Measuring Lorentz Contraction: Is it Real or Just an Illusion?

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The discussion centers on the reality of Lorentz contraction, a phenomenon predicted by Einstein's theory of relativity. Participants agree that while Lorentz contraction can be measured indirectly through experiments involving particle accelerators, there is no direct experimental verification of length contraction itself. The consensus is that length contraction is not an optical illusion but a measurable effect that arises from the relativistic effects of time dilation and the relativity of simultaneity. The conversation highlights the complexities of measuring length in different inertial frames and the implications for forces acting on moving objects.

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  • #31
1effect said:
The "contraction" is a function of the clock synchronization scheme and of the method of marking both ends of the object.
And it is interesting to trace where that contraction comes from as we generate the simultaneity shift by continuosly increasing the speed. If we consider a stationary ruler, and then we accelerate ourselves, we will accumulate a shift in the concept of "now" between the leading and trailing ends of the ruler (relative to the ruler's own concept). In our own frame, that will make the ruler shorten as its speed increases, which means we perceive the leading and trailing edges as having different accelerations. Of course, in the ruler's inertial frame, those accelerations are the same-- zero.

Now if we want to leave ourselves stationary, and accelerate the ruler instead, then to get the appropriate length contraction we will have to accelerate the leading and trailing ends differently, from the perspective of our own inertial frame. But here's the interesting part-- this time that will also mean that the accelerations are different as perceived in the frame of the rigid ruler. So to get an accelerated ruler to be rigid and maintain a fixed length in its own frame, it has to experience different accelerations along its length in its own frame. So in that sense I would say that even though we know the contraction has to be there due to the relativity of simultaneity, it is a "real" effect in the sense that it requires different accelerometer readings across a rigid object for that rigid object to maintain a constant length in its own frame.
 
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  • #32
I always thought of this problem like this:

Youre traveling close to C to a distant star 10 light years away. However due to the time dilation it would take the crew only 1 year to reach that star, using simple formula 10/1 we get 10 light years per year which is a big nono... therefore you have to make everything smaller to compensate this paradox.
 
  • #33
Schrödinger's Dog said:
And why is there even an issue?

Simple answer is, the question is meaningless or at least not relevant to the experiment.

Maybe the questioner is being a little naive and is assuming that the contraction is not a space/time one, and therefore the object literally gets contracted more than the rest and thus it could physically snap, instead of the space/time being contracted, in that objects frame of reference?

If that is the case again the question surely becomes meaningless?

I don't see your point. But to make things clearer, I know pretty well SR, let's say. And I know the answer to my question. I mean, I have two plates that attract each other and I put something that prevent them to schrink down. In the laboratory frame, the system is at equilibrium, and will stay so. And of course, the fact that someone is moving very fast wrt the lab, won't change anything to this.

My question, then, is simply: how the moving guy describes this system? You see, we more oftenly use SR as only a kinematical theory, whereas it also describes dynamics. And in my thought experiment there is dynamics because there are forces involved. So again, how it comes that the moving guy indeed sees a system at equilibrium with the distance between plates reduced by a factor gamma, whereas we (naively) expect the various forces in this system to be boosted by various powers of gamma? (for Casimir, gamma^4). I guess the answer lies in the transformation law of 4-forces, but... i would appreciate some hints!
 
  • #34
Jip,

And in my thought experiment there is dynamics because there are forces involved. So again, how it comes that the moving guy indeed sees a system at equilibrium with the distance between plates reduced by a factor gamma, whereas we (naively) expect the various forces in this system to be boosted by various powers of gamma? (for Casimir, gamma^4). I guess the answer lies in the transformation law of 4-forces, but... i would appreciate some hints!

Transformation of forces is discussed in this thread -

https://www.physicsforums.com/showthread.php?t=207419
 
  • #35
aachenmann said:
What's the reason that objects shrink when they move?
Is Lorentz-contraction an illusion or is it real?
Is there any experiment that verify moving objects really shrink?

The question is:
For n observers each moving at different speeds in the same direction, how can an object physically have n different lengths while being measured by these observers?
 
