D'Alembert Operator

#Math

$\displaystyle \Box =\partial ^{2}=\partial^{\mu}\partial_{\mu}=\eta^{\mu \nu}\partial_{\nu}\partial_{\mu}=\nabla ^{2}-\frac{1}{c^{2}}\frac{\partial^2 }{\partial t^{2}}$

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