The flavor of Special Relativity

At the heart of Special Relativity lies a simple idea: Physical laws are the same in any set of frames of reference that are in relative uniform motion.

If you take the constancy of the speed of light as a physical law, then lots of funky effects appear: Time Dilation, Length Contraction, Relative Simultaneity, etc. Light having a constant speed, independent of the frame of reference you measure it, is sometimes called the second postulate of the Special Theory of Relativity.

Frame of references are important and are not introduce for construction. When we talk about motion, we need a reference frame to describe the motion.

Take for example riding a train. When the train is arriving with passengers, from the station you see the train approaching. You are standing still, and you know this fact because you do not feel any other force (except our good old friend gravity). We can say that with respect of the station, the train is moving and changing its position.

Now from one of the passenger's point of view, he/she sees the station approaching. Our passenger is standing still in the train, in rest with respect to it (since the passenger is moving along with the train), and those not feel any force before the train starts to decelerate. This is one of those fancy trains...

Both points of view, whether the train or station were approaching are valid. Besides motion, relativity tells us that physics is the same in every frame of reference.

Speed is a measure of how positions changes along time. Galilean relativity assures us that according to our relative velocity, objects may be seen at rest (relative velocity is zero) or in motion (non-zero relative velocity). Since the speed of light is constant, then it has to be measure the same in every frame of reference relative to light. Hence, space and time have to arrange themselves in a certain way so that in any frame the speed of light is the same constant.

I consider this the thesis of Special Relativity: the study of space-time as a "dynamical" entity.