Is Lorentz Contraction Indistinguishable from Standard Relativity?

[Dale]

Universal generalization of "distance is a coordinate-dependent" is not logically true.

Yes, it is logically universally true, however I think you may be misunderstanding some of the terminology based on your comments.

1) The distance between objects in an inertial frame is constant and it is not the case that distance is a coordinate-dependent.

Whether or not something is constant is not at all the same thing as whether or not something is coordinate independent. For a quantity A to be constant simply means that dA/dt = 0. For a quantity to be coordinate-independent means that A=A' where A' is the coordinate transform of A. They are two completely independent concepts.

2) If there exists relative motion, then the stationary frame will calculate length contraction for the moving frame metrics.

Yes, you are correct. Mathematically, A≠A' where A is the distance in the stationary frame and A' is the distance in the moving frame. Therefore, as I said above, distance is coordinate-dependent (or "relative").

3) If there is an accelerating frame, then the frame will experience metric expansion in the direction of acceleration. However, the launch frame will calculate a constant distance for objects in the accelerating frame.

Again, A≠A', the distance is coordinate-dependent.

 

[cfrogue]

Yes, it is logically universally true, however I think you may be misunderstanding some of the terminology based on your comments.Whether or not something is constant is not at all the same thing as whether or not something is coordinate independent. For a quantity A to be constant simply means that dA/dt = 0. For a quantity to be coordinate-independent means that A=A' where A' is the coordinate transform of A. They are two completely independent concepts.Yes, you are correct. Mathematically, A≠A' where A is the distance in the stationary frame and A' is the distance in the moving frame. Therefore, as I said above, distance is coordinate-dependent (or "relative").Again, A≠A', the distance is coordinate-dependent.

Sorry, I do not see your terminology as descriptive enough.

And, strictly from a logical point of view, it does not satisfy condition 1. For me, distance is coordinate-dependent means just that. Because the coordinates can be the identity coords in an inertial frame, this does not meet condition 1.

In the stationary frame, Einstein said,
Let us take a system of co-ordinates in which the equations of Newtonian mechanics hold good.2 In order to render our presentation more precise and to distinguish this system of co-ordinates verbally from others which will be introduced hereafter, we call it the ``stationary system.''
http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

[JesseM]

I mean that the posted paper shows the nature of SR's acceleration that the accelerating frame sees an expanding metric in the direction of acceleration when compared to the instantaneous rest frame.

Again, I don't know what the phrase "expanding metric" is supposed to mean.

What does it mean to see a constant metric? Certainly the spacetime metric is the same in all frames, and in relativity the spacetime metric is the fundamental one. I suppose you can talk about a spatial metric in any given coordinate system, but what does it mean to say it's expanding? Are you just saying that the distance between the rockets expands? But of course even in classical Newtonian mechanics the distance between rockets will expand if they have different coordinate accelerations, yet in this situation I doubt you would talk about an "expanding metric". Do you have any exact definition for this phrase?

The metric I describe is only the one in the frame.

That doesn't really answer my questions above. Again, normally in relativity the metric is a spacetime metric which gives an invariant measure of spacetime "distance" along any path (along timelike worldlines, this is equivalent to the proper time), so the metric does not in any way depend on what frame you choose. I was asking if you were instead talking about a spatial metric, which I suppose would be defined in terms of the coordinate distance between points at a given moment in a given frame. Or does neither of these describe what you mean? A "metric" is always defining some measure of distance on a manifold, so what measure are you using?

 

[atyy]

I would be interested in your calculation.

This seems to be an area that has been left off from a rigorous analysis.

In the prior post, there are also some interesting possibilities after the acceleration is done.

Ok, I don't think I can do better than Bell, or Lorentz for that matter!

Let's try to set up electrostatic equilibrium of 3 charges in row (Q ... -q ... Q), and the distance between neighbouring charges is r. For the rightmost charge to be in equilibrium require Q/((2r)^2)=(q/r^2). If r is finite, then q=Q/4, but in which case r can be any finite value, so there is no unique equilibrium. In the case of relativistic quantum mechanics, I believe the ground state is unique, then one can do the calculation in the launch frame without appealing to the Lorentz symmetry of the laws. But that's more pain than I'd like to go through at the moment.

So basically Lorentz's "calculation in the launch frame" holds if we grant him quantum mechanics and a unique ground state."Consequently, if, neglecting the effects of molecular motion, we suppose each particle of a solid body to be in equilibrium under the action of the attractions and repulsions exerted be its neighbours, and if we take for granted the there is but one configuration of equilibrium, we may draw the conclusion that the system Σ' , if the velocity w is imparted to it, will of itself change into the system Σ. [emphasis mine]" http://en.wikisource.org/wiki/Electromagnetic_phenomena

 

[Dale]

And, strictly from a logical point of view, it does not satisfy condition 1. For me, distance is coordinate-dependent means just that. Because the coordinates can be the identity coords in an inertial frame, this does not meet condition 1.

Frankly, this response leaves me questioning your honesty. By this absurd line of reasoning the volume of your stereo does not depend on the amplifier because you can set the gain to 0 dB, or more directly that the linear system y=A.x is independent of A because A may be the identity matrix. This level and quality of argument is characteristic of trolls and crackpots.

The term "coordinate independent" means that a quantity does not change under any arbitrary coordinate transform; if something is not coordinate independent then it is "coordinate dependent". The term "Lorentz invariant" means that a quantity does not change under any arbitrary Lorentz transform; if something is not "Lorentz invariant" then it is "Lorentz variant" or "relative". Distance is coordinate dependent. I hope that is clear enough for you.

 

[Al68]

Universal generalization of "distance is a coordinate-dependent" is not logically true.

