In technical terms, we say that Einstein has understood that “absolute simultaneity” does not exist: there is no collection of events in the universe that exist “now.” The collection of all the events in the universe cannot be described as a succession of “nows,” of presents, one following the other; it has a more complex structure, illustrated in figure 3.2. The figure describes that which in physics is called “spacetime”: the set of all past and future events, but also those that are “neither-past-nor-future”; these do not form a single instant: they have a duration.
