Equação de Klein-Gordon

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

$(◻+\frac{{\hslash }^2{m}^2{c}^2}{2})$