¿Why one cannot push light but just in the normal direction to the ray? NAIVE QUESTION: I understand I cannot push a light ray (I mean, to sum speed vectors) in its propagation path, but ¿why I can certainly push it in the normal direction to the light ray? Is there some info on this subject anywhere in the web? Please, I am not looking for opinions nor comments, but just academic sites where the answer to this question have been scientifically developed. I have not found any info on this matter in any text on special relativity theory and I do not understand how anyone could talk about special relativity without previously explaning such anisotropy of light. many thanks for your help I am overwhelmed by this extremely simple question and by the fact that I have not found any info on this matter. Many thanks for any help, indeed.