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Today, many physicists use the expressions "particle" and "field" more or less synonymously. This is an obvious consequence of the transformations of the particle concept discussed in the last chapter. Particle physics started from the classical particle concept and arrived at a group theoretical concept which deals with the conserved dynamic quantities of fields. The quantum particles discussed in Sects. 6.2 and 6.4 are neither particles nor fields in a classical sense. They have wavelike and particle-like features. Quantum mechanical systems are described in terms of plane waves and more or less sharply localized wave packets. Quantized fields may be either in a sharp number state or in a state of well-defined phase but not in both states at once. For photons, the coherent states with minimum unsharpness of occupation number and phase are closest to the description of a classical field.

DOI: 10.1007/978-3-540-33732-4_7

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