It is a fact of life that not all events in our universe happen concurrently – instead, there is a certain order. Defining a time coordinate or defining time, the way physicists do it, is to define a prescription to associate with each event a number so as to reflect that order – if event B happens after event A, then the number associated with B should be larger than that associated with A. The first step of this definition is to construct a clock: Choose a simple process that repeats regularly. (What is “regular”? Luckily, in our universe, all elementary processes such as a swinging pendulum, the oscillations of atoms or of electronic circuits lead to the same concept of regularity.) As a second step, install a counter: A mechanism that, with every repetition of the chosen process, raises the count by one.

With this definition, one can at least assign a time (the numerical value of the counter) to events happening at location of the clock. For events at different locations, an additional definition is necessary: One needs to define simultaneity. After all, the statement that some far-away event A happens at 12 o’clock is the same as saying that event A and “our clock counter shows 12:00:00” are simultaneous. The how and why of defining simultaneity – a centre-piece of Einstein’s special theory of relativity – are described in the spotlight topic Defining “now”.

With all these preparations, physicists can, in principle, assign a time coordinate value (“a time”) to any possible events, and describes how fast or how slow processes happen, compared to that time coordinate.