# Is a length contraction just a visual thing?

#### absolute-space

Registered Member
Of the Lorentz transformation there is a particular thing mentioned of length contraction, is this contraction just from a visual perspective?

No. It's always there.

No. It's always there.
I am sorry I do not relate to how that answers my question in anyway. What do you mean it is always there, what is always there?

absolute-space:

Of the Lorentz transformation there is a particular thing mentioned of length contraction, is this contraction just from a visual perspective?
That's good question. To answer it, you need to understand the circumstances under which length contraction occurs. For simplicity, I only discuss special relativity here.

Length contraction is observed when the reference frame of the observer is changed. If you measure the length of an object that is stationary relative to you, the length you measure is what is known as the object's rest length. If you measure the length of the same object when it is moving past you at constant speed, you measure a shorter length than the rest length, due to relativistic effects.

You are asking whether this contraction is real or just a kind of visual illusion, in effect. The contraction that you measure for moving objects is the same however you measure it. The only thing that matters is your frame of reference as an observer - or, to put it another way, the relative speed between you and the object being measured. If you leave the object alone and just move yourself, you'll still see the length contraction of the object.

I think most physicists would be inclined to say that the length contraction of the object is a real effect.

My question to you, if you believe it is an illusion, is: what test would you propose to distinguish between a real contraction and an illusory one? If you can't suggest such a test, then the question itself may not be very meaningful.

absolute-space:

That's good question. To answer it, you need to understand the circumstances under which length contraction occurs. For simplicity, I only discuss special relativity here.

Length contraction is observed when the reference frame of the observer is changed. If you measure the length of an object that is stationary relative to you, the length you measure is what is known as the object's rest length. If you measure the length of the same object when it is moving past you at constant speed, you measure a shorter length than the rest length, due to relativistic effects.

You are asking whether this contraction is real or just a kind of visual illusion, in effect. The contraction that you measure for moving objects is the same however you measure it. The only thing that matters is your frame of reference as an observer - or, to put it another way, the relative speed between you and the object being measured. If you leave the object alone and just move yourself, you'll still see the length contraction of the object.

I think most physicists would be inclined to say that the length contraction of the object is a real effect.

My question to you, if you believe it is an illusion, is: what test would you propose to distinguish between a real contraction and an illusory one? If you can't suggest such a test, then the question itself may not be very meaningful.

I do not mention the length contraction as being an illusion, I asked whether or not it was a visual thing or a thing subject to the actual object decreasing in length. We do without doubt observe a ''visual'' length contraction of an object moving parallel away from a ''stationary'' observer who is at rest in an inertial reference frame. I also observe an object in motion parallel to another object in motion both travelling the same direction and speed will not observe any length contraction in regards of each others perspective of each other. Relatively both objects measure each others length even while in motion, as a rest length.

Analogy - Imagine observer (A) is standing on a train station platform, either side of the platform is a train, train (B) on the left track and train (C) on the right track, each train consists of five, fifty feet carriages.
Observer (A) measures both (B) and (C) to measure each train is 250ft in length while at relative rest length, relative to the stationary inertial reference frame of the observers.
Both trains start to accelerate at the same time at the same rate, both moving in the same direction away from (A) who remains relatively ''stationary'' to the reference frame of the platform. Observer (A) experiences the visual length contraction of both trains, each carriage decreasing greater in length the greater the acceleration.
However, observer D and observer E are on each train, they both are sitting in seat X, which is exactly the half way point of the initial measured rest length. Both observers D and E observe 125ft either side at rest length, in parallel motion both D and E still measure 125ft either side , relative to each other.

You may look in my direction with pulled faces, but to observe is to observe.

absolute-space:

I guess the point to make here is that there is no preferred reference frame in relativity.

The trains in your example are both stationary relative to observers D and E, so those observers see (measure) the rest lengths of the trains. On the other hand, D and E would see (measure) a contracted train platform.

Now, observer A sees contracted trains, but observer D sees a contracted platform. Which contraction is "real"? The answer is that both contractions are real, for the observers who measure them. A's frame of reference is no more special than D's frame of reference, so we can't say that A's observations are any more "real" than D's.

absolute-space:

I guess the point to make here is that there is no preferred reference frame in relativity.

The trains in your example are both stationary relative to observers D and E, so those observers see (measure) the rest lengths of the trains. On the other hand, D and E would see (measure) a contracted train platform.

Now, observer A sees contracted trains, but observer D sees a contracted platform. Which contraction is "real"? The answer is that both contractions are real, for the observers who measure them. A's frame of reference is no more special than D's frame of reference, so we can't say that A's observations are any more "real" than D's.

That could not be true, all observers by the power of communication can confirm that the rest lengths remained true. (A) can confirm to D and E that the platform as remained the same rest length, and likewise observer D and E can confirm both trains in motion have remained at rest length. All observers confirming that visual length contraction is the distant difference of angle of light relative to the observer and not a length contraction of the actual object.
I am surprised you view it differently when this seems apparent?

