I Why Is the Green Observer Always the Bent One in the Twin Paradox?

[PeterDonis]

if one twin stays on Earth while the other flies off in a rocket and comes back, I can't use those physical distinctions as a general rule to decide which one will be older when they meet? Sometimes it will be the Earth twin and sometimes it will be the rocket twin?

Since the Earth is a gravitating body, spacetime in this scenario is not flat, so you can't apply flat spacetime rules to it. Did you intend to have the Earth there? Or were you really intending to ignore gravity altogether and assume flat spacetime?

To answer the question as you ask it, with the Earth and gravity included, consider two possibilities:

(1) The rocket twin launches upward from Earth at less than escape velocity (we'll pretend there are no other gravitating bodies present anywhere, for simplicity) and then just let's his rocket coast until it reaches maximum height and falls back to Earth, landing at the same spot it took off from. The Earth twin is in a shielded bunker next to the launch point the whole time.

(2) The rocket twin launches upward from Earth at the same velocity as in #1 above, but uses his rocket to turn around long before it would reach maximum height if it were just coasting.

In #1 above, the rocket twin will have aged more. In #2, it depends on the details of when the turnaround happens.

 

[Orodruin]

But there is a specific rule for flat spacetime that says that the one that is not under the influence of a force has the longer worldline.

This is a direct result of the fact that finding the stationary world-lines of proper time results in the geodesic equations (when parametrised by proper time).

The OP seems confused in how a mathematical model relates to a physical reality. Just as I do not need to know anything about forces to know how long it takes to travel a distance d with acceleration a in classical mechanics, I do not need to know anything about forces to compute proper time. All I need is geometry and the geometry is physical every bit as much as any force (they are both mathematical descriptions of physical phenomena that we can relate to observations).

 

[Dale]

All I need is geometry and the geometry is physical every bit as much as any force (they are both mathematical descriptions of physical phenomena that we can relate to observations)

I agree but I just worry that you are overdoing it here. I think that @SamRoss made some good progress here already and probably doesn’t need the barrage of nitpicking he is getting from you and @PeterDonis.

He now understands the relationship between a bend in a worldline and the acceleration, which he didn’t before. He also rightly points out a previously known relationship between forces and acceleration. He also rightly infers the connection between forces and a bend in a worldline.

That is awesome! He identified and clearly articulated a specific problem, learned something new that addressed his conceptual problem, and correctly integrated the new concept with his existing knowledge. I wish all twin’s paradox threads were that productive.

He chooses to emphasize the force over the geometry, which is somewhat a matter of taste in the standard scenario and the scenarios where it is not a matter of taste are fairly advanced. It is also entirely understandable since the bulk of his physics experience has probably been very focused on forces.

Let him take the win! I think this is the best progress I have ever seen from the OP in a Twin Paradox thread.

 

[SamRoss]

If you are given the acceleration (kinematics), dynamics are irrelevant. Obviously, if you want to know why the object moved that way you need to do dynamics, but this is totally irrelevant to computing the time.

I want to know why the object moved that way!

Again, wrong, we are not trying to explain why an object follows a particular world-line.

Yes I am! I am trying to explain why an object follows a particular world line! Again, I think we're mainly in agreement, just emphasizing different parts of the problem. For me, what has always been puzzling about the twin paradox is that I would often hear it stated, including in the FAQ, without any reference to the dynamics, leading me to wonder if either observer could arbitrarily be chosen as the one with the curved world line. Obviously they can't. I am not and have never been confused by the idea that one observer has a longer proper time than the other, so long as I can trace the asymmetry in the proper time back to an asymmetry in the physical conditions.

As far as our issue of curvature being the definition of force - perhaps it would have been better for me to stick with the world "potential" but I got talked out of that in earlier posts so I switched to "force". Maybe I should just use "rocket fuel". Anyway, all I'm saying is that the trajectory of the world line follows from something physical. Different physical circumstances between the two observers leads to different world lines. From there, yes, we look to the geometry to calculate the proper times.

 

[SamRoss]

At the risk of preempting the end of the back-and-forth, I'd like to thank everyone who participated in this thread. This is exactly the type of rigorous, thoughtful debate I have come to expect and enjoy from PF!

