Quantum theory is a way of explaining the nature of matter, and how it behaves on the atomic, and even the subatomic, level. Closely related is quantum mechanics, which is a description of matter at the atomic and subatomic levels. The theory postulates that all matter is made up of individual, and quantifiable units. It was developed in 1900 by the German physicist Max Planck. He presented his theory to the German Physical Society. While much of the theory continues to stir great debate, and some controversy, there are parts that are nearly universally accepted based on the available evidence.
Quantum theory has been used in popular fiction to explain many different things. Popular television shows such as "Sliders" and "Quantum Leap" have generated mass interest. It even plays a role in Dan Brown's novel "Angels and Demons" during a portion explaining the existence and significance of antimatter. In most cases, this, and other works of fiction, look at one of two major theories related to the reality of the natural world in order to build their premises. These theories are respectively called the Copenhagen Interpretation, and the Many Worlds Theory.
The Many Worlds Theory postulates that once an object exists in any state, the universe around that object transforms into a number of parallel universes. The number of parallel universes is determined by the number of possible universes, in which it is possible for that object to exist. This number is impossible to know, at least as it stands right now with modern scientific instruments. At some point, instruments may be developed that can more explicitly show quantum theory principles.
The Copenhagen Interpretation of quantum theory suggests that a particle cannot be assumed to exist until it is measured. Then, once that takes place, it is whatever it is measured to be. While this seems like a straightforward suggestion, this interpretation of quantum theory indicates that a particle is actually in all possible states until it is measured and specifically observed.
This is often illustrated by using an example known as Schrodinger's cat. If a cat, in a hypothetical example, is taken and thrown into a box, it is safe to assume it is alive, at least for a certain period of time. If a vial of cyanide is thrown into that box, it may have broken and killed the cat. Or it may have been undamaged and spared the cat. As long as this is unknown, the Copenhagen Interpretation of quantum theory would postulate the cat is in both states, meaning alive and dead, at the same time.
One of the biggest critics of quantum theory is Albert Einstein. Einstein could not agree that a single particle could exist in more than one universe, as suggested in the Many Worlds Theory. The physicist, however, did accept some aspects of the theory. For example, he understood that energy could exist in quantifiable units, and suggested this was true not only of energy, but also of radiation.