Since the relationship is true in the rest frame of the second particle, it is true in any reference frame. E 1 , the energy of the first particle in the frame of the second particle, is a Lorentz scalar. Therefore, E 1 = γ γ 1 γ γ 2 m 1 c 2 − − γ γ 2 p 1 ⋅ ⋅ u 2 in any inertial reference frame, where E 1 is still the energy of the first particle in the frame of the second particle.