# I Time dilation again, Einstein or Resnick?

#### Raymond Potvin

It's not directly related to acceleration. It's related to changing your reference frame.

There are several ways to analyse this. One option is simply be to use a suitable IRF (inertial reference frame). The frame of the blue clock is the obvious candidate - but that probably won't convince you!

You could choose any other IRF. The one that the yellow clock has for the first half of its journey might be an interesting choice. In that frame, the yellow clock would be at rest for the outward journey, but then move to the left at a greater speed to catch the blue clock that moves to the left the whole time.

In that frame, you would find the same result for the clocks (both blue and yellow) when they meet up.
You're right, but we can also do that with the blue clock if we consider that it is the one that is moving, and we will still get reversed data. If we don't know which clock is moving, how can we predict which one will suffer time dilation? When you change reference frames, you're assuming that the yellow clock is accelerating, so why didn't you assume it was accelerating in the beginning?

#### PeroK

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You're right, but we can also do that with the blue clock if we consider that it is the one that is moving, and we will still get reversed data. If we don't know which clock is moving, how can we predict which one will suffer time dilation? When you change reference frames, you're assuming that the yellow clock is accelerating, so why didn't you assume it was accelerating in the beginning?
It's not to do with acceleration. But, you do know when you change reference frame. This could be due to acceleration. But, for example, there's a neat idea where the yellow clock simply transfers the time it reads to an identical clock moving in the opposite direction. That would do just as well.

In both cases, however the time of the return leg is measured, you know that you have changed reference frames.

Another idea is to use a neutral third IRF. In that IRF, the blue clock has the same velocity throughout but the yellow clock changes direction at some stage. There's no getting away from that. You can't pretend that the blue clock is changing direction when it isn't! And you can't pretend that the yellow clock isn't changing direction.

#### Raymond Potvin

It's not to do with acceleration. But, you do know when you change reference frame. This could be due to acceleration. But, for example, there's a neat idea where the yellow clock simply transfers the time it reads to an identical clock moving in the opposite direction. That would do just as well.

In both cases, however the time of the return leg is measured, you know that you have changed reference frames.

Another idea is to use a neutral third IRF. In that IRF, the blue clock has the same velocity throughout but the yellow clock changes direction at some stage. There's no getting away from that. You can't pretend that the blue clock is changing direction when it isn't! And you can't pretend that the yellow clock isn't changing direction.
It is a lot easier to consider that acceleration determines which clock is moving: we get the right result and eliminate all the other possibilities. Relativity is about how light moves between moving bodies, not about determining which one is moving, but if clocks can run slower than others, then they have to be the ones that are moving. Not accepting that logic means facing interminable discussions on relativity. On problems where it is impossible to tell which clock is moving, the discussion is always useless because no prediction can be made.

#### PeroK

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It is a lot easier to consider that acceleration determines which clock is moving: we get the right result and eliminate all the other possibilities. Relativity is about how light moves between moving bodies, not about determining which one is moving, but if clocks can run slower than others, then they have to be the ones that are moving. Not accepting that logic means facing interminable discussions on relativity. On problems where it is impossible to tell which clock is moving, the discussion is always useless because no prediction can be made.
Well, I'm sorry you feel that way. Physics Forums is a good place to learn about relativity. But not learning SR is, after all, your prerogative.

I've tried my best, but I'll have to leave you in your state of ignorance of the subject!

#### Janus

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Relativity is about how light moves between moving bodies, not about determining which one is moving, but if clocks can run slower than others, then they have to be the ones that are moving.
Relativity is not about how light moves, it is about the very nature of time and space and how it is measured. Light is used in many examples when discussing it as as a convenience due to the fact that it travels at c.

#### PAllen

The muon itself can be considered as a clock that starts with its creation and stops with its detection. We thus know where it starts and where it stops, and we know it is traveling at a relativistic speed, so we can make a calculation and discover that it lasts longer than in a lab. But we can't make such a calculation if we apply the same reasoning to the earth, because if we did, it is our clocks on earth that would be suffering time dilation, and the data from the atmospheric muon would be unexplainable.
Wrong on both counts. The earth frame description that the muon traveled for e.g. 10 microseconds is a comparison, implicitly, with two earth frame clocks - one at the creation event, one at the reception event. Per the muon, both of these clocks run slower than the muon clock, but they are out of synch with each other (relativity of simultaneity). Per the muon frame, the only possible explanation of the result is length contraction. Thus, ther is nothing invariant or preferred about the time dilation explanation compared to the length contraction explanation. This definitively NOT equivalent to twin differential aging.

#### Raymond Potvin

Hi Janus,
We use light in our discussions because we think it is the speed at which the information travels between bodies. It is from the light clock mind experiment that the whole relativity was erected, that the nature of space and time was conceptualized. It works because we can make predictions, so it is useful, but the interminable discussions about it on the scientific forums are not really useful. I made the present simulation of the twins paradox in expectation that I would make another one to show what would happen if we would change reference frames, but I finally decided not to make it, and to stick to the idea that the twin that was moving was the one that has accelerated. This way, if two clocks meet in space without knowing where they come from, we can know if one of them has traveled longer or faster while comparing them, but we could not have predicted it.

