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| {{String theory|cTopic=Theory}}
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| In [[theoretical physics]], '''T-duality''' is an equivalence of two physical theories, which may be either [[quantum field theory|quantum field theories]] or [[string theory|string theories]]. In the simplest example of this relationship, one of the theories describes [[string (physics)|strings]] propagating in an imaginary [[spacetime]] shaped like a circle of some radius <math>R</math>, while the other theory describes strings propagating on a spacetime shaped like a circle of radius <math>1/R</math>. The two theories are equivalent in the sense that all observable quantities in one description are identified with quantities in the dual description. For example, [[momentum]] in one description takes discrete values and is equal to the number of times the string [[winding number|winds]] around the circle in the dual description.
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| The idea of T-duality can be extended to more complicated theories, including [[superstring theory|superstring theories]]. The existence of these dualities implies that seemingly different superstring theories are actually physically equivalent. This led to the realization, in the mid-1990s, that all of the five consistent superstring theories are just different limiting cases of a single eleven-dimensional theory called [[M-theory]].
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| In general, T-duality relates two theories with different spacetime geometries. In this way, T-duality suggests a possible scenario in which the classical notions of geometry break down in a theory of [[Planck scale]] physics.<ref>Seiberg 2006</ref> The geometric relationships suggested by T-duality are also important in [[pure mathematics]]. Indeed, according to the [[SYZ conjecture]] of [[Andrew Strominger]], [[Shing-Tung Yau]], and [[Eric Zaslow]], T-duality is closely related to another duality called [[mirror symmetry (string theory)|mirror symmetry]], which has important applications in a branch of mathematics called [[enumerative geometry|enumerative algebraic geometry]].
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| ==Overview==
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| ===Strings and duality===
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| T-duality is a particular example of a general notion of [[string duality|duality]] in physics. The term ''duality'' refers to a situation where two seemingly different [[physical system]]s turn out to be equivalent in a nontrivial way. If two theories are related by a duality, it means that one theory can be transformed in some way so that it ends up looking just like the other theory. The two theories are then said to be ''dual'' to one another under the transformation. Put differently, the two theories are mathematically different descriptions of the same phenomena.
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| Like many of the dualities studied in theoretical physics, T-duality was discovered in the context of [[string theory]].<ref>Other dualities that arise in string theory are [[S-duality]], [[U-duality]], [[mirror symmetry (string theory)|mirror symmetry]], and the [[AdS/CFT correspondence]].</ref> In string theory, particles are modeled not as zero-dimensional points but as one-dimensional extended objects called [[string (physics)|strings]]. The physics of strings can be studied in various numbers of dimensions. In addition to three familiar dimensions from everyday experience (up/down, left/right, forward/backward), string theories may include one or more [[compact dimension]]s which are curled up into circles.
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| A standard analogy for this is to consider multidimensional object such as a garden hose.<ref>This analogy is used for example in Greene 2000, p.186</ref> If the hose is viewed from a sufficient distance, it appears to have only one dimension, its length. However, as one approaches the hose, one discovers that it contains a second dimension, its circumference. Thus, an ant crawling inside it would move in two dimensions. Such extra dimensions are important in T-duality, which relates a theory in which strings propagate on a circle of some radius <math>R</math> to a theory in which strings propagate on a circle of radius <math>1/R</math>.
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| ===Winding numbers===
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| {{main|Winding number}}
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| In mathematics, the [[winding number]] of a [[curve]] in the [[plane (mathematics)|plane]] around a given [[point (mathematics)|point]] is an [[integer]] representing the total number of times that curve travels counterclockwise around the point. The notion of winding number is important in the mathematical description of T-duality where it is used to measure the winding of strings around [[compactification (physics)|compact]] [[extra dimensions]].
