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My question might be completely irrelevant or stupid - sorry in advance if it's the case. I'm kinda lost with the time and space notions, and even formulating adequate questions is hard at that point.

But I'll give it a shot.

If time froze, the common conception would be that everything would stop moving.

But let's do it the other way around : if everything stopped moving, would time stop? Could we consider that time still moves forward if everyting in the universe was absolutely still?

If we can make no distinction between "times stops thus nothings moves" or "nothing moves thus time stops", then isn't it some kind of a clue that time and motion and the very same thing?

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    $\begingroup$ Related: physics.stackexchange.com/q/15371/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Oct 20, 2019 at 18:09
  • $\begingroup$ Motion isn't the only kind of change. $\endgroup$
    – PM 2Ring
    Commented Oct 21, 2019 at 12:26
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    $\begingroup$ If you were to ask this on the Philosophy stack exchange, you would find that this is an open question in the philosophy of time. $\endgroup$
    – Cort Ammon
    Commented Oct 22, 2019 at 1:15
  • $\begingroup$ Not my area of expertise BUT there are other examples in mechanics and physics of two quantities that are linked together but not the same thing. Stress and strain, B-fields and H-fields, etc. The fact that you can't have one without the other doesn't make them identical. $\endgroup$
    – Mariano G
    Commented Oct 7 at 12:46

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Time is dependent on events occurring. Without events, there's nothing to assign time component to. You are asking about motion, which is of course an event, but just one of many possible ones.

Let's take, for example, the current SI definition of a second - "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom".

Even if all motion has ceased, events such as this or nuclear decay, etc., will continue to occur.

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  • $\begingroup$ When I say "motion", I mean all possible motion, including atomic $\endgroup$
    – GaelF
    Commented Oct 21, 2019 at 11:01
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    $\begingroup$ In this case, of course, your premise will hold true. $\endgroup$
    – NotAName
    Commented Oct 29, 2019 at 8:42
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Not so much, your reasoning is on point here, however you must take into consideration that motion is described for example by kinematics. In kinematics every physical variable ( velocity, acceleration, distance) is a function of time.

by denoting: $$s(t)=...$$ $$v(t)=...$$ $$a(t)=...$$

we show that each of these quantities is dependent on the change of time.

Therefore If we were to remove time from these relations we wouldn't be able to create equations of motion, because motion is non existent without time.

if we were to go into quantum physics - then time can be thought of as fourth dimension….

So time and motion are distinct things, but they are deeply dependent on each other - that's how universe works.

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  • $\begingroup$ I think you irrefutably prove that motion depends on time, however you don't really tackle my question, which is more about the other way round : does time depend on motion as well? "Motion is non existent without time" : but is time non existent without motion? $\endgroup$
    – GaelF
    Commented Oct 21, 2019 at 7:44
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If there is no time, then there will be no motion but this does not mean that they are same. Motion depends on time.

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Philosophers can argue ad infinitum about the meaning of time, so I don't think there is an authoritative answer to your question. However, I'm on your side. I take the view that time is a measure of change, and it would be meaningless in a frozen universe (ie one in which there was no change any sort). That said, I suspect that a universe with no change is an impossibility, so the notion is a hypothetical one.

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Elapsed time is just a measure of how much some process has finished. If all processed in universe has stopped, then you would not have how to calibrate your time scale. Even there would be no of "you", because organic things are also just a complex set of processes. So no processes = no time = no measurer, either. But we can be happy that, such situation can't exist in reality. Even if you can reach absolute zero Kelvins temperature of system, it doesn't mean that such system stops motion of all it's particles in all ways. It does not - quantum fluctuations still remains at lowest temperature possible in nature. So it seems that movement is hard-coded into nature itself, albeit "time" concept at quantum fluctuations level has not much meaning in physics. If it would - we would know about universe startup a lot more than we know now. Maybe some day ...

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Your logic is faulty. Even assuming that both your statements are true they don't imply your conclusion. Two things that imply each other are not necessarily identical.

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If you include the motion of subatomic particles and the motion of light, we can't measure time without motion.

And of course we can't measure motion without time.

So whether they're different is a question that's outside of physics. If there isn't even theoretically an experiment we could do using unlimited resources that would tell the difference then it isn't a physics question.

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Clocks and sundials give measure to motion. Time gives measure to continuous motion and rates for change. Time is not measuring a thing called time. Time gives a name to a measure of motion but is not motion itself as many measures are ... eg length, mass, heat etc. In my opinion. BGH.

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No. Time, as Einstein found, is contingent on one's motion through space. As someone earlier in this discussion put it succinctly, without motion, there would be no time to reference or act as a residual effect. The idea that you freeze 'time' in order to prevent motion comes from the classical concept that time is an independent variable outside of space, aka 'the arrow of time' phenomena. However, given that time exists in unison with space ('spacetime') we know that if you were to slow downtime, that is, in essence, the same as extending space. Relativity teaches us that the faster you move through space, the slower you move through time. Hence, this means that you would only be moving faster through space ad infinitum until the point that time feels like it's frozen. Quite paradoxical, but that's how it works.

TL;DR: Time and motion are not the same. Time is an influential dimension of motion.

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Motion requires change in both space and time. So in that sense can we say that motion is space? I am more inclined to say that time and space both are constituents of motion.

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That is why it is called spacetime; since for movement to exist, there must be space. So the conclusion is that movement has the two characteristics: movement = spacetime.

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