Special theory of relativity  (Page 5/9)

In special theory of relativity, time and time rate in two inertial references are treated as different. If t and t' be the time recorded for a given event in two inertial references, then t may not equal to t'. We shall return to this topic again.

Absolute and stationary reference

There is no preferred inertial reference frame. This idea predates special relativity. It means that there is no absolute reference frame. Had there been an absolute reference, then we would have a fixed universal space in which all other objects (references) would be considered to be either in rest or moving. But the concept of space is a variant. In other words, the perception of space changes from one reference (say ground) to other reference (say moving train). If we drop a stone from a train, then the trajectory of the stone is a straight vertical line for an observer on the train. The same stone, however, is seen to follow a parabolic trajectory for an observer on the ground. Referring to these trajectories for a single motion of a stone, Einstein questioned the very concept of fixed space.

Trajectory of motion

Though there is no absolute reference, but the notion of a stationary reference is a powerful idea which stems from our life long perception of stationary objects in Earth's reference. Despite the fact that Earth is moving at about 107,278 km/hr (29.8 km/s) around Sun, we are generally not aware about it unless we make detailed observations about celestial objects. But as the concept of stationary system is ingrained in our perception, we employ this concept with great effect in the study of relative motion. We refer either of the moving systems as stationary in which an observer is making the measurements. Consider for example two spacecrafts moving with uniform velocities. We can refer either of spacecrafts as stationary and other as moving with a velocity which is measured from the referred stationary reference (spacecraft). There is no preference. In the case of a motion of a train, for example, we consider Earth as stationary and train as moving reference. There is no bar though that we consider moving train as stationary reference with the observer and Earth as moving reference in the opposite direction to the moving train.

The "rest" is local concept. The object like a house is at rest in Earth's reference. But the same object as seen from a spaceship is not stationary.

Constancy of speed of light

Constancy of speed of light has two different considerations in the study of relativity. In the first place, it is the central idea of special relativity. But besides this consideration, the constancy of light has other important consideration in that it is one of the measuring standards which can not be challenged in any inertial reference. This aspect is important as meaning of space and time in different inertial frames is not very explicit. We shall see subsequently that they are in fact entangled. Further, the distance (space) and time are relative and are therefore very subjective in conception and measurement.