  • #36
aachenmann said:
Lorentz got the right equations for the so-called Lorentz-transformation in special relativity, but his interpretations are very different from Einstein's. He believed atoms and molecules have changed because the electromagnetic field of a charge depends on the speed, but he could not explain time dilation while Einstein could explain both effects using SR without that kind of nonsense
What nonsense? Electric field of a moving point charge is definitely not the same as the field of a static point charge on the same spot!
If it was, then this was the recipy for instant information transfer!
The field has to travel between the charge and the sample point, so different points will "think" that a the moving charge is on diferent positions (which is not the current charge position).
 
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  • #37
Hello phyti.

"""The question is:
For n observers each moving at different speeds in the same direction, how can an object physically have n different lengths while being measured by these observers?"""

The authors i have read on the subject of SR all say that if you measure a moving object
using the correct method it will be shorter. To put it another way:- If you directly compare the length of a stick stationary with respect to you with a stick ( previously compared and found to be the same length in your frame ) moving relative to you it will be shorter. The same of course applies if the roles of the "moving" and "non-moving" frames are reversed. Each views the other as shorter than his own although both were previusly compared.

Of course we must assume that the history of the movement and accelerations of the sticks to get them to a position where we can directly compare them again has no bearing on the lengths.

If this applies to one moving frame it does of course apply to n different moving frames.

I am afraid i do not know if the practical difficulties of such an experiment have been overcome as i am not up to date with any recent ( less than 40 years ago ) results.

Although i believe these results will ( if not already verified ) be found to be correct by experimental verification i suppose until this is done it is open to question.

Mateinste.
 
  • #38
matheinste said:
I am afraid i do not know if the practical difficulties of such an experiment have been overcome as i am not up to date with any recent ( less than 40 years ago ) results.

Although i believe these results will ( if not already verified ) be found to be correct by experimental verification i suppose until this is done it is open to question.

Mateinste.

Hi Mate,

The difficulties continue to persist, to date there is no experimental verification of length contraction.
 
  • #39
matheinste said:
Hello phyti.

"""The question is:
For n observers each moving at different speeds in the same direction, how can an object physically have n different lengths while being measured by these observers?"""

The authors i have read on the subject of SR all say that if you measure a moving object
using the correct method it will be shorter. To put it another way:- If you directly compare the length of a stick stationary with respect to you with a stick ( previously compared and found to be the same length in your frame ) moving relative to you it will be shorter. The same of course applies if the roles of the "moving" and "non-moving" frames are reversed. Each views the other as shorter than his own although both were previusly compared.

Of course we must assume that the history of the movement and accelerations of the sticks to get them to a position where we can directly compare them again has no bearing on the lengths.

If this applies to one moving frame it does of course apply to n different moving frames.

I am afraid i do not know if the practical difficulties of such an experiment have been overcome as i am not up to date with any recent ( less than 40 years ago ) results.

Although i believe these results will ( if not already verified ) be found to be correct by experimental verification i suppose until this is done it is open to question.

Mateinste.

You can't make direct comparison of a static object to an identical moving object. When you do remote measurements, and assume you are not moving, the measurements will actually be longer! This can be shown with basic math calculations.

The reply to the original post was to show the object can't simultaneously have different lengths, therefore the differences are measurement/calculation results.
 
  • #40
Hello phyti.

The experiment, not yet performed, would not rely on remote measurement but on direct comparison.

According to SR and accepted by many ( of course this does not constitute a proof but a pointer in that direction ) an object can have different lengths when viewed in different inertial frames. You either accept this or do not accept the predictions of SR. There is no obligation to do so.

Matheinste.
 
  • #41
Welcome Pheinstein,

Do you mean Einstein's Special Theory of Relativity has not been fully verified and is still in a pickle?
I don't think he means that. Time dilation has been tested many times and found to agree with predictions of SR. It's a difficult experiment to verify measured length contraction.
 
  • #42
The reason this has not been measured is that there is no general unique prescription for measuring a length that does not involve some assumptions about rigidity and responses to forces,i.e., the details of how the various objects came to be moving as they are. You can, however, measure a time using a clock without making any assumptions, so we build a picture of what lengths mean from empirical observations of clocks. Length contraction is just part of the axiomatic structure of relativity, which accept because it is elegant and effective, and no competing theory comes close. It is as matheinste said, there is no "obligation".
 
