# Quick question - has length contraction actually been experimentally confirmed?

1. Aug 31, 2010

### jeebs

Hi,
I am aware that time dilation has been observed experimentally with atomic clocks on satellites etc. but after a few google searches I have not found anything about experimental confirmation of length contraction. Has this been observed at all? If so, how was it tested?
thanks.

2. Aug 31, 2010

### Staff: Mentor

I would say that the bunch length has been demonstrated to contract in particle accelerators (otherwise they would not work as expected). But I know that is not universally accepted as a demonstration of length contraction.

3. Sep 1, 2010

### Austin0

Hi DaleSpam Could you provide a simple conceptual explanation of bunch length and how it demonstrates contraction. I have sought this before and researched the web but find myself still unsure of the concepts. If what I remember is correct it relates to implse timing for acceleration but I couldn't really grasp the connection to contraction.
Thanks

4. Sep 1, 2010

### Staff: Mentor

In a particle accelerator you don't accelerate an individual particle, you accelerate a whole bunch of particles. Each of the particles in the bunch has the same charge, so they repel electromagnetically quite strongly, and one of the things that you have to do in designing an accelerator experiment is figure out how densely you can pack them given the limitations of your equipment. This is often done in the rest frame of the bunch where the repulsive interactions are easier to calculate. If you do not consider length contraction then you get the wrong answer about how many particles you can pack into the bunch.

5. Sep 1, 2010

### Austin0

Does this then mean what is being contracted is the electrostatic field associated with the electron
so they can acutally bunch closer together than would be possible at low velocities???
Thanks for keeping it simple :-)

6. Sep 1, 2010

### Staff: Mentor

Yes. Similarly (in principle) with a solid material, the intermolecular EM fields contract so the rod is shorter.

7. Sep 1, 2010

### Austin0

Hold on there pardner , what happened to "contraction is purely kinematic , it does not make sense to talk obout real or not ....or to look for causality or physical implications with the Lorentz effects" QUotes here being generic, not refering to you specifically [at least maybe not]??? :-)

Thanks

8. Sep 1, 2010

### DaveC426913

There's a frame that is moving relativistically relative to us. Everything in that frame is length-contracted, yes, even the electrostatic field.

9. Sep 1, 2010

### Austin0

I wasn't making any distinction between electrostatic fields and other contraction , only making a comment, partially a joke , related to the fact that DaleSpam made contraction sound pretty darn physical and causal. I.e. Things contract because the EM fields contract.

10. Sep 1, 2010

### Staff: Mentor

I don't understand the point you are trying to make. What is surprising to you here? All lengths contract, regardless of whether the length is the length of a rod or some characteristic EM field.

11. Sep 1, 2010

### yuiop

What if we had some sort of accelerator device in a lab of radius r meters, and some rods with rest lengths of Pi meters each and what if we could comfortably fit more than 2*r rods around the perimeter if they were moving at high enough velocity. Would that be physically real enough for you?

12. Sep 2, 2010

### dpeagler

I'm relatively new to relativity at this depth, but I was wondering why the EM fields contract due to relativistic effects. Obviously it's not because the values of [\mu]0 or [\epsilon]0 have varied, otherwise the frame wouldn't be a reference frame, correct?

Thanks for any information.

13. Sep 2, 2010

### Austin0

Sorry DaleSpam I should probably not indulge in my ideosyncratic humor.
There was absoutely nothing I found surprising and I appreciated your clear and direct explication which made perfect sense to me.

