We have studied the elements of Einstein's special theory of relativity, and can now put them together into a more panoramic view. He began by assuming the truth of two principles, both drawn from experience and experiments: the principle of relativity; and the constant speed of light. From these two central principles, Einstein and his followers deduced a series of stunning consequences, of which we have met several in turn during the previous chapters:

1. • time dilation

2. • relativity of simultaneity

3. • length contraction

4. • symmetry of effects

5. • relativistic mass increase

6. • energy-mass conversion

7. • celestial speed limit

8. • invariance of the spacetime interval.

These are predictions about what observation and measurement will discover, that is, about phenomena and appearances. We have not explored the details of the arguments Einstein gave for deducing these effects from his principles. It is enough here to state that they are consequences of the principles and have been confirmed by experiments.

These two principles and their predicted consequences together form the theory: Einstein's special theory of relativity. Note that this deals only with measurements made by equipment moving inertially (say, carried by a coasting spaceship). Einstein removed this restriction in his general theory, which we will examine in Chapter 13. Clearly, there is much that is puzzling and mysterious about the special theory of relativity. Why is the speed of light, unlike all other moving things, constant? Why are steady speeds undetectable? Why are physical laws the same regardless of speed? Why do distances and durations and masses depend on speed? Einstein's theory does not answer these questions.