point. This rather strangely implies that all the information about the object and the motions of the particles within it is lost. A black hole appears to break a basic principle of the universe in which the total disorder of the universe can only increase. A very careful analysis suggests that the black hole does absorb all the information that has been supplied to it. A black hole can only absorb a finite amount of information known which is proportional to its surface area. This is known as the Berkenstein bound. The black hole is obliged to radiate any additional information that is received in excess of this limit. When Leonard Susskind and Gerard t’Hooft conducted a detailed analysis of the emitted particles they found that the mathematics was identical to that used in “brane” theory leading to the conclusion that the black hole could be described as a complicated vibration in brane theory.

The holographic principle states that the entropy (or information) of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its boundary.

One corallarty of the holographic principle is that matter can not be broken up into an infinite number of constinstutuent particles. According to the Bekenstein bound which is through to apply to all particles a volume of space can only hold a finite amount of information. Partiles can only contain internal particles up to a limit imposed by this information bound.

Although the holographic principle sounds strange it is providing theoretical physicists with genuine insight and marks a major advance in theoretical physics.

Reference

Susskind, L. (1995). "The world as a hologram". Journal of Mathematical Physics 36 (11): 6377–6371. 1995JMP....36.6377S. 10.1063/1.531249