D'Alembert operator
In special relativity, electromagnetism and wave theory, the d'Alembert operator (represented by a box: ), also called the d'Alembertian or the wave operator, is the Laplace operator of Minkowski space. The operator is named for French mathematician and physicist Jean le Rond d'Alembert. In Minkowski space in standard coordinates (t, x, y, z) it has the form:
Here is the 3dimensional Laplacian and is the inverse Minkowski metric with , , for . Note that the μ and ν summation indices range from 0 to 3: see Einstein notation. We have assumed units such that the speed of light . Some authors also use the negative metric signature of [− + + +] with .
Lorentz transformations leave the Minkowski metric invariant, so the d'Alembertian is a Lorentz scalar. The above coordinate expressions remain valid for the standard coordinates in every inertial frame.
Contents
Alternate notations
There are a variety of notations for the d'Alembertian. The most common is the symbol : the four sides of the box representing the four dimensions of spacetime and the which emphasizes the scalar property through the squared term (much like the Laplacian). This symbol is sometimes called the quabla (cf. nabla symbol). In keeping with the triangular notation for the Laplacian sometimes is used.
Another way to write the d'Alembertian in flat standard coordinates is . This notation is used extensively in quantum field theory where partial derivatives are usually indexed: so the lack of an index with the squared partial derivative signals the presence of the D'Alembertian.
Sometimes is used to represent the fourdimensional LeviCivita covariant derivative. The symbol is then used to represent the space derivatives, but this is coordinate chart dependent.
Applications
The wave equation for small vibrations is of the form
 where is the displacement.
The wave equation for the electromagnetic field in vacuum is
 where is the electromagnetic fourpotential.
The Klein–Gordon equation has the form
Green's function
The Green's function for the d'Alembertian is defined by the equation
where is the multidimensional Dirac delta function and and are two points in Minkowski space.
A special solution is given by the retarded Green's function which corresponds to signal propagation only forward in time
 ^{1}
where is the Heaviside step function.
See also
References
 ^ Siklos. "The causal Green’s function for the wave equation". Retrieved 2 January 2013.
External links
 Hazewinkel, Michiel, ed. (2001), "D'Alembert operator", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 Poincaré, Henri (1906). On the Dynamics of the Electron (July). Wikisource., originally printed in Rendiconti del Circolo Matematico di Palermo.
 Weisstein, Eric W., "d'Alembertian", MathWorld.

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