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

1. Nov 19, 2013

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?

2. Nov 19, 2013

Staff: 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.

3. Nov 19, 2013

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? "

4. Nov 19, 2013

Staff: Mentor

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.

5. Nov 19, 2013

universal_101

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).

6. Nov 19, 2013

ZapperZ

Staff Emeritus
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.

7. Nov 19, 2013

stevendaryl

Staff Emeritus
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.

8. Nov 19, 2013

Staff: Mentor

Born rigidity is a definition. I don't know what kind of experiment you are talking about.

9. Nov 19, 2013

universal_101

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.
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).

10. Nov 19, 2013

yuiop

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.

11. Nov 19, 2013

bcrowell

Staff Emeritus
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.

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.

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.

Last edited by a moderator: May 6, 2017
12. Nov 19, 2013

stevendaryl

Staff Emeritus
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.

13. Nov 19, 2013

PAllen

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.

14. Nov 19, 2013

Meir Achuz

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.

15. Nov 19, 2013

stevendaryl

Staff Emeritus
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.)

16. Nov 19, 2013

universal_101

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)

Last edited: Nov 19, 2013
17. Nov 19, 2013

Meir Achuz

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'.

18. Nov 19, 2013

stevendaryl

Staff Emeritus
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.

19. Nov 19, 2013

Meir Achuz

20. Nov 19, 2013