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A modern and more general definition uses the Lagrangian (or equivalently the Hamiltonian ) of a system. Usually, (or ) of a system describing an interaction can be separated into a kinetic part and an interaction part : (or ).In field theory, always contains 3 fields terms or more, expressing for example that an initial electron (field 1) interacts with a photon (field 2) producing the final state of the electron (field 3). In contrast,

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