Experiment 8: Mr. Einstein is on a railroad car moving
to the left with velocity v, and on his car are two light bulbs that, from
his perspective, come on simultaneously. To confirm this, he could also
rig some sort of detector that would go off only if both beams of light
arrive at his position simultaneously.
Question: What will Mrs. Einstein see?
Answer: She will agree that both beams of light reach Mr. Einstein at the same time. However, since from her point of view the light on the right has greater distance to travel, she will see the light on the right come on first!
Conclusion: From the above experiments we see that events which may be simultaneous for one observer can happen in a different sequence for another observer. This leads us to the startling conclusion that there is no such thing as a universal "now" for which everyone will agree on what happens "now". That is, I can see two events as happening "now" while another observer will see one event happening "now" with the other event yet to occur!