 

[PeroK]

I want to know why the object moved that way!
Yes I am! I am trying to explain why an object follows a particular world line! Again, I think we're mainly in agreement, just emphasizing different parts of the problem. For me, what has always been puzzling about the twin paradox is that I would often hear it stated, including in the FAQ, without any reference to the dynamics, leading me to wonder if either observer could arbitrarily be chosen as the one with the curved world line. Obviously they can't. I am not and have never been confused by the idea that one observer has a longer proper time than the other, so long as I can trace the asymmetry in the proper time back to an asymmetry in the physical conditions.

As far as our issue of curvature being the definition of force - perhaps it would have been better for me to stick with the world "potential" but I got talked out of that in earlier posts so I switched to "force". Maybe I should just use "rocket fuel". Anyway, all I'm saying is that the trajectory of the world line follows from something physical. Different physical circumstances between the two observers leads to different world lines. From there, yes, we look to the geometry to calculate the proper times.

Well, this is why the paradox presented without acceleration cuts through all this meaningless soul-searching!

SR fundamentally is a theory of space and time and the four-dimensional geometry thereof. Rocket fuel is about as irrelevant as what the twins ate for their breakfast.

 

[PeterDonis]

so long as I can trace the asymmetry in the proper time back to an asymmetry in the physical conditions

And this general form of reasoning is fine: there must always be something physically different about the twins that leads to their different proper times. But "force" will not always be the relevant asymmetry in the physical conditions.

 

[Orodruin]

I want to know why the object moved that way!

No you do not. Not if all you are interested in is to compute the time. The dynamics are irrelevant for that. It is pure kinematics.

Yes I am! I am trying to explain why an object follows a particular world line!

Again, no, you are not. It is completely and utterly irrelevant for computing the proper time.

For me, what has always been puzzling about the twin paradox is that I would often hear it stated, including in the FAQ, without any reference to the dynamics, leading me to wonder if either observer could arbitrarily be chosen as the one with the curved world line.

Again, no! The curvature is a geometric property! It has nothing at all to do with dynamics. Dynamics might explain why an object follows a curved world-line, but it has nothing to do with the proper time along that world-line, which is an inherently geometric property.

Anyway, all I'm saying is that the trajectory of the world line follows from something physical.

So what? It is irrelevant for the computation of the proper time.

From there, yes, we look to the geometry to calculate the proper times.

Again. You do not need dynamics for proper times. If you need information about A to compute B, then you need A. If you do not need A to compute B, then it is irrelevant for B. Dynamics are irrelevant for proper time calculation.

so long as I can trace the asymmetry in the proper time back to an asymmetry in the physical conditions.

But the different physical conditions that are relevant to the asymmetry are all geometrical. Your personal desire to explain why something moves along a particular world-line is irrelevant here. Geometry is all there is to it if proper time is all we are asking about - as is the case in differential ageing. The relevant thing is the difference between the world-lines, not why the twins follow the different world-lines.

 

[SamRoss]

Let's say there are two observers, A and B. They start out at the same point in space-time. Next, their distance from each other increases at a constant rate. After that, their distance from each other decreases at a constant rate until they are back at the same position. Can you tell me which observer experienced a greater proper time?

 

[Herman Trivilino]

Summary: I read through "A Geometrical View of Time Dilation and the Twin Paradox" from the FAQ but I still have questions.

Why is the green guy always the bent one

Because he has the distinction of being the traveling twin. Somebody has to be the one to turn around, otherwise there would be no reunion and hence no way to compare ages.

 

[Dale]

Can you tell me which observer experienced a greater proper time?

Nope, additional information is required. The accelerometer readings would be sufficient.

 

[SamRoss]

Nope, additional information is required. The accelerometer readings would be sufficient.

This post was actually in response to a post from Orodruin. I probably should have included a quote. It makes more sense in that context.

 

[PeroK]

Let's say there are two observers, A and B. They start out at the same point in space-time. Next, their distance from each other increases at a constant rate. After that, their distance from each other decreases at a constant rate until they are back at the same position. Can you tell me which observer experienced a greater proper time?