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| For example, the image below shows several examples of curves in the plane, illustrated in red. Each curve is assumed to be [[closed manifold|closed]], meaning it has no endpoints, and is allowed to intersect itself. Each curve has an [[Orientability|orientation]] given by the arrows in the picture. In each situation, there is a distinguished point in the plane, illustrated in black. The ''winding number'' of the curve around this distinguished point is equal to the total number of counterclockwise [[Turn (geometry)|turns]] that the curve makes around this point.
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| {| align="center" border=0 cellpadding=0
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| |-valign="center"
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| |<math>\cdots</math>
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| |align="center"| [[Image:Winding Number -2.svg|80px]]
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| |align="center"| [[Image:Winding Number -1.svg|80px]]
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| |align="center"| [[Image:Winding Number 0.svg|80px]]
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| |-valign="top" style="height:3em"
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| |align="center"|−2
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| |align="center"|−1
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| |align="center"|0
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| |-valign="center"
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| |align="center"| [[Image:Winding Number 1.svg|80px]]
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| |align="center"| [[Image:Winding Number 2.svg|80px]]
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| |align="center"| [[Image:Winding Number 3.svg|80px]]
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| |<math>\cdots</math>
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| |-valign="top"
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| |align="center"|1
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| |align="center"|2
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| |align="center"|3
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| |}
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| When counting the total number of turns, counterclockwise turns count as positive, while clockwise turns counts as [[negative number|negative]]. For example, if the curve first circles the origin four times counterclockwise, and then circles the origin once clockwise, then the total winding number of the curve is three. According to this scheme, a curve that does not travel around the distinguished point at all has winding number zero, while a curve that travels clockwise around the point has negative winding number. Therefore, the winding number of a curve may be any integer. The pictures above show curves with winding numbers between −2 and 3:
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| ===Quantized momenta===
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| The simplest theories in which T-duality arises are [[two-dimensional]] [[sigma model]]s with circular target spaces. These are simple quantum field theories that describe propagation of strings in an imaginary spacetime shaped like a circle. The strings can thus be modeled as curves in the plane that are confined to lie in a circle, say of radius <math>R</math>, about the [[origin (mathematics)|origin]]. In what follows, the strings are assumed to be closed (that is, without endpoints).
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| Denote this circle by <math>S_R^1</math>. One can think of this circle as a copy of the [[real line]] with two points [[quotient space|identified]] if they differ by a multiple of the circle's circumference <math>2\pi R</math>. It follows that the state of a string at any given time can be represented as a function <math>\varphi(\theta)</math> of a single real parameter <math>\theta</math>. Such a function can be expanded in a [[Fourier series]] as
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| :<math>\varphi(\theta)=mR\theta+x+\sum_{n\neq0}c_ne^{in\theta}</math>.
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| Here <math>m</math> denotes the winding number of the string around the circle, and the constant mode <math>x=c_0</math> of the Fourier series has been singled out. Since this expression represents the configuration of a string at a fixed time, all coefficients (<math>x</math> and the <math>c_n</math>) are also functions of time.
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| Let <math>\dot{x}</math> denote the time derivative of the constant mode <math>x</math>. This represents a type of [[momentum]] in the theory. One can show, using the fact that the strings considered here are closed, that this momentum can only take on discrete values of the form <math>\dot{x}=n/R</math> for some integer <math>n</math>. In more physical language, one says that the momentum spectrum is ''quantized''.
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| ===An equivalence of theories===
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| In the situation described above, the total energy, or [[Hamiltonian function|Hamiltonian]], of the string is given by the expression
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| :<math>H=(mR)^2+\dot{x}^2+\sum_n|\dot{c}_n|^2+n^2|c_n|^2</math>. | |
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| Since the momenta of the theory are quantized, the first two terms in this formula are <math>(mR)^2+(n/R)^2</math>, and this expression is unchanged when one simultaneously replaces the radius <math>R</math> by <math>1/R</math> and exchanges the winding number <math>m</math> and the integer <math>n</math>. The summation in the expression for <math>H</math> is similarly unaffected by these changes, so the total energy is unchanged. In fact, this equivalence of Hamiltonians descends to an equivalence of two quantum mechanical theories: One of these theories describes strings propagating on a circle of radius <math>R</math>, while the other describes string propagating in a circle of radius <math>1/R</math> with momentum and winding numbers interchanged. This equivalence of theories is the simplest manifestation of T-duality.
