In this example I make 2 assumptions 1. if an object is moving through space at time T0 it will be at point x,y,z,T0 and at a different time T1 it will be at a different position x,y,z,T1. The other assumption I make is that light is non ballistic. see the associated pics Referring to fig 1 There is a box that may be moving through space. In the box is a strobe light at the red square, a mirror at the blue square and an observer with a clock at the green square. All objects are at rest wrt the box. The strobe flashes briefly once. Light will travel from the strobe to the observer. Light will also travel to the mirror reflect off the mirror to the observer. So the observer will see two flashes of light separated by a time period. Using the clock the observer measures the period between the two flashes of light. In fig 1 the light source is to the left of the direction of travel, the mirror on the right. With this configuration the observer fires the strobe and measures the time period he sees between the two flashes of light. Lets say he gets a reading of X The observer now changes the position of the mirror and the light. He places the mirror where the light source was and the lgiht source where the mirror was. The observer then places himslf at the psoition he was in in fig 1. The only thing that changes is the position of the mirror and the light all distances between object are the same as in fig 1. With this configuration the observer fires the strobe and measures the time period he sees between the two flashes of light. I calculate that the observer will not get a reading of X. Why is this or am I wrong?