# Lost in Langevin's language

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Here in English https://en.wikisource.org/wiki/Translation:The_Evolution_of_Space_and_Time
And in French https://fr.wikisource.org/wiki/L’Évolution_de_l’espace_et_du_temps

Quoted from page 45

[ 45 ] We can call pairs in space those pairs of events that have just been considered and to which the order of succession in time has no absolute sense, but are spatially distant in an absolute way.
It is noteworthy that, while the spatial distance of two events can not be canceled, it reaches a minimum precisely for reference systems in which the two events are simultaneous.
Hence the following statement:
The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

This statement contains, as a particular case, what is called the Lorentz contraction, that is to say, the fact that the same ruler considered by different groups of observers, some resting, others in motion relative to it, is shorter for those who see it passing by as for those who are attached to it. We have already seen that the length of a ruler for observers who see it passing by, is defined by the distance in space of two simultaneous positions (for those observers) on both ends of the ruler. According to the preceding this distance will be shorter for those observers than for all others, especially those attached to the ruler.
We also easily understand how the Lorentz contraction can be reciprocal, that is to say how two rulers that are equal when at rest, appear mutually shortened when they slide against one another, and observers attached to one of the rulers will see the other one shorter than its own. This reciprocity holds, because observers associated with the two rulers in motion relative to each other don't define simultaneity the same way.

(enhancing by me)
The first bold part is understood by me as the contrary of the second bold part.

(I checked the original text in French and it is not an error in translation)

Where is my error?

Edited by michel123456

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#### Posted Images

They seem consistent to me:

1. It is noteworthy that, while the spatial distance of two events can not be canceled, it reaches a minimum precisely for reference systems in which the two events are simultaneous.

In other words: the spatial distance of two events is a minimum frames which the two events are simultaneous.

2. The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

In other words: the spatial distance of two events, in the frame where they are simultaneous, is shorter than any other (i.e. it is the minimum)

Does that help?

Sorry. Ignore that. I misread the question. (Ironic!) Will have another go...

OK. It is quite subtle. I don't know if I can explain it ...

The distance between two events (remember an event is a point in space-time, not just space) is not the same as the length of a ruler, where the end points are only defined as (relative) spatial coordinates; in other words, they are not events. Also, the space-time distance between the events is invariant: the same for all observers.

Perhaps one way to visualise this is the fact the the ruler is, as you say, shorter when perceived from the moving frame of reference. So if that shorter ruler is used to measure the spatial distance between the two events, then that distance will be measured as greater (because you are using a shorter ruler).

Hopefully someone else can do a better job....

Edited by Strange
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Hopefully someone else can do a better job....

I'll give it a shot

I admit the Langevin text is a bit confusing at first reading.

It is noteworthy that, while the spatial distance of two events can not be canceled, it reaches a minimum precisely for reference systems in which the two events are simultaneous.

Hence the following statement:

The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

Bear in mind that in the above case the two events to be measured are not simultaneous in the measuring frame.

If you measure -with your ruler at rest- the moving train (and mark off the two train events that are simultaneous for the train frame), you will measure a LONGER train.

Lenth contraction is a different story. See below:

This statement contains, as a particular case, what is called the Lorentz contraction, that is to say, the fact that the same ruler considered by different groups of observers, some resting, others in motion relative to it, is shorter for those who see it passing by as for those who are attached to it.

Measuring the moving train that exists in our 3D space of simultaneous events means measuring distance between two events of the train that are not simultaneous for the train frame.

Train station observer and train passenger measure different 3D sections of simultaneous events through 4D spacetime.

Edited by VandD
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[..]

OK. It is quite subtle. I don't know if I can explain it ...

The distance between two events (remember an event is a point in space-time, not just space) is not the same as the length of a ruler, where the end points are only defined as (relative) spatial coordinates; in other words, they are not events. Also, the space-time distance between the events is invariant: the same for all observers.

Perhaps one way to visualise this is the fact the the ruler is, as you say, shorter when perceived from the moving frame of reference. So if that shorter ruler is used to measure the spatial distance between the two events, then that distance will be measured as greater (because you are using a shorter ruler).

Hopefully someone else can do a better job....

I think that that is correct; I'll try to elaborate with a specific, hypothetical example with ultra fast moving rulers.

[edit: VandD beat me to this, but it can still be helpful to explain the same idea in different ways.

And by chance I understood the intention of Langevin a little differently, so that here I do not present "a different story" but, as it should, "a particular case" of the same story.]