Take any inertial reference frame (IRF). Let $v_A(t)$ and $v_B(t)$ be the velocities of A and B in that IRF, with $t$ the coordinate time in that IRF. In general, the proper time experienced by A and B during the period $t = 0$ to $t = T$ is:
$$\tau_A = \int_0^T \sqrt{1- \frac{v_A^2}{c^2}} dt, \ \ \tau_B = \int_0^T \sqrt{1- \frac{v_B^2}{c^2}} dt$$
There are two important points:
a) the proper time depends only on the velocity profile of A and B.
b) If A and B meet twice (i.e. their paths cross twice), then $\tau_A, \tau_B$ (their proper time between those two events) is invariant across all IRF's.

There's another general point I'd like to make. It is very difficult or almost impossible to learn SR from the twin paradox. But, if you learn SR, then you can understand the twin paradox.

 

[Orodruin]

Let's say there are two observers, A and B. They start out at the same point in space-time. Next, their distance from each other increases at a constant rate. After that, their distance from each other decreases at a constant rate until they are back at the same position. Can you tell me which observer experienced a greater proper time?

No, because you have insufficiently defined the geometry. It is exactly the same in regular geometry: You draw two curves. First the distance between them increases, then it decreases. Which curve is longer? You do not know because your specification is incomplete. You need to specify which curve has a kink in it because that kink is part of the geometry.

Your information needs to be sufficient to identify both the spacetime geometry (in SR assumed to be Minkowski space) and the world-lines as geometrical objects in that spacetime.

 

[Federation 2005]

Acceleration is everything -- and it's the only thing. That comes straight out of the action principle for the law of inertia. For the action principle, you use the negative of proper time as the action. So "least action" -- which is what gives you inertial motion -- means "greatest proper time". So, as a consequence, that means that for any two trajectories that cross paths at the start and end of an interval, the one which has more inertial motion and has spent more tine in 0G will have less action -- meaning more proper time; while the one which has less inertiality in its motion (and more acceleration) will have more action (and less proper time)

It should actually be possible to take the function a = A(s), that maps one's proper time s to one's acceleration a in one's rest frame acceleration and derive, from it, the trajectory as a function of coordinate position and time. That, is, one should be able to prove a theorem like this:

Theorem: Given the initial position r(0) = 0, and initial velocity v(0) = V, with proper time s set to 0 at t = 0, and given the acceleration (in one's instantaneous rest frame) a = A(s) as a function of proper time, then there is a unique trajectory r(t) for which each of these are true. Assume that A is continuous.

Let T > 0 be the end time, S the proper time and (without any real loss of generality) assume r(T) = 0. Then T - S can be expressed entirely as a functional of A such that (i) T - S > 0 if A is non-zero for any proper time between 0 and S (inclusive); (ii) T - S = 0 if A is 0 between proper times 0 and S.

Proof:
Exercise and your next published paper. The details are very hairy; lots of coupled differential equations that need to have explicit solutions by quadrature found, before you can get an actual (integral) expression for T - S in terms of the function A.

For instantaneous changes in velocity (which are technically illegal since they are unphysical) you have to use delta functions for A and that WILL produce a non-zero contribution (as you can see by treating the delta function as a limit of smooth functions and examining the limiting value of T - S as you allow the smooth functions to approach the delta function).

Reference:
The twin paradox: the role of acceleration
J. Gamboa, F. Mendex, M. B. Paranjape, Benoit Sirois
https://arxiv.org/abs/1807.02148
This sets the story straight and gets rid of all the folklore myths that are STILL being perpetuated even by professional and mainstream physicists on this issue; and the faulty analyses usually seen for this problem; that contribute to the misunderstanding of the issue.

