*** Had a chance to rethink this and still believe it makes sense, so I need somebody to put me straight :-) *** 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 ? Now, in the stationery observer's frame, we can add a mirror perpendicular to the tube and a distance from the end of it, to reflect the signal straight back along the tube. Space dilation should squash the dimensions of the mirror in the same way as the tube, changing it from a diagonal across a box at y * y, giving 45 degrees, to a diagonal acroos a box of dimension 0.707y * y, giving an angle of 54.74 degrees, but in the opposite direction to the tube. In the 'moving' observer's frame, the light still travels down the tube as described above, but then hits a mirror pointing away from the tube. The signal bounces off nowhere near the tube. This does rather assume that light hitting a moving mirror rebounds at the same angle as light hitting a stationery mirror. Is this correct ? What's wrong with the above ? Both can't be true, so there must be an error in the 'logic' somewhere ? Thanks.