In other pages in this series we have seen that there is direct and measurable evidence for time dilation. One example from the many available, that of atomic clocks carried on aircraft, has shown that moving clocks do indeed run more slowly than stationary ones, just as predicted by Einstein. For the moment though it is sufficient to say that scientific proof is only available in models, and never in reality. Of course, as Einstein was happy to admit, the only real way to test a theory is by carrying out experiments in reality. A Light Clock.
Clocks exist in many forms.
Candles with marks on them to show how long they have been burning. Sundials that project the Sun's shadow onto graduated clock faces. Clockwork clocks that gradually release the energy stored in wound springs. Clocks that measure the vibrational frequency of crystals such as quartz, or even atoms. Most clocks measure how many times a repetitive action is carried out.