= Particle
{disambiguate=physics}
= Particle
{synonym}
A <particle> is an entity described as a localized constituent in an appropriate physical model. In <classical mechanics>, a <point particle> has a position and trajectory while its internal size is neglected. This approximation can describe an extended object when the omitted structure does not affect the question being studied.
In <quantum mechanics>, a <quantum particle> has a state described by a <wave function> or a more general state description, with probabilistic measurement outcomes. <Quantum field theory> describes particle states as excitations of fields. A <proton> can be treated as a particle in one experiment while its <quark> and <gluon> structure matters in another. The particle description therefore specifies what is being resolved, rather than asserting that every particle is a structureless classical point.
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