It is existentially quantified by the following.

1) The distance between objects in an inertial frame is constant and it is not the case that distance is a coordinate-dependent.

2) If there exists relative motion, then the stationary frame will calculate length contraction for the moving frame metrics.

3) If there is an accelerating frame, then the frame will experience metric expansion in the direction of acceleration. However, the launch frame will calculate a constant distance for objects in the accelerating frame.

These are all different behaviors based on the particular SR motion and thus the phrase "distance is a coordinate-dependent" is not a universally descriptive phrase.

1) and 2) above are not generally true. The distance between the ships in this case being constant in the launch frame is specific to the scenario, just to make the point simple.

The distance between the ships being length contracted (constant instead of increasing as in the co-moving frame) in the launch frame is due to relative velocity, which increases with time. Greater relative velocity equals greater contraction, which is why, since it is stipulated that the distance between the ships remains constant in the launch frame, it must increase with time in the co-moving frame.

 

[cfrogue]

Again, I don't know what the phrase "expanding metric" is supposed to mean.

Yea, normally one uses a metric space to determine a metric I agree.

What I mean is that the distance within the accelerating frame between objects, the two ships in this example, expands as the acceleration continues.

Once the acceleration stops, the normal at rest frame measurements snap back.

That doesn't really answer my questions above. Again, normally in relativity the metric is a spacetime metric which gives an invariant measure of spacetime "distance" along any path (along timelike worldlines, this is equivalent to the proper time), so the metric does not in any way depend on what frame you choose. I was asking if you were instead talking about a spatial metric, which I suppose would be defined in terms of the coordinate distance between points at a given moment in a given frame. Or does neither of these describe what you mean? A "metric" is always defining some measure of distance on a manifold, so what measure are you using?

Yes, I am only talking about a spatial metric without time.

I have not thought about what it looks like because I assume it is handled by GR.

But, what seems to be clear is that the distance bewteen the two ships increases as the acceleration continues.

Once the accelerations stops, the distance between the ships returns back to the original d.
Is this what you calculate?

 

[cfrogue]

Ok, I don't think I can do better than Bell, or Lorentz for that matter!

Let's try to set up electrostatic equilibrium of 3 charges in row (Q ... -q ... Q), and the distance between neighbouring charges is r. For the rightmost charge to be in equilibrium require Q/((2r)^2)=(q/r^2). If r is finite, then q=Q/4, but in which case r can be any finite value, so there is no unique equilibrium. In the case of relativistic quantum mechanics, I believe the ground state is unique, then one can do the calculation in the launch frame without appealing to the Lorentz symmetry of the laws. But that's more pain than I'd like to go through at the moment.

So basically Lorentz's "calculation in the launch frame" holds if we grant him quantum mechanics and a unique ground state."Consequently, if, neglecting the effects of molecular motion, we suppose each particle of a solid body to be in equilibrium under the action of the attractions and repulsions exerted be its neighbours, and if we take for granted the there is but one configuration of equilibrium, we may draw the conclusion that the system Σ' , if the velocity w is imparted to it, will of itself change into the system Σ. [emphasis mine]" http://en.wikisource.org/wiki/Electromagnetic_phenomena

This analysis does not account for the "push" from the back ship.
If one ship were pulling a string without the 2nd ship, this would be complete above.

However, the back ship is accelerating into the string.

Thus, I would assume from the launch frame, there exists an equilibrium at the center of the string.

I am not sure if I am thinking this through correctly.

 

[cfrogue]

Frankly, this response leaves me questioning your honesty. By this absurd line of reasoning the volume of your stereo does not depend on the amplifier because you can set the gain to 0 dB, or more directly that the linear system y=A.x is independent of A because A may be the identity matrix. This level and quality of argument is characteristic of trolls and crackpots.

The term "coordinate independent" means that a quantity does not change under any arbitrary coordinate transform; if something is not coordinate independent then it is "coordinate dependent". The term "Lorentz invariant" means that a quantity does not change under any arbitrary Lorentz transform; if something is not "Lorentz invariant" then it is "Lorentz variant" or "relative". Distance is coordinate dependent. I hope that is clear enough for you.

I am perfectly fine with your views.

I simply would not use your terminology for my stated reasons.

If you want to call me names for that, so be it.

 

[atyy]

This analysis does not account for the "push" from the back ship.
If one ship were pulling a string without the 2nd ship, this would be complete above.

However, the back ship is accelerating into the string.

Thus, I would assume from the launch frame, there exists an equilibrium at the center of the string.

I am not sure if I am thinking this through correctly.

Yes, Lorentz's and Bell's arguments only say that the moving equilibrium length in the launch frame of the string should be shorter than its stationary equilibrium length in the launch frame. What we know from the specification of the ships' acceleration that if the string does not break when it is moving, it must occupy a greater length in the launch frame than its moving equilibrium length in the launch frame, so it must be stressed at some point - but it does not tell us which at which point it is stressed, nor where the string eventually breaks.

 

[cfrogue]

Yes, Lorentz's and Bell's arguments only say that the moving equilibrium length in the launch frame of the string should be shorter than its stationary equilibrium length in the launch frame. What we know from the specification of the ships' acceleration that if the string does not break when it is moving, it must occupy a greater length in the launch frame than its moving equilibrium length in the launch frame, so it must be stressed at some point - but it does not tell us which at which point it is stressed, nor where the string eventually breaks.

