= Quantum field theory
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Quantum field theory describes <quantum states> and interactions using fields whose smeared values act as operators. Free-field modes behave as <quantum harmonic oscillators> or fermionic occupation modes; their excitations supply the particle description. This allows <creation operators> and <annihilation operators> to describe processes in which particle number changes.
A relativistic quantum field theory combines this framework with <special relativity>, <Lorentz invariance> and locality conditions such as <microcausality>. Nonrelativistic many-body systems also have useful quantum field theories, for example a <weakly interacting Bose gas> described by a bosonic field. The framework is therefore broader than a list of elementary-particle models.
<Correlation functions> predict responses and connect through appropriate limits to <scattering amplitudes>. <Perturbative quantum field theory> expands these quantities using <Feynman diagrams>, while strongly interacting problems may require other methods. Composite fields and arbitrarily short distances require attention to operator definitions and <renormalization>, rather than treating every field product as an ordinary pointwise multiplication.
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