Special Relativity originally dealt with the symmetries of the electromagnetic field and their consequences for experiments and for the interpretation of space and time measurements. It arose at the end of the nineteenth century from the difficulties in understanding the properties of light when this light was tested by observers at rest or in relative motion. Its name originated from the surprise that many of the concepts of classical non-relativistic physics refer to a frame of reference (‘observer’) and are true only relative to that frame.

The symmetries mentioned above show up as transformation properties with respect to Lorentz transformations. It was soon realized that these transformation properties have to be the same for all interacting fields, they have to be the same for electromagnetic, mechanic, thermodynamic, etc. systems. To achieve that, some of the ‘older’ parts of the respective theories had to be changed to incorporate the proper transformation properties. Because of this we can also say that Special Relativity shows how to incorporate the proper behaviour under Lorentz transformation into all branches of physics. The theory is ‘special’ in that only observers moving with constant velocities with respect to each other are on equal footing (and were considered in its derivation).

Although the words ‘General Relativity’ indicate a similar interpretation, this is not quite correct. It is true that historically the word ‘general’ refers to the idea that observers in a general state of motion (arbitrary acceleration) should be admitted, and therefore arbitrary transformation of coordinates should be discussed.