I'm confused again by a relativistic situation. On train A a mirror is placed atop another, with distance h between them. A light beam is bouncing back and forth between them. Now 1 light-hour away, spaceship B is traveling almost towards A at half the speed of light. not traveling in a directly straight line to point A, but maybe a few meters to the side of A. This way the line that the light beam appears to travel is a diagonal line to B. This diagonal has height h and width whatever it is, depending on the angle of B and its speed and what not, as long as it's greater than zero, we'll call it w. Basic Pythagorean theorem gives the distance that the light beam travels between mirror hits: sqrt(h^2+w^2) Now the doppler effect should make A's clock seem to be moving faster than it actually is. Does this mean that for B, it sees the light beam as traveling faster than the speed of light? the reason is that if B were at rest relative to A, the light beam would cover a distance of h in t amount of time. (traveling at c). But when moving towards A, it covers what appears a greater distance sqrt(h^2+w^2) in even quicker time (doppler shift), requiring a velocity faster than c? What am I missing?