 

[PeroK]

Acceleration is everything -- and it's the only thing. That comes straight out of the action principle for the law of inertia. For the action principle, you use the negative of proper time as the action. So "least action" -- which is what gives you inertial motion -- means "greatest proper time". So, as a consequence, that means that for any two trajectories that cross paths at the start and end of an interval, the one which has more inertial motion and has spent more tine in 0G will have less action -- meaning more proper time; while the one which has less inertiality in its motion (and more acceleration) will have more action (and less proper time)

It should actually be possible to take the function a = A(s), that maps one's proper time s to one's acceleration a in one's rest frame acceleration and derive, from it, the trajectory as a function of coordinate position and time. That, is, one should be able to prove a theorem like this:

Theorem: Given the initial position r(0) = 0, and initial velocity v(0) = V, with proper time s set to 0 at t = 0, and given the acceleration (in one's instantaneous rest frame) a = A(s) as a function of proper time, then there is a unique trajectory r(t) for which each of these are true. Assume that A is continuous.

Let T > 0 be the end time, S the proper time and (without any real loss of generality) assume r(T) = 0. Then T - S can be expressed entirely as a functional of A such that (i) T - S > 0 if A is non-zero for any proper time between 0 and S (inclusive); (ii) T - S = 0 if A is 0 between proper times 0 and S.

Proof:
Exercise and your next published paper. The details are very hairy; lots of coupled differential equations that need to have explicit solutions by quadrature found, before you can get an actual (integral) expression for T - S in terms of the function A.

For instantaneous changes in velocity (which are technically illegal since they are unphysical) you have to use delta functions for A and that WILL produce a non-zero contribution (as you can see by treating the delta function as a limit of smooth functions and examining the limiting value of T - S as you allow the smooth functions to approach the delta function).

Reference:
The twin paradox: the role of acceleration
J. Gamboa, F. Mendex, M. B. Paranjape, Benoit Sirois
https://arxiv.org/abs/1807.02148
This sets the story straight and gets rid of all the folklore myths that are STILL being perpetuated even by professional and mainstream physicists on this issue; and the faulty analyses usually seen for this problem; that contribute to the misunderstanding of the issue.

This is nonsense.

 

[Dale]

Take any inertial reference frame (IRF). Let $v_A(t)$ and $v_B(t)$ be the velocities of A and B in that IRF, with $t$ the coordinate time in that IRF. In general, the proper time experienced by A and B during the period $t = 0$ to $t = T$ is:
$$\tau_A = \int_0^T \frac{1}{\sqrt{1- \frac{v_A^2}{c^2}}} dt, \ \ \tau_B = \int_0^T \frac{1}{\sqrt{1- \frac{v_B^2}{c^2}}} dt$$

Shouldn't this be the inverse of the integrand?

 

[PeroK]

Shouldn't this be the inverse of the integrand?

Thanks. Fixed.

 

[Ibix]

So, as a consequence, that means that for any two trajectories that cross paths at the start and end of an interval, the one which has more inertial motion and has spent more tine in 0G will have less action -- meaning more proper time; while the one which has less inertiality in its motion (and more acceleration) will have more action (and less proper time)

Twin A accelerates with some proper acceleration A for some proper time T, waits one (proper) year, accelerates at A in the opposite direction for proper time 2T, waits another year, accelerates in the first direction at A for proper time T, then waits. Twin B does the exact same thing, except waiting two years of proper time between acceleration phases. Which one will be younger when B returns?

They've spent the same coordinate time inertial and the same coordinate time non-inertial, at least in one frame. They've spent the same proper time accelerating under the same acceleration. The only thing that's different is the proper time spent moving inertially. So your method seems to say only that the one with less proper time is the one with less proper time.

 

[Dale]

Acceleration is everything -- and it's the only thing. ...

Reference:
The twin paradox: the role of acceleration
J. Gamboa, F. Mendex, M. B. Paranjape, Benoit Sirois
https://arxiv.org/abs/1807.02148

The claim is clearly incorrect as it violates the clock hypothesis. The clock hypothesis has been experimentally tested and validated for accelerations up to 10^18 g. See: Bailey et al., “Measurements of relativistic time dilation for positive and negative muons in a circular orbit,” Nature 268 (July 28, 1977) pg 301.

Note, the reference that you provide is unpublished and therefore is not suitable for PF. Furthermore, in the end claims with experimental support, like Bailey's, always outweigh purely theoretical claims, like Gamboa's.

