d'Alembert Operator

$\square ={\partial }^2={\partial }^{\mu }{\partial }_{\mu }={\eta }^{\mu \nu }{\partial }_{\nu }{\partial }_{\mu }={\nabla }^2-\frac{1}{c^2}\frac{{\partial }^2}{\partial t^2}$

• ${\eta }^{\mu \eta }$ is the Minkowski Metric
• The last result is sometimes opposite in other texts