If the trains are 250ft in rest length and even in motion the trains are still observed by D and E to be 250ft, then seemingly obvious the train remains 250ft?

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That could not be true, all observers by the power of communication can confirm that the rest lengths remained true. (A) can confirm to D and E that the platform as remained the same rest length, and likewise observer D and E can confirm both trains in motion have remained at rest length.
Don't forget their rulers have shrunk too!

Don't forget their rulers have shrunk too!
An interesting observation but seemingly far fetched considering distance and lengths. How can a length of an object in linear motion contract? what force do you suggest imposes a force on the front of the object in motion to compress the length?

In the atmosphere we could allow for air resistance, but in space there is apparently no resistance or objects would not comply with Newtons Law of motion. To explain and prove an object in motion contracts, we would have to prove that the rear of the length is trying to overtake the front of the length or the front of the length was being forced backwards and not moving as fast as the rear, in either instant , it sounds very unlikely.
I accept such as the expansion and contraction of objects due to thermodynamics, this however being an isotropic affect of volume as opposed to just length. So surely if an objects length in motion contracts of the actual object, then why do we not see an expansion of height? We observe the oblate belly of the Earth and a height contraction creating a length expansion, the physics suggest the object contracting in length should also expand in height?

Does the train tunnel magically grow in height to allow the train through?

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An interesting observation but seemingly far fetched considering distance and lengths. How can a length of an object in linear motion contract?
Special Relativity.
what force do you suggest imposes a force on the front of the object in motion to compress the length?
There is no force causing the object to contract in length. I think you need to brush up on Special Relativity so that you can formulate better questions and objections.

... The contraction that you measure for moving objects is the same however you measure it. ...
Yes, but not all measurment processes are valid and measuring the contracted length is not easy.

For example if as the head of the train is just passing your poisiton (and you are some distance from the tracks so your camera can photograph the train's full length) you may conclude it is longer than its contracted length.

The time for the light reflecting off the end of the train to get to the open shuter is greater than that reflected by the locomotive front. So to arrive when the shutter is open, it had to start its trip earlier. I.e. Before the locomotive front was directly in front of the camera position.

You could have stuck little posts every cm along side the track, each with different "bar codes" on them. After the train is long gone you measure the distance between the two posts the photograph shows the front and rear of the train were next too, and get a wrong measurment of the contracted length.

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... Does the train tunnel magically grow in height to allow the train through?
No. the tunnel length is contracted too by its motion past the observer - same amount as the train is.

An interesting observation but seemingly far fetched considering distance and lengths. How can a length of an object in linear motion contract? what force do you suggest imposes a force on the front of the object in motion to compress the length?

In the atmosphere we could allow for air resistance, but in space there is apparently no resistance or objects would not comply with Newtons Law of motion. To explain and prove an object in motion contracts, we would have to prove that the rear of the length is trying to overtake the front of the length or the front of the length was being forced backwards and not moving as fast as the rear, in either instant , it sounds very unlikely.
I accept such as the expansion and contraction of objects due to thermodynamics, this however being an isotropic affect of volume as opposed to just length. So surely if an objects length in motion contracts of the actual object, then why do we not see an expansion of height? We observe the oblate belly of the Earth and a height contraction creating a length expansion, the physics suggest the object contracting in length should also expand in height?

Does the train tunnel magically grow in height to allow the train through?

There is no force and no compression involved. It would be absurd if there were. Think about it: it would mean that if someone moving rapidly past you happened to glance in your direction and noted that you appeared contracted in the direction of motion, then you would suddenly feel all squashed! Whereas if they did not, you wouldn't. Ridiculous, huh? And, as there is no compression, there is no increase in height when an object is perceived to be contracted in length due to the effect.

The effect is due to a perceived contraction of one dimension of space itself, in the direction of the relative motion. Consequently it affects everything moving at the speed in question, relative to another frame of reference, including the rulers used to measure lengths. As Origin says, you need to read about special relativity to understand this. It is not at all easy to explain in just words, without diagrams to help.

It would be absurd if there were. Think about it: it would mean that if someone moving rapidly past you happened to glance in your direction and noted that you appeared contracted in the direction of motion, then you would suddenly feel all squashed! Whereas if they did not, you wouldn't. Ridiculous, huh? And, as there is no compression, there is no increase in height when an object is perceived to be contracted in length due to the effect.

The effect is due to a perceived contraction of one dimension of space itself, in the direction of the relative motion. Consequently it affects everything moving at the speed in question, relative to another frame of reference, including the rulers used to measure lengths. As Origin says, you need to read about special relativity to understand this. It is not at all easy to explain in just words, without diagrams to help.
It is interesting that you say I need to read special relativity, when my question is obviously related to special relativity, I did not imagine length contraction in my head, I read about it. It is rather presumptuous of you to think that I have not looked at and into special relativity.