The latest paper on the subject says:

Bell’s paradox was that his intuition told him the cable would
break, yet there was no change in the distance between the ships in system S.
He suggested resolving the paradox by stating that a cable between the ships
would shorten due to the contraction of a physical object proposed by Fitzgerald
and Lorentz, while the distance between the ships would not change. This
resolution however contradicts special relativity which allows no such difference
in any measurement of these two equal lengths.

http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

 

[atyy]

The latest paper on the subject says:

Bell’s paradox was that his intuition told him the cable would
break, yet there was no change in the distance between the ships in system S.
He suggested resolving the paradox by stating that a cable between the ships
would shorten due to the contraction of a physical object proposed by Fitzgerald
and Lorentz, while the distance between the ships would not change. This
resolution however contradicts special relativity which allows no such difference
in any measurement of these two equal lengths.

http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

I believe that paper's own analysis is right, but I think it's assessment of Bell's analysis is wrong - ie. I think Bell was essentially right (but I also don't think the paper understands Bell's analysis, and so misrepresents it).

 

[JesseM]

Yea, normally one uses a metric space to determine a metric I agree.

What I mean is that the distance within the accelerating frame between objects, the two ships in this example, expands as the acceleration continues.

Once the acceleration stops, the normal at rest frame measurements snap back.

What kind of accelerating frame are you thinking of? The simplest way to define a non-inertial rest frame for an object which accelerates is to have the frame's time coordinate match up with the object's proper time along its worldline, and then say that at any given event on the object's worldline, the non-inertial frame's definition of simultaneity and distance at the time of that event should match up with the object's instantaneous inertial rest frame at that event on its worldline. In this case, since the distance to the back ship increases in the instantaneous inertial rest frame of the front ship, if we define the front ship's non-inertial rest frame in this way, the distance will increase in the non-inertial frame. Then once the two ships stop accelerating, the non-inertial frame's definition of distance will match up with that of their inertial rest frame. Is that something close to what you meant?

But, what seems to be clear is that the distance bewteen the two ships increases as the acceleration continues.

Once the accelerations stops, the distance between the ships returns back to the original d.
Is this what you calculate?

What do you mean by "the original d"? If you define the non-inertial frame of one of the ships in the way I suggested, it won't return to the d seen in the launch frame, once they stop accelerating the distance between them in the non-inertial frame will be equal to the distance between them in their rest frame. At no point will the distance between them in the non-inertial frame contract, it'll just stop expanding once they've both stopped accelerating. If you want to use a different type of non-inertial frame you have to explain how it defines distance, time, and simultaneity.

 

[cfrogue]

1) The distance between objects in an inertial frame is constant and it is not the case that distance is a coordinate-dependent.

2) If there exists relative motion, then the stationary frame will calculate length contraction for the moving frame metrics.

1) and 2) above are not generally true. The distance between the ships in this case being constant in the launch frame is specific to the scenario, just to make the point simple.

The accelerating ships are not an inertial frame and so the above logic does not apply.

The distance between the ships being length contracted (constant instead of increasing as in the co-moving frame) in the launch frame is due to relative velocity, which increases with time. Greater relative velocity equals greater contraction, which is why, since it is stipulated that the distance between the ships remains constant in the launch frame, it must increase with time in the co-moving frame.

I do not agree with this.

The integral for the solution only involves the accelerating frame and the instantaneous at rest frame, S'.

The launch frame is not part of the decision process in this paper.
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

The launch frame exists as a separate entity that only concludes the distance does not change between the ships but does not participate in the integral for the solution.

 

[cfrogue]

I believe that paper's own analysis is right, but I think it's assessment of Bell's analysis is wrong - ie. I think Bell was essentially right (but I also don't think the paper understands Bell's analysis, and so misrepresents it).

This is why discussion occurs.

So, based on this paper, can you explain the Bell position and how it is consistent with this paper?

Or, are they logically inconsistent?

 

[cfrogue]

What kind of accelerating frame are you thinking of? The simplest way to define a non-inertial rest frame for an object which accelerates is to have the frame's time coordinate match up with the object's proper time along its worldline, and then say that at any given event on the object's worldline, the non-inertial frame's definition of simultaneity and distance at the time of that event should match up with the object's instantaneous inertial rest frame at that event on its worldline. In this case, since the distance to the back ship increases in the instantaneous inertial rest frame of the front ship, if we define the front ship's non-inertial rest frame in this way, the distance will increase in the non-inertial frame. Then once the two ships stop accelerating, the non-inertial frame's definition of distance will match up with that of their inertial rest frame. Is that something close to what you meant?.

This is what I meant.

What do you mean by "the original d"? If you define the non-inertial frame of one of the ships in the way I suggested, it won't return to the d seen in the launch frame, once they stop accelerating the distance between them in the non-inertial frame will be equal to the distance between them in their rest frame. At no point will the distance between them in the non-inertial frame contract, it'll just stop expanding once they've both stopped accelerating. If you want to use a different type of non-inertial frame you have to explain how it defines distance, time, and simultaneity.

d for me is the distance between the ships from the POV of the ships before accelerating.

Now, I have a question.

Will the ships return to their original distance between them before acceleration once the acceleration stips?

 

[JesseM]

d for me is the distance between the ships from the POV of the ships before accelerating.

Now, I have a question.

Will the ships return to their original distance between them before acceleration once the acceleration stips?

Not if we define the ship's non-inertial frame in the way I did above. After all, the definition says that for an event on the ship's worldline after that ship has stopped accelerating, the ship's definition of distance at that moment will match the definition of distance in its current inertial rest frame, not the definition of distance in the original inertial frame they were at rest in before accelerating (the launch frame).

 

[JesseM]

The accelerating ships are not an inertial frame and so the above logic does not apply.

You can analyze the ships from the perspective of an inertial frame. As I told you on the GPS thread, you can use any frame you like to analyze any physical situation you like, you could use an accelerating frame to analyze a collection of inertial objects or an inertial frame to analyze a collection of accelerating objects, the choice of what frame to use is never forced on you by the motion of any physical objects.

