superselected

Superselection is a phenomenon in physics, particularly in the context of quantum mechanics and axiomatic quantum field theory. It occurs when a physical system can exist in multiple states, but transitions between these states are forbidden, effectively making some states "unavailable".

This concept was first introduced in the 1930s by John Wheeler and in 1966 by Daniel Compton, with the term "superselection" being coined by Robert J. Capilla and John A. Wheeler in 1969. Initially used in the context of spin and statistics, it has since been applied to broader areas, including quantum field theory and cosmology.

In superselection, the system's properties are categorized into different sectors or classes. Transitions between these sectors are rigorously forbidden, preventing the system from accessing certain states. This restriction arises from a combination of quantum mechanical principles, such as the properties of certain operators and the adherence to certain mathematical structures.

As a result of superselection, the accessible states of the system are reduced, effectively creating a new, constrained Hilbert space. This phenomenon is closely related to related concepts, including quantization and projection operator techniques. Researchers believe that superselections may be connected to certain phenomena in particle physics, possibly relating to mass generation or the nature of elementary particles.