There is an abundance of clocks in our life; personal clocks like our wristwatch and smart phone, public clocks like in train stations and on TV and radio broadcasting channels; scientific clocks in laboratories including the atomic standard clock. Still, in our experience time is an endless one-dimensional succession of seconds, hours, days, years, centuries… .
This is so because all our clocks are in sync: we force clocks to show the same time always and everywhere. Loosely speaking, this means that they all display the same reading at the same time, even if they do not all display the same reading (for example they are in different time zones). The synchronization really refers to the fact that given one reading, we know the readings of all the others at that moment.
The basic principles of synchronization are simple ones:
- Clocks to be synchronized are fixed (stationary) relative to each other. If a clock moves, there is a difference between its reading and that of other clocks (due to relativity).
- Everybody must agree on simultaneity; that is, if two events are simultaneous for one, then they are simultaneous for all. This defines the common clock and the common time.
In our high-tech society the technical details of synchrony are quite involved, although synchronizing clocks at the level of microseconds is well understood and is achieved by swapping handshake signals. However, at the level of high precision (nanoseconds and beyond), the notion of ‘the same time’ must be analyzed very carefully, and requires concepts of (general) relativity.
- The question which events are simultaneous for entities in arbitrary motion and influenced by gravity is a subtle one, still subject to discussion amongst physicists.
- Due to gravity, clocks that are stationed over a wide area of Earth cannot agree on the timing of events to better than some tens of nanoseconds.