- Sine-Gordon equation
The

**sine-Gordon equation**is a nonlinear hyperbolicpartial differential equation in 1+1 dimensions involving thed'Alembert operator and the sine of the unknown function. It was originally considered in the nineteenth century in the course of study of surfaces of constant negative curvature. This equation attracted a lot of attention in the 1970s due to the presence ofsoliton solutions.**Origin of the equation and its name**There are two equivalent forms of the sine-Gordon equation. In the (real) "space-time coordinates", denoted ("x","t"), the equation reads

:$,\; phi\_\{tt\}-\; phi\_\{xx\}\; +\; sinphi\; =\; 0.$

Passing to the "light cone coordinates" ("u", "v"), akin to "asymptotic coordinates" where

: $u=frac\{x+t\}2,\; quad\; v=frac\{x-t\}2,$

the equation takes the form:

:$varphi\_\{uv\}\; =\; sinvarphi.$

This is the original form of the sine-Gordon equation, as it was considered in the nineteenth century in the course of investigation of surfaces of constant

Gaussian curvature "K" = −1, also calledpseudospherical surface s. Choose a coordinate system for such a surface in which the coordinate mesh "u" = const, "v" = const is given by the asymptotic lines parameterized with respect to the arc length. Thefirst fundamental form of the surface in these coordinates has a special form: $ds^2\; =\; du^2\; +\; 2cosvarphi\; du\; dv\; +\; dv^2,$

where "φ" expresses the angle between the asymptotic lines, and for the

second fundamental form , "L" = "N" = 0. Then theCodazzi-Mainardi equation expressing a compatibility condition between the first and second fundamental forms results in the sine-Gordon equation. The study of this equation and of the associated transformations of pseudospherical surfaces in the 19th century by Bianchi and Bäcklund led to the discovery ofBäcklund transformation s.The name "sine-Gordon equation" is a pun on the well-known

Klein–Gordon equation in physics::$phi\_\{tt\}-\; phi\_\{xx\}\; +\; phi\; =\; 0.$

The sine-Gordon equation is the

Euler–Lagrange equation of theLagrangian :$mathcal\{L\}\_\{mathrm\{sine-Gordon(phi)\; :=\; frac\{1\}\{2\}(phi\_t^2\; -\; phi\_x^2)\; +\; cosphi.$

If you Taylor-expand the

cosine :$cos(phi)\; =\; sum\_\{n=0\}^infty\; frac\{(-phi\; ^2)^n\}\{(2n)!\}$

and put this into the Lagrangian you get the Klein-Gordon Lagrangian plus some higher order terms

:$mathcal\{L\}\_\{mathrm\{sine-Gordon(phi)\; -\; 1\; =\; frac\{1\}\{2\}(phi\_t^2\; -\; phi\_x^2)\; -\; frac\{phi^2\}\{2\}\; +\; sum\_\{n=2\}^infty\; frac\{(-phi^2)^n\}\{(2n)!\}$

:::::::$=\; 2mathcal\{L\}\_\{mathrm\{Klein-Gordon(phi)\; +\; sum\_\{n=2\}^infty\; frac\{(-phi^2)^n\}\{(2n)!\}.$

**Soliton solutions**An interesting feature of the sine-Gordon equation is the existence of

soliton and multisoliton solutions.**1-soliton solutions**The sine-Gordon equation has the following 1-

soliton solutions::$phi\_\{mathrm\{soliton(x,\; t)\; :=\; 4\; arctan\; e^\{m\; gamma\; (x\; -\; v\; t)\; +\; delta\},$

where $gamma^2\; =\; frac\{1\}\{1\; -\; v^2\}$

The 1-soliton solution for which we have chosen the positive root for $gamma$ is called a "kink", and represents a twist in the variable $phi$ which takes the system from one solution $phi=0$ to an adjacent with $phi=2pi$. The states $phi=0(\; extrm\{mod\}2pi)$ are known as vacuum states as they are constant solutions of zero energy. The 1-soliton solution in which we take the negative root for $gamma$ is called an "antikink". The form of the 1-soliton solutions can be obtained through application of a Bäcklund transform to the trivial (constant vacuum) solution and the integration of the resulting first-order differentials:

:$\{phi^prime\}\_\{u\}\; =\; phi\_u+2etasinleft(frac\{phi^prime+phi\}\{2\}\; ight),$:$\{phi^prime\}\_\{v\}\; =\; -phi\_v+frac\{2\}\{eta\}sinleft(frac\{phi^prime-phi\}\{2\}\; ight)mathrm\{,\; with\; \}phi\; =\; phi\_0\; =\; 0$ for all time.

