d'Alembert operator in Einstein notation Updated 2025-07-16
Given the function :the operator can be written in Planck units as:often written without function arguments as:Note how this looks just like the Laplacian in Einstein notation, since the d'Alembert operator is just a generalization of the laplace operator to Minkowski space.
Klein-Gordon equation in Einstein notation Updated 2025-07-16
The Klein-Gordon equation can be written in terms of the d'Alembert operator as:so we can expand the d'Alembert operator in Einstein notation to: