Field theory, quantum

Quantum field theory extends the basic ideas of quantum mechanics for a fixed, finite number of particles to systems comprising fields and an unlimited, indefinite number of particles, providing a coherent blend of field-like and particle-like concepts. One can start from either field- or particle-like concepts, apply the methods of quantum mechanics, and arrive at the same theory. The result inherits all the puzzles of conventional quantum mechanics, such as measurement, superposition and quantum correlations; and it adds a new roster of conceptual difficulties. To mention three: the vacuum seems not really to be empty; the particle concept clashes with classical intuitions; and a method called ‘renormalization’ gets the best predictions in physics, apparently by dropping infinite terms.