Editing Electron (section)

=== Fundamental properties ===
The [[invariant mass]] of an electron is approximately {{physconst|me|round=3|after=,}} or {{physconst|me_Da|round=3|after=.|ref=no}} Due to [[mass–energy equivalence]], this corresponds to a rest energy of {{physconst|mec2|round=2|ref=no}} ({{physconst|mec2_MeV|round=3|ref=no}}). The ratio between the mass of a [[proton]] and that of an electron is about 1836.<ref name=nist_codata_mu>
{{cite web
 | title = CODATA value: proton-electron mass ratio
 | url = https://physics.nist.gov/cgi-bin/cuu/Value?mpsme
 | work = 2006 CODATA recommended values
 | publisher = [[National Institute of Standards and Technology]]
 | access-date = 2009-07-18
 | df = dmy-all
 | archive-date = 2019-03-28
 | archive-url = https://web.archive.org/web/20190328001314/https://physics.nist.gov/cgi-bin/cuu/Value?mpsme
 | url-status = live
}}</ref><ref name=Zombeck2007>
{{cite book
 | last = Zombeck
 | first = M.V.
 | year = 2007
 | title = Handbook of Space Astronomy and Astrophysics
 | url = https://books.google.com/books?id=tp_G85jm6IAC&pg=PA14
 | edition = 3rd
 | page = 14
 | publisher = Cambridge University Press
 | isbn = 978-0-521-78242-5
 | access-date = 2020-08-25
 | archive-date = 2022-02-04
 | archive-url = https://web.archive.org/web/20220204082414/https://books.google.com/books?id=tp_G85jm6IAC&pg=PA14
 | url-status = live
}}</ref> Astronomical measurements show that the [[proton-to-electron mass ratio]] has held the same value, as is predicted by the Standard Model, for at least half the [[age of the universe]].<ref>
{{cite journal
 | last = Murphy | first = M.T.
 | year = 2008
 | title = Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe
 | journal = [[Science (journal)|Science]]
 | volume = 320 | issue = 5883 | pages = 1611–1613
 | doi = 10.1126/science.1156352
 | pmid = 18566280
 |bibcode = 2008Sci...320.1611M |arxiv = 0806.3081 | s2cid = 2384708
 |display-authors=etal
}}</ref>

Electrons have an [[electric charge]] of {{val|-1.602176634|e=-19|ul=C}},<ref name="CODATA">The original source for CODATA is 
{{cite journal
 | last1 = Mohr | first1 = P.J.
 | last2 = Taylor | first2 = B.N.
 | last3 = Newell | first3 = D.B.
 | year = 2008
 | title = CODATA recommended values of the fundamental physical constants
 | journal = [[Reviews of Modern Physics]]
 | volume = 80 | pages = 633–730
 | doi = 10.1103/RevModPhys.80.633 | bibcode=2008RvMP...80..633M
 | issue = 2
 |arxiv = 0801.0028 | citeseerx = 10.1.1.150.1225
}}
: Individual physical constants from the CODATA are available at: 
{{cite web
 | url = https://physics.nist.gov/cuu/
 | title = The NIST Reference on Constants, Units and Uncertainty
 | publisher = [[National Institute of Standards and Technology]]
 | access-date = 2009-01-15
 | archive-date = 2009-01-16
 | archive-url = https://web.archive.org/web/20090116162522/http://physics.nist.gov/cuu/
 | url-status = live
}}</ref> which is used as a standard unit of charge for subatomic particles, and is also called the [[elementary charge]]. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign.<ref>
{{cite journal
 | last1 = Zorn | first1 = J.C.
 | last2 = Chamberlain | first2 = G.E.
 | last3 = Hughes | first3 = V.W.
 | year = 1963
 | title = Experimental Limits for the Electron–Proton Charge Difference and for the Charge of the Neutron
 | journal = [[Physical Review]]
 | volume = 129 | issue = 6 | pages = 2566–2576
 | doi = 10.1103/PhysRev.129.2566
 |bibcode = 1963PhRv..129.2566Z
}}</ref> The electron is commonly symbolized by {{subatomicParticle|electron}}, and the positron is symbolized by {{subatomicParticle|positron}}.<ref name="raith" /><ref name="CODATA" />

