When students are first introduced to special relativity, the first thing they are taught is the derivation of the gamma factor and time-dilation, and it goes something like this: Suppose there are 2 observers in space, let's call them Alice and Bob. Let's assume Alice is stationary from our perspective, and Bob, who is holding a light clock in his hands, is traveling past Alice at some finite speed. Since the speed of light is constant, the time taken for the light beam to bounce between the 2 mirrors is longer for Bob from Alice perspective.... But doesn't this thought experiment assume that light has mass, and therefore inertia? Because if light has no inertia, how can it travel diagonally (to match Bob's speed) from Alice point of view? If light is emitted in a direction perpendicular to the direction of motion of Bob, shouldn't the light beam go straight up, and miss the mirror that has moved ahead?