The time dilation of a clock, when measured in an inertial frame, depends only on the speed of the clock in that frame, not on the direction of its velocity, nor its acceleration, nor any higher derivatives.

 

[Ibix]

Note, the reference that you provide is both unpublished and therefore is not suitable for PF.

As far as I can tell on a quick read, the paper simply does detailed calculations of the stay-at-home's proper time between the end of "during the outbound inertial leg" according to the outbound inertial frame and the beginning of "during the inbound inertial leg" according to the inbound inertial frame, using Rindler coordinates. I'm not sure I've seen the calculations done explicitly, but it's hardly revolutionary. That there is proper time unaccounted for if one just uses two inertial frames is textbook stuff and has certainly appeared in posts here. It's just that calculating it explicitly (rather than taking the total proper time for the stay-at-home and subtracting the time accounted for "during" the inertial phases) requires a lot of maths.

 

[stevendaryl]

As I said later in that quote - I don't remember my exact wording - something else must be specified in order for that freedom to disappear. Your own reply implies this. The freedom will go away only if I specify the lengths of the lines I want you to draw. If I don't tell you how long I want the lines to be then you can do whatever you want. In the twin paradox, unless you say which twin was under the influence of the force then you won't be able to justify picking one specific twin as the one with the shorter world line.

That's not true. Let's stick to the blank piece of paper for a while. There is an intrinsic notion of the distance between two points on a piece of paper. This notion of distance determines which paths are straight and which ones are curved. The same is true in Special Relativity. There is an intrinsic notion of something like distance between spacetime points. This is the spacetime metric. The metric determines which paths through spacetime are inertial and which ones are accelerated. This is geometric, prior to any notion of forces.

Now, once you put physics in, it is true in Special Relativity that accelerated paths require an expenditure of energy, while inertial paths do not. However, it isn't the forces that determine which paths are straight, but the metric that determines which paths require force to maintain them.

Think about a perfectly smooth spherical planet, with no friction. If you shove an object, and then let the object continue in a force-free manner, the object will follow "great circle" routes. A great circle route on a sphere is the shortest distance between two points on the sphere, if you're traveling along the surface of the sphere.

It's the geometry of spacetime that determines what are the inertial paths.

 

[stevendaryl]

Are we doing pure math or physics? How can we specify the lengths of the lines without looking at what's actually going on in the situation? How can we justify making one line longer or shorter than the other unless there is some physical distinction between the two observers in the real world?

It's hard to make sense of your question. Yes, there is a physical distinction between an accelerated path and an inertial path, in the same way that there is a physical distinction between a straight line and a curved line on a piece of paper. However, you don't need to know the history of the line on the paper to know whether it's curved. You don't need to know what kind of forces produced the line. Whether it's curved or not is a geometric fact about the line.

Geometry is physical.

 

[stevendaryl]

Let's say there are two observers, A and B. They start out at the same point in space-time. Next, their distance from each other increases at a constant rate. After that, their distance from each other decreases at a constant rate until they are back at the same position. Can you tell me which observer experienced a greater proper time?

It's the analogous question for two curves on a piece of paper. You have two points on the paper, $A$ and $B$. You draw two different curves connecting $A$ and $B$. Which curve is longer? The length of a curve is an intrinsic fact about the curve. You look at each curve independently to know how long it is. If you're only told how the distance between the two curves change, then you can't figure out from that how long each is.

 

[Ibix]

@SamRoss - I think the point being made here is that there is no difference between the statements that "an object was accelerated" and "an object has a curved worldline". They are the same statement.

It should actually be possible to take the function a = A(s), that maps one's proper time s to one's acceleration a in one's rest frame acceleration and derive, from it, the trajectory as a function of coordinate position and time.

Assuming you have the initial position and velocity in the coordinate frame, then yes. You just integrate. If you don't have the initial velocity (or at least some equivalent boundary condition) then no.

The proper time in terms of the coordinates still depends only on the velocity (and the metric).

 

[SamRoss]

No, because you have insufficiently defined the geometry... Your information needs to be sufficient to identify both the spacetime geometry (in SR assumed to be Minkowski space) and the world-lines as geometrical objects in that spacetime.

