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All of the ways I have heard of to measure the passage of time involve measuring some sort of motion (e.g. vibrations of a cesium atom, movement of the hands on a clock, etc.).

Can the passage of time be measured without reference to any motion?

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  • $\begingroup$ Motion is not enough, time can only be measured if there is thermodynamic disequilibrium. $\endgroup$
    – CuriousOne
    Mar 18, 2016 at 2:07

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Motion is relative. So even if you think you aren't moving, someone else will think you are moving.

So you just have to accept that something measuring time might be moving. And you have to figure out how to correctly handle that.

And it turns out the path of the moving object in 4d spacetime affects how it measures the metric along the path. And that (the metric along the path) is what a clock measures. Not time.

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  • $\begingroup$ Time is a completely local phenomenon. There is no need for a second coordinate system to measure time. $\endgroup$
    – CuriousOne
    Mar 18, 2016 at 2:08
  • $\begingroup$ For there to be a measurement doesn't there need to be a change? And for there to be a change doesn't there have to be more than one coordinate? $\endgroup$ Mar 18, 2016 at 2:39
  • $\begingroup$ @BillAlsept: Let me soften it down a little: all current theories use a pre-geometry which attaches an individual clock to every spacetime point that can be identified by an observer. This is probably not a good way of thinking about time for the very reason you mention. Maybe something like loop quantum gravity can put away with the local time concept in a self-consistent way, I just don't know enough about it. $\endgroup$
    – CuriousOne
    Mar 18, 2016 at 3:11
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To "measure" the passing of time requires that something change (a second hand moves, a crystal vibrates, a counter increments, water falls, light travels, etc). If nothing is changing, then there is no way for you to tell the difference between $T_1$ and $T_2$. Even if you're watching paint dry, superficially motionless, you'll still be mentally counting seconds, and your brain will change as you count chimpanzees.

Now, is what I mean by "change" the same as what you mean by "motion"? Can something change with no motion whatsoever? Although a philosopher might come up with a different answer, as an engineer I would say that all change involves motion. If you look at radioactivity, evaporation, chemical decay, or even gravity waves, all fundamentally involve something moving.

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Short answer is no.

Time in itself is not an entity. It is a measure of events in terms of other type of events. The events involve motion at some level (visible, or not). For example, if you have a blinking clock, (instead of oscillating, or rotating), there is no visible motion but there is some kind of underlying motion that regulates the sequence of blinks. The interval itself is in terms of underlying events. Thus, time is a measurement, and it must involve some events, which must have motion/action at core.

Therefore, measurement of time (it's flow) is not possible without involving events (motion).

That also implies that there can not be motion without measurement (flow) of time. Because, detection of motion, implicitly involves other events which take form of flow of time.

We can say, that time flow, and motion individually do not make sense. They both exist together, or neither does. Needless to say, an observer is involved.

The direction of time flow is same as the sequence of events used to measure its flow. Which happens to be always forward, per perception of any observer.

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  • $\begingroup$ Since motion is 100% relative your claim there can not be motion without measurement also means there can not be lack of motion without measurement since motion or lack of motion is relative. $\endgroup$
    – Timaeus
    Mar 18, 2016 at 6:39
  • $\begingroup$ @Timaeus, "there can not be lack of motion with measurement", because, the motion has to be in the measurement process. The question is about any motion including that inside clock/measurements. I agree that - to detect lack of a particular motion, you need time measurement to say, this much time has passed and nothing moved. That is a particular relative motion in terms of other smaller events. The smaller events themselves are result of some kind of motion, not necessarily the one you are proving a lack of. That is why the next part - they exist together. $\endgroup$
    – kpv
    Mar 18, 2016 at 13:32
  • $\begingroup$ Motion is relative, so if you think motion is contingent then lack of motion has to be contingent too, since motion isn't absolute. I quoted something you wrote that was not only mystical instead of just merely untrue. You can't divide up the world into motion and not motion and claim that one is natural and the other requires something because it's relative. Inertial motion and being at rest must be equally natural because the difference is a matter of perspective. $\endgroup$
    – Timaeus
    Mar 18, 2016 at 14:59
  • $\begingroup$ The line you referred to, very next sentence describes the context of detection. There is nothing mystical or untrue. The perspective can certainly be different. $\endgroup$
    – kpv
    Mar 18, 2016 at 15:12
  • $\begingroup$ The claim there can not be motion without [...] is based on the false idea that being at rest is natural and things only get to move if special things happen. This is a mystical idea (since motion is relative) and one we have to disabuse students of if they come in with it and we must make sure they don't get this wrong (and mystical) idea from us. But you are actually pushing the idea that rest is more natural, rest is a matter of perspective. You might think you are saying one thing, but the thing you actually wrote is bad, very very bad. And based on wrong ideas that rest is natural. $\endgroup$
    – Timaeus
    Mar 18, 2016 at 15:19

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