### Everyone Measures the Same Speed of Light

A pulse of light travels from the left side of the screen toward two observers. The lower observer is making measurements in an intertial frame that is stationary relative to the frame of the movie. The upper observer is making measurements using an intertial frame that is moving to the left at a velocity of 0.4 times the speed of light. Each observer measures the speed of light by noting the time that the pulse passes each of the two clocks (say times t1 and t2). The speed can then be determined by taking the distance between clocks (say L) divided by the time difference t2-t1. That is v = L/(t2-t1).  Note that even though the upper observer is moving toward the light pulse, this observer measures the same time interval between clocks as the lower obsever, and thus the same speed of light.   The explanation for this is that even though the right clock of the upper observer receives the light pulse before the right clock of the lower observer, from the frame of the lower observer (and the movie), the upper observer's clocks are not synchronized.  Of course, the upper observer would argue that her clocks are synchronized, but the lower observers clock are not.  Although the observers disagree about whose clocks are synchronized, they both measure the same speed of light.