Why Does the Ship's Twin Observe Earth's Clock Jump in the Twin Paradox?

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In the discussion about the twin paradox, the ship's twin observes Earth's clock jump from 3.6 years to 16.4 years due to the Doppler effect during an instantaneous turnaround. The ship's twin does not see their own clock jump because it continues to run normally, while the perceived time difference for Earth is a result of the shift in simultaneity. The conversation emphasizes that one cannot directly observe the clock but only the light signals, which can lead to confusing interpretations of time. The discussion also touches on the complexities of calculating distances and time in different frames, highlighting the importance of understanding relativity's implications on simultaneity. Ultimately, the paradox raises questions about how time is perceived differently in varying frames of reference.
  • #31
Al68 said:
If we consider the Earth to be the object that changes direction, won't we have to draw the ship's worldline as vertical and get completely different results?
Yes. That was my point. (It doesn't have to be vertical because the slope of the line depends on what frame we're using, but it's a straight line in all inertial frames, and it's vertical in the ship's own frame, which would be the most convenient frame to use).
 
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  • #32
Hello Al68.

Quote:-

---If we consider the Earth to be the object that changes direction, won't we have to draw the ship's worldline as vertical and get completely different results?--

Yes if the Earth changes direction, that is undergoes the acceleration and the ship does not, then the results will be different but then the scenario is different. In that case the ship's worldline will be a vertical straight line in a standard spacetime diagram.

Which body ghanges direction, in the basic stay at home and traveller case is the all important point. There are more complicated examples but it all comes down to the same thing.

In all such scenearios it all comes down to that the clock carried by the one who deviates most from a straight line on the spacetime diagram will show the shortest proper time. Proper time by definition being the time shown by a clock in which it remains at rest. Bear in mind that a clock is always at rest relative to itself moving inertially or not.

Matheinste.
 
  • #33
matheinste said:
Hello Al68.

Quote:-

---If we consider the Earth to be the object that changes direction, won't we have to draw the ship's worldline as vertical and get completely different results?--

Yes if the Earth changes direction, that is undergoes the acceleration and the ship does not, then the results will be different but then the scenario is different. In that case the ship's worldline will be a vertical straight line in a standard spacetime diagram.

Which body ghanges direction, in the basic stay at home and traveller case is the all important point. There are more complicated examples but it all comes down to the same thing.

In all such scenearios it all comes down to that the clock carried by the one who deviates most from a straight line on the spacetime diagram will show the shortest proper time. Proper time by definition being the time shown by a clock in which it remains at rest. Bear in mind that a clock is always at rest relative to itself moving inertially or not.

Matheinste.

That sounds like, although the "twins paradox" scenario is resolved as presented, all we have to do is redefine the scenario, define the turnaround point as a distance measured in the ship's frame, consider that the Earth accelerated relative to the ship, and have the Earth twin age less, since SR only cares about coordinate acceleration and not "proper" acceleration. What am I missing?

Thanks,
Al
 
  • #34
Hello Al68.

Quote:-

--consider that the Earth accelerated relative to the ship, and have the Earth twin age less,---

The Earth twin would age less if the Earth accelerated and the ship did not, but in these scenarios it is not usually the Earth that accelerates because this is not a very realisteic option and so would not help to pose the twins "paradox". As normally stated the twins "paradox" is meant to show a real difference in age from a realistic scenerio and present this as a paradox, which we all know can be resolved with SR. If we made the Earth in effect the "traveller" then we use an impractical and unrealistic, though not impossible scenario which lessens the effect of the "paradox" as usually given by making it wholly improbable anyway to the learner of SR at who it is aimed as an example for study.

Quote:--

--SR only cares about coordinate acceleration and not "proper" acceleration.---

The whole point here is that one accelerates and the other does not. I don't know how you define proper acceleration and coordinate acceleration, but what we care about in this scenario is absolute acceleration as detected by an accelerometer. Only one, the ship or the Earth experiences this in our present example. As normally proposed it is the ship which experiences the acceleration. If the Earth experiences it and the ship does not then,as you say, the result is reversed and the Earth twin ages less.

