Editing Classical field theory (section)

== Unification attempts ==
{{Main|Classical unified field theories}}

Attempts to create a unified field theory based on [[classical physics]] are classical unified field theories. During the years between the two World Wars, the idea of unification of [[gravity]] with [[electromagnetism]] was actively pursued by several mathematicians and physicists like [[Albert Einstein]], [[Theodor Kaluza]],<ref name=kal>{{cite journal |last=Kaluza |first=Theodor |date=1921 |title=Zum Unitätsproblem in der Physik |journal=Sitzungsber. Preuss. Akad. Wiss. Berlin. (Math. Phys.) |pages=966–972 |bibcode=1921SPAW.......966K }}</ref> [[Hermann Weyl]],<ref>{{cite journal |author=Weyl, H. |title=Gravitation und Elektrizität |journal=Sitz. Preuss. Akad. Wiss. |year=1918 |pages=147–159|doi=10.1007/978-3-663-19510-8_11 |bibcode=1918SPAW.......465W |isbn=978-3-663-19372-2 }}</ref> [[Arthur Eddington]],<ref>{{cite book |author=Eddington, A. S. |title=The Mathematical Theory of Relativity, 2nd ed. |publisher=Cambridge Univ. Press |year=1924 }}</ref> [[Gustav Mie]]<ref>{{cite journal |author=Mie, G. |title=Grundlagen einer Theorie der Materie |journal=Annalen der Physik |year=1912 |volume=37 |pages=511–534 |doi=10.1002/andp.19123420306 |issue=3|bibcode = 1912AnP...342..511M |url=https://zenodo.org/record/1424223 }}</ref> and Ernst Reichenbacher.<ref>{{cite journal |author=Reichenbächer, E. |title=Grundzüge zu einer Theorie der Elektrizität und der Gravitation |journal=Annalen der Physik |year=1917 |volume=52 |pages=134–173 |doi=10.1002/andp.19173570203 |issue=2|bibcode = 1917AnP...357..134R |url=https://zenodo.org/record/1424315 }}</ref>

Early attempts to create such theory were based on incorporation of [[electromagnetic fields]] into the geometry of [[general relativity]]. In 1918, the case for the first geometrization of the electromagnetic field was proposed in 1918 by Hermann Weyl.<ref name=Tilman>{{Citation| last = Sauer| first = Tilman| author-link = Sauer Tilman| chapter = Einstein’s Unified Field Theory Program| date = May 2014| editor1-last = Janssen| editor1-first = Michel | editor2-last = Lehner| editor2-first = Christoph | title = The Cambridge Companion to Einstein| publisher = Cambridge University Press| publication-date = May 2014| isbn = 9781139024525}}</ref>
In 1919, the idea of a five-dimensional approach was suggested by [[Theodor Kaluza]].<ref name=Tilman/> From that, a theory called [[Kaluza-Klein Theory]] was developed. It attempts to unify [[gravitation]] and [[electromagnetism]], in a five-dimensional [[space-time]].
There are several ways of extending the representational framework for a unified field theory which have been considered by Einstein and other researchers. These extensions in general are based in two options.<ref name=Tilman/> The first option is based in relaxing the conditions imposed on the original formulation, and the second is based in introducing other mathematical objects into the theory.<ref name=Tilman/> An example of the first option is relaxing the restrictions to four-dimensional space-time by considering higher-dimensional representations.<ref name=Tilman/> That is used in [[Kaluza-Klein Theory]]. For the second, the most prominent example arises from the concept of the [[affine connection]] that was introduced into [[General relativity|the theory of general relativity]] mainly through the work of [[Tullio Levi-Civita]] and [[Hermann Weyl]].<ref name=Tilman/>

Further development of [[quantum field theory]] changed the focus of searching for unified field theory from classical to quantum description. Because of that, many theoretical physicists gave up looking for a classical unified field theory.<ref name=Tilman/> Quantum field theory would include unification of two other [[Fundamental interactions|fundamental forces of nature]], the [[strong nuclear force|strong]] and [[weak nuclear force]] which act on the subatomic level.<ref>{{cite journal |last=Gadzirayi Nyambuya|first=Golden|title=Unified Field Theory – Paper I, Gravitational, Electromagnetic, Weak & the Strong Force| journal=Apeiron |date=October 2007|volume=14|issue=4|page=321|url=http://redshift.vif.com/JournalFiles/V14NO4PDF/V14N4GAD.pdf |access-date=30 December 2017}}</ref><ref>{{cite journal|last1=De Boer|first1=W.|title=Grand unified theories and supersymmetry in particle physics and cosmology|journal=Progress in Particle and Nuclear Physics|date=1994|volume=33| pages=201–301 |url=http://www-ekp.physik.uni-karlsruhe.de/~deboer/html/Lehre/Susy/deboer_review3.pdf|access-date=30 December 2017|arxiv=hep-ph/9402266|bibcode=1994PrPNP..33..201D|doi=10.1016/0146-6410(94)90045-0|s2cid=119353300}}</ref>