Quick question - has length contraction actually been experimentally confirmed?

[starthaus]

That is certainly not the case. In the launch frame it is attributed to length contraction. In the accelerating frame it is essentially attributed to time dilation (which leads to different acceleration profiles as mentioned by Passionflower). And in an inertial frame other than the launch frame it can be attributed to relativity of simultaneity.

Definitely, see here

Length contraction itself is not semantic, but the question of whether or not length contraction is "real" is purely semantic. As I said before, it is a coordinate-dependent measurable effect. Whether or not such effects are "real" depends on your definition of "real".

The main point is that the thought experiment put forward by Bell cannot be considered an experimental verification of length contraction since, amongst other issues, is not physically implementable.

 

[Dale]

Amount of length contraction is frame dependent, yes.

Exactly. At the event of the string breaking there exists a reference frame where the length contraction is 0, therefore it cannot possibly be a frame-invariant explanation for why the string breaks.

If there is a force gage at the tether mount then for all frames the prediction for the reading of the force gage will be the same.

Yes, the reading of the force gauge is an invariant scalar. Force gauges do not measure length contraction.

Part of this point of view is that loss-of-simultaneity and length-contraction are regarded as one and the same thing. ...

We have these different expressions for talking about relativistic phenomena: 'relativity of simultaneity' , 'length contraction'. If those are treated as different phenomena then there's room for the view that questioning level of physical reality is semantic.

So I have learned something about my own point of view: I regard loss-of-simultaneity-effect and 'length-contraction-effect as one and the same thing.

It is a non-standard point of view, certainly. Length contraction, time dilation, and relativity of simultaneity each refer to a different feature of the Lorentz transform. I think that you can say that a ball is red and a ball is bouncy without saying that bouncy is red. To me that is what it seems like you are doing with the above statement.

 

[Cleonis]

If there is a force gage at the tether mount then for all frames the prediction for the reading of the force gage will be the same.

Yes, the reading of the force gauge is an invariant scalar.

This emphasizes the point that only invariant properties qualify to figure in the physical explanation.
The amount of length contraction is frame dependent; the very occurrence of length contraction effects is the common factor.

So I have learned something about my own point of view: I regard loss-of-simultaneity-effect and 'length-contraction-effect as one and the same thing.

[...] Length contraction, time dilation, and relativity of simultaneity each refer to a different feature of the Lorentz transform. I think that you can say that a ball is red and a ball is bouncy without saying that bouncy is red.

We have that time dilation, relativity of simultaneity and length contraction are inseparable.

The difference with elasticity and color of a ball is clear; elasticity and color are independent. But there is no such thing as time dilation, and/or relativity of simultaneity, and/or length contraction occurring independently.

For example, we can refer to the temperature of a gas, or to the internal kinetic energy of a gas. While those are different units, they are inseparable properties.

 

[Dale]

the very occurrence of length contraction effects is the common factor.

As I already said, at the event of the breaking there is always a frame where the string is not length contracted. So length contraction is not a common factor in all frames.

 

[Cleonis]

[...] at the event of the breaking there is always a frame where the string is not length contracted. So length contraction is not a common factor in all frames.

This raises the question: is Bell's spaceship scenario about a process or about a single state? If it would be about a single state then presenting the scenario would involve only single slice of time. But of course Bell's spaceship scenario is about a process, it's about a development over time.

So let's examine the perspective of a frame that at the beginning has such a velocity relative to the spaceships that it's co-moving with the tether at the point in time that it breaks. In that frame the tether length is initially length contracted. As its velocity relative to the chosen frame decreases its length contraction decreases. As mapped in the chosen frame the two spaceships do not start accelerating simultaneously. Does the length contraction decrease fast enough to make up for the non-simultaneous start of acceleration? No it doesn't, the tether breaks.

Depending on what inertial frame of reference is chosen the narrative comes out differently, in how length contraction and shift of simultaneity proceed over time.
My underlying assumption is that SR is by nature a causal theory; a theory in which there is a one-on-one relation between cause and effect. Having multiple narratives means a deeper interpretation must exist, of a more abstract nature.

Then again, others may prefer a philosophy where our narratives are regarded as heuristic devices only. In such a philosophy what counts is whether machines that have been constructed are operating as designed (Do we get the density of particle bunches that we predicted to be achievable?), and no deeper interpretation is expected.

 

[Passionflower]

So let's examine the perspective of a frame that at the beginning has such a velocity relative to the spaceships that it's co-moving with the tether at the point in time that it breaks. In that frame the tether length is initially length contracted. As its velocity relative to the chosen frame decreases its length contraction decreases. As mapped in the chosen frame the two spaceships do not start accelerating simultaneously. Does the length contraction decrease fast enough to make up for the non-simultaneous start of acceleration? No it doesn't, the tether breaks.

Depending on what inertial frame of reference is chosen the narrative comes out differently, in how length contraction and shift of simultaneity proceed over time.
My underlying assumption is that SR is by nature a causal theory; a theory in which there is a one-on-one relation between cause and effect.

Reality knows of no frames, frames are human constructs and not physical at all.

Having multiple narratives means a deeper interpretation must exist, of a more abstract nature.

You got it exactly backwards. The creation of frames creates the false contradictions the reality is simple and unambiguous.

 

[Cleonis]

You got it exactly backwards.

Your disagreement is not with me. You are attributing a point of view to me that isn't mine.

Just to make sure: you have read my comment in https://www.physicsforums.com/showpost.php?p=2866583&postcount=22" of this thread?

 

[Passionflower]

Your disagreement is not with me. You are attributing a point of view to me that isn't mine.

The words I quoted are yours aren't they?

