Editing M-theory (section)

===Kaluza–Klein theory===

{{main article|Kaluza–Klein theory}}

In the early 20th century, physicists and mathematicians including Albert Einstein and [[Hermann Minkowski]] pioneered the use of four-dimensional geometry for describing the physical world.<ref>Yau and Nadis 2010, p. 9</ref> These efforts culminated in the formulation of Einstein's general theory of relativity, which relates gravity to the geometry of four-dimensional spacetime.<ref name="Yau and Nadis 2010, p. 10">Yau and Nadis 2010, p. 10</ref>

The success of general relativity led to efforts to apply higher dimensional geometry to explain other forces. In 1919, work by [[Theodor Kaluza]] showed that by passing to five-dimensional spacetime, one can unify gravity and [[electromagnetism]] into a single force.<ref name="Yau and Nadis 2010, p. 10"/> This idea was improved by physicist [[Oskar Klein]], who suggested that the additional dimension proposed by Kaluza could take the form of a circle with radius around {{nowrap|10<sup>−30</sup>}} cm.<ref>Yau and Nadis 2010, p. 12</ref>

The [[Kaluza–Klein theory]] and subsequent attempts by Einstein to develop [[unified field theory]] were never completely successful. In part this was because Kaluza–Klein theory predicted a particle (the [[Graviscalar|radion]]), that has never been shown to exist, and in part because it was unable to correctly predict the ratio of an electron's mass to its charge. In addition, these theories were being developed just as other physicists were beginning to discover quantum mechanics, which would ultimately prove successful in describing known forces such as electromagnetism, as well as new [[nuclear force]]s that were being discovered throughout the middle part of the century. Thus it would take almost fifty years for the idea of new dimensions to be taken seriously again.<ref>Yau and Nadis 2010, p. 13</ref>