I don't think you are missing anything you may just have the wrong idea about acceleration. Acceleration is absolute and physical and coordinate independent.

Matheinste.
 
  • #35
Al68 said:
That sounds like, although the "twins paradox" scenario is resolved as presented, all we have to do is redefine the scenario, define the turnaround point as a distance measured in the ship's frame, consider that the Earth accelerated relative to the ship, and have the Earth twin age less, since SR only cares about coordinate acceleration and not "proper" acceleration. What am I missing?
I don't get why you think you can replace one problem with another that doesn't look anything like the original.

The original problem specifices three events and three frames. If you change any of that, it's a different problem.
 
  • #36
matheinste said:
Hello Al68.

Quote:-

--consider that the Earth accelerated relative to the ship, and have the Earth twin age less,---

The Earth twin would age less if the Earth accelerated and the ship did not, but in these scenarios it is not usually the Earth that accelerates because this is not a very realisteic option and so would not help to pose the twins "paradox". As normally stated the twins "paradox" is meant to show a real difference in age from a realistic scenerio and present this as a paradox, which we all know can be resolved with SR. If we made the Earth in effect the "traveller" then we use an impractical and unrealistic, though not impossible scenario which lessens the effect of the "paradox" as usually given by making it wholly improbable anyway to the learner of SR at who it is aimed as an example for study.

Quote:--

--SR only cares about coordinate acceleration and not "proper" acceleration.---

The whole point here is that one accelerates and the other does not. I don't know how you define proper acceleration and coordinate acceleration, but what we care about in this scenario is absolute acceleration as detected by an accelerometer. Only one, the ship or the Earth experiences this in our present example. As normally proposed it is the ship which experiences the acceleration. If the Earth experiences it and the ship does not then,as you say, the result is reversed and the Earth twin ages less.

I don't think you are missing anything you may just have the wrong idea about acceleration. Acceleration is absolute and physical and coordinate independent.

Matheinste.

By coordinate acceleration I mean change in relative velocity. By proper acceleration I mean as measured by an accelerometer, like you're talking about. But the biggest thing I see that "causes" the ship's twin to age less in the twins paradox is the simple fact that he didn't travel as far relative to Earth as the Earth twin did relative to the ship, each as measured in his own frame. Simple common sense tells me that at 0.8c, a shorter trip equals less elapsed time (t=d/v in any frame). The resolution's conclusion just follows this stipulation. It doesn't resolve the big picture "clock paradox" for scenarios which may be different. Some of the resolutions say that acceleration is the key to the problem, but they claim this as an axiom without showing why this is true. After all, the Earth does accelerate (change velocity) relative to the ship. By definition coordinate acceleration equals change in relative velocity per unit time.

Just as a side note, most on this board are probably aware that Einstein believed the "clock paradox" was unresolvable in SR. He was fully aware of the (now) common resolutions and rejected them. Any thoughts on why he believed this?

Thanks,
Al
 
  • #37
Fredrik said:
I don't get why you think you can replace one problem with another that doesn't look anything like the original.

The original problem specifices three events and three frames. If you change any of that, it's a different problem.
Well, because I'm interested in the big picture "clock paradox", not just a single scenario whose conclusion only applies to narrowly defined conditions.

And I'm curious why Einstein thought it was unresolvable in SR, even after being fully aware of the now common resolutions. Since the reason probably isn't that Einstein "didn't understand SR", "didn't understand how simultaneity works", etc.

I'm sure that when Einstein presented the "clock paradox" and said that the ship's twin should be able to claim that the Earth twin aged less, he obviously didn't mean by using the same specified three events and three frames.

Thanks,
Al
 
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  • #38
Hello Al68.

Quote:-

---Some of the resolutions say that acceleration is the key to the problem, but they claim this as an axiom without showing why this is true----

To start with this point. To get back to the smaller picture let us talk about the role of acceleration in the standard twin paradox. This has been explained very well in many threads in this forum. I may be able to rephrase a few things but better people than me seem not have been able to make it clear to you so I don’t hold out much hope but I will try.

