This is our final article on teaching special relativity in the first week of an introductory physics course.1–3 One of the profound changes in our view of the world was Einstein's discovery of the lack of simultaneity. He illustrated this result with a thought experiment in which we observe a railroad car passing by us. We see the two ends of the car struck simultaneously by lightning bolts, but to someone riding inside the car, the lightning strikes were not simultaneous. My difficulty with this thought experiment is that while doing calculations, I have to go back and forth between two imagined points of view. To avoid this, I actually perform an experiment that involves two simultaneous events. then all we have to imagine is how the experiment looks to someone moving by us. Not only does the order of the two events depend on the direction of motion of the observer, but we can demonstrate that if information could travel faster than the speed of light, we could get answers to questions that have not yet been thought of.

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