Special relativity / Elementary Tour part 1: Relative to whom?

The prime example of a situation governed by special relativity is a region far, far away in the depths of space, far away from all stars and planets (and their gravitational influence). Imagine that, in this dark void, there are freely moving space stations, drifting along without any acceleration or rotation. On each of these stations sits an observer, with his own clocks and his own measuring rods, measuring times and distances. In addition, each such observer has a fully equipped physics lab on board, where he or she can perform a variety of experiments to explore the laws of physics. This is the kind of observer Einstein talks about, an observer in a free, unaccelerated frame of reference. Such frames of reference (and such observers) are commonly called inertial frames of reference (and inertial observers).

Two inertial observers move relative to each other

Einstein’s special relativity is all about observers like these. As the inertial observers move relative to each other, which of the statements they make are relative, and which are absolute?

If the answers were valid only in deep space, that would be a somewhat academic question. But many of the laws that one can derive for observers in empty space can be adapted to observers here on earth, for instance to scientists studying particles in a particle accelerator. In fact, researchers have been able to test many of the predictions of special relativity in ordinary laboratory experiments.