Take a ship and the Earth at rest relative to each other. Now in all that follows the words ship and Earth wherever they appear in the text can be interchanged. Because they are not in relative motion with regards each other let us for our purposes consider them at rest. In a standard spacetime diagram we can represent their common worldline as a vertical line, corresponding to the time axis.

Let the ship moves off in any direction, for our convenience we will show this displacement as being along the horizontal axis in our spacetime diagram. It does not matter whether to the left or to the right. The worldline of the ship will therefore be a straight line let us say to the right, and its angle upwards to the horizontal will be proportional to its velocity which may be anything less than c.
After some time the ship halts and heads back to the left at some speed making again an tilted upward straight line this time to the right to the left until it reaches the Earth again. We will assume that the initial acceleration, the deceleration at the turnaround point, the acceleration back towards Earth and the final deceleration at Earth to be very high for a very short period, this of course being only a thought experiment. We do this to make the time spent in the acceleration phases as short as possible, in theory infinitesimally small. It has in fact no effect on the outcome.

In spacetime the greater the spacetime distance traversed, and on a spacetime diagram the longer the line(s) representing the path taken, the SHORTER the proper time experienced by anything traveling along this path. So the earth, traveling in a straight line, travels the shortest distance and therefore its clocks record the LONGEST POSSIBLE time. This is contrary to common sense but is accounted for by the time dimension being involved in the calculation. I would show the mathematics but I am not yet proficient in latex. Remember spactime distance is not the same thing as spatial distance. The actual speeds and distances are immaterial, the principle is simply that the longest spacetime diagram path has the shortest proper time, that is shows less ageing. Proper time is of course what the ship and Earth experience themselves.

Anyhow to make the path deviate from the vertical axis of the spacetime diagram requires a change of velocity, that is an acceleration. Every time the spacetime path changes direction, as it must in this scenario, an acceleration is involved. It is the change in path that causes the difference in proper times and for this change in path an acceleration is needed. The acceleration does not cause the clock differences but is needed to alter the spacetime path.

The point of turnaround of the ship can be anywhere but in the scenario we have chosen it is somewhere to the right on the spacetime diagram to the right of the vertical worldline of the earth.

There have been authors who have claimed that GR is necessary to resolve the paradox but it is generally accepted that GR is not needed. As to Einstein, what can I say, I don’t know the facts.

This has all been posted before and anyway it is the best I can do and only hope it is accurate. If anybody spots any mistakes would they please point them out and we can put them right rather than let Al think we are in disagreement over the point.

Matheinste.
 
  • #39
matheinste said:
Hello Al68.

Quote:-

---Some of the resolutions say that acceleration is the key to the problem, but they claim this as an axiom without showing why this is true----

To start with this point. To get back to the smaller picture let us talk about the role of acceleration in the standard twin paradox. This has been explained very well in many threads in this forum. I may be able to rephrase a few things but better people than me seem not have been able to make it clear to you so I don’t hold out much hope but I will try.
..
Let the ship moves off in any direction, for our convenience we will show this displacement as being along the horizontal axis in our spacetime diagram. It does not matter whether to the left or to the right. The worldline of the ship will therefore be a straight line let us say to the right, and its angle upwards to the horizontal will be proportional to its velocity which may be anything less than c.
Thanks for your reply. Your post is very clear, and everything you said is clear to me. It's what everyone is omitting that I'm asking about. Why must we attribute the deviation from the initial common worldline to the observer who underwent proper acceleration? Why can't we just as rightly (according to SR) attribute this deviation to the other observer and draw the Earth's worldline to the right when it's velocity relative to the ship changes (coordinate acceleration)?

Why can't we just randomly choose which observer to draw vertically on the spacetime diagram? I know that sounds like a silly question, but that very question is part of the reason Einstein pursued GR.

Maybe the answer is so intuitively obvious that nobody considers it necessary to mention, but when I ignore my intuition and use only Einstein's 1905 SR paper, I can't find the answer. (And neither could he).

I understand what happens after we decide who's worldline to draw vertically, that's just simple math. But it's that decision that seems, for lack of a better word, Newtonian.

