Editing String theory (section)

=== Background independence ===
{{main|Background independence}}

One of the fundamental properties of Einstein's general theory of relativity is that it is [[background independence|background independent]], meaning that the formulation of the theory does not in any way privilege a particular spacetime geometry.<ref>[[#Smolin|Smolin]], p. 81</ref>

One of the main criticisms of string theory from early on is that it is not manifestly background-independent. In string theory, one must typically specify a fixed reference geometry for spacetime, and all other possible geometries are described as perturbations of this fixed one. In his book ''[[The Trouble With Physics]]'', physicist [[Lee Smolin]] of the [[Perimeter Institute for Theoretical Physics]] claims that this is the principal weakness of string theory as a theory of quantum gravity, saying that string theory has failed to incorporate this important insight from general relativity.<ref>[[#Smolin|Smolin]], p. 184</ref>

Others have disagreed with Smolin's characterization of string theory. In a review of Smolin's book, string theorist Joseph Polchinski writes

{{quote|[Smolin] is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them... In string theory, it has always been clear that the physics is background-independent even if the language being used is not, and the search for a more suitable language continues. Indeed, as Smolin belatedly notes, [AdS/CFT] provides a solution to this problem, one that is unexpected and powerful.<ref name="Polchinski 2007"/>}}

Polchinski notes that an important open problem in quantum gravity is to develop holographic descriptions of gravity which do not require the gravitational field to be asymptotically anti-de Sitter.<ref name="Polchinski 2007" /> Smolin has responded by saying that the AdS/CFT correspondence, as it is currently understood, may not be strong enough to resolve all concerns about background independence.<ref>{{cite web|url=http://www.kitp.ucsb.edu/joep/links/on-some-criticisms-of-string-theory/lee-smolins-response |title=Response to review of The Trouble with Physics by Joe Polchinski|access-date=December 31, 2015 |url-status=dead |archive-url=https://web.archive.org/web/20151105234427/http://www.kitp.ucsb.edu/joep/links/on-some-criticisms-of-string-theory/lee-smolins-response |archive-date=November 5, 2015 |author=Smolin, Lee |date=April 2007|publisher=kitp.ucsb.edu}}</ref>