Let's say that according to frame B, we have two identical bodies with the same invariant mass, say 1 kg, each travelling in opposite directions at .1 c, where v1 = .1 c and v2 = -.1 c, which then collide and stick together. Since the frame is homogeneous and the bodies are identical, they will become stationary within frame B, correct? Now let's look at what frame A will observe. Frame A measures frame B to have a relative speed of v = .6 c. By applying relativistic addition of speeds, the speeds frame A measures of the two bodies, then, are v1' = (v + v1) / (1 + v v1 / c^2) = .660377358 c v2' = (v + v2) / (1 + v v2 / c^2) = .531914893 c According to A, then, the total momentum of the system of two bodies before the collision is p1 + p2 = (m v1') / sqrt(1 - (v1'/c)^2) + (m v2') / sqrt(1 - (v2'/c)^2) = .879408087 kg m / sec + .628148633 kg m / sec = 1.507556721 kg m / sec The final momentum after the collision is p3 = ((2m) v) / sqrt(1 - (v/c)^2) = 1.5 kg m / sec Relativistic momentum doesn't appear to be conserved. Why not?