*** Please ignore. Just noticed my error !!! *** Ok. Having been beaten senseless untill I can see how the maths behind Einstein's Mirrors Experiment works, another question to help with my journey. http://en.wikipedia.org/wiki/File:Time-dilation-002.svg The above diagram is a representation of the setup for the experiment. As pointed out in another thread, a speed for the moving observer of (1/sqrt(2))c, or 0.707c, generates a 45 degree angle for the path of the light signal in the stationary observer's frame. As the stationary observer, we know where this beam is heading. If we place an opaque tube in line with the expected path, the signal will travel straight through the tube and hit the final mirror. In the moving observer's frame, a 45 degree tube approaching at 0.707c, wouldn't allow the signal to pass straight through it to the bottom mirror. However, as length dilation is equivalent to the time dilation, the tube, rather than being defined by an x * x box, giving a 45 degree angle, is defined by a 0.707x * x box, giving a 54.74 degree angle. The signal now meets the tube at exactly the right time for it to travel along the angled tube and still hit the mirror directly below. Apologies. This is going somewhere, hopefully. Is this right so far ?