First post on PF! Anyways, I'm a high school student so please forgive my lack of knowledge in some areas :) So I was reading about special relativity and I understand the ideas behind both postulates. It's the application of those postulates that confuses me. For example, please let me know if my idea of time dilation is correct. So if a person is sitting on a train with a flashlight and turns it on, the speed of that beam of light will be 186,282 mps. The additional movement of the train is not added to the speed of light because nothing (with mass) can travel faster than light (because of infinite energy). So since the speed of light cannot change, light accounts for for the added velocity of the train by slowing down time itself. Is that correct? Because the way I think of it, a stationary flashlight and a moving flashlight (both are turned on at the same position) will give off a beam of light towards an object that has the same speed. Since speed = distance / time, and both speed and distance are the same for both flashlights, time is the only variable that can change for the moving flashlight. Is this correct? But then this doesn't explain why the stationary flashlight also experiences time dilation. If I have this part correct, then I'll ask the REAL question I had (which is related to low speeds as mentioned in the title).