# What is the proof/validity of Born rigidity in Special relativity?

#### Trojan666ru

Relativity theory always find some ridiculous explanations like "born rigidity or relativity of simultaneity" when in face paradoxes
What is the experimental validity of such predictions?

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#### Dale

Mentor
So far SR has withstood every experimental test within its domain of validity.

Perhaps you should spend some time reading through the sticky on experimental evidence for relativity before using words like "ridiculous", it just makes you look like an uninformed crackpot.

#### Trojan666ru

Sorry I've already red most of the experimental evidences that support relativity, but that doesn't prove "born rigidity" happen in SR. What if "the experimental evidences that support relativity has some other kind of explanations? "

#### Nugatory

Mentor
What if "the experimental evidences that support relativity has some other kind of explanations? "
That would be a more interesting question if someone were to find a reasonable alternative explanation to consider, or a significant hole in the way that relativity explains the experimental evidence. So far, no one has found either... And it's not for want of looking.

#### universal_101

Sorry I've already red most of the experimental evidences that support relativity, but that doesn't prove "born rigidity" happen in SR. What if "the experimental evidences that support relativity has some other kind of explanations? "
Born rigidity was introduced in SR, so that the objects can go under Length Contraction (due to inertial relative motion w.r.t an observer, i.e. Lorentz contraction) without breaking the objects in the process(i.e. no permanent damage done) and at the same time maintaining the rigidity(i.e. born rigidity).

The bitter part of this introduction is that we don't have any experimental proof of Length contraction(directly implied), nor of the increase in the density due to length contraction.

The above definition of born rigidity does not include accelerating objects, i.e. every non-point object undergoes permanent deformation along the line of motion when accelerated(i.e. according to theory, the born rigidity breaks every time any real object accelerates).

And same as the inertial Lorentz contraction mentioned before, there is again, NO Experimental evidence (direct or implied).

#### ZapperZ

Staff Emeritus
2018 Award
Born rigidity was introduced in SR, so that the objects can go under Length Contraction (due to inertial relative motion w.r.t an observer, i.e. Lorentz contraction) without breaking the objects in the process(i.e. no permanent damage done) and at the same time maintaining the rigidity(i.e. born rigidity).

The bitter part of this introduction is that we don't have any experimental proof of Length contraction(directly implied), nor of the increase in the density due to length contraction.

The above definition of born rigidity does not include accelerating objects, i.e. every non-point object undergoes permanent deformation along the line of motion when accelerated(i.e. according to theory, the born rigidity breaks every time any real object accelerates).

And same as the inertial Lorentz contraction mentioned before, there is again, NO Experimental evidence (direct or implied).
Try designing particle accelerator structures without incorporating such effects in the design! For example in FELs, from the electron bunch frame, the spacing of the undulator/wiggler is severely contracted. Without taking into such account, the description in that frame and lab frame will be completely off! It works when you use it, it doesn't work when you don't. To me, as an experimentalist, that is a darn strong evidence for its validity.

Furthermore, this appears to be a common misunderstanding of what this effect. It isn't the object that is contracting. It is the spatial dimension!

http://arxiv.org/abs/0906.1919

Zz.

#### stevendaryl

Staff Emeritus
Sorry I've already red most of the experimental evidences that support relativity, but that doesn't prove "born rigidity" happen in SR. What if "the experimental evidences that support relativity has some other kind of explanations? "
Born rigidity is not something that can be proved or disproved--it's a definition: An object is "Born rigid" if it always has the same length, as measured in its own instantaneous rest frame. Real objects are not perfectly Born rigid. If you push on one end of a long rod, it will compress a little. If you pull on the other end, it will stretch a little. Rigidity is an idealization.

#### Dale

Mentor
that doesn't prove "born rigidity" happen in SR
Born rigidity is a definition. I don't know what kind of experiment you are talking about.

