English: (a) By symmetry, two streams of particles moving in opposite directions leaves a central particle undisturbed. (b) In the frame of the top stream, the bottom stream has more kinetic energy and the spacing between the particles of the bottom stream are Lorentz-contracted, both resulting in an increased gravitational attraction on the central particle. The physical situation has not changed simply because of the change in frames. The central particle because of a velocity-dependent repulsion from the bottom stream, i.e. gravitomagnetism.