A thought Experiment in Time Dilation

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Discussion Overview

This discussion revolves around a thought experiment involving time dilation as it relates to a rapidly spinning disc. Participants explore the implications of relativistic effects on different points of the disc, particularly comparing the aging of the edge versus the center. The conversation touches on theoretical aspects of physics, specifically special relativity, and the potential observable effects of time dilation in such a scenario.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant proposes spinning a disc made of super strong material at billions of RPM in a vacuum to observe time dilation effects, questioning whether the edge would "disappear" due to these effects.
  • Another participant suggests that while clocks on the edge of the disc would tick more slowly compared to those at the center, they would not disappear, referencing previous experiments with Mossbauer rotors that observed relativistic effects.
  • Some participants argue that the edge of the disc would age slower than the center, using the example of radioactive isotopes to illustrate this point.
  • There is a discussion about the complexity of describing a rotating disc in special relativity and how it relates to general relativity, with one participant noting that GPS satellite clocks experience similar relativistic effects.
  • Several participants express curiosity about the visual implications of time dilation, questioning whether one could observe the edge of the disc slipping back in time.
  • One participant challenges the notion of the edge disappearing, emphasizing that it would still exist but would be aging at a different rate compared to the center.
  • Another participant raises the philosophical question of what clocks are actually measuring, whether it is time itself or our perception of it.

Areas of Agreement / Disagreement

Participants generally disagree on the interpretation of time dilation effects, particularly regarding the idea of the edge of the disc disappearing. While there is consensus that clocks on the edge would tick more slowly, the implications of this on physical presence and visibility remain contested.

Contextual Notes

Some participants note the limitations of the thought experiment, including the practical impossibility of achieving the proposed speeds and the complexities involved in applying special relativity to a rotating disc.

just dave
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TL;DR
Something I thought of years ago that no one has answered.
Ok so here's my idea,it came to me while watching a record album spin on my turntable(I believe it was Dark Side of the Moon). Technically this exp. could be done now but the results would take a while.
You take a disc of some super strong material attach it to a ultra high RPM motor and put it in a vacuum chamber.Then spin it up to billions of rpm.Now a point near the center of this disc will not be moving very much but on the edge it's moving a lot.
Depending on the size (say it's 10 ft. in circumference) and it's speed (say 1 billion rpm) the edge of the disc is moving at 17% of c. So time slows down more on the edge of the disc then the center.Does the edge of the disc begin to disappear due to Time Dilation? How far back in time would it have to go before you noticed any effects?
Now I know there's no material strong enough to withstand the centrifugal forces or a motor that can rev up that high,that is why I called it a thought exp.This exp. could be done now but at a much slower speed and would have to run for years.
So what do you think?
 
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just dave said:
Summary:: Something I thought of years ago that no one has answered.

Ok so here's my idea,it came to me while watching a record album spin on my turntable(I believe it was Dark Side of the Moon). Technically this exp. could be done now but the results would take a while.
You take a disc of some super strong material attach it to a ultra high RPM motor and put it in a vacuum chamber.Then spin it up to billions of rpm.Now a point near the center of this disc will not be moving very much but on the edge it's moving a lot.
Depending on the size (say it's 10 ft. in circumference) and it's speed (say 1 billion rpm) the edge of the disc is moving at 17% of c. So time slows down more on the edge of the disc then the center.Does the edge of the disc begin to disappear due to Time Dilation? How far back in time would it have to go before you noticed any effects?
Now I know there's no material strong enough to withstand the centrifugal forces or a motor that can rev up that high,that is why I called it a thought exp.This exp. could be done now but at a much slower speed and would have to run for years.
So what do you think?

What are you expecting to happen?

All others things being equal, the edge of the disc would age slower than the centre. E.g. if you had a radioactive isotope on the edge and at the centre, the isotope on the edge would decay more slowly.
 
just dave said:
Summary:: Something I thought of years ago that no one has answered.

