Acceleration and Inertia

Abstract

In the first three chapters, we saw which effects steady, straight motion has on space, time, and mass. The part of the theory of relativity dealing with these phenomena is the theory of special relativity. Now we go one step further and allow the speed to change with time, that is, we allow for acceleration. The simplest case is rotation with constant speed, as on a merry-go-round. Then school geometry ceases to be true, because rods shrink differently in different directions. Clocks act even more weirdly, leading to the famous twin paradox, which we explain in detail. Because space itself bends when we accelerate, space depends on time via the changing speed, and becomes space-time: for example, the “shortest path between two points in space” of school geometry no longer has a definite meaning.