The gamma matrices are denoted by γ, in which again μ = 0, 1, 2, 3, and there are a number of representations to select from. The matrix γ is not necessarily the 4 × 4 identity matrix. The expression i ℏ ℏ γ γ μ μ ∂ ∂ μ μ + m c ≡ ≡ i ℏ ℏ ( γ γ 0 ∂ ∂ 0 + γ γ 1 ∂ ∂ 1 + γ γ 2 ∂ ∂ 2 + γ γ 3 ∂ ∂ 3 ) + m c ( 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 ) is a 4 × 4 matrix operator which acts on 4-component spinor fields.