D'Alembert operator explained

In special relativity, electromagnetism and wave theory, the d'Alembert operator (denoted by a box:

\Box ), also called the d'Alembertian, wave operator, box operator or sometimes quabla operator^[1] (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, it has the form

\begin{align} \Box & = \partial ^{\mu} \partial _{\mu} = η ^{\mu\nu} \partial _{\nu} \partial _{\mu} = (1)/(c ^{2}) (\partial ^{2})/(\partial t ^{2}) - (\partial ^{2})/(\partial x ^{2}) - (\partial ^{2})/(\partial y ^{2}) - (\partial ^{2})/(\partial z ^{2}) \\ & = (1)/(c ^{2}) {\partial ^{2} \over \partial t ^{2}} - \nabla ^{2} = (1)/(c ^{2}) {\partial ^{2} \over \partial t ^{2}} - \Delta ~~. \end{align}

Here

\nabla ^{2} : = \Delta is the 3-dimensional Laplacian and is the inverse Minkowski metric with η _{00} = 1 , η _{11} = η _{22} = η _{33} = -1 , η _{\mu\nu} = 0 for \mu ≠ \nu .Note that the and summation indices range from 0 to 3: see Einstein notation.

(Some authors alternatively use the negative metric signature of, with

η _{00} = -1 , η _{11} = η _{22} = η _{33} = 1 .)

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.

The box symbol and alternate notations

There are a variety of notations for the d'Alembertian. The most common are the box symbol

\Box (Unicode:) whose four sides represent the four dimensions of space-time and the box-squared symbol \Box ^{2} which emphasizes the scalar property through the squared term (much like the Laplacian). In keeping with the triangular notation for the Laplacian, sometimes \Delta _{M} is used.

Another way to write the d'Alembertian in flat standard coordinates is

\partial ^{2} . 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

\nabla 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

\Box _{c} u\left(x,t\right) \equiv u _{tt} - c ^{2 u} _{xx} = 0 ~, where is the displacement.

The wave equation for the electromagnetic field in vacuum is

\Box A ^{\mu} = 0 where is the electromagnetic four-potential in Lorenz gauge.

The Klein–Gordon equation has the form

\left(\Box + (m ^{2c} ^{2})/(\hbar ^{2}) \right) \psi = 0 ~.

Green's function

The Green's function,

G\left(\tilde{x} - \tilde{x}'\right) , for the d'Alembertian is defined by the equation \Box G\left(\tilde{x} - \tilde{x}'\right) = \delta\left(\tilde{x} - \tilde{x}'\right)

where

\delta\left(\tilde{x} - \tilde{x}'\right) is the multidimensional Dirac delta function and \tilde{x} and \tilde{x}' 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^[2]

G\left(\vec{r}, t\right) = (1)/(4 \pi r) \Theta(t) \delta\left(t - (r)/(c) \right)

where

\Theta is the Heaviside step function.

See also

• Four-gradient
• d'Alembert's formula
• Klein–Gordon equation
• Relativistic heat conduction
• Ricci calculus
• Wave equation
• One-way wave equation

External links

• • Translation:On the Dynamics of the Electron (July). Poincaré. Henri. 1906., originally printed in Rendiconti del Circolo Matematico di Palermo.

Notes and References

1. Book: Theoretische Physik . 2015 . 978-3-642-54618-1 . Aufl. 2015 . Berlin, Heidelberg . 899608232 . Bartelmann . Matthias . Feuerbacher . Björn . Krüger . Timm . Lüst . Dieter . Rebhan . Anton . Wipf . Andreas .
2. Web site: S. Siklos. The causal Green's function for the wave equation. 2 January 2013. 30 November 2016. https://web.archive.org/web/20161130174612/http://www.damtp.cam.ac.uk/user/stcs/courses/fcm/handouts/wave_equation.pdf. dead.