I know this is a silly question, but I'm only asking this because I honestly can't seem to find a good definition of time that doesn't include another word that relies on the definition (or logical understanding rather) of time.

For example, in many dictionaries the definition of time is as follows:

Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them.

But the concept of 'past', 'present', and 'future' (mostly the first and last) rely heavily on time.

My best shot at a definition of time is this:

Time is the dimension on which the evolution of state of a system is allowed to occur.

Is this question senseless? Is the notion of time best left axiomatic? Do you know of a good/better definition of time? (The more concise the better)

  • 4
    $\begingroup$ Time is that which a suitable clock shows. Here is a piece of good advice: look into a physics textbook if you want to learn about physics, don't use the dictionary. The same is true for chemistry, biology or anything else. Dictionaries are very poor sources of information about anything, sometimes including language. $\endgroup$
    – CuriousOne
    Jan 2, 2015 at 5:37
  • $\begingroup$ Ah that definition is actually pretty satisfying. $\endgroup$ Jan 2, 2015 at 5:47
  • 1
    $\begingroup$ Yes, it is, right until you start musing about what "suitable clock" is supposed to mean... :-) I hope you got my hint, though: what is important is the fact that one can find "clocks" in nature that show some numbers that have the properties of "time" (i.e. they keep increasing monotonously) and several of them, if kept close enough together, even if they are different in make show pretty much the same time all of the time. $\endgroup$
    – CuriousOne
    Jan 2, 2015 at 5:55
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/17056/2451 , physics.stackexchange.com/q/15371/2451 and links therein. $\endgroup$
    – Qmechanic
    Jan 2, 2015 at 9:28

3 Answers 3


Well , here is my experimentalist answer:

Time is a necessary parameter in order to describe observed changes in three dimensional space, dx/dt, dy/dt, dz/dt. If there were no observable changes in the (x,y,z) contour map of the world (including us as contours also) there would be no time parametrization needed.

These changes are an experimental fact and to start with the day and night clock was used to define the parameter. Clocks can be anything that consistently reproduces periodically the same (x,y,z) for a specific location/point.

This is classical time. Special relativity and even more General Relativity are a different story with much more sophisticated mathematical modeling.


Physics uses a system of units, of which duration, length, and mass are examples.

The goal of a physical prediction is to use these units and a fundamental concept of "betweenness". And the predictions go something like, if you measure this length then the measurement would be between the 2.14 meter marking and the 2.16 meter marking. Another prediction might be the flash occurs between the 3:14:56 reading of the clock and the 3:14:58 reading of the clock. It is the exact same concept of betweenness in the two cases. And since the modern definition of the meter is actually based on clocks, they truly are the same thing now.

Every measurement is predicted as being between one fiduciary marking/reading and another. So clocks and their reading are how we measure units of time, and always as a prediction as being between two markings or readings. Distances are measured by rulers. When you have a theory, you have to ultimately figure out a way to get a prediction about something we can measure.

  • $\begingroup$ Clocks only run in one direction, yardsticks can be moved forth and back... there is not even a remote resemblance between time and space (even though certain geometric theories of spacetime gloss over the obvious and are deceptive to students who don't pay attention). The dependence between measurables and theories is the other way around, of course. What comes first is the observation of quantities that can be measured and then we fit a theory to them, not the other way round. $\endgroup$
    – CuriousOne
    Jan 2, 2015 at 6:59

There are two kinds of time not to be confounded one with each other:

  1. Time is proper time. Mass is transporting energy through time, we may call it "persistence" or "aging" of an object, as synonyms of time. Time is measured by any clock. A clock becomes "suitable" if we can make some synchronization with our own clock, but any clock is measuring time. A clock cannot measure consecutively in two time directions, it would be no clock.

  2. We can use our own clock or any other clock to evaluate simultaneity of events, that means we are comparing the moment of an event with our clock of reference by categories such as "before", "after" or "simultaneous". Simultaneity is relative and is proper to each clock.

In summary we have to distinguish between proper times of objects which cannot be part of the same coordinate system and coordinate time of one clock of reference which puts all events in one coordinate system.

Time is linked to space by the spacetime mechanism wherein it is considered as a kind of fourth dimension. But apart from this mechanism there is no similitude between space and time.


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