I mentioned Mach's principle in an earlier post, but nothing came of it. I realize that when the ship fires it's thrusters, Earth's velocity changes relative only to the ship, while the ship's velocity changes relative to earth, and relative to every other single body in the universe. This is obviously not considered important by anyone, since nobody brought it up. Is it relevant?
There have been authors who have claimed that GR is necessary to resolve the paradox but it is generally accepted that GR is not needed. As to Einstein, what can I say, I don’t know the facts.
Well, he was one of those authors, although most consider his GR resolution to be flawed.

Thanks,
Al
 
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  • #40
Al68,
Why can't we just randomly choose which observer to draw vertically on the spacetime diagram? I know that sounds like a silly question, but that very question is part of the reason Einstein pursued GR.

Because one of them has a curved worldline which cannot be transformed into a straight vertical line by Lorentz transformation.

It sort of follows that to handle force-free acceleration, you need to curve the axes.

M
 
  • #41
Mentz114 said:
Al68,

Why can't we just randomly choose which observer to draw vertically on the spacetime diagram? I know that sounds like a silly question, but that very question is part of the reason Einstein pursued GR.
Because one of them has a curved worldline which cannot be transformed into a straight vertical line by Lorentz transformation.
I'm asking about the decision that led to us to consider that one of them has a curved worldline, not how we treat the problem afterward.

I don't know how else to put it.

Thanks,
Al
 
  • #42
Hello again Al68.

Quote:-

----Why must we attribute the deviation from the initial common worldline to the observer who underwent proper acceleration? Why can't we just as rightly (according to SR) attribute this deviation to the other observer and draw the Earth's worldline to the right when it's velocity relative to the ship changes (coordinate acceleration)?

Why can't we just randomly choose which observer to draw vertically on the spacetime diagram? I know that sounds like a silly question, but that very question is part of the reason Einstein pursued GR.----

A vertical worldline in a spacetime diagram represents a body in inertial motion, for our purposes this is the same as being at rest. If a body accelerates it undergoes a spatial displacement with changing time which would have to be represented by a sloping line to the left or right in the spacetime diagram.

I do not know for sure, this being a quick answer, but I suppose with some corresponding alterations to the slope of the spatial displacement axis, that is horizontal axis, and a corresponding alteration to the other objects worldline, some sort of modified spacetime diagram may be possible in which an accelerated observer may be given a vertical worldline. I would be interested in any answers to this which may be forthcoming. But why complicate things. The outcome remains the same as far as time differentials are concerned.

As for Mach’s principle, I think there would be no difference I the posing of or resolution of the paradox if nothing else existed in the universe. Acceleration would still be absolute.

I wrote this while before Mentz114 posted his last reply. He says all that needs to be said on that point and i suspect that what he has said about the Lorentz transformation answers my question about a modified spacetime diagram. Not possible ??

Mateinste.
 
  • #43
Hello Al68

In answer to the curved worldline—

A spacetime diagram plots distance or displacement against time. For constant velocity this plot or graph is a straight line. For accelerated motion it is not.

Matheinste.
 
  • #44
Al68 said:
Well, because I'm interested in the big picture "clock paradox", not just a single scenario whose conclusion only applies to narrowly defined conditions.
It's instructive to consider a scenario defined by three time-like straight lines chosen at random (except that no two of them are parallel), where we imagine physical observers traveling on those world lines and comparing their clocks at the events where two lines meet. This scenario contains everything that's relevant from the standard twin "paradox". (Note that there's no acceleration).

Al68 said:
And I'm curious why Einstein thought it was unresolvable in SR, even after being fully aware of the now common resolutions. Since the reason probably isn't that Einstein "didn't understand SR", "didn't understand how simultaneity works", etc.
A person who understands SR would never believe that this is unresolvable in SR, and I do believe that Einstein understood SR.

Al68 said:
I'm sure that when Einstein presented the "clock paradox" and said that the ship's twin should be able to claim that the Earth twin aged less, he obviously didn't mean by using the same specified three events and three frames.
I don't know how he presented it (or even that he did), so I can only assume that if he said anything like that, he must have meant that a very naive application of the time dilation formulas which completely ignores any other effects due to relativity of simultaneity leads to the result that the Earth twin aged less (and also that the Earth twin aged more).
 
