501KiB, 1600x1564, velocities-speed-of-light-Invariance-Arrows-train[1].jpg
If one object A were traveling at 90% the speed of light and another object B were traveling at 90% the speed of light in the opposite direction, how fast would they appear to be going from a 3rd perspective C, e.g. if A and B passed each other right in front of C?
