What is time?
Time is a critical aspect of many forms of knowledge. So, what is time? At a simplistic level, time is simply an ordering of moments at which events occur or could occur. Given any two events or moments, they either occur simultaneously, or one before the other and one after the other. Events can also partially overlap, so we technically should speak of the sub-events of when each event begins and when they end. For the purpose of this discussion it is those indivisible sub-events that are of significance.
Traditionally, we "measure" time by choosing some periodic and regular event and simply counting the occurrences of that event.
This begs the question of what an "event" is. At a simplistic level, an event is motion or propagation that is detectable. Put another way, an event is a detectable change in position or appearance. Motion includes chemical reactions, biological processes, and physical state changes. At the microscopic level motion may simply be large numbers of particles (molecules, atoms, subatomic particles) that move so that a macroscopic change is detectable. We need to use the term detectable in a theoretical sense of could something be detectable rather than the practical sense of whether we do in fact have usable tools to accomplish the detection. Time is the fact that the change occurred (an event), separate from our observation of the change (the event). Propagation includes a field or force or "wave" of some sort that travels some distance. At a human scale we depend on propagation to sense that motion has occurred, but in those cases it is the change in motion that signifies time rather than the role of propagation in aiding us in sensing the change in motion.
Rather than using transient events, we can also look for changes in conditions and look at the interval of time over which that condition is true. Ultimately, that is the same as looking at the pair of events, one of which corresponds to the start of the condition and the other of which corresponds to the end of the condition. Obviously events of some other form must be transpiring during that interval in order for us to judge that the interval of time is occurring.
So, we use time to measure and specify when an event occurred (or a condition changed) as well as the length of the interval between two events or the duration of a condition.
Now, what about the gaps between the occurrences of events, does time exist and "pass" (or "flow") in those gaps? If no detectable physical process can occur during the gap, I would say that time does not exist for the duration of the gap. The trivial answer is that since we can not detect the presence of the gap (i.e., two events seem to occur one right after the other), then there actually isn't any gap.
What is the smallest detectable motion or propagation that is detectable? Don't know. Is that the true unit of time? Maybe. Since we do not know that unit, we traditionally use some larger unit, typically the second, and refer to fractions of that unit. Conceptually, we can divide that unit ever finer so that conceptually we are referring to intervals which are smaller than the smallest intervals of motion and propagation that theoretically could be detectable, or at least have detected so far. Conceptually, we can do the math to refer to hypothetical moments of time which never existed as detectable events. But from a practical, human perspective, that is not much of an issue.
Humans also have their minds and even without observing events in the outside world can envision the passage of time. One's breathing or beating heart or blinking eyes can provide the periodic events needed to detect and measure the passage of time. Mental state changes (how fast can you think or count?) can also supply the events needed to detect the passage of time. At this human level, there is probably a cognitive "unit" of time which is the smallest interval that a typical human mind can detect. There is probably a smaller unit which would be the smallest unit of chemical and electrical and biological activity that a device could detect, but that is no longer the "human" unit of time.
At a macroscopic level, the motion of the sun, sunrise and sunset, movement and phases of the moon and stars, turns of seasons, movements of migratory animals, life cycle changes, sun spot cycles, comets, and other natural phenomena provide periodic event sequences needed to detect and measure the passage of time.
We can also measure time in a social sense in terms of ages and eras, times when particular technologies or values or modes of behavior are prevalent. Once again, events and changes are used to "measure" the rise and fall of a socially significant period of time.
And, we can also similarly measure the natural world in terms of geological and biological events and conditions that change.
Then there is time at the cosmic scale, with the birth, evolution, and death of stars, galaxies, and other cosmic structures. Millions and billions of years.
And we also have time at the sub-atomic level that is of concern to physicists, but that is a rather distinct discussion.
Ultimately, we measure all of this on a single, combined scale ranging over:
- Fractions of seconds, down to a billionth or even a billionth of a billionth of a second
- Millions of years
- Billions of years
A future topic is: What does time mean? Does time itself have any meaning, apart from the meaning of events that we are measuring? Or, is time itself inherently, and almost by definition meaningless? Maybe not. We can certainly refer to time as an "object of discourse", which would of course have meaning. But, time itself, distinct from any events that might be occurring would seem rather meaningless. On the other hand, if we define time in terms of events occurring, maybe time is always implicitly linked to some meaning, in particular the meaning of the events that mark time. We might even go so far as to suggest that time carries meaning (i.e., time is the carrier of meaning), since without the medium of time, the events could not be transpiring and having their meaning.