# Localization

Observers discuss systems under observation (SUOs) in terms of relatively localized points in space and time, conceptual objects called *events* located in a conceptual arena called *space-time* in classical mechanics and *spacetime* in special relativity (SR) and general relativity (GR).

# Temporal ordering

There is a sense of temporal *order* in the universe: observers classify events as earlier, simultaneous, or later, relative to some chosen reference event.

# Temporal architecture

This is the verbal description of the various relationships between events, observers, and SUOs and depends on the mathematical model used. For example, there is an obvious difference between linear time and cyclic time. Another important architectural difference involves the *space-time* and *spacetime* concepts discussed in Chapter 5. Space-time has the traditional architecture used by all human observers sitting in any laboratory, measuring laboratory time with clocks and locating events in three-dimensional space using rulers. On the other hand, spacetime need not be split into two such components in this way: it has an architecture of a four-dimensional continuum independent of any observer, a model introduced by Minkowski in 1908 and reviewed in Chapter 19. We discuss temporal architecture further in Chapter 13.