The interval of time between two events is the elapsed time between the two. This interval is called a duration of time. It is measured as the number of ticks of a clock; a duration is a number with a unit such as seconds. But what about a moment in time, can it be defined, does it exist? In informal conversation, an instant or moment is a very short duration. In physics, however, an instant is even shorter. It is instantaneous; it has zero duration.
There is a deep philosophical dispute about whether points of time actually exist, just as there is a similar dispute about whether spatial points actually exist. The dispute began when Plato wrote “This queer thing, the instant, …occupies no time at all….”. Some philosophers wish to disallow point-events and point-times. They want to make do with intervals, and want an instant always to have a positive duration.
- Points of time cannot be detected. Nobody could ever measure time that finely, even indirectly.
- Nevertheless, in physics it is generally assumed that any interval of time is a linear continuum of the points of time that compose it.