A thought experiment: X is a particle moving horizontally 0.8c to right, L is a light beam moving vertically to up, and O is an observer at rest. With precise timing in test arrangement, light beam hits X directly from below and gets absorbed by X (all beam's energy transfers to kinetic energy, for simplicity). As the light beam has only upwards momentum in the reference frame of O, O would expect that the collision changes only the vertical velocity of X and the horizontal velocity remains unchanged. However, in the reference frame of X, light beam doesn't approach directly from below, but diagonally from bottom-right. In collision, beam's momentum would split to horizontal and vertical component, therefore changing also the relative horizontal velocity between O and X, which doesn't change in the reference frame of O. As the two reference frames seem to disagree (at least in my understanding), is there some absolute way to solve this? Thanks in advance.