A rod PQ of proper length 1. lies along the X-axis and moves towards the positive x direction with speed = with respect to the ground (see figure), where is the speed of light in vacuum. An observer on the ground measures the positions of P and Q at different times tp and to respectively in the ground frame, and finds the difference betwen them to be What is the value orto - tp? L 3c L Sc L 2L 3c 0 P Q 9L 10 .