# Implications of the statement Acceleration is not relative

by GregAshmore
Tags: implications, statement
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P: 4,786
 Quote by GregAshmore Given: G1. Earth and rocket are both at rest at same position. Earth clock and rocket clock are synchronised. G2. At time 0.0, rocket fires a pulse. G3. Earth and rocket separate at relative velocity 0.8c. G4. At distance 10 units from Earth, as measured in Earth frame, rocket fires a pulse. G5. Earth and rocket approach at relative velocity -0.8c. G6. Upon reaching Earth, rocket fires a pulse, coming to rest on Earth. G7. Gravitational effects of mass are to be ignored.
Here is a spacetime diagram for Earth's inertial rest frame:

Earth is shown as the wide blue line and the rocket as a wide red line. Dots along each path indicate one unit of elapsed Proper Time and I have marked most of them. I have also marked the four events that you indicated.

Here is a diagram to show the rocket at rest:

Both Earth and the rocket send a signal to the other one every unit of Proper Time. These signals provide the information that accounts for the visualization that you mentioned at the end of your post. The diagrams make it obvious that the situation is not symmetrical between the Earth and the rocket. They also make it clear that either diagram will provide all the information to determine the visualization of either observer.

You should track a few of the signals, noting the Proper Time (according to the dots) they were sent and received and then go to the other diagram and confirm the same information.

I didn't necessarily use the same coordinates that you used but, again, this will have no bearing on any outcome. The Coordinate Times are not significant when comparing between frames, only the Proper Times matter.
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 P: 3,188 It may be useful to elaborate a little on Langevin's discussion about the fact that acceleration has an absolute sense, as he meant it in a slightly different way than those people in this forum to which your first post relates; however Langevin gave the "twin" example for exactly this purpose, to illustrate the "absolute" effects of acceleration. The way he meant it is made clear by his description (as well as by the text that precedes it, but that's too long to cite here): Only a uniform velocity relative to it cannot be detected, but any change of velocity, or any acceleration has an absolute sense. [..] the laws of electromagnetism are not the same in respect to axes attached to this [accelerated] material system as in respect to axes in collective uniform motion of translation. We will see the appearance of this absolute character of acceleration in another form. [..] For [..] observers in uniform motion [..]l the proper time [..] will be shorter than for any other group of observers associated with a reference system in arbitrary uniform motion. [..] We can [..] say that it is sufficient to be agitated or to undergo accelerations, to age more slowly, [..] Giving concrete examples: [..] This remark provides the means for any among us who wants to devote two years of his life, to find out what the Earth will be in two hundred years, and to explore the future of the Earth, by making in his life a jump ahead that will last two centuries for Earth and for him it will last two years, but without hope of return, without possibility of coming to inform us of the result of his voyage, since any attempt of the same kind could only transport him increasingly further [in time]. For this it is sufficient that our traveler consents to be locked in a projectile that would be launched from Earth with a velocity sufficiently close to that of light but lower, which is physically possible, while arranging an encounter with, for example, a star that happens after one year of the traveler's life, and which sends him back to Earth with the same velocity. Returned to Earth he has aged two years, then he leaves his ark and finds our world two hundred years older, if his velocity remained in the range of only one twenty-thousandth less than the velocity of light. The most established experimental facts of physics allow us to assert that this would actually be so. [etc.] - starting p.47 of http://en.wikisource.org/wiki/The_Ev...Space_and_Time
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 Quote by ghwellsjr I don't know why you would say this. Are you referring to the Earth's inertial rest frame? There are an infinite number of ways to construct spacetime diagrams for your scenario, all with different velocities with respect to each other and all just as valid and all producing the same final clock readings and the same things that each observer actually sees.
I was too general in my wording. I only meant that Events A, B, and C have been placed, there is only one way to place Event D.