I do not agree with this.

The integral for the solution only involves the accelerating frame and the instantaneous at rest frame, S'.

The launch frame is not part of the decision process in this paper.
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

Do any of the equations in that paper make use of a non-inertial frame? Skimming the paper, it seemed like everything was analyzed from the perspective of different inertial frames.

 

[cfrogue]

Not if we define the ship's non-inertial frame in the way I did above. After all, the definition says that for an event on the ship's worldline after that ship has stopped accelerating, the ship's definition of distance at that moment will match the definition of distance in its current inertial rest frame, not the definition of distance in the original inertial frame they were at rest in before accelerating (the launch frame).

Are you arguing that the ships will be at a distance d' > d of the original launch frame distance after the acceleration ceases?

If so, you have produced a paradox.

 

[cfrogue]

You can analyze the ships from the perspective of an inertial frame. As I told you on the GPS thread, you can use any frame you like to analyze any physical situation you like, you could use an accelerating frame to analyze a collection of inertial objects or an inertial frame to analyze a collection of accelerating objects, the choice of what frame to use is never forced on you by the motion of any physical objects.

Do any of the equations in that paper make use of a non-inertial frame? Skimming the paper, it seemed like everything was analyzed from the perspective of different inertial frames.

The paper focuses on the accelerating frame and an instanteneous at rest frame.

The launch frame is incidental.

 

[JesseM]

Are you arguing that the ships will be at a distance d' > d of the original launch frame distance after the acceleration ceases?

In the non-inertial frame I defined, yes.

If so, you have produced a paradox.

How?

The paper focuses on the accelerating frame and an instanteneous at rest frame.

Again, I didn't see anywhere in the paper where they referred to an accelerating frame. Can you provide a quote, or point to an equation that you think involves quantities measured in an accelerating frame?

 

[atyy]

This is why discussion occurs.

So, based on this paper, can you explain the Bell position and how it is consistent with this paper?

Or, are they logically inconsistent?

Bell's argument is logically consistent with the argument in the paper.

Let's start in the launch frame. In that frame we know that Maxwell's equations hold. Maxwell's equations tell us the electric field for an electric charge that is stationary in the launch frame, as well as one that is moving in the launch frame.

The field of a moving charge is "flattened" compared with the field of a stationary charge. This can be seen from http://farside.ph.utexas.edu/teaching/jk1/lectures/node26.html, Eq 265. It can also be obviously seen pictorially from http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html by comparing the "linear" case (ie. constant velocity) with v=0 and with v=0.9.

Because of this flattening, if we imagine that electrical forces are one of the things that hold particles in a string together, we may imagine that Maxwell's equations predict that the equilibrium length of a moving string is shorter than that of a stationary string. If this is correct, then all we have made use of is Maxwell's equations in the launch frame. This is not totally correct because classical electrostatics does not predict a unique equilibrium configuration, and also because a string is held together not just by classical electrical forces, but also by quantum mechanics (eg. Pauli exclusion principle, the uniqueness of the ground state) and not just classical electrostatics. With these additional assumptions, then we can predict that a string's moving equilibrium length is shorter than its stationary equilibrium length.

An essential part of these heuristics was the flattening of the electric field of a moving charge using only Maxwell's equations in the launch frame. So a crucial question in seeing if the calculations in the launch frame and in other frames match up is: is this flattening predicted by using Maxwell's equations in the rest frame of the moving charge, then Lorentz transforming to the launch frame? The answer is yes.

 

[Al68]

1) and 2) above are not generally true. The distance between the ships in this case being constant in the launch frame is specific to the scenario, just to make the point simple.

The accelerating ships are not an inertial frame and so the above logic does not apply.

Huh? That makes no sense whatsoever. I was referring to your statement that "1) The distance between objects in an inertial frame is constant and it is not the case that distance is a coordinate-dependent." and "3) If there is an accelerating frame, then the frame will experience metric expansion in the direction of acceleration. However, the launch frame will calculate a constant distance for objects in the accelerating frame." None of those statements are true generally for obvious reasons.

The distance between the ships being length contracted (constant instead of increasing as in the co-moving frame) in the launch frame is due to relative velocity, which increases with time. Greater relative velocity equals greater contraction, which is why, since it is stipulated that the distance between the ships remains constant in the launch frame, it must increase with time in the co-moving frame.

I do not agree with this.

The integral for the solution only involves the accelerating frame and the instantaneous at rest frame, S'.

The launch frame is not part of the decision process in this paper.
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

The launch frame exists as a separate entity that only concludes the distance does not change between the ships but does not participate in the integral for the solution.

Seriously? You are now objecting to anyone referring to the launch frame? None of your response even remotely explains why you "do not agree with this".

Are you arguing that the ships will be at a distance d' > d of the original launch frame distance after the acceleration ceases?

Of course the ships will be farther apart in a co-moving inertial frame than in the launch frame. Why on Earth would they "snap back" to their original distance in any frame just because they kill their engines? Magic?

 

[cfrogue]

In the non-inertial frame I defined, yes.

How?

Because, the distance between the accelerating frame will exceed that of the launch frame.

Again, I didn't see anywhere in the paper where they referred to an accelerating frame. Can you provide a quote, or point to an equation that you think involves quantities measured in an accelerating frame?

Yea, it does not directly refer to it but does infer it.

But, what do you think this means?

We denote the spaceships as L and R, each having acceleration a(t) in the positive x direction in system S
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

 

[cfrogue]

Huh? That makes no sense whatsoever. I was referring to your statement that "1) The distance between objects in an inertial frame is constant and it is not the case that distance is a coordinate-dependent." and "3) If there is an accelerating frame, then the frame will experience metric expansion in the direction of acceleration. However, the launch frame will calculate a constant distance for objects in the accelerating frame." None of those statements are true generally for obvious reasons.