Does the edge of the disc begin to disappear due to Time Dilation?
No. clocks on the edge go slow but they don’t disappear. This experiment has been done using Mossbauer rotors, not as fast as you mention, but fast enough to observe relativistic effects on the edge.
 
PeroK said:
What are you expecting to happen?

All others things being equal, the edge of the disc would age slower than the centre. E.g. if you had a radioactive isotope on the edge and at the centre, the isotope on the edge would decay more slowly.
I want to know would you see the edge start to disappear as it slips back in time,in effect,seeing Time Dilation right before your eyes. I think that would be pretty cool no?
 
just dave said:
Does the edge of the disc begin to disappear due to Time Dilation? How far back in time would it have to go before you noticed any effects?
Clocks on the rim of the disc would tick slowly, that's all (about 1 second per 1.01s as measured by stationary clocks in your example). There's no "going back in time" here.

It's worth noting that describing a rotating disc in special relativity can get quite complicated. In fact, developing understanding of it was one of the threads that led to general relativity.
 
Dale said:
No. clocks on the edge go slow but they don’t disappear. This experiment has been done using Mossbauer rotors, not as fast as you mention, but fast enough to observe relativistic effects on the edge.
Do you have a website or link to this experiment?
 
Ibix said:
Clocks on the rim of the disc would tick slowly, that's all (about 1 second per 1.01s as measured by stationary clocks in your example). There's no "going back in time" here.

It's worth noting that describing a rotating disc in special relativity can get quite complicated. In fact, developing understanding of it was one of the threads that led to general relativity.
Not exactly going back but slowing down compared to the rest of the disc.
 
just dave said:
Do you have a website or link to this experiment?
The reference is
Kuendig, Phys. Rev. 129 no. 6 (1963), pg 2371.
But I don’t have a link
 
just dave said:
I want to know would you see the edge start to disappear as it slips back in time,in effect,seeing Time Dilation right before your eyes. I think that would be pretty cool no?

Why would it disappear? It would simply in physical terms age less than the centre.
 
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  • #10
Dale said:
The reference is
Kuendig, Phys. Rev. 129 no. 6 (1963), pg 2371.
But I don’t have a link
Abstract is here: https://journals.aps.org/pr/abstract/10.1103/PhysRev.129.2371. You have to pay for the article - but some publishers are doing special access while COVID19 is around. Dunno if that applies here.
 
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  • #11
just dave said:
Not exactly going back but slowing down compared to the rest of the disc.
Clocks on the rim will tick slowly, that's all.

It's more complex because gravity is involved, but GPS satellite clocks have a tiny correction due to relativistic effects, including this same effect. They don't fade from view or anything.
 
  • #12
PeroK said:
Why would it disappear? It would simply in physical terms age less than the centre.
Because it is slipping out of the present as we watch it.Time is continuing at a "normal" unaccelerated pace for us and the center of the disc.
 
  • #13
just dave said:
Because it is slipping out of the present as we watch it.Time is continuing at a "normal" unaccelerated pace for us and the center of the disc.
That's not what time dilation is about.
 
  • #14
just dave said:
Because it is slipping out of the present as we watch it.Time is continuing at a "normal" unaccelerated pace for us and the center of the disc.
Why would that make it disappear? It is still there, just at a different time according to its clock than according to ours.

If you and I meet and immediately look at our watches and they show different times that doesn’t mean that we didn’t meet, just that my watch said our meeting happened at a different time than your watch said.
 
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  • #15
Ibix said:
Clocks on the rim will tick slowly, that's all.

It's more complex because gravity is involved, but GPS satellite clocks have a tiny correction due to relativistic effects, including this same effect. They don't fade from view or anything.
Yes I know and elementary particles live longer when traveling at relativistic speeds. What are clocks actually measuring though? Time or our idea of time?
 
  • #16
just dave said:
What are clocks actually measuring though? Time or our idea of time?
What is the difference?

Whatever they are measuring it doesn’t involve things disappearing because it goes slow.
 