#### PeroK

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Hi Janus,
We use light in our discussions because we think it is the speed at which the information travels between bodies. It is from the light clock mind experiment that the whole relativity was erected, that the nature of space and time was conceptualized. It works because we can make predictions, so it is useful, but the interminable discussions about it on the scientific forums are not really useful. I made the present simulation of the twins paradox in expectation that I would make another one to show what would happen if we would change reference frames, but I finally decided not to make it, and to stick to the idea that the twin that was moving was the one that has accelerated. This way, if two clocks meet in space without knowing where they come from, we can know if one of them has traveled longer or faster while comparing them, but we could not have predicted it.
If you build a new clock on board a spaceship that previously accelerated, would your new clock know that it was "really" time dilated and know that it was different from a clock built on Earth?

#### Raymond Potvin

Wrong on both counts. The earth frame description that the muon traveled for e.g. 10 microseconds is a comparison, implicitly, with two earth frame clocks - one at the creation event, one at the reception event. Per the muon, both of these clocks run slower than the muon clock, but they are out of synch with each other (relativity of simultaneity). Per the muon frame, the only possible explanation of the result is length contraction. Thus, ther is nothing invariant or preferred about the time dilation explanation compared to the length contraction explanation. This definitively NOT equivalent to twin differential aging.
In my simulation, the yellow clock's width is half the width of the blue one. If I wouldn't have made that correction, the yellow one would have ticked less than 16 times, so it would have suffered more time dilation than what the data tells us. Without contraction, the muon would simply last longer than expected, and the same would be happening to the clocks on earth if we would change reference frames.

#### Raymond Potvin

If you build a new clock on board a spaceship that previously accelerated, would your new clock know that it was "really" time dilated and know that it was different from a clock built on Earth?
If we know that the ship has accelerated in a given direction for a while, then we know its speed in this direction with regard to the point where it started accelerating. If a clock was left at that point, we could thus know how much time dilation our clock actually suffers compared to that clock, but if we encounter another clock en route, nothing can help us to predict the two readings.

#### jbriggs444

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If a clock was left at that point, we could thus know how much time dilation our clock actually suffers compared to that clock,
You are dropping a clock off at a point that you pass by? [Or that passes by you]. How will that help determine anything? How will you compare your clock against the left-behind clock when both are receding into the distance with respect to one another?

#### Raymond Potvin

You are dropping a clock off at a point that you pass by? [Or that passes by you]. How will that help determine anything? How will you compare your clock against the left-behind clock when both are receding into the distance with respect to one another?
Hi jbriggs!
Leaving a clock behind in space means put it outside the ship and accelerate away from it. If you return to the point after a while and the clock is still there and has not accelerated, you know your own clock will have dilated.

#### jbriggs444

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Hi jbriggs!
Leaving a clock behind in space means put it outside the ship and accelerate away from it. If you return to the point after a while and the clock is still there and has not accelerated, you know your own clock will have dilated.
No, you do not. You know that you have differential aging.

#### Raymond Potvin

No, you do not. You know that you have differential aging.
Well, if you know you will be younger than somebody left with the clock, then you know your clock will confirm that you are younger.

#### PAllen

In my simulation, the yellow clock's width is half the width of the blue one. If I wouldn't have made that correction, the yellow one would have ticked less than 16 times, so it would have suffered more time dilation than what the data tells us. Without contraction, the muon would simply last longer than expected, and the same would be happening to the clocks on earth if we would change reference frames.
I have no interest in your simulation. The physics of muons reaching the ground is well eatablished and explained by special relativity. To the extent that you are proposing an alternative personal theory, that is not an allowed discussion topic here and is almost certainly wrong as well.

Also, note in the muon scenario there is no change of reference frames. The muon is inertial for its whole existence, as is the earth to a reasonable approximation. Any explanation involving change of reference frame is not just wrong but wholly irrelevant.

#### Raymond Potvin

I have no interest in your simulation. The physics of muons reaching the ground is well eatablished and explained by special relativity. To the extent that you are proposing an alternative personal theory, that is not an allowed discussion topic here and is almost certainly wrong as well.

Also, note in the muon scenario there is no change of reference frames. The muon is inertial for its whole existence, as is the earth to a reasonable approximation. Any explanation involving change of reference frame is not just wrong but wholly irrelevant.
My simulation is not off topic, and it is not a personal theory either. It is just a fabulous tool to study relativity. But I admit that I didn't need to rely on the reference frame principle to make it, and I admit that assuming acceleration helps us to determine which clock is moving is not written in the book. Can you tell me why you don't like simulations?

