The world offers many things that you could use for measuring time: many processes from day and night, though pendulum, to human pulses, provide synchronised repetition, which is essential.
Light clocks
are a robust choice because of their simplicity. That’s because the universal speed provides a reliable basis for generating ticks: the ticks are reflections of light off a pair of mirrors. Reliable ticking is the basis for measurement, so start by constructing a simple clock – a pulse of light bouncing between mirrors.
This Alice’s clock. What makes the clock hers is that it moves along with Alice: it records Alice-time
. The clock is co-moving with Alice: any other clock co-moving with her will tick at the same rate: including the biochemical clocks that control her physical changes. Although the light clock is simple, it does report Alice-time
: reporting her increasing age.
Next, consider a clock with a variable separation of the mirrors to explore the clock's workings. Constant (universal) speed and increased separation of mirrors reduce the tick rate.
Such clocks are simple in principle and preferred so that nothing is hidden.