  • #43
Mentz114 said:
Welcome Pheinstein,


I don't think he means that. Time dilation has been tested many times and found to agree with predictions of SR. It's a difficult experiment to verify measured length contraction.

Of course I don't mean that. Relativity is one of the most tested theories, see here for a comprehensive set of tests.
 
  • #44
Pheinstein said:
Do you mean Einstein's Special Theory of Relativity has not been fully verified and is still in a pickle?

No, this is not what I mean.
 
  • #45
Jip said:
I don't see your point. But to make things clearer, I know pretty well SR, let's say. And I know the answer to my question. I mean, I have two plates that attract each other and I put something that prevent them to schrink down. In the laboratory frame, the system is at equilibrium, and will stay so. And of course, the fact that someone is moving very fast wrt the lab, won't change anything to this.

My question, then, is simply: how the moving guy describes this system? You see, we more oftenly use SR as only a kinematical theory, whereas it also describes dynamics. And in my thought experiment there is dynamics because there are forces involved. So again, how it comes that the moving guy indeed sees a system at equilibrium with the distance between plates reduced by a factor gamma, whereas we (naively) expect the various forces in this system to be boosted by various powers of gamma? (for Casimir, gamma^4). I guess the answer lies in the transformation law of 4-forces, but... i would appreciate some hints!

Sorry I misunderstood what you meant, but it looks like you've got an answer anyway.
 
  • #46
Ken G said:
The reason this has not been measured is that there is no general unique prescription for measuring a length that does not involve some assumptions about rigidity and responses to forces,i.e., the details of how the various objects came to be moving as they are. You can, however, measure a time using a clock without making any assumptions, so we build a picture of what lengths mean from empirical observations of clocks. Length contraction is just part of the axiomatic structure of relativity, which accept because it is elegant and effective, and no competing theory comes close. It is as matheinste said, there is no "obligation".
I tend to agree with the things that I have read from KENNY G,(a little like a magician's name). I once told a practicing physicist the following and he was shocked, incredulous and a little disgusted with my nerve at the same time. I remember from a book on Einstein that he said that (approx.) things that you could not measure were irrelevant. I know I'm going to catch hell for that statement but that's the gist that I remember of the statement. However, I don't think he meant that they weren't important. I disagree: if you play the game you must accept all the rules. As was mentioned, special relativity needs the length contraction to work, so there is an "obligation" to buy into it.
 
  • #47
But I must mention that he also the physicist that "stone walled" me when I asked if there was no "ether" then how was light propagated from the sun?(I was a undergraduate, be merciful).
 
  • #48
I agree with Normouse. Length contraction is a logical consequence of Einstein clock synchronization and the invariance of c. It is not obligatory only in the same sense that logic and reason are not obligatory.
 
  • #49
Why is Einstein clock synchronization obligatory?
 
  • #50
Einstein clock synchronization

Ken G said:
Why is Einstein clock synchronization obligatory?

It is not compulsory. There are other ways to synchronize distant clocks: everyday clock synchronization, internal clock synchronization, slow clock transport...
 
  • #51
Bernhard is correct, Einstein clock synchronication is a good convention, not something obligatory. But if you use it and if c is constant, then length contraction is a logical consequence, not an option.

The reason that the synchronization convention is important in this discussion is that length contraction refers to the interval between two events that occur simultaneously in some reference frame. You have to choose some synchronization convention for that.
 
  • #52
DaleSpam said:
length contraction refers to the interval between two events that occur simultaneously in some reference frame.
Why do you claim that?

Suppose the distance, as measured by a two-way light signal, between A and B is D then a traveler traveling between A and B will always measure the distance to be < D. If the traveler's speed is << c the measured distance is practically speaking D, however if his speed approaches c the distance will approach 0.
 
  • #53
That is an interesting take on length contraction, I haven't heard it described in terms of two-way light propagation instead of simultaneous measurements.

Are A and B here events or worldlines? I am guessing you mean worldlines.
 
