Space-time symmetry is broken
  Space-time intervals corresponding to different events on the worldline of any ponderable object (for example a clock) are time-like. In consequence, in the analysis of any space-time experiment involving clocks only the region for $c\Delta t \ge 0$ between the line $\Delta x = 0$ and the light cone projection $c\Delta t = \Delta x$ of the $c\Delta t$ versus $\Delta x$ Minkowski plot is physically relevant. This breaks the manifest space-time symmetry of the plot. A further consequence is the unphysical nature of the `relativity of simultaneity' and `length contraction' effects of conventional special relativity theory. The only modification of space-time transformation laws in passing from Galilean to special relativity is then the replacement of universal Newtonian time by a universal (position independent) time dilation effect for moving clocks. 
J.H. Field
physics.gen-ph