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| == Superstrings ==
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| [[File:StringTheoryDualities.jpg|thumb|right|350px|A diagram of string theory dualities. Yellow arrows indicate [[S-duality]]. Blue arrows indicate T-duality.]]
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| Up until the mid 1990s, physicists working on string theory believed there were five distinct versions of the theory: [[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 different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries.
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| In the mid 1990s, physicists noticed that these five string theories are actually related by highly nontrivial dualities. One of these dualities is T-duality. For example, it was shown that type IIA string theory is equivalent to type IIB string theory via T-duality and also that the two versions of heterotic string theory are related by T-duality.
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| The existence of these dualities showed that the five string theories were in fact not all distinct theories. In 1995, at the string theory conference at [[University of Southern California]], [[Edward Witten]] made the surprising suggestion that all five of these theories were just different limits of a single theory now known as [[M-theory]].<ref>Witten 1995</ref> Witten's proposal was based on the observation that different superstring theories are linked by dualities and the fact that type IIA and E<sub>8</sub>×E<sub>8</sub> heterotic string theories are closely related to a gravitational theory called eleven-dimensional [[supergravity]]. His announcement led to a flurry of work now known as the [[second superstring revolution]].
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| == Mirror symmetry ==
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| {{main|Mirror symmetry (string theory)}}
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| [[Image:Calabi yau.jpg|left|thumb|200px|A three-dimensional projection of a six-dimensional [[Calabi–Yau manifold]]. ]]
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| In string theory and [[algebraic geometry]], the term "[[mirror symmetry (string theory)|mirror symmetry]]" refers to a phenomenon involving complicated shapes called [[Calabi-Yau manifolds]]. These manifolds provide an interesting geometry on which strings can propagate, and the resulting theories may have applications in [[particle physics]].<ref>Candelas et al. 1985</ref> In the late 1980s, it was noticed that such a Calabi-Yau manifold does not uniquely determine the physics of the theory. Instead, one finds that there are ''two'' Calabi-Yau manifolds that give rise to the same physics.<ref>Dixon 1988; Lerche, Vafa, and Warner 1989</ref> These manifolds are said to be "mirror" to one another. This mirror duality is an important computational tool in string theory, and it has allowed mathematicians to solve difficult problems in [[enumerative geometry]].<ref>Zaslow 2008</ref>
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| [[File:torus cycles.png|thumb|175px|right|A [[torus]] is the [[cartesian product]] of two circles.]]
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| One approach to understanding mirror symmetry is the [[SYZ conjecture]], which was suggested by [[Andrew Strominger]], [[Shing-Tung Yau]], and [[Eric Zaslow]] in 1996.<ref>Strominger, Yau, and Zaslow 1996</ref> According to the SYZ conjecture, mirror symmetry can be understood by dividing a complicated Calabi-Yau manifold into simpler pieces and considering the effects of T-duality on these pieces.<ref>Yau and Nadis 2010, p.174</ref>
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| The simplest example of a Calabi-Yau manifold is a [[torus]] (a surface shaped like a donut). Such a surface can be viewed as the [[cartesian product|product]] of two circles. This means that the torus can be viewed as the [[union (set theory)|union]] of a collection of longitudinal circles (such as the red circle in the image). There is an auxiliary space which says how these circles are organized, and this space is itself a circle (the pink circle). This space is said to ''parametrize'' the longitudinal circles on the torus. In this case, mirror symmetry is equivalent to T-duality acting on the longitudinal circles, changing their radii from <math>R</math> to <math>1/R</math>.
