( ◻ + m 2 c 2 ℏ 2 ) ψ ( x , t ) = 0 {\displaystyle \left(\Box +{\frac {m^{2}c^{2}}{\hbar ^{2}}}\right)\psi (x,t)=0} ( ◻ = ∂ 2 c 2 ∂ t 2 ) {\displaystyle \left(\Box ={\frac {\partial ^{2}}{c^{2}\partial t^{2}}}\right)}
∂ 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 }