 Quote by ghwellsjr No, they are not symmetric. The rocket sees the Earth move away at 0.8c for a distance of 3.333 units and then remain at that distance for some time and then come back to the rocket. But the Earth sees the rocket move away at 0.8c to a distance of 10 units and then immediately start coming back. It doesn't matter what spacetime diagram you use to depict your scenario, they are all just as valid and produce the same results. The only thing that is different in them are the values of the coordinates (and the geometric shapes of the worldlines).
Again, poor choice of words. "Similar" would have been better than "symmetry".
P: 221
 Quote by TrickyDicky Right there, that's what I mean. This is the asymetry that is not so easy to explain. And maybe the OP naively thinks that if there is an absolute acceleration it would imply a rate of change of absolute velocity but that can't be because there is no such a thing as absolute velocity in relativity. At this point I guess I should wait for the OP to confirm if this gets any close to his line of thought.
Well, I did have a thought that might resolve to something like what you say. As I recall, someone defined proper acceleration as the derivative of proper velocity with respect to proper time. I don't understand how there can be proper velocity at all, because proper time is the elapsed time at "the same place". If position never changes with respect to time, it would seem that proper velocity must be zero. I did not say anything about it because my objection was much more basic than that, and I figured there would be a logical explanation for it if and when I get that far.
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 Quote by DaleSpam I think that this is probably one of the key topics of modern relativity. The key concepts of relativity, both special and general, are geometrical (Minkowski geometry for SR and pseudo-Riemannian geometry for GR). Just like you can take a piece of paper and draw geometrical figures and discuss many things, such as lengths and angles, without ever setting up a coordinate system. The same thing is possible in relativity. The "piece of paper" is spacetime which has the geometrical structure of a manifold. The "geometrical figures" are worldlines, events, vectors, etc. that represent the motion of objects, collisions, energy-momentum, etc. In this geometrical approach, the twin scenario is simply a triangle, and the fact that the travelling twin is younger is simply the triangle inequality for Minkowski geometry. In a coordinate-independent sense, the traveler's worldline is bent, and that in turn implies that his worldline is necessarily shorter as a direct consequence of the Minkowski geometry. Now, on top of that underlying geometry, you can optionally add coordinates. Coordinates are simply a mapping between points in the manifold (events in spacetime) and points in R4. The mapping must be smooth and invertible, but little else, so there is considerable freedom in choosing the mapping. It is possible to choose a mapping which maps straight lines in spacetime to straight lines in R4, such mappings are called inertial frames. It is also possible to choose a mapping which maps bent lines in spacetime to straight lines in R4, a non-inertial frame. Such a mapping does nothing to alter the underlying geometry. The bent lines are still bent in a coordinate-independent geometrical sense, but because it simplifies the representation in R4 it can still be useful on occasion in order to simplify calculations. Because the mapping is invertible, in many ways it doesn't matter if you are talking about points in the manifold or points in R4. So you talk about things being "at rest" based on R4, and things happening "simultaneously" based on R4, and many other things. However, it is occasionally important to remember the underlying geometry. I hope this helps.
It does.

 Quote by DaleSpam It has, in fact, been made clear to you that SR can handle the twins paradox. What obviously hasn't been made clear to you is why. Your repetition of bald assertions that have already been contradicted is unhelpful. It wastes your time in repeating it and it wastes our time in repeating our responses. It also irritates those (maybe only me) who feel like their well-considered and helpful responses have been completely ignored by you.
Not ignored; not understood. Annoying either way, when the reason for not understanding is a failure to work out misunderstandings on paper before making statements.

Another factor on my side was that I thought you did not understand exactly what I was troubled by. Working through the twin paradox, looking for an answer to what troubled me, also led me to understand why SR is valid for solving the problem, at least with respect to kinematics. (I don't say SR isn't valid with respect to dynamics, only that I don't know enough to say it is.) I know that what I did wrt the twin paradox is at the most elementary level. But for me, it was like the transition from saying "ga ga, goo goo" to standing up on two feet and taking a step or two (before stumbling). Hopefully I will be less annoying in future.
P: 221
 Quote by harrylin It depends on what they supposedly mean with that. In normal use of the word "in rest", in the context of SR calculations, the rocket cannot claim to be all the time in rest. This is because it's not all the time in rest in any inertial frame.
I struggled with the issue of whether the rocket is at rest throughout, or not. Part of the struggle has to do with the definition of "frame". If one defines frame to be an inertial frame, the rocket is not at rest while accelerating, while changing inertial frames. However, if one recognizes non-inertial frames, then the rocket is at rest throughout. If a frame is the same thing as the coordinate system whose origin is coincident with an observer, then there is indeed such a thing (in the abstract) as a non-inertial frame. You will note that I hadn't fully thought this through when I posted the calculations: I made sure to say "coordinate system" instead of "frame" when describing the setup. Edit: And it was George's clarification which helped me see this.