  • #54
In Lorentz's early version of relativity (that was superceded by Einstein's Special Relativity) an object moving relative to the hypothetical ether was length contracted by a factor (gamma) that was a function of it's velocity relative to the ether. In this early viewpoint the contraction was a real physical manifestation that was a result of a physical interaction between the moving object and the ether. Imagine we had identical twins, Adam and Bob. If Adam was moving relative to the ether while Bob was stationary wrt the ether, then in the Lorentz version Adam really would be physically contracted. However, the rulers carried by Adam would also be length contracted and his clock would be advancing at a slower rate and that would cause Adam to measure Bob as being length contracted. The end result is that neither Adam nor Bob could actually tell who is really length contracted and who is really stationary with respect to the ether. Einstein argued that since there is no way to detect the ether, then it could be dispensed with and this is the modern mainstream view.

In the lorentz version, the logic system that is applied can be summerized as:

If (length of A) > (length of B) then (length of B) < (length of A)

If (time interval A) > (time interval B) then (time interval B) < (time interval A)

Now while Lorentz logic seams reasonable and intuitive the above statements concern presumed intrinsic physical values but gloss over what is actually measured.

In the Einstein version the relationship is :

If (A measures length A>B) then (B measures length B>A)

If (A measures time interval A>B) then (B measures time interval B>A)

The Einstein version is slightly non-intuitive, but is mathematically correct and at the end of the day we should concern ourselves only with what we can actually measure rather than what we imagine is "really" going on.

The Lorentz version says that if two objects have relative motion, then both objects can not be stationary wrt the ether at the same time and so one of the objects must "really" be shorter than the other. The Einstein version makes clear that we have no way of measuring which object is "really" shorter and the argument becomes philosophical rather than scientific.
 
  • #55
Hello kev.

In answer to #54 with regards to your last comment. Whether or not an object local to us is moving relative to us or not i think can be decided and accepted as fact.The length of an object not moving relative to us is defined in SR as its proper length. Perhaps we can think of that as its real length because for us that is what it would be. We can measure it and if we had no knowledge of realative length contraction no question would ever occur about its reality. If a moving object is contracted relative to our non moving object then we can argue that this length is really shorter or not but surely there is an objective reality, as far as anything can be considered real, about the proper length as defined defined and measured by us.

Matheinste.
 
  • #56
Hello again kev.

In answer to my #55 on second reading it does not say anything different than your last comment. Sorry.

Matheinste.
 
  • #57
kev said:
The Einstein version is slightly non-intuitive, but is mathematically correct and at the end of the day we should concern ourselves only with what we can actually measure rather than what we imagine is "really" going on.
I agree and have made the same point with regard to both the many-worlds interpretation of quantum mechanics (popular on this forum) and the interpretation of expanding space in cosmology (popular almost everywhere). Einstein's logic sounds more convincing in relativity, for some reason.
 
  • #58
Can just someone confirm that I am (approximetly :) correct? I expected correction and I am quiet surprised to not get one. :)

Youre traveling close to C to a distant star 10 light years away. However due to the time dilation it would take the crew only 1 year to reach that star, using simple formula 10/1 we get 10 light years per year which is a big nono... therefore you have to make everything smaller to compensate this paradox.
 
  • #59
Tachyonie said:
Can just someone confirm that I am (approximetly :) correct? I expected correction and I am quiet surprised to not get one. :)
I think your conclusion is correct. One of the assumptions of relativistic mechanics (that is used to derive Lorentz transformation) is that the speed of object B in the system of object A is the same as the speed of object A in the system of B. If B travels a distance of L in a time t in the system A, then equalization of relative velocities gives equation:

L/t=L'/t'

or L'/L=t'/t (=1/gamma)

Length contraction is a necessary consequence of time dilation.
 
  • #60
Tachyonie said:
Can just someone confirm that I am (approximetly :) correct? I expected correction and I am quiet surprised to not get one. :)

Hi,

Just like to add that no one measures the speed of the spaceship to be 10c when they use their own clocks and their own rulers. The only person that might come to that conclusion would be a person measuring the distance using rulers at rest with the star frame and clocks at rest with the spaceship frame.

(By the way, I am not disagreeing with your statement ;)
 
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