You and I are not communicating.

You cannot refute 1 and 2, if so do that please.

Seriously? You are now objecting to anyone referring to the launch frame? None of your response even remotely explains why you "do not agree with this". Of course the ships will be farther apart in a co-moving inertial frame than in the launch frame. Why on Earth would they "snap back" to their original distance in any frame just because they kill their engines? Magic?

I showed the math, you show yours.

 

[cfrogue]

None of your response even remotely explains why you "do not agree with this". Of course the ships will be farther apart in a co-moving inertial frame than in the launch frame. Why on Earth would they "snap back" to their original distance in any frame just because they kill their engines? Magic?

More directly, if you support the proposition that the accelerating ships will retain their > d positions after the acceleration stops, you will find that the launch frame will completely disagree with your assessment since the ships maintained their distance.

So, where is your math to reconcile this?

 

[JesseM]

Because, the distance between the accelerating frame will exceed that of the launch frame.

What's paradoxical about that? The distance in the non-inertial frame began to grow larger than d from the moment they began accelerating. Anyway, you don't even need to consider a non-inertial frame here; just using the Lorentz transformation and their x(t) functions in the launch frame, you can easily show that the distance between them in other inertial frames can be larger than d. Different inertial frames always disagree on the distance between a pair of objects, just like they disagree on the time between a pair of events, that's just a feature of how the Lorentz transformation works.

Yea, it does not directly refer to it but does infer it.

How so? Do you think any of the variables in the equations they use refer to quantities in a non-inertial frame?

But, what do you think this means?

We denote the spaceships as L and R, each having acceleration a(t) in the positive x direction in system S
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v2.pdf

That quote is from p. 4, and if you look at the context it's clear that S is what we have been calling the "launch frame". Do you disagree?

 

[matheinste]

More directly, if you support the proposition that the accelerating ships will retain their > d positions after the acceleration stops, you will find that the launch frame will completely disagree with your assessment since the ships maintained their distance.

So, where is your math to reconcile this?

Try reading the proposed scenario. The constancy of the distance between the ships in the launch frame is not a natural occurrence but a stipulation in the set up of the problem.

Apart from that there is a logical inconsistency in what you say. You say the ships will "snap back" to the original distance. This, apart from being ridiculous, assumes that they acquired a greater distance through the acceleration, which although true, refutes your argument for constancy.

Matheinste.

 

[cfrogue]

Try reading the proposed scenario. The constancy of the distance between the ships in the launch frame is not a natural occurrence but a stipulation in the set up of the problem.

Apart from that there is a logical inconsistency in what you say. You say the ships will "snap back" to the original distance. This, apart from being ridiculous, assumes that they acquired a greater distance through the acceleration, which although true, refutes your argument for constancy.

Matheinste.

Fine, I will argue my point from SR here.

Any logic that assumes the ships will have a greater distance after acceleration stops from the accelerating frame will contradict the launch frame that sees the distance as constant and then contracted by the normal LT calculations. You must have logic and equations to support this position.

I want to see all this.

 

[JesseM]

Fine, I will argue my point from SR here.

Any logic that assumes the ships will have a greater distance after acceleration stops from the accelerating frame will contradict the launch frame that sees the distance as constant and then contracted by the normal LT calculations. You must have logic and equations to support this position.

I want to see all this.

What do you mean when you say the launch frame "sees the distance as constant and then contracted by the normal LT calculations"? The launch frame sees the distance as constant at all times, even after the acceleration stops, so why do you say "then contracted"? Do you mean the distance seen in the launch frame once the ships stop accelerating is contracted relative to the distance between them in the ships' new inertial rest frame? Of course that's true, it's why the distance is greater in the ships' rest frame!

 

[atyy]

I guess cfrogue's confusion comes from something like:

1) In SR it is not moving objects that Lorentz contract, but space itself.
2) The distance between the ships is moving, so it should Lorentz contract, since space itself contracts.
3) Yet this moving distance remains the same, thus we have obtained a contradiction.

Well, something must be wrong above. I myself never use this space itself contracts or that time itself dilates, so I don't really have an intuition where the error occurs, but maybe cfrogue can confirm that I have summarized his confusion correctly, and someone else can write the correct version of the above statements 1-3.

 

[matheinste]

Fine, I will argue my point from SR here.

Any logic that assumes the ships will have a greater distance after acceleration stops from the accelerating frame will contradict the launch frame that sees the distance as constant and then contracted by the normal LT calculations. You must have logic and equations to support this position.

I want to see all this.

Why, so you can ignore them.

Matheinste.

 

[Al68]

More directly, if you support the proposition that the accelerating ships will retain their > d positions after the acceleration stops, you will find that the launch frame will completely disagree with your assessment since the ships maintained their distance.

So, where is your math to reconcile this?

Supporting that proposition is equivalent to supporting the proposition that the ships retain their velocity relative to earth, since their "> d positions" are due only to relative velocity.

Maybe this will be more clear:

1) The distance between the ships remained constant in the launch frame.
2) The distance between the ships increased in the new co-moving ship frame.
3) The distance between the ships after acceleration is shorter in the launch frame than it is in the co-moving frame.

The math is simple:

Distance between the ships in launch frame equals distance between the ships in co-moving frame times sqrt(1-v^2/c^2). It's that simple.

The distance between the ships will be different in the launch frame than in a co-moving frame as long as there is relative velocity between them.

