Editing Elementary charge (section)

== Quantization ==
{{See also|Partial charge}}
''Charge quantization'' is the principle that the charge of any object is an [[integer]] multiple of the elementary charge. Thus, an object's charge can be exactly 0&nbsp;''e'', or exactly 1&nbsp;''e'', −1&nbsp;''e'', 2&nbsp;''e'', etc., but not  {{sfrac|1|2}}&nbsp;''e'', or −3.8&nbsp;''e'', etc. (There may be exceptions to this statement, depending on how "object" is defined; see below.)

This is the reason for the terminology "elementary charge": it is meant to imply that it is an indivisible unit of charge.

=== Fractional elementary charge ===
There are two known sorts of exceptions to the indivisibility of the elementary charge: [[quark]]s and [[quasiparticle]]s.
* [[Quark]]s, first posited in the 1960s, have quantized charge, but the charge is quantized into multiples of {{nowrap|{{sfrac|1|3}}&thinsp;''e''}}. However, quarks cannot be isolated; they exist only in groupings, and stable groupings of quarks (such as a [[proton]], which consists of three quarks) all have charges that are integer multiples of ''e''. For this reason, either 1&nbsp;''e'' or {{nowrap|{{sfrac|1|3}} ''e''}} can be justifiably considered to be "the [[quantum]] of charge", depending on the context. This charge commensurability, "charge quantization", has partially motivated [[Grand Unified Theory#Motivation|grand unified theories]].
* [[Quasiparticle]]s are not particles as such, but rather an [[emergence|emergent]] entity in a complex material system that behaves like a particle. In 1982 [[Robert B. Laughlin|Robert Laughlin]] explained the [[fractional quantum Hall effect]] by postulating the existence of fractionally charged [[quasiparticle]]s. This theory is now widely accepted, but this is not considered to be a violation of the principle of charge quantization, since quasiparticles are not [[elementary particles]].

=== Quantum of charge ===
All known [[elementary particle]]s, including quarks, have charges that are integer multiples of {{sfrac|1|3}}&nbsp;''e''. Therefore, the "[[quantum]] of charge" is {{sfrac|1|3}}&nbsp;''e''. In this case, one says that the "elementary charge" is three times as large as the "quantum of charge".

On the other hand, all ''isolatable'' particles have charges that are integer multiples of ''e''. (Quarks cannot be isolated: they exist only in collective states like protons that have total charges that are integer multiples of ''e''.) Therefore, the "quantum of charge" is ''e'', with the proviso that quarks are not to be included. In this case, "elementary charge" would be synonymous with the "quantum of charge".

In fact, both terminologies are used.<ref>''Q is for Quantum'', by John R. Gribbin, Mary Gribbin, Jonathan Gribbin, page 296, [https://books.google.com/books?id=zBsDkgI1uQsC&pg=RA1-PA296 Web link]</ref> For this reason, phrases like "the quantum of charge" or "the indivisible unit of charge" can be ambiguous unless further specification is given. On the other hand, the term "elementary charge" is unambiguous: it refers to a quantity of charge equal to that of a proton.

=== Lack of fractional charges ===
[[Paul Dirac]] argued in 1931 that if [[magnetic monopole]]s exist, then electric charge must be quantized; however, it is unknown whether magnetic monopoles actually exist.<ref>{{cite journal|doi=10.1146/annurev.ns.34.120184.002333|doi-access=free|title=Magnetic Monopoles|year=1984|last1=Preskill|first1=J.|journal=[[Annual Review of Nuclear and Particle Science]]|volume=34|issue=1|pages=461–530|bibcode=1984ARNPS..34..461P}}</ref><ref>{{cite news |title=Three Surprising Facts About the Physics of Magnets |url=https://www.space.com/42685-physics-of-magnets-surprising-facts.html |access-date=17 July 2019 |work=Space.com |date=2018 |language=en}}</ref> It is currently unknown why isolatable particles are restricted to integer charges; much of the [[string theory landscape]] appears to admit fractional charges.<ref>{{cite journal |last1=Schellekens |first1=A. N. |title=Life at the interface of particle physics and string theory |journal=Reviews of Modern Physics |date=2 October 2013 |volume=85 |issue=4 |pages=1491–1540 |doi=10.1103/RevModPhys.85.1491|arxiv=1306.5083 |bibcode=2013RvMP...85.1491S |s2cid=118418446 }}</ref><ref>{{cite journal |last1=Perl |first1=Martin L. |last2=Lee |first2=Eric R. |last3=Loomba |first3=Dinesh |title=Searches for Fractionally Charged Particles |journal=[[Annual Review of Nuclear and Particle Science]] |date=November 2009 |volume=59 |issue=1 |pages=47–65 |doi=10.1146/annurev-nucl-121908-122035| doi-access=free|bibcode=2009ARNPS..59...47P }}</ref>

{{see also|Anomaly (physics)#Anomaly cancellation}}