I don't say that I fully understand the concept of "absolute proper acceleration" being compatible with "no absolute space". However, it seems to me that this is something that needs to be considered with the dynamics of SR. Kinematically, the spacetime diagram shows that the rocket is at rest in its non-inertial frame.

That, by the way, is the objection that I felt was being dismissed--the call for consideration of the case in which the rocket is at rest. If I (now, or finally) understand George correctly, that objection is intentionally dismissed by Taylor and Wheeler. Indeed, when the objection is raised, it could be shown, using the spacetime diagram that is already under consideration, that the rocket is at rest throughout the episode. What the objector wants (or at least, what I wanted) was for the symmetrical diagram to be drawn, because he thinks that this is the only spacetime diagram in which the rocket is at rest. That thought is based on a misconception of the spacetime diagram--a misconception which I began to perceive as I thought more about the spacetime diagrams I drew for the pole-in-barn paradox.

 Quote by harrylin While that is often said, it is only half true, and therefore misleading. The dynamics is inherent by the prescription of reference to inertial frames for the Lorentz transformations. If one purely considered kinematics only then the situation would be symmetrical.
The "one exception" (which I did not explicitly point out later, I realize now) is the rocket being non-inertial while accelerating. That is a dynamic phenomenon.
P: 221
my_wan said,
 Gravity turns this relationship on its head. Note the force felt when accelerating. When on the surface of a gravitational mass, like Earth, the principle of equivalence tells us this weight we feel is the same force we feel when accelerating. Now if you jump off a roof then while accelerating toward the ground you feel no force, effectively weightless. Hence, in your frame of reference you are at rest, i.e., not accelerating, but the Earth is accelerating toward you. Yet everybody in the Universe can agree that your speed is changing, even if not by how much. In this case proper gravity is absolute, while how much gravity and coordinate paths are relative. So in this case gravity is absolute, but the coordinate acceleration due to gravity is relative.
I take it that the bold text above is what harrylin refers to as the modern argument...

 Quote by harrylin Thanks for bringing that up, as it is exactly that modern argument that 1916GR denies; and I had the impression that GregAshmore noticed that point, that it's basically that issue that he discovered. Einstein tried to relativise acceleration by relativising gravitation, so that it's a matter of free opinion if a rocket accelerates or not. Nowadays few people accept that view. That argument fails in the first version by Langevin, see my earlier remarks as well as elaborations by others.
No, I was not disagreeing with Einstein's contention that it is a matter of free opinion as to whether the rocket accelerates or not.

My understanding is that, even according to modern ideas, it is indeed a matter of free opinion as to whether the rocket accelerates--if one considers acceleration in the usual sense of "rate of change in velocity as measured with respect to a set of coordinates". The statement was, "Coordinate acceleration is relative". (Edit to clarify.)

It is proper acceleration which is absolute; but one may be at rest in a coordinate system while experiencing proper acceleration.

There is at least one person in the universe who will disagree with the claim that the rocket is accelerating: the observer in the rocket who is convinced that he is at rest. (This, I think, is along the lines of another comment on the claim, posted by someone else.)

Still, I don't know anything about how the dynamics of the "resting while accelerating rocket" work, so I'm not making any statement of my own opinion on this issue. I'm only giving my understanding of what I have been told.
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P: 17,534
 Quote by GregAshmore My understanding is that, even according to modern ideas, it is indeed a matter of free opinion as to whether the rocket accelerates--if one considers acceleration in the usual sense of "rate of change in velocity as measured with respect to a set of coordinates". The statement was, "Coordinate acceleration is relative". (Edit to clarify.)
Yes.

