SRT is a classical physical theory and as a such deals with description of physical reality by numbers. The presumption used in any classical theory is that physical reality CAN be described by numbers. From one side this prsumption based on the believe that physical reality is unique and from the other side on the fact that the numbers are unique.
- SRT introduced 4 dimensional supernumbers to describe moving objects.
- SRT found 4-metric tensor: before we knew only that the proper length of the stright line between the points A(x=0,y=0) and B(x,y) is sqrt(x2+y2). Now we know that the proper length of the stright line between the points A(t=0,x=0) and B(t,x) is sqrt(x2-c2t2) or proper time is sqrt(c2t2-x2).
- According to the requirements of 4D description Einstein improved Newton's dynamics equation to the Relativistic Dynamics Equation.
These three are the big advantages and they represent the second face of Janus. There is no relativity here. We did not care in 3D geometry that different observers can use different 3D coordinat systems. We have here the same option: not to care about other observers also. All physics with all "relativistic effects" can be described in just one 4D coordinate system. There is a little disadvantage: to manage the second face of Janus we have to be able to manage math on a very high level. At least we have to know Riemann's N-dimensional geometry which in fact is a mathematical theory of n-dimensional supernumbers. This is why the second face looks not so attractive and normally hides behind the first face. But the second face is a real thing while tne first face is a fake...