All events we witness are labeled by a series of numbers, three to tell
us where it happened,
and one to determine *when* it happened. All in all four numbers are
needed. These numbers are determined by some measuring devices such as
measuring rods and clocks.

According to Newton (see Sect. 4.3.1)
the properties of measuring rods and clocks can be
made completely independent of the system which they measure (if it does
not look like that, you can buy a higher quality device which will
satisfy this criterion). But Einstein showed this is *not* the case:
even Cartier watches slow down when compared to Seiko watches when they
move with respect to each other. Even high density steel beams will be
measured to be shorter than wimpy papers when their relative velocity
is non-zero.

The measurements obtained by two observers in motion relative to each other
are not identical, but they *are* related. For example, the times
measured by two clocks are related by the time-dilation formula given
earlier. Suppose observer A measures the location and time at which an
event occurs: spider-man ran the 100 yard dash in 3 seconds flat. Now
observer B, moving with respect to A, wants a description of this feat
in his own coordinates. In order to find how many yards
spider man ran *as measured
by B* this observer needs to know his velocity with respect to A, the
distance spidy ran as measured by A (100 yds) and how long did he take
as measured by A's clock (3 sec); it is *not* enough to know the
distance and relative velocity, the *time* it took is also needed.

The fact that in order to compare results from different observers both
position and time are required is completely foreign to Newtonian
mechanics. Yet this is the way the universe is organized. Far from being
independent, space (that is, position) and time are interlinked. In
fact, the mathematical description of the Special Theory of Relativity
is most naturally expressed by combining space and time into one object:
*space-time*. A point in space-time determines the position *
and* time of occurrence of an event.

Within Special Relativity space-time is unaltered by whatever is in it. There are rules that state how the measurements of two observers are related, but these rules are unaltered by the objects (and beings) that populate space-time, they are the same whether we look at a pea, an elephant or a star millions of times more massive than the Sun. Space and time are still the arena where Nature unfolds.

We will see when we describe the General Theory of Relativity (Chap. 7) that space-time is far from being this imperturbable object where things just happen, it is in fact a dynamical system which affects and is affected by the matter in it. The development of our ideas of space and time from being independent of each other and imperturbable, to being meshed into space-time system, to being a dynamical object is one of the most profound developments derived from the general and special theories of relativity.