You say ''
There is no force and no compression involved. It would be absurd if there were. Think about it: it would mean that if someone moving rapidly past you happened to glance in your direction and noted that you appeared contracted in the direction of motion, then you would suddenly feel all squashed! Whereas if they did not, you wouldn't. Ridiculous, huh? And, as there is no compression, there is no increase in height when an object is perceived to be contracted in length due to the effect.

The effect is due to a perceived contraction of one dimension of space itself, in the direction of the relative motion. Consequently it affects everything moving at the speed in question, relative to another frame of reference, including the rulers used to measure lengths. As Origin says, you need to read about special relativity to understand this. It is not at all easy to explain in just words, without diagrams to help.
It is interesting that you say I need to read special relativity, when my question is obviously related to special relativity, I did not imagine length contraction in my head, I read about it. It is rather presumptuous of you to think that I have not looked at and into special relativity.

You say ''There is no force and no compression involved'', then can you please explain how a length of the actual object can possibly contract ?

I have here a ruler, it is 30cm in length, I mark two points A and B on a piece of paper to mark the length of the 30cm ruler. To contract the length of the ruler, I have to apply two points of centripetal pressure, one from either end of the ruler . I can clearly observe the length contraction relative to the marked points on the paper, I also observe the linear length is now a curve.

Please explain how this differs? when to contract anything or expand anything , there has to be work involved for the displacement of point particles in the object.

Please explain how this differs? when to contract anything or expand anything , there has to be work involved for the displacement of point particles in the object.
They are not displaced. The place is different for different people.

I can clearly observe the length contraction relative to the marked points on the paper,
The length won't change, as long as the paper is at rest with respect to the ruler.

They are not displaced. The place is different for different people.

The length won't change, as long as the paper is at rest with respect to the ruler.
In all due respect, I think you missed the point of applying physics and the relationship between force and an objects form. For an object to change form there has to be work being done, do you suggest an object could possibly contract in length without there being any work done within the internals or external of the object?

By what ''magic'' do you perceive this effect occurs then ?

Also, and yes the length of the rulers start and end point doe's contract relative to the original marks on the paper, creating an XY curve and if the point particles did not displace like you suggest, then apparently there is no contraction of the length because nothing is displaced of the object.

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Also, and yes the length of the rulers start and end point doe's contract relative to the original marks on the paper, - -
Not according to relativity theory. Not if the paper is at rest alongside the ruler.
For an object to change form there has to be work being done, - -
Change compared with what? Compared with itself, and everything at rest around it, your ruler hasn't changed at all.

It is interesting that you say I need to read special relativity, when my question is obviously related to special relativity, I did not imagine length contraction in my head, I read about it. It is rather presumptuous of you to think that I have not looked at and into special relativity.

You say ''There is no force and no compression involved'', then can you please explain how a length of the actual object can possibly contract ?

I have here a ruler, it is 30cm in length, I mark two points A and B on a piece of paper to mark the length of the 30cm ruler. To contract the length of the ruler, I have to apply two points of centripetal pressure, one from either end of the ruler . I can clearly observe the length contraction relative to the marked points on the paper, I also observe the linear length is now a curve.

Please explain how this differs? when to contract anything or expand anything , there has to be work involved for the displacement of point particles in the object.

No. The length of the ruler and the paper - and any other object moving with them - appear contracted, when viewed by someone relative to whom they are moving. But to someone at rest relative to these objects, the lengths are unchanged. There is thus no "displacement" and no work done. It is merely an effect of how things appear when seen from a viewpoint in motion relative to the objects.

The reason I (and Origin) said you needed to read about Special Relativity is because (a) from your questions it is clear that you do not understand it and (b), as I have pointed out, it is not something that either Origin or I can easily explain in words to you on this forum. Special Relativity is notoriously hard to explain in any case and, to make matters worse, we cannot draw diagrams here. If you have already read about Special Relativity and you still don't get it, what can I say? All I can suggest it that you read it again, more carefully, or choose a different source and see if that helps. (Don't try Wiki, though: their article strikes me as rather formal and full of distracting terminology).

Length contraction can be viewed as a kind of perspective illusion, as long as you recognize that illusions are still showing you something real (just unexpected).

In all due respect, I think you missed the point of applying physics and the relationship between force and an objects form. For an object to change form there has to be work being done, do you suggest an object could possibly contract in length without there being any work done within the internals or external of the object? By what ''magic'' do you perceive this effect occurs then ?
Length contraction is not magic, it is physics. Length contraction (and time dilation) is a direct consequence of the speed of light being the maximum speed that a signal can be transmitted and that all observers measure the speed of light in a vacuum at c.
If you were able to fly to star 10 light year away at 0.8c you would say that you traveled for 6 years and a distance of 6 light years. To you the entire universe had contracted! To someone that was stationary to you they would say that you traveled 10 light years in 10 years.

So who is right? They both are correct of course. How can they both be right? Because there is no absolute space (apologies to your name) or absolute time so there is no prefered frame. It is all relative - get it?