 

[JesseM]

If cfrogue wants some more detailed math, here it is:

Say that in the launch frame, the left ship is at position x=0 at time t=0, and the right ship is at position x=d at time t=0. At t=0 they both begin to accelerate with constant coordinate acceleration a in the launch frame, and they both stop accelerating at time t=t1 in the launch frame. Then prior to t1, v(t) for both ships will be given by v(t) = a*t, and x(t) for the left ship will be given by x(t) = (1/2)*a*t^2 while x(t) for the right ship will be given by x(t) = (1/2)*a*t^2 + d. So at time t1 when they stop acceleration, both ships will have a velocity in the launch frame of a*t1, and the position of the left ship will be (1/2)*a*t1^2 while the position of the right ship will be (1/2)*a*t1^2 + d. After that they both move at constant velocity v1 = a*t1, so after t1 x(t) for the left ship will be given by x(t) = v1*t - v1*t1 + (1/2)*a*t1^2 while x(t) for the right ship will be given by x(t) = v1*t - v1*t1 + (1/2)*a*t1^2 + d.

Now we can use the formula I derived in post 134 of the twins thread, which tells us that if we pick the event E of the left ship stopping acceleration, and pick another event E' on the worldine of the right ship which is simultaneous with E in the ship's own inertial rest frame, in the launch frame where the right ship is moving at speed v1 and was at a distance of d from E when it happened, the coordinate time between E and E' must be (d*v1/c^2)*gamma^2, so if E occurred at time t1, E' occurred at time t = t1 + (d*v1/c^2)*gamma^2. Plugging this time into the x(t) for the right ship, E' must have occurred at a position of x = v1*[t1 + (d*v1/c^2)*gamma^2] - v1*t1 + (1/2)*a*t1^2 + d = (d*v1^2/c^2)*gamma^2 + (1/2)*a*t1^2 + d.

So, coordinates of E in the launch frame:
x = (1/2)*a*t1^2, t = t1.
Since v1 = a*t1, the coordinates of E can be rewritten as:
x = (1/2)*v1*t1, t = t1

Coordinates of E' in the launch frame:
x = (d*v1^2/c^2)*gamma^2 + (1/2)*a*t1^2 + d, t = t1 + (d*v1/c^2)*gamma^2.

Since v1 = a*t1, the coordinates of E' can be rewritten as:
x = (d*v1^2/c^2)*gamma^2 + (1/2)*v1*t1 + d, t = t1 + (d*v1/c^2)*gamma^2

And remember, E was an event on the worldline of the left ship--the event of it stopping its acceleration--and E' was an event on the worldline of the right ship--the event that happened simultaneously with E in the right ship's inertial rest frame once it finished accelerating. So, now we can figure out the x' and t' coordinates of E and E' in the rest frame of the ships after they stop accelerating, using the Lorentz transformation, and the difference in x' coordinates of the two events will be the distance between the ships in their final inertial rest frame once the left ship stops accelerating.

The coordinates of E in the ship's final inertial frame are:

x' = gamma*((1/2)*v1*t1 - v1*t1) = gamma*(-1/2)*(v1*t1)

t' = gamma*(t1 - (1/2)*t1*v1^2/c^2)

The coordinates of E' in the ship's final inertial frame are:

x' = gamma*([(d*v1^2/c^2)*gamma^2 + (1/2)*v1*t1 + d] - v1*[t1 + (d*v1/c^2)*gamma^2])
= gamma*(d + (-1/2)*v1*t1)

t' = gamma*([t1 + (d*v1/c^2)*gamma^2] - v1/c^2*[(d*v1^2/c^2)*gamma^2 + (1/2)*v1*t1 + d])
= gamma*(t1 + gamma^2*(d*v1/c^2)*[1 - v1^2/c^2] + (-1/2)*t1*v1^2/c^2 - d*v1/c^2)
= gamma*(t1 + [1/(1 - v1^2/c^2)]*(d*v1/c^2)*[1 - v1^2/c^2] - (1/2)*t1*v1^2/c^2 - d*v1/c^2)
= gamma*(t1 - (1/2)*t1*v1^2/c^2)

So, you can see here from direct calculation that both E and E' do have the same t' coordinate in their final rest frame, regardless of whether you trusted my derivation in post 134 of the twins thread. You can also see that the difference in x' coordinate between E and E' is gamma*d, so this must be the distance between the two ships in their final rest frame, which is greater than the distance between them in the launch frame by a factor of gamma.

 

[cfrogue]

What's paradoxical about that? The distance in the non-inertial frame began to grow larger than d from the moment they began accelerating. Anyway, you don't even need to consider a non-inertial frame here; just using the Lorentz transformation and their x(t) functions in the launch frame, you can easily show that the distance between them in other inertial frames can be larger than d. Different inertial frames always disagree on the distance between a pair of objects, just like they disagree on the time between a pair of events, that's just a feature of how the Lorentz transformation works.

That was not the issue.

Does the distance remain after the acceleration.

If so, then the accelerating frame will believe the distance has grown and the launch frame after the acceleration will believe the distance has contracted as d/gamma.

But, the launch frame and prior accelerating frame will disagree about this d.

 

[cfrogue]

I guess cfrogue's confusion comes from something like:

1) In SR it is not moving objects that Lorentz contract, but space itself.
2) The distance between the ships is moving, so it should Lorentz contract, since space itself contracts.
3) Yet this moving distance remains the same, thus we have obtained a contradiction.

Well, something must be wrong above. I myself never use this space itself contracts or that time itself dilates, so I don't really have an intuition where the error occurs, but maybe cfrogue can confirm that I have summarized his confusion correctly, and someone else can write the correct version of the above statements 1-3.

Actually, I am concerned about the distance between the ships after the accelerations stops from both POV's.

 

[cfrogue]

That quote is from p. 4, and if you look at the context it's clear that S is what we have been calling the "launch frame". Do you disagree?

