This concludes Part I of this book. I hope you have been able to grasp an outline of what relativity is all about. Let us summarize what we have learned:

• The “Special Theory of Relativity” was constructed by Einstein to resolve the mystery of the speed of light. Einstein's solution was that the concept of simultaneity depended on the frame of reference. And the rule that relates the observations from different frames was given by the Lorentz transformation.

The predictions of Special Relativity such as time dilation and Lorentz contraction are as infamous as they are famous. The reason for the notoriety is due to the apparent paradoxical nature of the prediction: say we have two frames, A and B, moving relative to each other. According to Special Relativity, the observer in frame A will observe the clock in frame B to run slower than the clock in frame A, and the ruler in frame B to be shorter than the ruler in frame A. The observer in frame B will observe the exact opposite. Now how can both points of view be true at the same time?

Of course, the two points of view are NOT true at the same time. They are both true because they are NOT at the same time.