 Quote by GregAshmore It is proper acceleration which is absolute; but one may be at rest in a coordinate system while experiencing proper acceleration.
Correct.

 Quote by GregAshmore There is at least one person in the universe who will disagree with the claim that the rocket is accelerating: the observer in the rocket who is convinced that he is at rest. (This, I think, is along the lines of another comment on the claim, posted by someone else.)
It seems like you get the distinction between coordinate and proper acceleration.
P: 3,188
 Quote by GregAshmore [..] As I recall, someone defined proper acceleration as the derivative of proper velocity with respect to proper time. I don't understand how there can be proper velocity at all, because proper time is the elapsed time at "the same place". If position never changes with respect to time, it would seem that proper velocity must be zero. I did not say anything about it because my objection was much more basic than that, and I figured there would be a logical explanation for it if and when I get that far.
I somewhat agree with that; I quickly looked around for definitions and if I see it correctly the definition by Smoot of "proper acceleration" relates to the coordinate acceleration with respect to an instantaneously co-moving inertial frame (thus there is a change of velocity from zero to non-zero) and that makes sense to me.
In contrast, for me the definition of "proper acceleration" as given in Wikipedia is a misnomer for what I would call apparent gravitation. - http://en.wikipedia.org/wiki/Proper_acceleration
It may be that such different definitions bugged you (they did bug me in earlier discussions).
P: 3,188
 Quote by GregAshmore I struggled with the issue of whether the rocket is at rest throughout, or not. Part of the struggle has to do with the definition of "frame". If one defines frame to be an inertial frame, the rocket is not at rest while accelerating, while changing inertial frames. However, if one recognizes non-inertial frames, then the rocket is at rest throughout.
Of course it's always possible to be at rest relative to oneself and to define non-inertial reference frames and coordinate systems. However such a reference frame isn't what is implied with "at rest" in the context of SR, which relates the physics to inertial frames: you have remarked that yourself. And saying that something is in rest in a frame in which we define it to be in rest ("the rocket is at rest in its non-inertial frame") is simply meaningless.
 [..] I don't say that I fully understand the concept of "absolute proper acceleration" being compatible with "no absolute space".
Not sure to parse that correctly; some of the text of Langevin that I omitted argues for absolute space. That's a matter of opinion.
 [..] it could be shown, using the spacetime diagram that is already under consideration, that the rocket is at rest throughout the episode.
Once more, "at rest" in SR normally means at rest in an inertial coordinate system that one chooses as "rest system". In no SR space diagram is the rocket continuously "at rest", because it is moving during part of the voyage no matter which inertial system one chooses as "rest" system.
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Quote by GregAshmore
Quote by ghwellsjr
 Quote by GregAshmore There is only one way to construct the spacetime diagram, due to the unique non-inertial behavior of the rocket.
I don't know why you would say this. Are you referring to the Earth's inertial rest frame? There are an infinite number of ways to construct spacetime diagrams for your scenario, all with different velocities with respect to each other and all just as valid and all producing the same final clock readings and the same things that each observer actually sees.
I was too general in my wording. I only meant that Events A, B, and C have been placed, there is only one way to place Event D.
Actually, Event B is of no consequence in the definition of your scenario. As long as we know the rocket's speed and where (or when) it turns around, that defines the end point.

However, I'd like you to consider an issue related to the one you just raised here and that is, how does the rocket know when to turn around? The rocket cannot know from any direct measurement when the Earth has traveled 10 units away in the rocket frame. That distance is the difference between the spacial components of Event B and C which are simultaneous in the Earth frame but which have a distance between them of 16.67 in the rocket frame. Even if the rocket had instant access to the remote information, it would still have to do some calculation if it's based on distance to determine when to fire its rockets.

