Is Global Simultaneousness Compatible with Special Relativity?

Special relativity theory abandoned the notion of absolute space and time. For many physicists, it also abandoned the notion of global time, i.e., a unique time coordinate that could be assigned to the overall universe. On the other hand, with theories and discussions on the dynamical development of the overall universe, the notion of a global state, which is associated with global time, seems to be very useful, if not unavoidable. This paper discusses to what extent the notions of global states, global time and global simultaneousness can be defined such that they are compatible with Einstein’s relativity theories. The results show that the establishment of a global coordinate system requires, in addition to the choice of suitable space and time coordinates the selection of a physical object with an assumed uniform velocity.