Definitions

# Sine-Gordon equation

The sine-Gordon equation is a nonlinear hyperbolic partial differential equation in 1+1 dimensions involving the d'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 of soliton 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

$, phi_\left\{tt\right\}- phi_\left\{xx\right\} + sinphi = 0.$

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

$u=frac\left\{x+t\right\}2, quad v=frac\left\{x-t\right\}2,$

the equation takes the form:

$varphi_\left\{uv\right\} = sinvarphi.$

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 called pseudospherical surfaces. 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. The first 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 the Codazzi-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 of Bäcklund transformations.

The name "sine-Gordon equation" is a pun on the well-known Klein–Gordon equation in physics:

$phi_\left\{tt\right\}- phi_\left\{xx\right\} + phi = 0.$

The sine-Gordon equation is the Euler–Lagrange equation of the Lagrangian

$mathcal\left\{L\right\}_\left\{mathrm\left\{sine-Gordon\right\}\right\}\left(phi\right) := frac\left\{1\right\}\left\{2\right\}\left(phi_t^2 - phi_x^2\right) + cosphi.$

If you Taylor-expand the cosine

$cos\left(phi\right) = sum_\left\{n=0\right\}^infty frac\left\{\left(-phi ^2\right)^n\right\}\left\{\left(2n\right)!\right\}$

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

$mathcal\left\{L\right\}_\left\{mathrm\left\{sine-Gordon\right\}\right\}\left(phi\right) - 1 = frac\left\{1\right\}\left\{2\right\}\left(phi_t^2 - phi_x^2\right) - frac\left\{phi^2\right\}\left\{2\right\} + sum_\left\{n=2\right\}^infty frac\left\{\left(-phi^2\right)^n\right\}\left\{\left(2n\right)!\right\}$

$= 2mathcal\left\{L\right\}_\left\{mathrm\left\{Klein-Gordon\right\}\right\}\left(phi\right) + sum_\left\{n=2\right\}^infty frac\left\{\left(-phi^2\right)^n\right\}\left\{\left(2n\right)!\right\}.$

## 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:

$phi_\left\{mathrm\left\{soliton\right\}\right\}\left(x, t\right) := 4 arctan e^\left\{m gamma \left(x - v t\right) + delta\right\},$

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

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 $phi=0\left(textrm\left\{mod\right\}2pi\right)$ 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:

$\left\{phi^prime\right\}_\left\{u\right\} = phi_u+2betasinleft\left(frac\left\{phi^prime+phi\right\}\left\{2\right\}right\right),$
$\left\{phi^prime\right\}_\left\{v\right\} = -phi_v+frac\left\{2\right\}\left\{beta\right\}sinleft\left(frac\left\{phi^prime-phi\right\}\left\{2\right\}right\right)mathrm\left\{, with \right\}phi = phi_0 = 0$ 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. Here we take a clockwise (left-handed) twist of the elastic ribbon to be a kink with topological charge $vartheta_\left\{textrm\left\{K\right\}\right\}=-1$. The alternative counterclockwise (right-handed) twist with topological charge $vartheta_\left\{textrm\left\{AK\right\}\right\}=+1$will be an antikink.

### 2-soliton solutions

Multi-soliton solutions can be obtained through continued application of the Bäcklund transform to the 1-soliton solution, as prescribed by a Bianchi lattice relating the transformed results. The 2-soliton solutions of the sine-Gordon equation show some of the characteristic features of the solitons. The traveling sine-Gordon kinks and/or antikinks pass through each other as if perfectly permeable, and the only observed effect is a phase shift. Since the colliding solitons recover their velocity and shape such kind of interaction is called an elastic collision.

Another interesting 2-soliton solutions arise from the possibility of coupled kink-antikink behaviour known as a breather. There are known three types of breathers: standing breather, traveling large amplitude breather, and traveling small amplitude breather.

### 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 $Delta_\left\{textrm\left\{B\right\}\right\}$ is given by:

$Delta_\left\{B\right\}=frac\left\{2textrm\left\{arctanh\right\}sqrt\left\{\left(1-omega^\left\{2\right\}\right)\left(1-v_\left\{textrm\left\{K\right\}\right\}^\left\{2\right\}\right)\right\}\right\}\left\{sqrt\left\{1-omega^\left\{2\right\}\right\}\right\}$

where $v_\left\{textrm\left\{K\right\}\right\}$ is the velocity of the kink, and $omega$ is the breather's frequency. If the old position of the standing breather is $x_\left\{0\right\}$, after the collision the new position will be $x_\left\{0\right\}+Delta_\left\{textrm\left\{B\right\}\right\}$.

## Related equations

The sinh-Gordon equation is given by

$varphi_\left\{xx\right\}- varphi_\left\{tt\right\} = sinhvarphi,$

This is the Euler–Lagrange equation of the Lagrangian

$mathcal\left\{L\right\}=\left\{1over 2\right\}\left(varphi_t^2-varphi_x^2\right)-coshvarphi,$

Another closely related equation is the elliptic sine-Gordon equation, given by

$varphi_\left\{xx\right\} + varphi_\left\{yy\right\} = sinvarphi,$

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 (or Wick rotation) y = it.

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 states in addition to the solitons and breathers.

## Supersymmetric 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.

## 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.