1 c 2 ∂ 2 ∂ t 2 ψ − ∇ 2 ψ + m 2 c 2 ℏ 2 ψ = 0 {\displaystyle {\frac {1}{c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}\psi -\nabla ^{2}\psi +{\frac {m^{2}c^{2}}{\hbar ^{2}}}\psi =0}
