From the 1999 lecture programme:
Early clocks relied on the flow of some substance, such as water or sand, to measure the flow of time itself.
However, there is nothing around us that can flow evenly enough to measure time accurately. It was later realised that a more reliable method would be to use "inverse time", in other words oscillations or waves. If we could find a system with some natural oscillation period, then an agreed number of these oscillations could be counted in order to define a unit of time. One such system which was used early on is the swinging pendulum.
In fact this dependence on oscillations, as opposed to the flow of some substance, is something which lives with us to this very day in our most advanced clocks. Anyone who has forgotten to wear their watch on a particular day will know how difficult it can be to judge how much time has passed.
However it turns out that our bodies are remarkably good at judging "inverse time" or oscillations, especially those associated with light and sound waves. In particular, our bodies are excellent detectors of the colours and music (or noise!) we experience all around us. It is 200 years since Thomas Young put forward his wave theory of light.
Today, we are literally able to sculpt both light and sound waves to create optic-fibre communication, perform intricate laser surgery and ultrasound scans, and design fantastic special effects for our own entertainment. From an artistic perspective, it also appears that the music we most like contains time patterns which obey certain universal laws, despite our often quite different musical tastes!
BBC / Royal Institution