#### universal_101

Try designing particle accelerator structures without incorporating such effects in the design! For example in FELs, from the electron bunch frame, the spacing of the undulator/wiggler is severely contracted. Without taking into such account, the description in that frame and lab frame will be completely off! It works when you use it, it doesn't work when you don't. To me, as an experimentalist, that is a darn strong evidence for its validity.
Theoretical Explanation of FELs, comes under the category of 'suit yourself', the so called rigorous derivation of ${λ_w}/{γ^2}$ from Maxwell's Equations, is based on jumping frames(it is because of this theoretical inconsistency that most texts do not include the derivation) to get the square of the gamma. That is, the only reason the case is studied by jumping frames, so as to make theory(Length contraction) compatible with experimental results.
Furthermore, this appears to be a common misunderstanding of what this effect. It isn't the object that is contracting. It is the spatial dimension!

http://arxiv.org/abs/0906.1919
According to the linked paper, the inertial Length contraction is nothing but illusion, i.e. its an apparent measured effect which we should not worry much about. In other words the length has meaning only in the proper frames.
And, just as the ‘shortening’ of a stick that is rotated in three dimensions is an illusion, we now can see that the ‘shortening’ of a stick that is rotated in four dimensions by a Lorentz transformation is also illusory.
Now, intuitively/logically the definition of inertial length contraction(according to the paper) does not match very well with the explanation of FELs using the inertial Length contraction(frames jumping), because the former is a measured/apparent effect(illusion), whereas later is used to explain non illusory effects(radiation).

#### yuiop

...
And same as the inertial Lorentz contraction mentioned before, there is again, NO Experimental evidence (direct or implied).
I think length contraction is implied by observations of the number of muons that make it from the top of the atmosphere to sea level. The average lifetime of a muon is very short and they would decay before they reached sea level, if the thickness of the atmosphere was not length contracted from their point of view.

Length contraction is also implied by the MMX experiment. If the arm parallel to the motion was not length contracted by the right amount, the famous "null result" would not have been obtained.

#### bcrowell

Staff Emeritus
Gold Member
Born rigidity was introduced in SR, so that the objects can go under Length Contraction (due to inertial relative motion w.r.t an observer, i.e. Lorentz contraction) without breaking the objects in the process(i.e. no permanent damage done) and at the same time maintaining the rigidity(i.e. born rigidity).
No, this is a complete misunderstanding of Born rigidity. Born rigidity is a definition, not a prediction of SR. Born rigidity does not exist in reality. It's of no interest for an object moving inertially and without rotation. It is not a property of an object (even a hypothetical, idealized object) but rather a theoretically possible motion of an object on which external forces are exerted according to a plan that has to have been constructed before the motion even began.

The bitter part of this introduction is that we don't have any experimental proof of Length contraction(directly implied), nor of the increase in the density due to length contraction.
This is total nonsense. Length contraction is part of the Lorentz transformation, and Lorentz invariance is one of the most accurately tested theories in the history of science. See, e.g., http://relativity.livingreviews.org/Articles/lrr-2005-5/ [Broken]

The above definition of born rigidity does not include accelerating objects, i.e. every non-point object undergoes permanent deformation along the line of motion when accelerated(i.e. according to theory, the born rigidity breaks every time any real object accelerates).
More nonsense.

And same as the inertial Lorentz contraction mentioned before, there is again, NO Experimental evidence (direct or implied).
More nonsense.

Furthermore, this appears to be a common misunderstanding of what this effect. It isn't the object that is contracting. It is the spatial dimension!

http://arxiv.org/abs/0906.1919
This is an interesting and subtle point that is not going to get the discussion it deserves in a thread as contaminated with nonsense as this one. Franklin's interpretation is fine, but it's not the only correct one. Bell's interpretation differs markedly from Franklin's, and Bell isn't betraying his "misunderstanding" of SR. Everybody agrees on the mathematics and the experimental validation of the mathematics, but the interpretation is a matter of philosophy, not science.

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#### stevendaryl

Staff Emeritus
According to the linked paper, the inertial Length contraction is nothing but illusion, i.e. its an apparent measured effect which we should not worry much about. In other words the length has meaning only in the proper frames.
I don't quite agree with the use of the word "illusion". Length has a very precise (if frame-dependent) definition: the length of an object at time t is the spatial distance between the locations of the two endpoints at time t. It's not an illusion that a moving object has a shorter length than the same object when at rest, it's true. (Assuming relativity is correct.)