To me, a much cleaner way to specify the Twin Paradox is to state the Proper Time on the traveler's clock when he should fire the rockets to turn around. This has the advantage that it doesn't require the specification of any reference frame. In fact, it doesn't even require fully specifying any events since we don't care about the spatial component. So if we know how long it takes for the traveler to get to the turnaround point and we know how fast he is traveling, those two parameters fully specify the complete Twin Paradox scenario (assuming of course that he is returning at the same speed he left at). I fully explained this in the thread you linked to in your opening post.
P: 3,188
 Quote by GregAshmore my_wan said, [..] I take it that the bold text above is what harrylin refers to as the modern argument...
Quite so; with the demotion of pseudo gravitational fields one returns to Langevin's argument that acceleration has "absolute" effects that everyone can observe -even for the case that the accelerometer reading remains zero.
 [..]My understanding is that, even according to modern ideas, it is indeed a matter of free opinion as to whether the rocket accelerates--if one considers acceleration in the usual sense of "rate of change in velocity as measured with respect to a set of coordinates". The statement was, "Coordinate acceleration is relative". (Edit to clarify.) It is proper acceleration which is absolute; but one may be at rest in a coordinate system while experiencing proper acceleration.
One can always choose a coordinate system to be always at rest in; once more, that is meaningless for the physics. However, it is certainly true that proper acceleration (both definitions of it) is quantitatively agreed upon by all. If that's all you wanted to understand, then you have certainly achieved your goal.
 There is at least one person in the universe who will disagree with the claim that the rocket is accelerating: the observer in the rocket who is convinced that he is at rest. (This, I think, is along the lines of another comment on the claim, posted by someone else.) [..]
That person, if indeed he denies to be changing his state of motion, will have to explain the physical causes for the observed effects; and I mistakenly thought that you were contemplating the different physical explanations that are given in the literature. If and when you come to that point, you may want to read earlier comments and references that were provided in this discussion.
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 Quote by GregAshmore That, by the way, is the objection that I felt was being dismissed--the call for consideration of the case in which the rocket is at rest. If I (now, or finally) understand George correctly, that objection is intentionally dismissed by Taylor and Wheeler. Indeed, when the objection is raised, it could be shown, using the spacetime diagram that is already under consideration, that the rocket is at rest throughout the episode. What the objector wants (or at least, what I wanted) was for the symmetrical diagram to be drawn, because he thinks that this is the only spacetime diagram in which the rocket is at rest.
T&W never consider non-inertial frames or a frame in which the traveler is at rest, at least not when discussing the Twin Paradox. What they are dismissing is the explanation that I gave in post #23 of the thread you linked to in your OP. I considered three Inertial Reference Frames which included different rest states of the two twins. I attempted to show that any one of them was adequate to explain everything about the scenario, which is pretty much the classic way of explaining the Twin Paradox. But they think they have a better way which involves any observer calculating the spacetime interval between pairs of events. I don't think that helps at all but since they are writing the book, they get to decide when the objectors are happy. But real objectors, like you, remained unhappy.
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 Quote by harrylin Once more, "at rest" in SR normally means at rest in an inertial coordinate system that one chooses as "rest system". In no SR space diagram is the rocket continuously "at rest", because it is moving during part of the voyage no matter which inertial system one chooses as "rest" system.
True. But no one is claiming that the rocket is at rest in an inertial frame--not even the twin in the rocket. The twin in the rocket feels the unbalanced force of the rocket engine, and he knows (or would know upon reunion) that the twin on earth feels no such force. Even without the formal definitions of inertial and non-inertial, the rocket twin would recognize that his situation is fundamentally different than that of his twin. Fully aware of that difference, he claims that he is at rest throughout the episode.

I don't see how the claim is disproved by pointing out that the rocket changes inertial frames during the firing of the engine. The change of inertial frames only confirms what everyone knows: the rocket is non-inertial. From the earth twin's point of view, the rocket is in an inertial frame, accelerates, and comes to rest in another inertial frame. The rocket twin disagrees with this assessment. He can point to the spacetime diagram (which the earth twin accepts as valid) and show that he remains at rest in his own frame, even while not at rest in any one inertial frame. To prove the rocket twin wrong, it must either be shown that his frame moved with respect to some absolute position marker, or that the laws of dynamics are violated if he does not move. There is no absolute position marker, and the laws of dynamics are not considered in my analysis. [If these statements are wrong, at least they are not bald statements; I've done my homework. ]
P: 221
 Quote by ghwellsjr T&W never consider non-inertial frames or a frame in which the traveler is at rest, at least not when discussing the Twin Paradox. What they are dismissing is the explanation that I gave in post #23 of the thread you linked to in your OP. I considered three Inertial Reference Frames which included different rest states of the two twins. I attempted to show that any one of them was adequate to explain everything about the scenario, which is pretty much the classic way of explaining the Twin Paradox. But they think they have a better way which involves any observer calculating the spacetime interval between pairs of events. I don't think that helps at all but since they are writing the book, they get to decide when the objectors are happy. But real objectors, like you, remained unhappy.
I read that post in its entirety before opening this thread. I didn't catch on to what you were doing because in each of the diagrams the rocket twin is spoken of as moving for part of the trip.

