It’s important that all the clocks in a reference frame be synchronized, meaning that all clocks in the reference frame have the same reading at any one instant of time. Thus we need a method of synchronization. One idea that comes to mind is to designate the clock at the origin as the master clock. We could then carry this clock around to every clock in the lattice, adjust that clock to match the master clock, and finally return the master clock to the origin.

This would be a perfectly good method of clock synchronization in Newtonian mechanics, where time flows along smoothly, the same for everyone. But we’ve been driven to reexamine the nature of time by the possibility that time is different in reference frames moving relative to each other. Because the master clock would move, we cannot assume that the moving master clock would keep time in the same way as the stationary clocks.

We need a synchronization method that does not require moving the clocks. Fortunately, such a method is easy to devise. Each clock is resting at the intersection of meter sticks, so by looking at the meter sticks, the assistant knows, or can calculate, exactly how far each clock is from the origin. Once the distance is known, the assistant can calculate exactly how long a light wave will take to travel from the origin to each clock. For example, light will take $1.00\mu \text{s}$ to travel to a clock 300 m from the origin.

To synchronize the clocks, the assistants begin by setting each clock to display the light travel time from the origin, but they don’t start the clocks. Next, as FIGURE 36.13 shows, a light flashes at the origin and, simultaneously, the clock at the origin starts running from $t=0\text{s}$. The light wave spreads out in all directions at speed c. A photodetector on each clock recognizes the arrival of the light wave and, without delay, starts the clock. The clock had been preset with the light travel time, so each clock as it starts reads exactly the same as the clock at the origin. Thus all the clocks will be synchronized after the light wave has passed by.

FIGURE 36.13 Synchronizing clocks.