I would reserve the word "illusion" to apply in a case where someone is given misleading information that leads him to make an incorrect conclusion. For example, someone holding his hand up in such a way that an elephant seems to be sitting in his hand--in actuality, the elephant is far away, and seems smaller by perspective. For a more physics-relevant illusion, moving objects appear rotated, because of the Terrell rotation.

#### PAllen

According to the linked paper, the inertial Length contraction is nothing but illusion, i.e. its an apparent measured effect which we should not worry much about. In other words the length has meaning only in the proper frames.

Now, intuitively/logically the definition of inertial length contraction(according to the paper) does not match very well with the explanation of FELs using the inertial Length contraction(frames jumping), because the former is a measured/apparent effect(illusion), whereas later is used to explain non illusory effects(radiation).
This is one of the few weak points of a pretty good paper. A simple counter argument is there is nothing illusory about rotating an object in 3-space. You can fit long, thin, objects through small openings because of the ability to rotate them. Similarly, the reality of spacetime rotation (=length contraction) produces phenomena that would not otherwise be possible. For example, the muon reaching the ground, in the muon's frame.

#### Meir Achuz

Homework Helper
Gold Member
Bell does 'misunderstand' SR, because he wants to use both Lorentz's (and Fitzgeralds0 notion that there is a real contraction of the object AND Einstein's contraction due the transformation between frames. You can argue for one or the other, but they can't be combined.

#### stevendaryl

Staff Emeritus
This is total nonsense. Length contraction is part of the Lorentz transformation, and Lorentz invariance is one of the most accurately tested theories in the history of science.
I think that it is correct to say that Lorentz contraction has not been directly observed in the way that time dilation has been. I mean, we don't have any objects of accurately known rest length that have been accelerated to high enough speed to measure their contraction. Not that there is very much doubt about the reality of Lorentz contraction (There is, as far as I know, no alternative to relativity that agrees with all known experiments but does not predict length contraction.)

#### universal_101

I think length contraction is implied by observations of the number of muons that make it from the top of the atmosphere to sea level. The average lifetime of a muon is very short and they would decay before they reached sea level, if the thickness of the atmosphere was not length contracted from their point of view.

Length contraction is also implied by the MMX experiment. If the arm parallel to the motion was not length contracted by the right amount, the famous "null result" would not have been obtained.
I didn't say one cannot imply inertial length contraction, instead I said it cannot be directly implied. (For example, I can also think of different theories that implies something else or may be nothing). The quote that you used from my post is about the length contraction due to acceleration for non-point objects(i.e. real effects)

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#### Meir Achuz

Homework Helper
Gold Member
The Lorentz contraction of SR has not been tested experimentally, and probably can't be, but it is implied by all the other experimental successes of SR. SR would have to be radically changed (destroyed) if Lorentz contraction (in the SR sense) were dropped. Many things in physics have that kind of 'evidence'.

#### stevendaryl

Staff Emeritus
Bell does 'misunderstand' SR, because he wants to use both Lorentz's (and Fitzgeralds0 notion that there is a real contraction of the object AND Einstein's contraction due the transformation between frames. You can argue for one or the other, but they can't be combined.
I don't think that Bell misunderstands it. I'm not exactly sure what you mean by those two alternatives (1) contraction is real, and (2) contraction is due to transformation between frames, but they don't seem incompatible to me.

Homework Helper
Gold Member

#### nitsuj

I think that it is correct to say that Lorentz contraction has not been directly observed in the way that time dilation has been. I mean, we don't have any objects of accurately known rest length that have been accelerated to high enough speed to measure their contraction. Not that there is very much doubt about the reality of Lorentz contraction (There is, as far as I know, no alternative to relativity that agrees with all known experiments but does not predict length contraction.)
I think it is correct to say that the "observed" time dilation is the reciprocal? of length contraction. (SR)Time dilation doesn't somehow exist on it's own.

