Originally Posted by **MBW** In my view, the key points of the theory are:
1) The Principle of Relativity - The laws of physics are the same (i.e. invariant) in all inertial frames of reference.
2) Invariant Light Speed.
3) The __concept__ of a 4 dimensional SpaceTime.
All the rest (e.g. time dilation) is a consequence of these 2 basic postulates,
(Invariant Laws, Constant Speed of Light)
expressed in the context of a 4 dimensional SpaceTime. |

Your "3)" is something that pre-dates relativity theory. It is present in all of physics since physicists began using three dimensional geometry and time. What special relativity theory does is take the way that we conceptualize space and time (through the ideal processes of measurement and the ideal Newtonian mechanics and Maxwell's electrodynamics behavior of physical objects at a specific location) and use the other two principles to show that one cannot make a determination of the nature of spatial coordinates without at the same time making a determination of the nature of time coordinates. This ends us implying that a 4D spacetime geometry is the underlying geometry of a 3D space + 1D time geometry that can be produced in a number of ways. This then forces us to write physical laws in certain ways to ensure that they can be expressed in a way that allows them to be applied to any (special) system of coordinates such that the laws can be easily translated from one system to another.

(General relativity then accepts this behavior at the level of geometry for adjacent points, but allows complete freedom of coordinate choice. This then requires additional restrictions on the way that physical laws are expressed so that they can be applied to any system of coordinates.

So I would say that the underlying principles of special relativity are:

1) The Principle of Relativity - Changing the system of coordinate one used, as long as they are coordinates in which one can do physics, does not change physics.

2) Invariant Light Speed - the speed of light between coordinate points is the same in all coordinate systems in which one can do physics. [This relies also on the assumption of most, if not all, of Maxwell's electrodynamics.]

3) Those coordinate systems in which it is possible to do physics are those coordinate systems in which Newtonian mechanics correctly describes motion.