  • #45
Al68 said:
Why must we attribute the deviation from the initial common worldline to the observer who underwent proper acceleration? Why can't we just as rightly (according to SR) attribute this deviation to the other observer and draw the Earth's worldline to the right when it's velocity relative to the ship changes (coordinate acceleration)?

Why can't we just randomly choose which observer to draw vertically on the spacetime diagram? I know that sounds like a silly question, but that very question is part of the reason Einstein pursued GR.
I think you're missing an important thing here. We're not trying to find the best possible theory to handle this scenario. We're just trying to find out what special relativity says about it. And it's clear from any formulation of special relativity that straight lines have a very special significance.

Al68 said:
Maybe the answer is so intuitively obvious that nobody considers it necessary to mention,
It's not obvious that there are no better theories, but we're not looking for a better theory. The twin paradox is about finding the mistake in an incorrect application of the rules of SR.

Al68 said:
I understand what happens after we decide who's worldline to draw vertically, that's just simple math. But it's that decision that seems, for lack of a better word, Newtonian.
"Minkowskian", or "special relativistic" would be pretty good ways to say it.

Al68 said:
I mentioned Mach's principle in an earlier post, but nothing came of it. I realize that when the ship fires it's thrusters, Earth's velocity changes relative only to the ship, while the ship's velocity changes relative to earth, and relative to every other single body in the universe. This is obviously not considered important by anyone, since nobody brought it up. Is it relevant?
Mach's principle may be important to someone who knows SR and is trying to find a theory that includes gravity. Such a researcher might decide early on to only consider theories that satisfy some version of Mach's principle and throw away all other theories without further consideration. This is a lot like deciding to only consider theories where coordinate transformations between inertial frames with a common origin preserve the light-cone at the origin, when trying to find SR. (Only not as powerful, because light-cone preservation is almost the entire theory of SR).
 
  • #46
Al68,

I'm asking about the decision that led to us to consider that one of them has a curved worldline, not how we treat the problem afterward.

I don't know how else to put it.
I thought I'd answered that. We didn't choose which one to treat inertially, and which one non-inertially.The twin who travels non-inertially nominates themselves. Acceleration is absolute.

M

[edit] This is what Matheinste and Fredrik are saying also, I think.
 
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  • #47
matheinste said:
Hello Al68

In answer to the curved worldline—

A spacetime diagram plots distance or displacement against time. For constant velocity this plot or graph is a straight line. For accelerated motion it is not.

Matheinste.
Why not?
 
  • #48
Fredrik said:
A person who understands SR would never believe that this is unresolvable in SR, and I do believe that Einstein understood SR.
Einstein did consider it unresolvable in SR. And said so, and tried to resolve it in GR.
 
  • #49
Herllo Al68.

The plot or graph of constant velocity against time is a straight line. For accelerated motion it is not.

You ask why not!

This is basic mathematics and physics of motion. If you do not know this you really should learn it as it is at a very basic level and if you do not understand this you have no chance of understanding anything in physics involving motion.

Matheinste.
 
  • #50
Mentz114 said:
Al68,


I thought I'd answered that. We didn't choose which one to treat inertially, and which one non-inertially.The twin who travels non-inertially nominates themselves. Acceleration is absolute.

M

[edit] This is what Matheinste and Fredrik are saying also, I think.

Sure proper acceleration is absolute, but coordinate acceleration is not.

And I know we didn't choose which twin travels non-inertially, but we did choose to treat the inertial frame differently.
 
  • #51
Hello Al68.

Quote

---And I know we didn't choose which twin travels non-inertially, but we did choose to treat the inertial frame differently. ----

Because it is different.

Matheinste.
 
  • #52
Al68,

And I know we didn't choose which twin travels non-inertially, but we did choose to treat the inertial frame differently.
It is different.
M
 
  • #53
Hello Al68.

Regarding proper acceleration and coordinate acceleration, something i know little about, i believe that in flat spacetime, the sort of spacetime that SR deals with, they are the same and so coordinate acceleration is not relevant. A spacetime diagram's axes, that is coordinate system, are linear indicating a flat spacetime.

Matheinste.
 
