Editing Edward Witten (section)

===M-theory===
By the mid 1990s, physicists working on [[string theory]] had developed five different consistent versions of the theory. These versions are known as [[type I string|type I]], [[type IIA string|type IIA]], [[type IIB string|type IIB]], and the two flavors of [[heterotic string]] theory ([[special orthogonal group|SO(32)]] and [[E8 (mathematics)|E<sub>8</sub>×E<sub>8</sub>]]). The thinking was that of these five candidate theories, only one was the actual correct [[theory of everything]], and that theory was the one whose low-energy limit matched the physics observed in our world today.<ref name="Rickles 2016">{{cite book | last=Rickles | first=Dean | title=A Brief History of String Theory | publisher=Springer | date=2016-08-23 | isbn=978-3-662-50183-2}}</ref>

Speaking at [[Strings (conference)|Strings]] '95 conference at [[University of Southern California]], Witten made the surprising suggestion that these five string theories were in fact not distinct theories, but different limits of a single theory, which he called [[M-theory]].<ref>{{Cite conference 
| publisher=University of Southern California
| place=Los Angeles
| series=Future Perspectives in String Theory
| date = 13–18 March 1995
| author= Witten, E.
| title= Some problems of strong and weak coupling
|url=http://physics.usc.edu/Strings95/program.html|access-date=2023-02-01|website=physics.usc.edu}}</ref><ref>{{cite journal |last1=Witten |first1=Edward |year=1995 |title=String theory dynamics in various dimensions |journal=[[Nuclear Physics B]] |volume=443 |issue=1 |pages=85–126 |doi=10.1016/0550-3213(95)00158-O|arxiv = hep-th/9503124 |bibcode = 1995NuPhB.443...85W |s2cid=16790997 }}</ref> Witten's proposal was based on the observation that the five string theories can be mapped to one another by certain rules called [[String duality|dualities]] and are identified by these dualities. It led to a flurry of work now known as the [[second superstring revolution]].<ref name="Rickles 2016"/>