Wouldn’t quantum mechanics suggest that it’s not even theoretically possible for all motion to stop?
Assuming it is possible, it may be happening repeatedly. We’d have no way to know.
Autolycus said:
Why must it be a human ear+brain? If a tree falls in a forest and there is no human to hear it, does it make a sound? Sure - the bear who knocked it over heard it. The bird that was sitting in it and had to move heard it. The chipmonk that nearly went splat heard it.
But CookingWithGas’s point is that it hinges on the definition of “sound” used. If sound is a signal received by an ear+brain, that is one definition, if sound is the transmission of air pressure waves in certain wavelengths, that is another definition.
What if I put a microphone in the otherwise empty woods (no ears+brains around), then let the tree fall? Does the microphone record a sound, or does the microphone not record anything? Or does the microphone record a signal that isn’t a sound until you play the recording later and it reaches your ear+brain?
Now it’s just silly semantics. [/hijack]
A romp in the sack with yours truly, of course.
Well thinking about this … what if all motion stopped in our universe but another part of the multiverse still had movement? In that case time would have stopped for us relative to the other part of the multiverse for some measuable period.
Related question - since we seem to have a consensus that there is no time without motion, or at least no meaning to time in that case - what about a cubic measure of a vacuum blocked off from all radiation? Assume no electromagnetic feild penetrates. If there is no motion within that volume, then is there no time? Or is the out to claim that it is impossible to net have some quantum events popping in and out of the space so therefore events do occur and so there is, therefore, time?
Relative to what reference frame?
This question is sort of like asking, “If I disappeared and were immediately and imperceptibly replaced by a copy identical in every way, would I still be me?” It implies a bunch of totally impossible conditions, fails to define its terms, and doesn’t really define what would count as an answer.
I’m not trying to rank on the OP—it’s an interesting subject for speculation. But the problem is that the nature of time isn’t a universally-agreed-upon certainty. At all!
If we knew enough about time to ask this question meaningfully, we’d already have the answer. (Though I admit that’s not very helpful. :D)
Fair enough, but I am of the opinion that time would continue to “march on” if all matter stopped.
Reason being… EM waves (e.g. light) existed before all matter stopped. The EM waves would still exist during the time period when all matter has stopped. Time must exist for EM waves to exist.
Please correct me if I’m wrong about this.
So light continues to propagate while all matter stops? Okay, but you realize that’s totally impossible, right? When you stop all the matter, where does all that kinetic energy go? How about the kinetic energy of a tungsten light bulb filament? Does it stop throwing off light?
If the light keeps going while we freeze everything else, what happens when the sunlight from 7 minutes ago (before we froze the sun) hits a black car seat at the equator— does light energy no longer impart heat (and hence motion; kinetic energy) to the things it strikes?
My point is that the question requires so much special pleading that you might as well embed any answer you want in the given conditions you’re starting with. You need to specify what time is to ask the question, and whatever you specify will, trivially and unhelpfully, be the answer.
I feel like the real question here is, “Does time exist independently of our ability to measure it?” or maybe “Does time exist with nothing to change over it?”
Those are both good questions, but they’re sufficiently contentious that you might have better luck in GD than GQ.
That said, my personal belief is… “no.” 
I supposed I should concede that, given what GD threads are like, this might imply a fairly radical redefinition of the concept of “luck” as well.
It’s worse than that. Time doesn’t exist anyway.
Without motion, would atomic isotopes continue to decay? If “all motion stops”, the particles released by the decay wouldn’t go anywhere; but when motion restarted, would there be an accumulation of particles suddenly free to move?
DSeid said:
According to cosmologists, time is embedded in our universe, it makes no sense to talk about time outside the universe. Therefore, whatever is happening in some other multiverse component is independent and irrelevant to what occurs within this universe. YMMV.
The Seventh Deadly Finn said:
I hate to bring this up, but it’s like the plane on a treadmill question. First you have to define how you interpret the question. Once that is out of the way, then the answer is trivial. But people disagree over how to define the question. Ergo, the debate is neverending.
Crafter_Man said:
You seem to be trying to examine a specific case where all matter is constrained but energy is not. But you are projecting the speculation without any idea of how to establish said conditions. So the answer is undefinable. I realize this is a speculative topic anyway, but you are introducing an additional constraint that seems a self-contradiction. The OP was about motion stopping. By definition, no EM would be moving, either.
I was going to raise this point. If all motion everywhere stopped, how would you be able to measure the duration of the effect as being five minutes long? As opposed to five seconds or five years?
If, for example, I told you that there had been a period of absolute motionlessness that occurred at exactly noon EST yesterday, how could you determine how long that period of motionlessness had lasted?
Another vote for “there’s no detectable difference between ‘stopping time’ and ‘stopping all motion’” and “stopping time doesn’t mean anything or at least it’s by definition undetectable”.
Really, the only interesting case is “stopping time/motion in some places but not others”. But that wasn’t the question.
I do not believe that there is any cosmological consensus that “time is embedded in our universe” nor that it would make “no sense to talk about time outside the universe” (assuming that our universe was part of a multiverse). It might be talking about time behaving in a different way or the same way (instead of not at all) and if so it would be an external frame of reference, if one could somehow measure between pocket universes.
Time only occurs if there is a relative change in the distribution of matter/energy within the fabric of space. That change in distribution is what most people mean when they talk about about “motion.”
A moment in time is a given distribution.
Without motion there is no time.
I think maybe Crafter_Man was getting at something more fundamental about the properties of light specifically.
e.g. That a photon’s velocity can’t be reduced to zero without destroying it.
Or maybe about time from the light’s own reference frame?
Of course, not moving over a dt of zero is not the same thing as zero velocity… But it all depends on the terms of the OP.
I think the OP is interesting, but ends up being like the “plane on a conveyor belt” question: it depends on which physical laws we turn off in the hypothetical.