The electron has an intrinsic [[angular momentum]] or spin of {{sfrac|''ħ''|2}}.<ref name="CODATA" /> This property is usually stated by referring to the electron as a [[spin-1/2]] particle.<ref name="raith" /> For such particles the spin magnitude is {{sfrac|''ħ''|2}},<ref name=Gupta2001 /> while the result of the measurement of a [[Projection (mathematics)|projection]] of the spin on any axis can only be ±{{sfrac|''ħ''|2}}. In addition to spin, the electron has an intrinsic [[Electron magnetic moment|magnetic moment]] along its spin axis.<ref name="CODATA" /> It is approximately equal to one [[Bohr magneton]],<ref name=Hanneke />{{efn|Bohr magneton:
: <math>\textstyle\mu_{\mathrm{B}}=\frac{e\hbar}{2m_{\mathrm{e}}}</math>}} which is a physical constant that is equal to {{physconst|muB|after=.}} The orientation of the spin with respect to the momentum of the electron defines the property of elementary particles known as [[helicity (particle physics)|helicity]].<ref name="anastopoulos">
{{cite book
 | last = Anastopoulos
 | first = C.
 | year = 2008
 | title = Particle Or Wave: The Evolution of the Concept of Matter in Modern Physics
 | url = https://books.google.com/books?id=rDEvQZhpltEC&pg=PA261
 | publisher = Princeton University Press
 | pages = 261–262
 | isbn = 978-0-691-13512-0
 | access-date = 2020-08-25
 | archive-date = 2021-01-07
 | archive-url = https://web.archive.org/web/20210107160318/https://books.google.com/books?id=rDEvQZhpltEC&pg=PA261
 | url-status = live
}}</ref>

The electron has no known [[preon|substructure]].<ref name="prl50">
{{cite journal
 | last1 = Eichten | first1 = E.J.
 | last2 = Peskin | first2 = M.E.
 | last3 = Peskin | first3 = M.
 | year = 1983
 | title = New Tests for Quark and Lepton Substructure
 | journal = [[Physical Review Letters]]
 | volume = 50 | pages = 811–814 | issue = 11
 | doi = 10.1103/PhysRevLett.50.811 | bibcode=1983PhRvL..50..811E
 | osti = 1446807
 | s2cid = 119918703
}}</ref><ref>
{{cite journal
 | last = Gabrielse | first = G.
 | year = 2006
 | title = New Determination of the Fine Structure Constant from the Electron ''g'' Value and QED
 | journal = [[Physical Review Letters]]
 | volume = 97 | pages = 030802(1–4)
 | doi = 10.1103/PhysRevLett.97.030802
 | pmid = 16907491
 | bibcode=2006PhRvL..97c0802G
 | issue = 3
 | s2cid = 763602
 |display-authors=etal
}}</ref> Nevertheless, in [[condensed matter physics]], [[spin–charge separation]] can occur in some materials. In such cases, electrons 'split' into three independent particles, the [[spinon]], the [[orbiton]] and the [[Holon (physics)|holon]] (or chargon). The electron can always be theoretically considered as a bound state of the three, with the spinon carrying the spin of the electron, the orbiton carrying the orbital degree of freedom and the chargon carrying the charge, but in certain conditions they can behave as independent [[quasiparticles]].<ref name=bbc>{{cite web |url=https://news.bbc.co.uk/1/hi/england/8227861.stm |title=UK {{pipe}} England {{pipe}} Physicists 'make electrons split' |work=BBC News |date=2009-08-28 |access-date=2016-07-11 |archive-date=2017-08-31 |archive-url=https://web.archive.org/web/20170831102806/http://news.bbc.co.uk/1/hi/england/8227861.stm |url-status=live }}</ref><ref>[https://www.sciencedaily.com/releases/2009/07/090730141607.htm Discovery About Behavior Of Building Block Of Nature Could Lead To Computer Revolution] {{Webarchive|url=https://web.archive.org/web/20190404130054/https://www.sciencedaily.com/releases/2009/07/090730141607.htm |date=2019-04-04 }}. ''Science Daily'' (July 31, 2009)</ref><ref name=gov>{{cite web |last=Yarris |first=Lynn |url=https://www.lbl.gov/Science-Articles/Archive/ALS-spinons-holons.html |title=First Direct Observations of Spinons and Holons |publisher=Lbl.gov |date=2006-07-13 |access-date=2016-07-11 |archive-date=2022-02-24 |archive-url=https://web.archive.org/web/20220224105553/https://www2.lbl.gov/Science-Articles/Archive/ALS-spinons-holons.html |url-status=live }}</ref>