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| The SYZ conjecture generalizes this idea to the more complicated case of six-dimensional Calabi-Yau manifolds like the one illustrated above. As in the case of a torus, we can divide a six-dimensional Calabi-Yau manifold into simpler pieces, which in this case are [[3-torus|3-tori]] (three-dimensional objects which generalize the notion of a torus) parametrized by a [[3-sphere]] (a three-dimensional generalization of a sphere).<ref>More precisely, there is a 3-torus associated to every point on the three-sphere except at certain bad points, which correspond to singular tori. See Yau and Nadis 2010, pp.176--7.</ref> T-duality can be extended from circles to the three-dimensional tori appearing in this decomposition, and the SYZ conjecture states that mirror symmetry is equivalent to the simultaneous application of T-duality to these three-dimensional tori.<ref>Yau and Nadis 2010, p.178</ref> In this way, the SYZ conjecture provides a geometric picture of how mirror symmetry acts on a Calabi-Yau manifold.
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| ==Notes==
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| {{reflist|2}}
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| == See also ==
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| *[[S-duality]]
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| *[[U-duality]]
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| *[[Mirror symmetry (string theory)|Mirror symmetry]]
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| *[[AdS/CFT correspondence]]
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| ==References==
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| * {{cite journal |last1=Candelas |first1=Philip |last2=Horowitz |first2=Gary |last3=Strominger |first3= Andrew |last4=Witten |first4=Edward |year=1985 |title=Vacuum configurations for superstrings |journal=Nuclear Physics B |volume=258 |issue= |pages=46–74|bibcode = 1985NuPhB.258...46C |doi = 10.1016/0550-3213(85)90602-9 }}
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| * {{cite journal |last1=Dixon |first1=Lance |year=1988 |title=Some world-sheet properties of superstring compactifications, on orbifolds and otherwise |journal=ICTP Ser. Theoret. Phys. |volume=4 |pages=67–126}}
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| * {{cite book |last1=Greene |first1=Brian |title=The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory |year=2000 |publisher=Random House |isbn=978-0-9650888-0-0 }}
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| * {{cite journal |last1=Lerche |first1=Wolfgang |last2=Vafa |first2=Cumrun |last3=Warner |first3=Nicholas |year=1989 |title=Chiral rings in <math>N = 2</math> superconformal theories |journal=Nuclear Physics B |volume=324 |issue=2 |pages=427–474|bibcode = 1989NuPhB.324..427L |doi = 10.1016/0550-3213(89)90474-4 }}
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| * {{cite arXiv |last=Seiberg |first=Nathan |eprint=hep-th/0601234 |title=Emergent Spacetime |year=2006 }}
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| * {{cite journal |last1=Strominger |first1=Andrew |last2=Yau |first2=Shing-Tung |last3=Zaslow |first3=Eric |year=1996 |title=Mirror symmetry is T-duality |journal=Nuclear Physics B |volume=479 |issue=1 |pages=243–259|arxiv = hep-th/9606040 |bibcode = 1996NuPhB.479..243S |doi = 10.1016/0550-3213(96)00434-8 }}
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| * {{cite conference |title=Some problems of strong and weak coupling |last1=Witten |first1=Edward |date=March 13–18, 1995 |publisher=World Scientific |booktitle=Proceedings of Strings '95: Future Perspectives in String Theory }}
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| * {{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 }}
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| * {{Cite book| first1 = Shing-Tung | last1 = Yau | first2 = Steve | last2 = Nadis | year = 2010 | title = The Shape of Inner Space: String Theory and the Geometry of the Universe's Hidden Dimensions | publisher = Basic Books | isbn = 978-0-465-02023-2 }}
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| * {{Citation | last1=Zaslow | first1=Eric | contribution=Mirror Symmetry | year=2008 | title=The Princeton Companion to Mathematics | editor-last=Gowers | editor-first=Timothy}}
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| [[Category:Quantum field theory]]
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| [[Category:String theory]]
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