Exactly. My whole point is that too often when the twin paradox is described, the description seems to be incomplete, just as I have described it. The distance between two observers increases and then decreases - that's all you get - yet one of them turns out to be older. When I looked at the FAQ and saw a flat red line and a bent green line, I thought "Okay, from the point of view of the red guy, he stays still while the green guy leaves and then comes back. Why can't I flip that? Why can't I say that from the point of view of the green guy, he stays still and the red guy leaves and comes back?" Now, for someone who's more experienced with relativity, you might say that obviously there's some extra information, some dynamics, that were not explicitly mentioned in the build-up to the problem, but that's not obvious to everyone. Someone like me is left wondering, "Why can't I flip this diagram?" I strongly recommend some discussion be added to the FAQ concerning why it is not possible to flip the diagram as I did in my original post, to make it clear that something is different between the two observers. Furthermore, the distinction should be explained in terms of something more concrete than just the geometry itself. If it is possible to "see" the dynamical distinction somehow encoded into the geometry, as you clearly do, then that should be added as well.

 

[PeroK]

Re acceleration

Exactly. My whole point is that too often when the twin paradox is described, the description seems to be incomplete, just as I have described it. The distance between two observers increases and then decreases - that's all you get - yet one of them turns out to be older.

Please provide a reference where the paradox is presented like this. It's not the way it's ever been presented in my experience. It's usually shown as one twin blasting away from Earth in a rocket.

 

[Orodruin]

Acceleration is everything -- and it's the only thing.

No, it is not, as has been explained repeatedly in this thread already. Geometry is the only thing. Specifying the geometry and curves in the spacetime is part of the specification.

My whole point is that too often when the twin paradox is described, the description seems to be incomplete, just as I have described it. The distance between two observers increases and then decreases - that's all you get - yet one of them turns out to be older.

Sorry, but this is absurd. The typical specification of the twin paradox clearly states which twin turns around, which is the very physical statement you are ignoring that makes the situation well defined. You left this out in your description.

Why can't I flip that? Why can't I say that from the point of view of the green guy, he stays still and the red guy leaves and comes back?"

This again completely ignores that one path in spacetime has a bend and the other does not. This is like asking why you cannot make a curve with a bend in it straight by rotating the paper you drew it on. Again, everything is in the geometry.

 

[Ibix]

If it is possible to "see" the dynamical distinction somehow encoded into the geometry, as you clearly do, then that should be added as well.

A spacetime diagram is just a displacement-time graph (we take them a bit more literally in relativity is all). It's a plot of $x(t)$ for an object. The slope of the line is $dx/dt$, which is the velocity. The slope changing is therefore the same as the velocity changing - which is acceleration.

 

[SamRoss]

It's hard to make sense of your question.

Please provide a reference where the paradox is presented like this.

The typical specification of the twin paradox clearly states which twin turns around, which is the very physical statement you are ignoring that makes the situation well defined. You left this out in your description.

I think I just had an "aha!" moment. Tell me if I'm on the right track. In descriptions of different observers' reference frames, people often say things like, "Pretend Alice is holding a ruler in her hand that she uses to measure distances relative to her and Bob is holding a ruler to measure distances relative to him." Well, I didn't realize that they ever let go of their rulers! I thought that even when changing direction, the green guy would keep his ruler in his hand. That's why, to me, the diagram shown in the FAQ...

...describes a completely symmetric situation, with the red guy arbitrarily chosen as the one at rest. If the green guy continues to hold on to his ruler, then from his point of view the situation would look like what I put in my original post...

But he doesn't hold on to his ruler, does he? When he turns around, the ruler keeps going without him.

Here are three Minkowski diagrams showing the same scenario.

Ibix, I should have paid more attention to your post a while back. At first, when I saw this picture...

... it didn't seem to make sense. I didn't understand why the green line would ever leave the t' axis because I was not thinking of the moving reference frame as having an existence independent of the green guy. Reading through it again today, I realized I must be mistaken about the meaning of the diagrams. Now that I imagine him letting go of his ruler, it all makes sense! Please tell me I'm getting this right. The people don't hold on to their rulers forever, do they?