 

[Dale]

I'm with you on this Passionflower. I can't tell what position Cleonis is trying to take. Everything else that he says seems to contradict his post 22, but he keeps referring to it. Very confusing.

 

[Austin0]

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.

Does this mean that in the Ehrenfest scenario that the length contraction would be completely negated by the inertial forces resulting from rotation?

SInce you have a regard for realistic physics in hypothetical scenarios do you consider it realistic that any material could maintain structural integrity in the face of the outward inertial forces inherent in relativistic angular velocities??

 

[William Nelso]

Length contraction is real. You can compute the length of a moving ruler using E&M and Quantum Mechanics, without ever using Relativity or the Lorentz Transformation. Relativity comes from the physics, not the other way around.

 

[yuiop]

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.

This is just nonsense. The rods do not expand because they are not linked to each other. In the Ehrenfest paradox the train carriages are more rigid than the elastic links between the carriages and so the links have to expand to take up the extra space between the carriages at higher velocities. In the rod thought experiment, it is the gaps between the rods that expands as the rods contract and so you can fit additional rods in the gaps. I don't think Born rigidity is even relevant here. Born rigidity is a method of acceleration, rather than a property of a material. It does not matter how the rods are accelerated as long as they get to a final velocity and are allowed to stabilize to their length contracted length. It is only when we are considering methods of spinning up solid discs to relativistic speeds that applying Born rigid acceleration becomes a problem. For rods that are not connected to each other, it is not a problem.

If you wanted to make the Ehrenfest paradox a bit more like the rod thought experiment, you could remove one link so that the train is not connected all the way around, but occupies all the track and is is just touching at the point where the link has been removed. When the train gets to high enough velocity it will have length contracted sufficiently that you could (in principle but with considerable technical difficulty) slip another (high speed) carriage onto the track into the expanded gap.

 

[Passionflower]

I think this topic is not the place to discuss the Erhenfest paradox, but one comment: how can a rod remain Born rigid if it rotates? I thought only rods that accelerate in one direction can remain Born rigid? It seems to me that because the rod is spatially separated there will be some form of Thomas precession which will make Born rigidity impossible.

Am I wrong?

 

[Dale]

how can a rod remain Born rigid if it rotates? I thought only rods that accelerate in one direction can remain Born rigid? It seems to me that because the rod is spatially separated there will be some form of Thomas precession which will make Born rigidity impossible.

Am I wrong?

You are correct. Linear acceleration can be done without material strain (Born rigid), but there is no way to have angular acceleration without mechanical strain. Something must stretch.

 

[Austin0]

Length contraction is real. You can compute the length of a moving ruler using E&M and Quantum Mechanics, without ever using Relativity or the Lorentz Transformation. Relativity comes from the physics, not the other way around.

Does relative simultaneity enter into these calculations?
Or does anything comparable to relative simultaneity fall out of the calculatiosn??
Thanks

 

[yuiop]

I think this topic is not the place to discuss the Erhenfest paradox, ...

You are right that it is getting off topic, so I have started a new thread here https://www.physicsforums.com/showthread.php?p=2876211#post2876211 to discuss the issues raised by yourself, Dalespam and Starthaus.

 

[starthaus]

This is just nonsense. The rods do not expand because they are not linked to each other.

Err, you are talking about a "ring" of rods, how do you get them to move together as a whole?

In the Ehrenfest paradox the train carriages are more rigid than the elastic links between the carriages and so the links have to expand to take up the extra space between the carriages at higher velocities.

The Ehrenfest paradox I was referring to is the case of a disc, exactly as your attempt at the ring of rods.

In the rod thought experiment, it is the gaps between the rods that expands as the rods contract and so you can fit additional rods in the gaps.

The gaps expand?

I don't think Born rigidity is even relevant here.

You would, if you paid attention to the underlying physics. How do you plan to ramp up the speed of your "ring"?

 

[Meir Achuz]

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.

I came across this thread late in the game.
The quick answer is that length contraction has NOT "been observed at all".

Some of the controversy in this thread might be resolved by reading the first section of <http://arxiv.org/abs/0906.1919>.

 

[yuiop]

I came across this thread late in the game.
The quick answer is that length contraction has NOT "been observed at all".

Some of the controversy in this thread might be resolved by reading the first section of <http://arxiv.org/abs/0906.1919>.[/QUOTE]

I believe there are some misleading statements in that paper. One is their claim that Lorentz contraction would predict a positive result for the MM experiment. The other is this statement:

It should also be pointed out that another classically reasonable method of
measuring the length is to take a photograph of a moving object and compare
it with a photograph of the same object at rest. As Terrell[3] showed some time
ago, the photograph would show an object that is somewhat rotated, but of
the same shape and dimensions as it had at rest. Indeed, the photograph of a
moving sphere would show a sphere of the same size.

Although they mention a sphere at the end, they neglect to mention that the claim "the photograph would show an object that is somewhat rotated, but of the same shape and dimensions as it had at rest." is true if and only if the moving object is a sphere. It is not true for a rod for example and this shows the biased view of the authors.

There is another way to "photograph" length contraction. Set up an array of inject jet nozzles on one side of a narrow passage. Have the moving object pass through the passage and set off the ink jets nozzles simultaneously and briefly. The silhouette of the object left on the opposite side of the passage will be length contracted.

As for ... length contraction has NOT "been observed at all"... an example was given earlier in this thread of the observed length contraction in bunches of particles moving around a particle accelerator ring. Do you disagree with that?

 

[GiftOfPlasma]

I always thought that the most common form of length contraction was the distance between two charged particles. It's demonstrated by calculating out the electrodynamic forces on one charged particle wrt another. The distance changes depending on the frame of reference.