The issue of the radius of the electron is a challenging problem of modern theoretical physics. The admission of the hypothesis of a finite radius of the electron is incompatible to the premises of the theory of relativity. On the other hand, a point-like electron (zero radius) generates serious mathematical difficulties due to the [[self-energy]] of the electron tending to infinity.<ref>[[Eduard Shpolsky]], Atomic physics (Atomnaia fizika), second edition, 1951</ref> Observation of a single electron in a [[Penning trap]] suggests the upper limit of the particle's radius to be 10<sup>−22</sup>&nbsp;meters.<ref>
{{cite journal
 | last = Dehmelt | first = H.
 | year = 1988
 | title = A Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius
 | journal = [[Physica Scripta]]
 | volume = T22 | pages = 102–110
 | doi = 10.1088/0031-8949/1988/T22/016
 |bibcode = 1988PhST...22..102D | s2cid = 250760629
}}</ref>
The upper bound of the electron radius of 10<sup>−18</sup>&nbsp;meters<ref>{{cite web |author-link=Gerald Gabrielse |first=Gerald |last=Gabrielse |url=https://gabrielse.physics.harvard.edu/gabrielse/overviews/ElectronSubstructure/ElectronSubstructure.html |title=Electron Substructure |department=Physics |publisher=Harvard University |access-date=2016-06-21 |archive-date=2019-04-10 |archive-url=https://web.archive.org/web/20190410164332/https://gabrielse.physics.harvard.edu/gabrielse/overviews/ElectronSubstructure/ElectronSubstructure.html |url-status=dead }}</ref> can be derived using the [[uncertainty relation]] in energy. There ''is'' also a physical constant called the "[[classical electron radius]]", with the much larger value of {{val|2.8179|e=-15|u=m}}, greater than the radius of the proton. However, the terminology comes from a simplistic calculation that ignores the effects of [[quantum mechanics]]; in reality, the so-called classical electron radius has little to do with the true fundamental structure of the electron.<ref>
{{cite book
 | last = Meschede
 | first = D.
 | year = 2004
 | title = Optics, light and lasers: The Practical Approach to Modern Aspects of Photonics and Laser Physics
 | url = https://books.google.com/books?id=PLISLfBLcmgC&pg=PA168
 | publisher = [[Wiley-VCH]]
 | page = 168
 | isbn = 978-3-527-40364-6
 | access-date = 2020-08-25
 | archive-date = 2014-08-21
 | archive-url = https://web.archive.org/web/20140821185221/http://books.google.com/books?id=PLISLfBLcmgC&pg=PA168
 | url-status = live
}}</ref><ref name=HakenWolfBrewer2005 /><ref group="lower-alpha">The classical electron radius is derived as follows. Assume that the electron's charge is spread uniformly throughout a spherical volume. Since one part of the sphere would repel the other parts, the sphere contains electrostatic potential energy. This energy is assumed to equal the electron's [[Invariant mass#Rest energy|rest energy]], defined by [[special relativity]] (''E''&nbsp;=&nbsp;''mc''<sup>2</sup>).<br />
From [[electrostatics]] theory, the [[potential energy]] of a sphere with radius ''r'' and charge ''e'' is given by:
: <math>E_{\mathrm p} = \frac{e^2}{8\pi \varepsilon_0 r},</math>
where ''ε''<sub>0</sub> is the [[vacuum permittivity]]. For an electron with rest mass ''m''<sub>0</sub>, the rest energy is equal to:
: <math>\textstyle E_{\mathrm p} = m_0 c^2,</math>
where ''c'' is the speed of light in vacuum. Setting them equal and solving for ''r'' gives the classical electron radius.<br />
See: Haken, Wolf, & Brewer (2005).</ref>

There are [[elementary particle]]s that spontaneously [[Particle decay|decay]] into less massive particles. An example is the [[muon]], with a [[Exponential decay#Mean lifetime|mean lifetime]] of {{val|2.2|e=-6}}&nbsp;seconds, which decays into an electron, a muon [[neutrino]] and an electron [[neutrino#Antineutrinos|antineutrino]]. The electron, on the other hand, is thought to be stable on theoretical grounds: the electron is the least massive particle with non-zero electric charge, so its decay would violate [[charge conservation]].<ref>
{{cite journal
 | last = Steinberg | first = R.I.
 | year = 1999
 | title = Experimental test of charge conservation and the stability of the electron
 | journal = [[Physical Review D]]
 | volume = 61 | issue = 2 | pages = 2582–2586
 | doi = 10.1103/PhysRevD.12.2582
 |bibcode = 1975PhRvD..12.2582S |display-authors=etal
}}</ref> The experimental lower bound for the electron's mean lifetime is {{val|6.6|e=28}} years, at a 90% [[confidence interval|confidence level]].<ref name=bx2015 /><ref>
{{cite journal
 |last            = Beringer |first=J.
 |display-authors = etal
 |collaboration   = Particle Data Group
 |year            = 2012
 |title           = Review of Particle Physics: [electron properties]
 |journal         = [[Physical Review D]]
 |volume          = 86
 |issue           = 1
 |pages           = 010001
 |doi             = 10.1103/PhysRevD.86.010001
 |bibcode         = 2012PhRvD..86a0001B
 |url             = https://pdg.lbl.gov/2012/listings/rpp2012-list-electron.pdf
 |doi-access      = free
 |access-date     = 2022-02-24
 |archive-date    = 2022-01-15
 |archive-url     = https://web.archive.org/web/20220115063155/https://pdg.lbl.gov/2012/listings/rpp2012-list-electron.pdf
 |url-status      = live
}}</ref><ref>
{{cite journal
 | last1 =  Back | first1 = H.O. |display-authors=etal
 | year = 2002
 | title = Search for electron decay mode e → γ + ν with prototype of Borexino detector
 | journal = [[Physics Letters B]]
 | volume = 525 | issue = 1–2 | pages = 29–40
 | doi = 10.1016/S0370-2693(01)01440-X | doi-access = free
 | bibcode = 2002PhLB..525...29B
}}</ref>