Editing Point particle (section)

==In quantum mechanics==
[[Image:Quark_structure_proton.svg|thumb|200px|right|A proton is a combination of two [[up quark]]s and one [[down quark]], held together by [[gluon]]s.]]
In [[quantum mechanics]], there is a distinction between an [[elementary particle]] (also called "point particle") and a [[composite particle]]. An elementary particle, such as an [[electron]], [[quark]], or [[photon]], is a particle with no known internal structure. Whereas a composite particle, such as a [[proton]] or [[neutron]], has an internal structure.
However, neither elementary nor composite particles are spatially localized, because of the [[Uncertainty principle|Heisenberg uncertainty principle]]. The particle [[wavepacket]] always occupies a nonzero volume. For example, see [[atomic orbital]]: The electron is an elementary particle, but its quantum states form three-dimensional patterns.

Nevertheless, there is good reason that an elementary particle is often called a point particle. Even if an elementary particle has a delocalized wavepacket, the wavepacket can be represented as a [[quantum superposition]] of [[quantum state]]s wherein the particle is exactly localized. Moreover, the ''interactions'' of the particle can be represented as a superposition of interactions of individual states which are localized.  This is not true for a composite particle, which can never be represented as a superposition of exactly-localized quantum states. It is in this sense that physicists can discuss the intrinsic "size" of a particle: The size of its internal structure, not the size of its wavepacket. The "size" of an elementary particle, in this sense, is exactly zero.

For example, for the electron, experimental evidence shows that the size of an electron is less than {{val|e=-18|u=m}}.<ref>{{cite web | url=https://cerncourier.com/a/precision-pins-down-the-electrons-magnetism | title=Precision pins down the electron's magnetism | date=4 October 2006 }}</ref> This is consistent with the expected value of exactly zero. (This should not be confused with the [[classical electron radius]], which, despite the name, is unrelated to the actual size of an electron.)