above
(the world-lines being vertical, this distance can of course never vary with time). Now, since the distance of the two stones must remain the same for the moving observer, he also must measure this distance along a horizontal, hence along the space axis
. In other words, the space axis and the time axis are unique, or absolute.
It follows, of course, that since the space and time axes remain unchanged regardless of our motion along the embankment, all horizontals will represent events occurring simultaneously not only for an observer at rest on the embankment, but for all observers. We thus get the absolute nature of simultaneity.
Then again, since all observers measure time along the same direction, they will all recognise one same absolute distinction between past and future, regardless of their position and motion along the embankment. Thus, point-events below
([Fig. XIII]), will represent events that occurred on the embankment prior to zero hour; those point-events lying above the line