Suppose that each ruler is nominally 12 m long, and one ruler has powerful lasers at the points 1 m and 11 m. If the two events are marked ("at the same time" according to the laser system) by means of those lasers into the other, relatively moving ruler when the two rulers are aligned in the middle, then those marks will appear at for example 0.99 m and 11.01 m.

This result can be phrased in two ways, corresponding with the two statements of Langevin:

1. The 10 m (proper) distance between the lasers is marked as 10.02 m distance on the relatively moving ruler. And 10 m is less than the 10.02 m distance on the relatively moving ruler.

2. As that 10.02 m distance on the relatively moving ruler corresponds to the 10 m laser distance, the relatively moving ruler is measured as being length contracted.

Edited by Tim88
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I'll give it a shot

I admit the Langevin text is a bit confusing at first reading.

Bear in mind that in the above case the two events to be measured are not simultaneous in the measuring frame.

If you measure -with your ruler at rest- the moving train (and mark off the two train events that are simultaneous for the train frame), you will measure a LONGER train.

Lenth contraction is a different story. See below:

Measuring the moving train that exists in our 3D space of simultaneous events means measuring distance between two events of the train that are not simultaneous for the train frame. That length is shorter than the distance between two simultaneous train events measured from a frame at rest reltaive to train (a train passenger doing the measurement of his train ... obviously at rest relative to him).

Train station observer and train passenger measure different 3D sections of simultaneous events through 4D spacetime

removed quote marks

Edited by VandD
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I think that that is correct; I'll try to elaborate with a specific, hypothetical example with ultra fast moving rulers.

[edit: VandD beat me to this, but it can still be helpful to explain the same idea in different ways.

And by chance I understood the intention of Langevin a little differently, so that here I do not present "a different story" but, as it should, "a particular case" of the same story.]

Suppose that each ruler is nominally 12 m long, and one ruler has powerful lasers at the points 1 m and 11 m. If the two events are marked ("at the same time" according to the laser system) by means of those lasers into the other, relatively moving ruler when the two rulers are aligned in the middle, then those marks will appear at for example 0.99 m and 11.01 m.

This result can be phrased in two ways, corresponding with the two statements of Langevin:

1. The 10 m (proper) distance between the lasers is marked as 10.02 m distance on the relatively moving ruler. And 10 m is less than the 10.02 m distance on the relatively moving ruler.

2. As that 10.02 m distance on the relatively moving ruler corresponds to the 10 m laser distance, the relatively moving ruler is measured as being length contracted.

You must be trying to confuse me more.

1. means to me that proper length 10m is less than 10.02, thus that the ruler expands (and do not contract)

2. You seem saying in point 2 that the proper length on the ruler is 10.02???

quoting Langevin

It is noteworthy that, while the spatial distance of two events can not be canceled, it reaches a minimum precisely for reference systems in which the two events are simultaneous.

Hence the following statement:

The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

I would expect the first bold the be "maximum" instead of "minimum"

And the second bold to be "longer" instead of "shorter"

Isn't he talking about objects & observers at rest in the same FOR?

And isn't he talking about "spatial distance" and not spacetime interval?

Edited by michel123456
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You must be trying to confuse me more.

1. means to me that proper length 10m is less than 10.02, thus that the ruler expands (and do not contract)

2. You seem saying in point 2 that the proper length on the ruler is 10.02???

: No for the first, yes for the marked length in the second question. I'm afraid that here we face a technically very simple issue that I myself also found confusing except when adding a picture (first time I try this here with ASCII):

0 1 2 3 4 5 6 7 8 9 10 11 12 v ->

0 1 2 3 4 5 6 7 8 9 10 11 12

This is how it looks for the ruler with lasers; the top ruler is the "moving" ruler and the laser positions are indicated in red on the bottom ruler.

In this picture the 10 m nominal distance on the "stationary" ruler is marked by the lasers as a nominal distance of about 12 m on the "moving ruler" (fat black burn marks), and so the marked ruler is observed as being length contracted by the system with lasers.

Does that help?

PS: the main drawback of Langevin's presentation, apart of being rather long winded, is that it apparently was a speech* with little or no visual aids such as pictures, equations and calculations.

[..]

Isn't he talking about objects & observers at rest in the same FOR?

And isn't he talking about "spatial distance" and not spacetime interval?

He is talking about spacial distances between events that are measured differently with different FORs.

Edited by Tim88
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I would expect the first bold the be "maximum" instead of "minimum"

And the second bold to be "longer" instead of "shorter"

Isn't he talking about objects & observers at rest in the same FOR?

And isn't he talking about "spatial distance" and not spacetime interval?

