As stated in SR and the principle of equivalence, there is no privileged reference frame or reference frame at absolute rest. However, I went across the following idea that seems to be able to tell if a reference frame is moving or at rest. Please help me point out what is wrong in my idea: Below is a typical diagram in SR for time dilation: http://c5fktg.bay.livefilestore.com/y1pJJyR8RSkfOuKZEWNaoUE69kdG4zl7PVXMRoCmpRsgTty5zXwmtQ60SSVIwQlEmjrlGm9hPbVCOzGCvb0fBM7URyLQ4NR9sev/light_clock.GIF The top left picture shows a light clock at rest. Note that flash of light bulb emits light towards all directions. Let us say seven rays to be convenient - besides the vertical ray, there are three on the left and three on the right. The top right picture shows a light clock moving towards the right at about 1/4 c, so the 5th ray from the left will hit the mirror on the top and bounce back to hit the same position as the bulb. Now let us switch to observe it in the reference frame of the moving light clock. Since the 5th ray from the left will bounce up and down. Therefore, there are four rays on the left and only two on the right. So, my idea is: By measuring the difference of light ray density (or number of photons in unit area) on the left and right side, I can tell if the reference frame is at rest or moving, and the velocity of moving. Please tell me where I am wrong. Thanks!