The basis of my comment , not exactly a point, was the inconsistency between two interpretations of the Lorentz effects.
There are many threads where people ask about the causal mechanism of these effects and whether they are to be taken as real or not and whether motion is real or not.
There are many responces to these questions that go: Well this is not really a meaningful question
because there is no causality involved , that these effects are purely kinmatic effects diue to measurements in relative frames. That motion itself is purely relative with no physical implications whatever.
Well this position may wel be true but is not neccessarily consistent with interpretations that attribute real physicallity to contraction i.e. creating internal tension if occurring within constraints for eg. or in this context seeming to imply actual EM field changes resulting in lattice contraction. You may not actually maintain that view but it could be interpreted from your choice of words.
Once again I have no problem at all with this interprtation and tend to assume the reality of these effects and was basically kidding you on the assumption that you were well aware of all of the above, from many previous threads.
Thanks again for your clear responce

Last edited: Sep 2, 2010
14. Sep 2, 2010

### starthaus

Once you start accelerating the rods around the perimeter they expand since they are not Born rigid, so you could not fit in more than 2*r rods. This is not a valid thought experiment for demonstrating length contraction. The above is a variant of the Ehrenfest paradox and the resolution is the same. Direct experimental verification of length contraction (as opposed to the direct experimental verification of time dilation) is not possible with today's technology. Indirect verification , including the calculation for the length of particle beams and the explanation of the Michelson Morley experiment as viewed from a frame external to the moving Earth (like the Sun) are the only things we have available today.

15. Sep 2, 2010

### Staff: Mentor

I see what you are saying. I am always very leery of such discussions. I think that they hinge critically on the various persons' definitions of the words "real" and "physical", which nobody seems to be able to define unambiguously but everybody has a kind of gut feeling that their impression is right and obvious.

Length contraction (and all other relativistic effects) is a coordinate-dependent measurable effect. Some people think that it is measurable so it must be "real" and others think that it is coordinate-dependent so it must not be "real". So I try to avoid ambiguous words like "real" and use well-defined ones like "coordinate dependent" instead.

IMO, the question of the "reality" of length contraction is a purely semantic one which I don't take too seriously.

16. Sep 2, 2010

### matheinste

Yes it is a problem. When I was first introduced to SR I was, wisely, very much influenced by Rindler who in a couple of his earlier books says that length contraction is "real in every sense of the word" which is not very helpful. My sense of the word real may be very different from that of other people. Since learning a little more on the subject I would prefer to say that it is measurable, and that its effect is coordinate dependent because in its own rest frame nothing is happening to length, we are laying our coordinate system on the rest length and coming up with a smaller value.

On the other hand it is theoretically possible (if of course it exists) to demonstrate contraction, by a comparison using three "identical" rods alongside each other. This is probably nearer to most peoples idea of reality

But if we understand the mechanism and can make use of it in our physics then our idea of its "reality" is of no importance except from a philosophical viewpoint.

Matheinste.

17. Sep 2, 2010

### Staff: Mentor

I agree strongly!

18. Sep 4, 2010

### Austin0

I just want to say that I think your summation above is a masterpeice of rational equivocation and I mean that in a positive complimentary way. it doesn't directly contradict anyone and it neatly and succinctly removes you from the question.

But,,,,, although I don't have an answer, I do think there is a "real" question to be explored and and I don't think it is just semantic Just MHO

19. Sep 4, 2010

### Staff: Mentor

Well, if you can ever formulate the "real" question in a non-semantic way then I think that I would greatly enjoy the resulting discussion.

20. Sep 4, 2010

### Cleonis

I do think the question can and should be taken seriously.

There is the thought demonstation, first presented by Dewan and Beran, later retold by John Stewart Bell, usually referred to as 'Bell's spaceship paradox'. Two spaceships, connected by an unstretchable tether of length L, tether fully extended, are initially comoving. They synchronize their clocks. At an agreed point in time they commence acceleration, parallel to the tether, both accelerating at exactly the same G-count. For the tether to not break it would have to decrease the separation between the spaceships. However, since the spaceships meticulously maintain the same G-count the tether will snap.

There is only one physical factor that the breaking of the tether can be attributed to: length contraction.
Hence we have that the principles of SR imply that in specific circumstances length contraction leads to an irreversible result: a tether breaks.

With an irreversible consequence on the table I don't see a way of maintaining that the issue is purely semantic.

Last edited: Sep 4, 2010