The 1-soliton solutions can be visualized with the use of the elastic ribbon sine-Gordon model as discussed by "Dodd and co-workers". [

*Dodd RK, Eilbeck JC, Gibbon JD, Morris HC. "Solitons and Nonlinear Wave Equations". Academic Press, London, 1982.*] Here we take a clockwise (left-handed) twist of the elastic ribbon to be a kink with topological charge $vartheta\_\{\; extrm\{K=-1$. The alternative counterclockwise (right-handed) twist with topological charge $vartheta\_\{\; extrm\{AK=+1$will be an antikink.**3-soliton solutions**3-soliton collisions between a traveling kink and a standing breather or a traveling antikink and a standing breather results in a phase shift of the standing breather. In the process of collision between a moving kink and a standing breather,the shift of the breather $Delta\_\{\; extrm\{B$ is given by:

$Delta\_\{B\}=frac\{2\; extrm\{arctanh\}sqrt\{(1-omega^\{2\})(1-v\_\{\; extrm\{K^\{2\})\{sqrt\{1-omega^\{2\}$

where $v\_\{\; extrm\{K$ is the velocity of the kink, and $omega$ is the breather's frequency. If the old position of the standing breather is $x\_\{0\}$, after the collision the new position will be $x\_\{0\}+Delta\_\{\; extrm\{B$.

**Related equations**The

**sinh-Gordon equation**is given by:$varphi\_\{xx\}-\; varphi\_\{tt\}\; =\; sinhvarphi,$

This is the

Euler–Lagrange equation of theLagrangian :$mathcal\{L\}=\{1over\; 2\}(varphi\_t^2-varphi\_x^2)-coshvarphi,$

Another closely related equation is the

**elliptic sine-Gordon equation**, given by:$varphi\_\{xx\}\; +\; varphi\_\{yy\}\; =\; sinvarphi,$

where "φ" is now a function of the variables "x" and "y". This is no longer a soliton equation, but it has many similar properties, as it is related to the sine-Gordon equation by the

analytic continuation (orWick rotation ) "y" = i"t".The

**elliptic sinh-Gordon equation**may be defined in a similar way.A generalization is given by

Toda field theory .**Quantum version**In quantum field theory the sine-Gordon model contains a parameter. The particle spectrum consists of a soliton, an anti-soliton and a finite (possibly zero) number of breathers. The number of the breathers depends on the value of the parameter.

**In finite volume and on a half line**On can also consider the sine-Gordon model on a circle, on a line segment, or on a half line. It is possible to find boundary conditions which preserve the integrability of the model. On a half line the spectrum contains

boundary bound state s in addition to the solitons and breathers.**upersymmetric sine-Gordon model**A supersymmetric extension of the sine-Gordon model also exists. Integrability preserving boundary conditions for this extension can be found as well.

**Notes****References*** Polyanin AD, Zaitsev VF. "Handbook of Nonlinear Partial Differential Equations". Chapman & Hall/CRC Press, Boca Raton, 2004.

* Rajaraman R. "Solitons and instantons". North-Holland Personal Library, 1989.

**External links*** [

*http://eqworld.ipmnet.ru/en/solutions/npde/npde2106.pdf Sine-Gordon Equation*] at EqWorld: The World of Mathematical Equations.

* [*http://eqworld.ipmnet.ru/en/solutions/npde/npde2105.pdf Sinh-Gordon Equation*] at EqWorld: The World of Mathematical Equations.

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