#### PAllen

My simulation is not off topic, and it is not a personal theory either. It is just a fabulous tool to study relativity. But I admit that I didn't need to rely on the reference frame principle to make it, and I admit that assuming acceleration helps us to determine which clock is moving is not written in the book. Can you tell me why you don't like simulations?
I didn’t say I don’t like simulations in general. I just see no need to analyze yours for a simple SR problem, and question its value if it keeps leading you to false concepts such as there is an objective distinction as to what clock is moving, and that without this you seem to believe you can’t make predictions. These are horrible misunderstandings.

Besides the muon case, where there is no change of reference frame at all, and no accelaeration, and no meeting of clocks after separating, you should also be aware of the existence twin problems where the amount of acceleration and its profile is the same between twins, but one still ends up younger (what differs between the twins is when, on their own clock, the acceleration is applied).

#### PeroK

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My simulation is not off topic, and it is not a personal theory either.
You're obviously an intelligent guy, but you should understand that what you believe is not the theory of relativity, but a personal variation based on some aspects of SR and the concept of absolute motion (based on each object retaining a memory of all the accelerations it has ever had).

It's pointless to debunk your theory as I imagine you would not entertain any calculations aimed at that purpose - just as you are oblivious to anyone (including professional physicists) trying to explain that a central tenet of SR is that all motion is relative and that velocity-based time dilation is fully symmetrical.

In any case, you should be under no illusion that you have understood the theory of special relativity. You have not. You have constructed your own alternative personal theory.

#### Raymond Potvin

Besides the muon case, where there is no change of reference frame at all, and no accelaeration, and no meeting of clocks after separating, you should also be aware of the existence twin problems where the amount of acceleration and its profile is the same between twins, but one still ends up younger (what differs between the twins is when, on their own clock, the acceleration is applied).
It's so much simpler to add acceleration as an evidence that discriminates the possibilities. A twin cannot say it has accelerated when it has not, that's over complicating the problem for no practical advantage. The muon case is a bit different, but we also know it has been created in the upper atmosphere, that it has been accelerated towards us, and that it has decelerated in the detector.

#### PeroK

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It's so much simpler to add acceleration as an evidence that discriminates the possibilities. A twin cannot say it has accelerated when it has not, that's over complicating the problem for no practical advantage.
But, despite what you may suppose from the Internet, SR is not just about the twin paradox!

Also, where you are wrong is assuming that using the standard SR approach you either get no prediction or a wrong answer. You have claimed this several times.

In any case, it's still your own variation on SR.

#### Raymond Potvin

But, despite what you may suppose from the Internet, SR is not just about the twin paradox!

Also, where you are wrong is assuming that using the standard SR approach you either get no prediction or a wrong answer. You have claimed this several times.

In any case, it's still your own variation on SR.
I claimed that assuming a clock has accelerated doesn't affect the calculations, it only simplifies the understanding. If a prediction can be made while changing reference frames, then the same prediction can be made while only using the rf of the clock that has not accelerated. If no prediction can be made with SR, then no prediction can be made with a simulation either because nothing will help us to know which one of the clocks is moving.

#### jbriggs444

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I claimed that assuming a clock has accelerated doesn't affect the calculations, it only simplifies the understanding. If a prediction can be made while changing reference frames, then the same prediction can be made while only using the rf of the clock that has not accelerated. If no prediction can be made with SR, then no prediction can be made with a simulation either because nothing will help us to know which one of the clocks is moving.
No experimental results depend on what inertial reference frame you choose to adopt -- i.e. which of the clocks is initially seen as moving.

#### PeroK

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I claimed that assuming a clock has accelerated doesn't affect the calculations, it only simplifies the understanding. If a prediction can be made while changing reference frames, then the same prediction can be made while only using the rf of the clock that has not accelerated. If no prediction can be made with SR, then no prediction can be made with a simulation either because nothing will help us to know which one of the clocks is moving.
In post #75 I showed you how to solve the problem in a reference frame where both clocks move.

Did you understand that solution?

In any case, the principle of relativity says that you must get the same answer in all inertial reference frames. You do not have to use a reference frame where one clock remains at rest.

Your requirement to know which clock really moves is unnecessary.

#### Raymond Potvin

In post #75 I showed you how to solve the problem in a reference frame where both clocks move.
Did you understand that solution?
I said you were right, and I said a simulation would give the same result, but I also said that if we were considering that it is the yellow clock that was accelerating towards the blue one at the end, then we can also consider that it is the one that is accelerating away from it in the beginning, and that the problem was easier to figure out this way.

In any case, the principle of relativity says that you must get the same answer in all inertial reference frames. You do not have to use a reference frame where one clock remains at rest.

Your requirement to know which clock really moves is unnecessary.
It's not necessary if no prediction is necessary, otherwise it is. Without knowing that the yellow clock had to change directions, taking that clock as the rf would not have given the right result because you could have changed the direction of the blue one instead.

#### jbriggs444

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It's not necessary if no prediction is necessary, otherwise it is. Without knowing that the yellow clock had to change directions, taking that clock as the rf would not have given the right result because you could have changed the direction of the blue one instead.
That only requires knowing which clock changed directions, not knowing which clock (if either) was motionless throughout the exercise.

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