Along the North South axes we measure off on the graph paper equal distances for equal physical quantities of [ct] and along the East West it is the same for lengths in space in the one (x) direction. So what do these measurements of [ct] actually mean physically? If we were conducting a scientific experiment ,and the experimenter is situated at the origin how does he or she make these two measurements so as to uniquely quantify a point on the graph? Is "t" counted by ,theoretically any regular cyclic event such as someone very regularly tapping their finger or a tap dripping very regularly (or a good clock ,obviously)? Then after a number of cycles of this "time keeper" how is [ct] measured? Do you use a mirror and divide by two after you have measured the distance to it in exactly the same way as you have measured distances along the x-axis? If I have correctly described that physical process can I next ask about how the spacetime interval itself would be physically measured in a way corresponding to a [ct,x] point on the graph?