I am sure this question in some form gets posted here all the time again and again, but I somehow can not find it in the form I would recognize, so my appologies for probably posting it again. So one of the classics: Two light beams - A and B get launched from Earth to the opposite directions at the same moment from the same place. A rocket with some man inside of it launches in the direction of B beam at the same time the beams get launched and from the same place. Another man stays on earth at that particular spot. Let's assume light moves 1 megameter per second and rocket immediately accelerates to 0.5 megameters per second. .........=>...... A<----|--->B I kind of had a good and intuitively pleasing explanation on what happens with the rocket man, earth man and beam B. So after 1 earth man's second he says that beam is 1 megameter away from earth, but the rocket man objects that after 1 rocket man's seconds the beam is 1 megameter away from his ship - as his ship is already 0.5 megameters away from earth then it's 1.5 megameters away from earth and not 1. Here the slowing down of the rocket man's clock would be a very cool and easy to understand explanation of the phenomena that both see beam B leaving them with the same speed of 1 megameter per second, even though rocket man is chasing it. I kind of got the feeling that now I understand everything:) And then I started to think about beam A...:) Ok, just to be closer to the real situation I included into the calculations the time that is needed for the information about the location of both beams to be delivered to the ship and earth. So the picture from earths point of view after 1 second looks the following: 1) A is 0.5 megameters away from Earth (it actually is further but that information has not reached Earth yet) 2) B is also 0.5 megameters away from Earth 3) Rocket man should see beam B being 0.75 megameters away from the rocket, 4) Rocket man should see beam A being 0.25 megameters away from the rocket. So my common sense and the so often heard word simetry (not intuition of course) says that rocket man says the same replacing Rocket <> Earth and A <> B. And the results of the experiments with light say that rocket man in reality should see both beams being away the same distance all the time, i.e., there can be no such moment that the rocket man sees beam B being 0.75 megameters and beam A being 0.25 megameters away from the rocket. So - how do you grasp this from the Earth man's point of view? How is the rocket man misinterpreting the data? Is it so that from Earth man's point of view the rocket man sees everything behind him squeezed by ~0.375 and everything in front of him expanded by ~2 and what about the clock then... Or this question does not make sense due to some reason?