The fundamental element of relativity is called an event. An event is a physical activity that takes place at a definite point in space and at a definite instant of time. An exploding firecracker is an event. A collision between two particles is an event. A light wave hitting a detector is an event.

Events can be observed and measured by experimenters in different reference frames. An exploding firecracker is as clear to you as you drive by in your car as it is to me standing on the street corner. We can quantify where and when an event occurs with four numbers: the coordinates $(x,y,z)$ and the instant of time t. These four numbers, illustrated in FIGURE 36.11, are called the spacetime coordinates of the event.

FIGURE 36.11 The location and time of an event are described by its spacetime coordinates.

The spatial coordinates of an event measured in reference frames S and $\text{S}\prime$ may differ. It now appears that the instant of time recorded in S and $\text{S}\prime$ may also differ. Thus the spacetime coordinates of an event measured by experimenters in frame S are $(x,y,z,t)$ and the spacetime coordinates of the same event measured by experimenters in frame $\text{S}\prime$ are $(x\prime ,y\prime ,z\prime ,t\prime )$.

The motion of a particle can be described as a sequence of two or more events. We introduced this idea in the preceding section when we agreed to measure the velocity of a bicycle and then of a light wave by making measurements when the object passed the tree (first event) and when the object passed the lamppost (second event).