  • #54
Mentz114 said:
The twin who travels non-inertially nominates themselves. Acceleration is absolute.
I agree, but I would like to add that you can see that without thinking in terms of acceleration. Consider my example with three non-parallel time-like lines, chosen at random. Note that all 3 observers in this case will agree which of the three events is the latest, and also which is the earliest. Therefore, they will also agree which of the three events corresponds to the turnaround event. There's no acceleration in this scenario, but it's still clear that the funny thing that happens with simultaneity at the "turnaround" event is what resolves the naive paradox.

Al68 said:
Why not?
Because we're talking about special relativity, and that theory was constructed to satisfy the requirement that coordinate changes between inertial frames take straight lines to straight lines. This isn't mentioned explicitly, but Einstein's "postulates" don't make sense unless this is taken to be a part of what they mean.

In other words, the the world line of an inertial observer is straight by definition, in the theory that was used incorrectly to find the "paradox".

Al68 said:
Einstein did consider it unresolvable in SR. And said so, and tried to resolve it in GR.
I think it's more likely that you have misunderstood what he said. SR is just the theory of Minkowski space, which is just \mathbb R^4 with some functions. Both the functions and \mathbb R^4 can be explicitly constructed from the axioms of set theory. Therefore, if SR really contains a paradox, all of mathematics falls with it. Maybe not all of it, but we definitely lose the integers, so bye bye 1+1=2.

I have explained this lots of times in this forum. I think the fact that almost no one understands this means that there's something very wrong with the way SR is presented in all the standard texts.
 
  • #55
matheinste said:
Regarding proper acceleration and coordinate acceleration, something i know little about, i believe that in flat spacetime, the sort of spacetime that SR deals with, they are the same and so coordinate acceleration is not relevant.
They are the same in any inertial frame on Minkowski space, but we can easily imagine a global coordinate system such that an accelerating object e.g. has x=0 at all times.

I learned recently that some authors actually consider such a coordinate system a part of GR instead of a part of SR. I find that quite bizarre.
 
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  • #56
matheinste said:
Because it is different.
Mentz114 said:
It is different.
Agreed. And to provide one more detail: It's different...in special relativity, which is the theory we're working with here.
 
  • #57
matheinste said:
Herllo Al68.

The plot or graph of constant velocity against time is a straight line. For accelerated motion it is not.

You ask why not!

This is basic mathematics and physics of motion. If you do not know this you really should learn it as it is at a very basic level and if you do not understand this you have no chance of understanding anything in physics involving motion.

Matheinste.
I should give up now, but I can't resist pointing out that the Earth's velocity relative to the ship is not constant. So the coordinate acceleration of the Earth relative to the ship is not zero.

Thanks,
Al
 
  • #58
Hello Al68

Quote:-

---So the coordinate acceleration of the Earth relative to the ship is not zero.---

In flat spacetime coordinate acceleration and proper acceleration are the same. We are dealing with flat spacetime. If the Earth's proper acceleration is zero then its coordinate acceleration is zero. This is the case if we choose the ship to be the traveller. In this case an accelerometer on the Earth will show no acceleration so its acceleration is zero.

Its my bedtime. goodnight.

Matheinste.
 
  • #59
matheinste said:
In flat spacetime coordinate acceleration and proper acceleration are the same.

Well, coordinate acceleration could be defined relative to a reference frame co-moving with the ship's clock. Proper acceleration cannot.

Thanks,
Al
 
  • #60
Al68 said:
I should give up now, but I can't resist pointing out that the Earth's velocity relative to the ship is not constant. So the coordinate acceleration of the Earth relative to the ship is not zero.
This is true (if you're talking about a coordinate system with the ship at x=0 both before and after the turnaround), but as I said in #54 (in a different way), Minkowski space was chosen as the space-time for SR because it makes it obvious that a coordinate transformation from one inertial frame to another takes straight lines to straight lines. I think you will find that your "ship frame" (which you still haven't defined fully) will violate this requirement, no matter how you finish its definition.

Why do I say that you haven't defined the ship's frame? Because its world line only defines the time axis. You haven't defined a way to assign time coordinates to events that aren't on the time axis.
 

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