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{{Quantum book backlink|Quantum field theory}}
'''Quantum fields and particles''' are the central concepts of quantum field theory, in which the basic ingredients of nature are continuous fields defined over space-time and particles are understood as quantized excitations of those fields.<ref name="peskin">Peskin, M. E.; Schroeder, D. V. ''An Introduction to Quantum Field Theory'' (1995).</ref> This framework unifies quantum mechanics with special relativity and replaces the older picture of particles as isolated point-like objects moving independently.
[[File:Quantum_fields_particles_excitation_yellow.jpg|thumb|400px|Particles as excitations of quantum fields: localized quanta appear as ripples on underlying continuous fields.]]

== Fields as fundamental entities ==
In quantum field theory, the primary physical objects are fields such as scalar fields <math>\phi(x)</math>, spinor fields <math>\psi(x)</math>, and gauge fields <math>A_\mu(x)</math>.<ref name="weinberg">Weinberg, S. ''The Quantum Theory of Fields'' (1995).</ref> Each point in space-time is associated with field degrees of freedom, and the field extends continuously throughout the universe.

In this picture, what is usually called a “particle” is not a separate classical object but a discrete excitation of a field. For example, the electromagnetic field gives rise to photons, while the electron field gives rise to electrons and positrons.<ref name="schwartz">Schwartz, M. D. ''Quantum Field Theory and the Standard Model'' (2014).</ref>

== Quantization of fields ==
The transition from classical field theory to quantum field theory occurs through quantization, where the field variables are promoted to operators acting on a Hilbert space.<ref name="peskin"/>

For a bosonic field, the canonical commutation relation is
<math>
[\phi(x), \pi(y)] = i \delta^{(3)}(x - y)
</math>

where <math>\pi(y)</math> is the conjugate momentum field. For fermionic fields, anticommutation relations are used instead:
<math>
\{\psi_\alpha(x), \psi^\dagger_\beta(y)\} = \delta_{\alpha\beta}\delta^{(3)}(x-y)
</math>

These algebraic relations ensure the correct quantum statistics: bosons obey Bose–Einstein statistics, while fermions obey Fermi–Dirac statistics.<ref name="schwartz"/>

== Particles as field excitations ==
A quantized field can be decomposed into normal modes, each of which behaves like a quantum harmonic oscillator.<ref name="weinberg"/> The excitation quanta of these oscillators are interpreted as particles.

For example, a scalar field may be expanded as
<math>
\phi(x) = \int \frac{d^3p}{(2\pi)^3} \frac{1}{\sqrt{2E_{\mathbf{p}}}}
\left(a_{\mathbf{p}} e^{-ipx} + a^\dagger_{\mathbf{p}} e^{ipx}\right)
</math>

where <math>a^\dagger_{\mathbf{p}}</math> and <math>a_{\mathbf{p}}</math> are creation and annihilation operators. The operator
<math>
a^\dagger_{\mathbf{p}} |0\rangle
</math>
creates a one-particle state of momentum <math>\mathbf{p}</math> from the vacuum state <math>|0\rangle</math>.<ref name="peskin"/>

Thus the particle concept emerges naturally from the quantized field formalism.

== Fields, states, and Fock space ==
The full quantum system is described in a Fock space, which contains states with varying particle number.<ref name="schwartz"/> Multi-particle states are constructed by repeated action of creation operators on the vacuum:
<math>
a^\dagger_{\mathbf{p}_1} a^\dagger_{\mathbf{p}_2} |0\rangle
</math>

This formalism is especially important in relativistic physics because particle number need not be conserved: interactions may create or destroy particles as long as conservation laws such as energy, momentum, and charge are respected.

== Interactions between fields ==
Interactions are encoded in the Lagrangian density <math>\mathcal{L}</math>, which determines how fields evolve and couple to one another.<ref name="peskin"/>

The action is
<math>
S = \int d^4x \, \mathcal{L}
</math>

and the field equations follow from the principle of stationary action. A simple interacting field theory may include self-interaction terms or couplings between matter and gauge fields. In quantum electrodynamics, for example, the electron field couples to the electromagnetic field through the interaction structure built into the covariant derivative.<ref name="weinberg"/>

These couplings allow field excitations to scatter, annihilate, and create new excitations, which is how particle interactions are described in modern high-energy physics.

== Vacuum and fluctuations ==
In quantum field theory, the vacuum is not empty in a classical sense. It is the lowest-energy state of the field system, but it still contains quantum fluctuations.<ref name="schwartz"/> Even when no real particles are present, the fields remain dynamical and contribute to observable effects such as vacuum polarization and the Casimir effect.

This makes the vacuum itself a physical object with structure, rather than mere emptiness.

== Propagation and correlation functions ==
A powerful way to describe particles and fields is through correlation functions such as
<math>
\langle 0 | T\{\phi(x)\phi(y)\} | 0 \rangle
</math>

which measure how field excitations propagate between space-time points.<ref name="peskin"/> These quantities are directly related to propagators and scattering amplitudes and form the computational backbone of perturbative quantum field theory.

== Conceptual importance ==
The field-particle picture resolves several conceptual issues that arise when combining quantum theory with relativity. A purely particle-based description becomes inadequate because relativistic processes can create and destroy particles. Quantum field theory solves this by making fields fundamental and particles emergent.<ref name="weinberg"/>

This viewpoint underlies the Standard Model of particle physics and modern descriptions of electromagnetic, weak, and strong interactions.

=See also=
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

== References ==
{{reflist|3}}
{{Author|Harold Foppele}}

{{Sourceattribution|Quantum field theory (QFT) core|1}}

Physics:Quantum Fields and Particles - Revision history

imported>WikiHarold: Repair Quantum Collection B backlink template

imported>WikiHarold: Repair Quantum Collection B backlink template