What helped me was to realize that in the typical two-frame spacetime diagram, the world line of an inertial particle shows the particle both as moving and at rest. It is moving in one frame, and at rest in the other frame. Thus, the one spacetime diagram actually shows the case I wanted to see-the case in which the rocket twin considers himself at rest. The symmetrical diagram (which is invalid) is not needed.
P: 221
 Quote by ghwellsjr Actually, Event B is of no consequence in the definition of your scenario. As long as we know the rocket's speed and where (or when) it turns around, that defines the end point.
It would be of consequence for the person who mistakenly believes that because either object can be the one that appears to turn around, the spacetime diagram can be drawn with either object having the bent worldline. Event B is where the Earth would turn around. I chose to show at this juncture that the Earth cannot change inertial frames because it experiences no unbalanced force; because it is inertial.

 Quote by ghwellsjr However, I'd like you to consider an issue related to the one you just raised here and that is, how does the rocket know when to turn around? The rocket cannot know from any direct measurement when the Earth has traveled 10 units away in the rocket frame. That distance is the difference between the spacial components of Event B and C which are simultaneous in the Earth frame but which have a distance between them of 16.67 in the rocket frame. Even if the rocket had instant access to the remote information, it would still have to do some calculation if it's based on distance to determine when to fire its rockets. To me, a much cleaner way to specify the Twin Paradox is to state the Proper Time on the traveler's clock when he should fire the rockets to turn around. This has the advantage that it doesn't require the specification of any reference frame. In fact, it doesn't even require fully specifying any events since we don't care about the spatial component. So if we know how long it takes for the traveler to get to the turnaround point and we know how fast he is traveling, those two parameters fully specify the complete Twin Paradox scenario (assuming of course that he is returning at the same speed he left at). I fully explained this in the thread you linked to in your opening post.
It is a cleaner solution. Yet even now I feel the [vestigial] reflexive urge to tune it out because it says the rocket twin "travels", "turn[s] around", "return[s]". The doubter has been told that in relativity the rocket has the same right to be at rest as the Earth has. Language of motion applied to the rocket speaks so loudly that it drowns out the perfectly valid point that is being made.

For perspective, I have read two or three explanations of the twin paradox to my 30+ son. He has some technical training, has a job that requires him to evaluate contractual language. He had exactly my reaction, without me making any comment.
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P: 17,534
 Quote by harrylin I somewhat agree with that; I quickly looked around for definitions and if I see it correctly the definition by Smoot of "proper acceleration" relates to the coordinate acceleration with respect to an instantaneously co-moving inertial frame (thus there is a change of velocity from zero to non-zero) and that makes sense to me. In contrast, for me the definition of "proper acceleration" as given in Wikipedia is a misnomer for what I would call apparent gravitation. - http://en.wikipedia.org/wiki/Proper_acceleration
The two definitions are equivalent. The Smoot definition basically just pushes the use of accelerometers one step further. Instead of reading the proper acceleration directly off the accelerometer, you define an inertial frame by strapping accelerometers to your clocks and rods, ensuring that they read 0, and then reading the proper acceleration off the clocks and rods.
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P: 17,534
 Quote by harrylin That person, if indeed he denies to be changing his state of motion, will have to explain the physical causes for the observed effects;
Which he can do quite easily simply by stating the laws of physics in a covariant form and then using any coordinates where his state of motion does not change.

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