Equação de Klein-Gordon

$(◻+\frac{m^2{c}^2}{{\hslash }^2})\psi (x,t)=0$

onde

$(◻=\frac{{\partial }^2}{c^2\partial t^2}-{\mathrm{\nabla }}^2)$

então

$\frac{{\partial }^2\psi }{\partial t^2}=c^2{\mathrm{\nabla }}^2\psi -\frac{m^2{c}^4}{{\hslash }^2}\psi$