- Sine-Gordon equation
The sine-Gordon equation is a nonlinear hyperbolic
partial differential equationin 1+1 dimensions involving the d'Alembert operatorand 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 solitonsolutions.
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
Passing to the "light cone coordinates" ("u", "v"), akin to "asymptotic coordinates" where
the equation takes the form:
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 formof the surface in these coordinates has a special form
where "φ" expresses the angle between the asymptotic lines, and for the
second fundamental form, "L" = "N" = 0. Then the Codazzi-Mainardi equationexpressing 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 equationin physics:
The sine-Gordon equation is the
Euler–Lagrange equationof the Lagrangian
If you Taylor-expand the
and put this into the Lagrangian you get the Klein-Gordon Lagrangian plus some higher order terms
An interesting feature of the sine-Gordon equation is the existence of
solitonand multisoliton solutions.
The sine-Gordon equation has the following 1-
The 1-soliton solution for which we have chosen the positive root for is called a "kink", and represents a twist in the variable which takes the system from one solution to an adjacent with . The states 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 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:
:: 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 . The alternative counterclockwise (right-handed) twist with topological charge will be an antikink.
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 is given by:
The sinh-Gordon equation is given by
This is the
Euler–Lagrange equationof the Lagrangian
Another closely related equation is the elliptic sine-Gordon equation, given by
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" = i"t".
The elliptic sinh-Gordon equation may be defined in a similar way.
A generalization is given by
Toda field theory.
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.
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.
* 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.
* [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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