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Skill 2 of 13
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What exactly is time?

You might say: ‘time is duration’ or ‘time is how long something lasts’. But that’s circular; that’s like saying ‘time is how much time that has passed’. It doesn’t make much sense to use words like ‘duration’ and ‘how long it lasts’ before the idea of time has been invented. It does seem really hard to explain ‘time’.

But we can say that ‘time’ is what the moving cars show. And what the flying birds show. And what the withering flowers show. Because ‘time’ seems to be a word invented for change.

Time is that which a clock shows

Common saying[1,2]

There are many types of change:

  • We can have a change in temperature when something heats up,
  • a change in position when something moves,
  • a change in size when something deforms,
  • etc.

What all such examples have in common is that the old and new situations are “separated”. The ‘before’ and the ‘after’ do not overlap; they are not there simultaneously. We are able to distinguish between ‘before’ and ‘after’.

This “separation”, this distinguishing, is mysterious. Why does the world behave in such a way that situations are distinguishable? As difficult as it is to imagine a world where this is not the case, just as difficult is it to explain why it is the case.

If we can’t explain why, we can at least invent a name for this odd behaviour: time.

Note that not only can we distinguish between ‘before’ and ‘after’, we also obviously know that ‘before’ happens beforeafter’. Always. There is no turning back time. Time apparently has a fixed “direction”, in contrast to e.g. position:

You can go back to where you started but not to when you started.

We could call the “direction” an arrow of time. That arrow can be imagined as pointing from frame to frame in a movie.[3]


  1. How do I sum up speed through space and time to obtain c (in terms of units)?’ (web page, answer to forum post), user CuriousOne, Physics Stack Exchange, 2015, physics.stackexchange.com/a/193057/4962 (accessed May 1st, 2019)
  2. What is Time?’ (web page), Chanakaya, Society and You, 2019, www.societyandyou.co/what-is-time (accessed Sep. 15th, 2020)
  3. The Arrow of Time’ (web page), Luke Mastin, Exactly what is time?, 2014, www.exactlywhatistime.com/physics-of-time/the-arrow-of-time (accessed May 1st, 2019)