I presume by time dilation has been "observed" you are referring to differential aging, which itself isn't time dilation. Has time dilation even been observed, or is it merely calculated...just like length contraction?

#### universal_101

No, this is a complete misunderstanding of Born rigidity. Born rigidity is a definition, not a prediction of SR. Born rigidity does not exist in reality. It's of no interest for an object moving inertially and without rotation. It is not a property of an object (even a hypothetical, idealized object) but rather a theoretically possible motion of an object on which external forces are exerted according to a plan that has to have been constructed before the motion even began.
I didn't say born rigidity is a prediction of SR, ofcourse it is an idealized condition of an object so that the effects of acceleration on real objects can be ignored, and so only the geometrical effects are left to be analysed.

This is total nonsense. Length contraction is part of the Lorentz transformation, and Lorentz invariance is one of the most accurately tested theories in the history of science. See, e.g., http://relativity.livingreviews.org/Articles/lrr-2005-5/ [Broken]

More nonsense.

More nonsense.
Lorentz transformation is based on Lorentz in-variance of Electromagnetic phenomenon(Maxwell's equations) under relative motion. This implies being a transformation, LT is a set of transformation equations which give the same absolute result(For ex.- acceleration etc.) irrespective of the frame we choose to see the effects from. In other words, just as under the rotation of a stick in 3-D gives us different spatial co-ordinates (without changing the actual length), LT gives us different co-ordinates (spatial, temporal) for a same absolute property.

My point being co-ordinates does not mean anything, I can always choose a different co-ordinate system and get the same results. That is the length of the stick never changes.

This is an interesting and subtle point that is not going to get the discussion it deserves in a thread as contaminated with nonsense as this one. Franklin's interpretation is fine, but it's not the only correct one. Bell's interpretation differs markedly from Franklin's, and Bell isn't betraying his "misunderstanding" of SR. Everybody agrees on the mathematics and the experimental validation of the mathematics, but the interpretation is a matter of philosophy, not science.
Labeling is easy!, the more important point is not betraying the one's 'understanding'.

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#### universal_101

I don't quite agree with the use of the word "illusion". Length has a very precise (if frame-dependent) definition: the length of an object at time t is the spatial distance between the locations of the two endpoints at time t. It's not an illusion that a moving object has a shorter length than the same object when at rest, it's true. (Assuming relativity is correct.)

I would reserve the word "illusion" to apply in a case where someone is given misleading information that leads him to make an incorrect conclusion. For example, someone holding his hand up in such a way that an elephant seems to be sitting in his hand--in actuality, the elephant is far away, and seems smaller by perspective. For a more physics-relevant illusion, moving objects appear rotated, because of the Terrell rotation.
You are in direct contradiction with the author if you suggest that Inertial Length Contraction is true. Since it is an (just like simultaneity)measured effect and it does not imply the length of an object actually contracted {i.e. it produced strain or stress).

#### stevendaryl

Staff Emeritus
I presume by time dilation has been "observed" you are referring to differential aging, which itself isn't time dilation.
I was using those two synonymously. What do you mean by "time dilation"?

#### stevendaryl

Staff Emeritus
You are in direct contradiction with the author if you suggest that Inertial Length Contraction is true.
Then I think the author is wrong. As I said, "length" has a definition, and according to that definition, a moving object is contracted compared with a similarly constructed object at rest.

Since it is an (just like simultaneity)measured effect and it does not imply the length of an object actually contracted {i.e. it produced strain or stress).
Somebody made the analogy between Lorentz transformations and rotations. I think that's a very good analogy. If you have a two-dimensional object lying on the ground, you can characterize it by giving its East/West extent, and by giving its North/South extent. A rotated version of the same object will in general have a different East/West extent, and a different North/South extent. That doesn't mean that there are any additional stresses on the rotated version of the object. That's because the forces keeping the object together are rotationally invariant; they don't have a preferred direction in space. But it's not an ILLUSION that the rotated object has a different East/West extent.

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