The two movies below show a railroad car (boxcar) that is moving to
the right at half the speed of light. On the side of the track is
a telegraph pole. When the center of the boxcar is alligned with
the telegraph pole a spark jumps between the pole and boxcar.
This produces a flash of light, which travels towards both ends of the
When this scene is viewed from the reference frame of the
telegraph pole (upper movie), the light reaches the back end of the
boxcar before the front end. This is because the boxcar is
moving, so the light has a shorter distance to travel to get to the
back of the boxcar than to the front.
The movie below shows the same situation from
the frame of reference of the boxcar. In this frame the boxcar is
stationary, and the light has the same distance to travel to the two
ends of the car. Since the speed of light is the same in both
directions the light arrives at both ends of the car at the same
Thus, two events -- light reaching the back of
the boxcar and light reaching the front of the boxcar -- that are
simultaneous in one inertial frame (boxcar frame) are
simultaneous in another intertial frame (pole frame).