∂ 2 ψ ∂ t 2 = c 2 ∇ 2 ψ − m 2 c 4 ℏ 2 ψ {\displaystyle {\frac {\partial ^{2}\psi }{\partial t^{2}}}=c^{2}\nabla ^{2}\psi -{\frac {m^{2}c^{4}}{\hbar ^{2}}}\psi }
