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In Depth of Nuclear Engineering

In quantum mechanics, an energy level is said to be degenerate if it corresponds to two or more different measurable states of a quantum system. Conversely, two or more different states of a quantum mechanical system are said to be degenerate if they give the same value of energy upon measurement. The exchange interaction is a quantum mechanical effect between identical particles. (Actually, one should better speak only of the exchange energy, or the exchange term, to avoid the incorrect idea that this effect corresponds to a classical force or potential.) In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interacts and is the first theory where full agreement between quantum mechanics and special relativity is achieved.