I agree with you

 

[cfrogue]

Supporting that proposition is equivalent to supporting the proposition that the ships retain their velocity relative to earth, since their "> d positions" are due only to relative velocity.

Maybe this will be more clear:

1) The distance between the ships remained constant in the launch frame.
2) The distance between the ships increased in the new co-moving ship frame.
3) The distance between the ships after acceleration is shorter in the launch frame than it is in the co-moving frame.

The math is simple:

Distance between the ships in launch frame equals distance between the ships in co-moving frame times sqrt(1-v^2/c^2). It's that simple.

The distance between the ships will be different in the launch frame than in a co-moving frame as long as there is relative velocity between them.

The co-moving frame is gone after the acceleration discontinues.

Besides, one does not need a co-moving frame after acceleration is done since the two ships will be in the same frame at that point anyway.

Now, the question is using only ships1 and ship2, is the distance between them > d once the acceleration discontinues from their perspective?

 

[cfrogue]

If cfrogue wants some more detailed math, here it is:

Say that in the launch frame, the left ship is at position x=0 at time t=0, and the right ship is at position x=d at time t=0. At t=0 they both begin to accelerate with constant coordinate acceleration a in the launch frame, and they both stop accelerating at time t=t1 in the launch frame. Then prior to t1, v(t) for both ships will be given by v(t) = a*t, and x(t) for the left ship will be given by x(t) = (1/2)*a*t^2 while x(t) for the right ship will be given by x(t) = (1/2)*a*t^2 + d. So at time t1 when they stop acceleration, both ships will have a velocity in the launch frame of a*t1, and the position of the left ship will be (1/2)*a*t1^2 while the position of the right ship will be (1/2)*a*t1^2 + d. After that they both move at constant velocity v1 = a*t1, so after t1 x(t) for the left ship will be given by x(t) = v1*t - v1*t1 + (1/2)*a*t1^2 while x(t) for the right ship will be given by x(t) = v1*t - v1*t1 + (1/2)*a*t1^2 + d.

I am confused. The SR acceleration equations give the following for the launch frame.
v(t) = a t / sqrt[ 1 + (a t)^2 ]
x(t) = 1/a [ sqrt( 1 + (a t)^2 ) - 1 ]

 

[JesseM]

That was not the issue.

Does the distance remain after the acceleration.

In every inertial frame, as well as the non-inertial frame I defined, the distance at the moment the last ship stops accelerating (since in many frames they do not stop accelerating simultaneously) will also be the distance at a later time when they are traveling inertially.

If so, then the accelerating frame will believe the distance has grown and the launch frame after the acceleration will believe the distance has contracted as d/gamma.

Huh? Why do you think answering yes to "does the distance remain after the acceleration" would imply the launch frame thinks the distance contracts? Again, in any of these frames, the distance at the moment the last ship stops accelerating is the same as the distance later when both ships are moving inertially, so since the launch frame sees the distance as d throughout the acceleration, it will continue to see the distance as d once they are both moving inertially. If you define d' as the distance between the ships in their new inertial rest frame after they stop accelerating (which is the same as the final distance between them in the non-inertial frame I defined), then it is true that the d in the launch frame is equal to d'/gamma, but no one will measure a distance of d/gamma (again assuming d is defined as the constant distance between them in the launch frame while they were accelerating).

But, the launch frame and prior accelerating frame will disagree about this d.

Please don't mix your uses of the symbol d! If d refers specifically to the distance in the launch frame, then you shouldn't refer to the distance in the non-inertial frame as "this d". Anyway, it is certainly true that the final distance d in the launch frame is different than the final distance d' in the non-inertial frame, but if this is all you're saying, then again I don't see why you consider this a paradox.

That quote is from p. 4, and if you look at the context it's clear that S is what we have been calling the "launch frame". Do you disagree?

I agree with you

So do you also agree that nowhere in the paper do they use any variables or do any calculations that involve a non-inertial frame?

I am confused. The SR acceleration equations give the following for the launch frame.
v(t) = a t / sqrt[ 1 + (a t)^2 ]
x(t) = 1/a [ sqrt( 1 + (a t)^2 ) - 1 ]

That's for constant proper acceleration, I was assuming constant coordinate acceleration, which makes the math simpler. The Bell spaceship paradox doesn't specify anything about the detailed nature of the acceleration beyond the fact that both ships have identical coordinate acceleration at any given moment in the launch frame, so assuming constant coordinate acceleration (which implies increasing proper acceleration) is fine.

 

[cfrogue]

Huh? Why do you think answering yes to "does the distance remain after the acceleration" would imply the launch frame thinks the distance contracts? Again, in any of these frames, the distance at the moment the last ship stops accelerating is the same as the distance later when both ships are moving inertially, so since the launch frame sees the distance as d throughout the acceleration, it will continue to see the distance as d once they are both moving inertially. If you define d' as the distance between the ships in their new inertial rest frame after they stop accelerating (which is the same as the final distance between them in the non-inertial frame I defined), then it is true that the d in the launch frame is equal to d'/gamma, but no one will measure a distance of d/gamma (again assuming d is defined as the constant distance between them in the launch frame while they were accelerating).

How can you do this?

The launch frame does not know a d'.
How does the launch frame conclude d'?

Please don't mix your uses of the symbol d! If d refers specifically to the distance in the launch frame, then you shouldn't refer to the distance in the non-inertial frame as "this d". Anyway, it is certainly true that the final distance d in the launch frame is different than the final distance d' in the non-inertial frame, but if this is all you're saying, then again I don't see why you consider this a paradox.

Because, the launch frame will not longer know the correct value to perform d/gamma.
This is simple.

