Here, ◻ ◻ = ± ± η η μ μ ν ν ∂ ∂ μ μ ∂ ∂ ν ν is the wave operator and ∇ ∇ 2 is the Laplace operator. The speed of light c and Planck constant ℏ ℏ are often seen to clutter the equations, so they are therefore often expressed in natural units where c = ℏ ℏ = 1 .