In Fig. 5-7a, two parallel, infinitely long streams of massive particles have equal and opposite velocities − v and + v relative to a test particle at rest and centered between the two. Because of the symmetry of the setup, the net force on the central particle is zero. Assume ⁠ v ≪ ≪ c ⁠ so that velocities are simply additive. Fig. 5-7b shows exactly the same setup, but in the frame of the upper stream. The test particle has a velocity of + v, and the bottom stream has a velocity of +2 v. Since the physical situation has not changed, only the frame in which things are observed, the test particle should not be attracted towards either stream.