Editing Magnetic monopole (section)

== Historical background ==

=== Early science and classical physics ===

Many early scientists attributed the magnetism of [[lodestone]]s to two different "magnetic fluids" ("effluvia"), a north-pole fluid at one end and a south-pole fluid at the other, which attracted and repelled each other in analogy to positive and negative [[electric charge]].<ref>{{cite web |url=https://books.google.com/books?id=N1YEAAAAYAAJ&pg=PA352 |title=The Encyclopaedia Britannica: A Dictionary of Arts, Sciences, Literature and General Information |first=Hugh |last=Chisholm |date=June 26, 2018 |publisher=[Cambridge] University Press |via=Google Books}}</ref><ref>{{cite web |url=https://books.google.com/books?id=6rYXAAAAIAAJ&pg=PA424 |title=Principles of Physics: Designed for Use as a Textbook of General Physics |first=William Francis |last=Magie |date=June 26, 2018 |publisher=Century Company |via=Google Books}}</ref> However, an improved understanding of [[electromagnetism]] in the nineteenth century showed that the magnetism of lodestones was properly explained not by magnetic monopole fluids, but rather by a combination of [[electric current]]s, the [[electron magnetic moment]], and the [[magnetic moment]]s of other particles. [[Gauss's law for magnetism]], one of [[Maxwell's equations]], is the mathematical statement that magnetic monopoles do not exist. Nevertheless, [[Pierre Curie]] pointed out in 1894<ref>{{cite journal |author=Pierre Curie |title=Sur la possibilité d'existence de la conductibilité magnétique et du magnétisme libre |language=fr |trans-title=On the possible existence of magnetic conductivity and free magnetism |journal=Séances de la Société Française de Physique |place=Paris |pages=76–77 |year=1894 |url=https://archive.org/stream/sancesdelasocit19physgoog#page/n82/mode/2up }}</ref> that magnetic monopoles ''could'' conceivably exist, despite not having been seen so far.

=== Quantum mechanics ===

The [[quantum mechanics|quantum]] theory of magnetic charge started with a paper by the [[physicist]] [[Paul Dirac]] in 1931.<ref>{{cite journal |last=Dirac |first=Paul |author-link=Paul Dirac |title=Quantised Singularities in the Electromagnetic Field |journal=Proceedings of the Royal Society A |location=London |volume=133 |page=60 |year=1931 |issue=821 |doi=10.1098/rspa.1931.0130 |bibcode=1931RSPSA.133...60D |url=http://rspa.royalsocietypublishing.org/content/133/821/60 }}</ref> In this paper, Dirac showed that if ''any'' magnetic monopoles exist in the universe, then all electric charge in the universe must be [[charge quantization|quantized]] (Dirac quantization condition).<ref name=littlejohn>[http://bohr.physics.berkeley.edu/classes/221/0708/lectures/Lecture.2007.10.11.pdf Lecture notes by Robert Littlejohn], University of California, Berkeley, 2007–08</ref> The electric charge ''is'', in fact, quantized, which is consistent with (but does not prove) the existence of monopoles.<ref name=littlejohn/>

Since Dirac's paper, several systematic monopole searches have been performed. Experiments in 1975<ref name="PRL-35-487">{{cite journal |last1=Price |first1=P. B. |last2=Shirk |first2=E. K. |last3=Osborne |first3=W. Z. |last4=Pinsky |first4=L. S. |date=August 25, 1975 |title=Evidence for Detection of a Moving Magnetic Monopole |journal=Physical Review Letters |bibcode=1975PhRvL..35..487P |doi=10.1103/PhysRevLett.35.487 |volume=35 |issue=8 |pages=487–490 }}</ref> and 1982<ref name="PRL-48-1378">{{cite journal |last=Cabrera |first=Blas |date=May 17, 1982 |title=First Results from a Superconductive Detector for Moving Magnetic Monopoles |journal=Physical Review Letters |bibcode=1982PhRvL..48.1378C |doi=10.1103/PhysRevLett.48.1378 |volume=48 |issue=20 |pages=1378–1381}}</ref> produced candidate events that were initially interpreted as monopoles, but are now regarded as inconclusive.<ref>[[#References|Milton]] p. 60</ref> Therefore, whether monopoles exist remains an open question. Further advances in theoretical [[particle physics]], particularly developments in [[grand unified theories]] and [[quantum gravity]], have led to more compelling arguments (detailed below) that monopoles do exist. [[Joseph Polchinski]], a string theorist, described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen".<ref name=Polchinski>{{cite journal|title=Monopoles, Duality, and String Theory|first=Joseph|last=Polchinski|date=February 1, 2004|journal=International Journal of Modern Physics A|volume=19|issue=supp01|pages=145–154|doi=10.1142/S0217751X0401866X|arxiv=hep-th/0304042|bibcode=2004IJMPA..19S.145P|s2cid=831833}}</ref> These theories are not necessarily inconsistent with the experimental evidence. In some theoretical [[Scientific modelling|model]]s, magnetic monopoles are unlikely to be observed, because they are too massive to create in [[particle accelerator]]s (see {{slink||Searches for magnetic monopoles}} below), and also too rare in the Universe to enter a [[particle detector]] with much probability.<ref name=Polchinski/>

Some [[condensed matter physics|condensed matter systems]] propose a structure superficially similar to a magnetic monopole, known as a [[flux tube]]. The ends of a flux tube form a [[magnetic dipole]], but since they move independently, they can be treated for many purposes as independent magnetic monopole [[quasiparticle]]s. Since 2009, numerous news reports from the popular media<ref name=sciencedaily/><ref name=symmetrymagazine/> have incorrectly described these systems as the long-awaited discovery of the magnetic monopoles, but the two phenomena are only superficially related to one another.<ref name=TchernyshyovQuote>[https://physicsworld.com/a/magnetic-monopoles-spotted-in-spin-ices/ "Magnetic monopoles spotted in spin ices"], ''Physics World'', September 3, 2009. "Oleg Tchernyshyov at Johns Hopkins University [a researcher in this field] cautions that the theory and experiments are specific to spin ices, and are not likely to shed light on magnetic monopoles as predicted by Dirac."</ref><ref name=GibneyQuote>{{cite journal |last=Gibney |first=Elizabeth |date=29 January 2014 |title=Quantum cloud simulates magnetic monopole |journal=Nature |doi=10.1038/nature.2014.14612|s2cid=124109501 |quote=This is not the first time that physicists have created monopole analogues. In 2009, physicists observed magnetic monopoles in a crystalline material called spin ice, which, when cooled to near-absolute zero, seems to fill with atom-sized, classical monopoles. These are magnetic in a true sense, but cannot be studied individually. Similar analogues have also been seen in other materials, such as in superfluid helium.&nbsp;... Steven Bramwell, a physicist at University College London who pioneered work on monopoles in spin ices, says that the [2014 experiment led by David Hall] is impressive, but that what it observed is not a Dirac monopole in the way many people might understand it. 'There's a mathematical analogy here, a neat and beautiful one. But they're not magnetic monopoles.'}}</ref> These condensed-matter systems remain an area of active research. (See ''{{slink||"Monopoles" in condensed-matter systems}}'' below.)