In the absence of sources, Maxwell's equations reduce to: ∂ ∂ ν ν ∂ ∂ ν ν F α α β β = def ∂ ∂ 2 F α α β β = def 1 c 2 ∂ ∂ 2 F α α β β ∂ ∂ t 2 − − ∇ ∇ 2 F α α β β = 0 , which is an electromagnetic wave equation in the field strength tensor.