So do you also agree that nowhere in the paper do they use any variables or do any calculations that involve a non-inertial frame?

I agree with you.

That's for constant proper acceleration, I was assuming constant coordinate acceleration, which makes the math simpler. The Bell spaceship paradox doesn't specify anything about the detailed nature of the acceleration beyond the fact that both ships have identical coordinate acceleration at any given moment in the launch frame, so assuming constant coordinate acceleration (which implies increasing proper acceleration) is fine.

I do not understand what you mean with this.

 

[JesseM]

How can you do this?

The launch frame does not know a d'.
How does the launch frame conclude d'?

d' is not measured in the launch frame! What I wrote was "If you define d' as the distance between the ships in their new inertial rest frame after they stop accelerating"

Because, the launch frame will not longer know the correct value to perform d/gamma.
This is simple.

Frames are just coordinate systems and don't "know" anything about other frames, but intelligent beings such as ourselves can certainly relate distances in one frame to distances in another, that's what the length contraction equation l' = l/\sqrt{1 - v^2/c^2} does after all.

That's for constant proper acceleration, I was assuming constant coordinate acceleration, which makes the math simpler. The Bell spaceship paradox doesn't specify anything about the detailed nature of the acceleration beyond the fact that both ships have identical coordinate acceleration at any given moment in the launch frame, so assuming constant coordinate acceleration (which implies increasing proper acceleration) is fine.

I do not understand what you mean with this.

Do you understand the difference between coordinate acceleration and proper acceleration? Coordinate acceleration is just the rate the coordinate velocity is changing (derivative of v(t) in that frame), and coordinate velocity is just the rate the coordinate position is changing with coordinate time (derivative of x(t) in that frame). Proper acceleration at any given moment is defined as an object's coordinate acceleration in the object's instantaneous inertial rest frame at that moment, not in any other frame. If an object has constant proper acceleration, its coordinate acceleration in the launch frame is decreasing, as you can see from the http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html equation v = at / sqrt[1 + (at/c)^2]. But there is nothing in the statement of the Bell spaceship problem that says they must have constant proper acceleration, it only says their coordinate acceleration must be identical.

 

[cfrogue]

d' is not measured in the launch frame! What I wrote was "If you define d' as the distance between the ships in their new inertial rest frame after they stop accelerating"

Here, instead of all this background noise, do you believe this extra distance holds between the ships after the acceleration discontinues yes or no.

 

[JesseM]

Here, instead of all this background noise, do you believe this extra distance holds between the ships after the acceleration discontinues yes or no.

In any frame where the distance increases as they accelerate (i.e. not the launch frame), the distance will stay increased after the acceleration discontinues, yes. I already told you that in post 290:

Again, in any of these frames, the distance at the moment the last ship stops accelerating is the same as the distance later when both ships are moving inertially, so since the launch frame sees the distance as d throughout the acceleration, it will continue to see the distance as d once they are both moving inertially.

 

[Dmitry67]

That what Max Tegmark was talking about the 'mere labels': when we say

F=ma

We think that Force F equal to (m)*** multiplied by (a)cceleration.
The same formula

Ы=Я*Ж

wont tell you anything
But in TOE if won't be important.

 

[Al68]

Does the distance remain after the acceleration.

If so, then the accelerating frame will believe the distance has grown and the launch frame after the acceleration will believe the distance has contracted as d/gamma.

But, the launch frame and prior accelerating frame will disagree about this d.

Of course the distance will be different for different frames. That's what length contraction is.

(1) Constant distance between the ships in the launch frame is stipulated in the scenario.
(2) Also stipulated is that the velocity of the ships will increase relative to the launch frame.
(3) The ratio (gamma) between "ship frame distance" and "launch frame distance" increases with velocity, ie "Ship frame distance" > "launch frame distance" for any velocity > 0.

1+2+3= Distance between the ships in the ships' final inertial frame is greater than the distance between the ships in the launch frame.

Now, the question is using only ships1 and ship2, is the distance between them > d once the acceleration discontinues from their perspective?

Absolutely, assuming that d is the distance between the ships in the launch frame.

For example, if the ships are 600 km apart in the launch frame, accelerate to 0.8 c while maintaining the 600 km distance in the launch frame, then the distance between the ships in their inertial frame after they cut their engines will be 1000 km.

 

[cfrogue]

In any frame where the distance increases as they accelerate (i.e. not the launch frame), the distance will stay increased after the acceleration discontinues, yes. I already told you that in post 290:

Then the launch frame cannot correctly calculate LY once the acceleeration is finished since the launch frame still has the original d as the distance between the ships.

 

[cfrogue]

Of course the distance will be different for different frames. That's what length contraction is.

(1) Constant distance between the ships in the launch frame is stipulated in the scenario.
(2) Also stipulated is that the velocity of the ships will increase relative to the launch frame.
(3) The ratio (gamma) between "ship frame distance" and "launch frame distance" increases with velocity, ie "Ship frame distance" > "launch frame distance" for any velocity > 0.

1+2+3= Distance between the ships in the ships' final inertial frame is greater than the distance between the ships in the launch frame.
Absolutely, assuming that d is the distance between the ships in the launch frame.

Then how does the launch frame correctly apply LT after the acceleration.

Should the launch frame calculate a new d and abandon the old d?

 

[A.T.]

Then how does the launch frame correctly apply LT after the acceleration.

In the same way as during the acceleration, based on the current speed of the rockets.

Should the launch frame calculate a new d and abandon the old d?

Isn't d constant?

assuming that d is the distance between the ships in the launch frame

 

[cfrogue]

In the same way as during the acceleration, based on the current speed of the rockets.

No, because the distance between the ships from the launch frame does not agree with the ships assessment.