(Problem added in revision of 2nd edition). Express the components pμ of 4-momentum for a photon. Assume it is traveling in the x1 direction. Use natural units where speed of light c=1. (Hint: Use energy expressed in terms of frequency f and 3-momentum in terms of wave length λ.) Then show that even though pμ=0, (p)2=pμpμ=0. (Hint: Use speed of light expressed in terms of frequency and wave length.) Does this make sense in light of (2-55), given what we know about the photon mass? Then express pμ in terms of ω=2πf and wave number k=2π/λ, where ℏ=h/2π (= 1 in natural units).