Editing Quantum gravity (section)

=== String theory ===
{{Main|String theory}}
[[File:Calabi-Yau.png|thumb|Projection of a [[Calabi–Yau manifold]], one of the ways of [[Compactification (physics)|compactifying]] the extra dimensions posited by string theory]]

The central idea of string theory is to replace the classical concept of a [[point particle]] in quantum field theory with a quantum theory of one-dimensional extended objects: string theory.<ref>An accessible introduction at the undergraduate level can be found in {{Cite book
|last=Zwiebach
|first=Barton
|author-link= Barton Zwiebach
|title=A First Course in String Theory
|publisher=[[Cambridge University Press]]
|date=2004
|isbn=978-0-521-83143-7
}}, and more complete overviews in {{Cite book
|last=Polchinski
|first=Joseph
|author-link=Joseph Polchinski
|date=1998
|title=String Theory Vol. I: An Introduction to the Bosonic String
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-63303-1
}} and {{Cite book
|last=Polchinski
|first=Joseph
|author-link=Joseph Polchinski
|date=1998b
|title=String Theory Vol. II: Superstring Theory and Beyond
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-63304-8
}}</ref> At the energies reached in current experiments, these strings are indistinguishable from point-like particles, but, crucially, different [[Normal mode|modes]] of oscillation of one and the same type of fundamental string appear as particles with different ([[electric]] and other) [[charge (physics)|charges]]. In this way, string theory promises to be a [[theory of everything|unified description]] of all particles and interactions.<ref>{{Cite journal
|last=Ibanez
|first=L. E.
|title=The second string (phenomenology) revolution
|journal=[[Classical and Quantum Gravity]]
|volume=17
|issue=5
|date=2000
|pages=1117–1128
|arxiv=hep-ph/9911499
|doi=10.1088/0264-9381/17/5/321
|bibcode = 2000CQGra..17.1117I |s2cid=15707877
}}</ref> The theory is successful in that one mode will always correspond to a [[graviton]], the [[messenger particle]] of gravity; however, the price of this success is unusual features such as six extra dimensions of space in addition to the usual three for space and one for time.<ref>For the graviton as part of the string spectrum, e.g. {{Harvnb|Green|Schwarz|Witten|2012|loc=sec. 2.3 and 5.3}}; for the extra dimensions, ibid sec. 4.2.</ref>

In what is called the [[History of string theory#1984–1989: first superstring revolution|second superstring revolution]], it was conjectured that both string theory and a unification of general relativity and [[supersymmetry]] known as [[supergravity]]<ref>{{Cite book
|last=Weinberg
|first=Steven
|chapter=Chapter 31
|author-link=Steven Weinberg
|title=The Quantum Theory of Fields II: Modern Applications
|publisher=Cambridge University Press
|date=2000
|chapter-url=https://books.google.com/books?id=aYDDRKqODpUC
|isbn=978-0-521-55002-4
|url-access=registration
|url=https://archive.org/details/quantumtheoryoff00stev
}}</ref> form part of a hypothesized eleven-dimensional model known as [[M-theory]], which would constitute a uniquely defined and consistent theory of quantum gravity.<ref>{{Cite journal
|last=Townsend
|first=Paul K.
|title=Four Lectures on M-Theory
|journal=High Energy Physics and Cosmology
|volume=13
|series=ICTP Series in Theoretical Physics
|page=385
|date=1996
|arxiv=hep-th/9612121
|bibcode=1997hepcbconf..385T
}}</ref><ref>{{Cite journal
|last=Duff
|first=Michael
|author-link=Michael Duff (physicist)
|title=M-Theory (the Theory Formerly Known as Strings)
|journal=[[International Journal of Modern Physics A]]
|volume=11 |issue=32
|date=1996
|pages=5623–5642
|doi=10.1142/S0217751X96002583
|arxiv=hep-th/9608117
|bibcode=1996IJMPA..11.5623D
|s2cid=17432791
}}</ref> As presently understood, however, string theory admits a very large number (10<sup>500</sup> by some estimates) of consistent vacua, comprising the so-called "[[string landscape]]". Sorting through this large family of solutions remains a major challenge.