  • #17
Dale said:
Why would that make it disappear? It is still there, just at a different time according to its clock than according to ours.

If you and I meet and immediately look at our watches and they show different times that doesn’t mean that we didn’t meet, just that my watch said our meeting happened at a different time than your watch said.
We are not physically connected.The disc started out in the same time frame but now part of it is ageing more slowly than the rest of it.
 
  • #18
just dave said:
What are clocks actually measuring though?
Generally they are counting occurrences of some physical process: the orbit of the Earth round the Sun, the Moon round the Earth, the rotations of the Earth on its axis, the swings of a pendulum, the oscillations of a quartz crystal, the transitions of a caesium atom.
 
  • #19
just dave said:
We are not physically connected.The disc started out in the same time frame but now part of it is ageing more slowly than the rest of it.
It's still physically there though!
 
  • #20
just dave said:
We are not physically connected.The disc started out in the same time frame but now part of it is ageing more slowly than the rest of it.
That isn’t relevant.

Look, time dilation just doesn’t work this way. This has been done. There is no disappearing involved. Just discard that idea. I don’t know where you got it, but the world doesn’t work like that.
 
  • #21
PeroK said:
It's still physically there though!
Yes I get it but there would have to be some other effects on the rest of the disc then.
 
  • #22
[QUOTE="just dave said:
Yes I get it but there would have to be some other effects on the rest of the disc then.
Certainly. It will experience stress and eventually disintegrate - for the same reason the string breaks in Bell's Spaceship Paradox.
 
  • #23
DaveC426913 said:
Certainly. It will experience stress and eventually disintegrate - for the same reason the string breaks in Bell's Spaceship Paradox.
Assuming the material is strong enough to withstand the stresses from the high RPM. Would it fracture due to time dilation? Also I'm not familiar with Bell's Spaceship Paradox I'll have to look that up.
 
  • #24
just dave said:
Assuming the material is strong enough to withstand the stresses from the high RPM. Would it fracture due to time dilation? Also I'm not familiar with Bell's Spaceship Paradox I'll have to look that up.
From the length contraction.

The material along the circumference will shrink - pull away from itself.

Ah. Right. The circular configuration of Bells' Spaceship Paradox is called the Ehrenfest Paradox.
That's the one you want to look up.

And it looks like - in a realistic scenario - the disc might not actually shatter. (though the stresses due to length contraction are still present.)
 
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  • #25
just dave said:
Would it fracture due to time dilation?

No. Time dilation isn't a force and it doesn't cause any mechanical stress.
 
  • #26
Dale said:
That isn’t relevant.

Look, time dilation just doesn’t work this way. This has been done. There is no disappearing involved. Just discard that idea. I don’t know where you got it, but the world doesn’t work like that.
I guess I'm thinking more of going back in time, more like a time machine.
 
  • #27
just dave said:
Assuming the material is strong enough to withstand the stresses from the high RPM. Would it fracture due to time dilation? Also I'm not familiar with Bell's Spaceship Paradox I'll have to look that up.
In this context, all that happens is that clocks tick slowly, and length contraction means that the stresses on the disc are higher than a non-relativistic calculation would predict.
just dave said:
I guess I'm thinking more of going back in time, more like a time machine.
Seriously, all that happens is that the clocks on the disc tick slowly. It doesn't stop existing now or anything like that - you just have a thing that's younger than you would expect if you didn't know about time dilation.
 
  • #28
just dave said:
I guess I'm thinking more of going back in time, more like a time machine.

That isn't what time dilation is. No matter how many times you ask, that answer will not change.
 
  • #29
PeterDonis said:
No. Time dilation isn't a force and it doesn't cause any mechanical stress.
Even if time on the edge of the disc nearly stops? I would think that would have some physical consequence on the rest of the disc.
 
  • #30
just dave said:
Even if time on the edge of the disc nearly stops?

No.

just dave said:
I would think that would have some physical consequence on the rest of the disc.

It doesn't. Again, no matter how many times you ask, the answer will not change.
 

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