I just found out what relativity is and got some questions: Actually, only one: So, according to einstein, 1. Light travels the same speed independent of the source speed. 2. Its not possible to tell which object is moving compared to an other object. I got that if one object A is moving faster than a object B, and fires a laser vertical to the direction it moves, from B perspective the laser will travel a longer distance. Ok. As we cant even tell which object is moving, i could say that object B is moving and A is not. So, in the same situation, if object B fires a laser vertical to object A moving direction, from object A perspective the laser would travel a longer distance too. So, in the same situation, if booth fire the laser i could say that time for object A slows down compared to object B, and time for object B slows down compared to object A? Wouldnt it cancel the time dilation and nothing would happen? Where am i wrong? Im not trying to discuss about how the theory is wrong, im trying to figure out how it works.