Here we can directly compare the moving clock with synchronised stationary clocks. The stationary clocks tick twice while the moving clock ticks only once. This represents time dilation, that is time slowing down, by a factor of two.

Moving light clocks slow down - but do ordinary clocks slow down? According to the second postulate of special relativity they must: no experiment can tell you that you are moving absolutely. Imagine placing your favourite clock on top of the moving light clock, and moving with them both. The two clocks must keep the same time, otherwise this would tell you that you are moving, which violates the second postulate. Since the light clock slows, so must the other clock.

Unlike the relativistically simple light clock, the details of why other types of clocks slow down may be complicated. However, if the special relativistic postulates are true, then slow down it must.