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Concept
D'Alembert operator
Summary
In special relativity, electromagnetism and wave theory, the d'Alembert operator (denoted by a box: ), also called the d'Alembertian, wave operator, box operator or sometimes quabla operator (cf. nabla symbol) is the Laplace operator of Minkowski space. The operator is named after 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 3-dimensional Laplacian and ημν 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 c = 1.
(Some authors alternatively use the negative metric signature of (− + + +), with .)
Lorentz transformations leave the Minkowski metric invariant, so the d'Alembertian yields a Lorentz scalar. The above coordinate expressions remain valid for the standard coordinates in every inertial frame.
There are a variety of notations for the d'Alembertian. The most common are the box symbol (Unicode: ) whose four sides represent the four dimensions of space-time and the box-squared symbol which emphasizes the scalar property through the squared term (much like the Laplacian). 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 the box symbol is used to represent the four-dimensional Levi-Civita covariant derivative. The symbol is then used to represent the space derivatives, but this is coordinate chart dependent.
The wave equation for small vibrations is of the form
where u(x, t) is the displacement.
The wave equation for the electromagnetic field in vacuum is
where Aμ is the electromagnetic four-potential in Lorenz gauge.
Official source
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