Events are what “really happen,” but how do we learn about an event? That is, how do the experimenters in a reference frame determine the spacetime coordinates of an event? This is a problem of measurement.

We defined a reference frame to be a coordinate system in which experimenters can make position and time measurements. That’s a good start, but now we need to be more precise as to how the measurements are made. Imagine that a reference frame is filled with a cubic lattice of meter sticks, as shown in FIGURE 36.12. At every intersection is a clock, and all the clocks in a reference frame are synchronized. We’ll return in a moment to consider how to synchronize the clocks, but assume for the moment it can be done.

FIGURE 36.12 The spacetime coordinates of an event are measured by a lattice of meter sticks and clocks.

Now, with our meter sticks and clocks in place, we can use a two-part measurement scheme:

• The $(x,y,z)$ coordinates of an event are determined by the intersection of the meter sticks closest to the event.

• The event’s time t is the time displayed on the clock nearest the event.

You can imagine, if you wish, that each event is accompanied by a flash of light to illuminate the face of the nearest clock and make its reading known.

Several important issues need to be noted:

1. The clocks and meter sticks in each reference frame are imaginary, so they have no difficulty passing through each other.

2. Measurements of position and time made in one reference frame must use only the clocks and meter sticks in that reference frame.

3. There’s nothing special about the sticks being 1 m long and the clocks 1 m apart. The lattice spacing can be altered to achieve whatever level of measurement accuracy is desired.

4. We’ll assume that the experimenters in each reference frame have assistants sitting beside every clock to record the position and time of nearby events.

5. Perhaps most important, t is the time at which the event actually happens, not the time at which an experimenter sees the event or at which information about the event reaches an experimenter.

6. All experimenters in one reference frame agree on the spacetime coordinates of an event. In other words, an event has a unique set of spacetime coordinates in each reference frame.