I am having difficulty wrapping my mind around Einstein's relativity, and am wondering if someone can please help to enlighten me. My Physics teacher explained Einstein's relativity like this - say there were two spacecraft, one speeding forward and the other is standing still. If each spacecraft fired a photon beam forward, the measurement for the speed of the photon beam would be the same on each ship, because the speed of light is always constant. This is reconciled in the equation speed = distance over time, since time actually travels slower on the craft as the craft moves faster (though it's not experienced such on the craft). This makes sense - speed is distance over time, and if we know the speed of light, and we know the distance between the craft and the photon beam, then time is the variable. To get to the point, here is my question: What if you fired the photon beam off the BACK of the spaceship, so you're moving away from the photon beam? Time must still slow down, but if the speed of light is still constant, how can that be explained in terms of relative distance and time? That is - since you're moving away from the beam of light, intuitively one would think that the light beam would "stretch" relative to the craft and therefore appear to be moving faster than the speed of light - however we know that this isn't so. So what actually happens, from the perspective of the craft moving away from the light beam?