You don't seem to understand what I write here: http://www.scienceforums.net/topic/98501-lost-in-langevins-language/#entry943663

Why? Is it because you still don't understand relativity of simultaneity?

You don't know yet that different frames of reference involve different simultaneity?

Why is it you seem to make no progress at all understanding SR?

Edited by VandD
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You don't seem to understand what I write here: http://www.scienceforums.net/topic/98501-lost-in-langevins-language/#entry943663

Why? Is it because you still don't understand relativity of simultaneity?

You don't know yet that different frames of reference involve different simultaneity?

Why is it you seem to make no progress at all understanding SR?

I understand your explanation. I understand that moving lengths are contracted.

I don't understand Langevin's phrase.

Making some cuts in his words he seems to say that

"the spatial distance of two events reaches a minimum for reference systems in which the two events are simultaneous".

The "reference systems in which the two events are simultaneous" is understood by me as the FOR at rest.

So he seems to say that "the spatial distance of two simultaneous events reaches a minimum" in a FOR at rest.

Is that correct? I mean, do I understand the phrase correctly?

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This is how I understand Langevin:

I refer to Langevin's example at [40] of two objects dropped thru a hole in the floor of a car.
Let's now consider two holes at some distance.
The observer in the car carefully synchronized two clocks in order to drop the objects at the same time.

______________________
| |
| |
| c1 c2 |
/ \--- -------- ---/ \
\_/ o o \_/

If the car is moving, the observer on the ground notices that the clocks are not synchronized to him.
Therefore the object at c1 is dropped before the other, and the distance on the ground between the objects is longer than the distance between the holes in the car's floor.

______________________
| |
| |
| | ---->
/ \--- --------o---/ \
____________\_/___o____________\_/_____________________________________

______________________
| |
| |
| | ----->
/ \--- -------- ---/ \
__________________o__\_/____________o___\_/____________________________

But if the observer in the car defined the distance between the holes as 1m by mesuring the time for a light signal going on and back in 2/300000000 sec, the observer on the ground will say for him that's less than 1m. That's the ruler contraction.

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I understand your explanation. I understand that moving lengths are contracted.

I don't understand Langevin's phrase.

Making some cuts in his words he seems to say that

"the spatial distance of two events reaches a minimum for reference systems in which the two events are simultaneous".

The "reference systems in which the two events are simultaneous" is understood by me as the FOR at rest.

So he seems to say that "the spatial distance of two simultaneous events reaches a minimum" in a FOR at rest.

Is that correct? I mean, do I understand the phrase correctly?

The first phrase is intended to be more general I think, but as applied to the special case it looks correct to me. Maybe the issue is with what he means with "spatial distance", very similar to what happened in the muon discussion ("distance" vs "atmosphere").

See the sketch in my post #7 of the physical situation according to the rest system. [edit:] I'll paste it back in here, with slight improvement of precision:

moving ruler: 0 1 2 3 4 5 6 7 8 9 10 11 12 v ->

ruler in rest: 0 1 2 3 4 5 6 7 8 9 10 11 12

As I interpret his words and applied to length contraction, the spatial distance between the laser flash events (in red) is 10 m according to the ruler in rest. The distance between the same events is measured (marked) as 12 m on the moving ruler, as you see.

Langevin says then (for my example) that 10 m is the minimum distance determination possible, and all other frames that are in relative motion will measure a greater distance (e.g. 12 m). In the "moving" frame this is explained by observing that, according to the moving frame, the laser flashes were not simultaneous.

[edit: the relation between "not simultaneous" and "measuring a greater spatial distance" is nicely sketched by bvr in his post, although his interpretation of another phrase by Langevin is different from mine. And when browsing back to Langevin's p.40, don't forget that on that page he discusses classical mechanics, as introduction to the more complicated SR.]

This is obviously true in general for two events in space and time. By means of this example I thus "reverse engineered" the generalization with which Langevin started (personally I would have concluded with that). Slightly rephrasing:

The spatial distance of two events that are simultaneous in an inertial reference system, is shorter according to that system than according to another system that is in relative motion to the first.

Edited by Tim88
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I understand your explanation. I understand that moving lengths are contracted.

I don't understand Langevin's phrase.

Making some cuts in his words he seems to say that

"the spatial distance of two events reaches a minimum for reference systems in which the two events

Let's call these two events A and B. More specific: event A is "front train is hit by a bird", event B is "rear of train is hit by lightning"

are simultaneous".

The "reference systems in which the two events are simultaneous" is understood by me as the FOR at rest.

So he seems to say that "the spatial distance of two simultaneous events reaches a minimum" in a FOR at rest.

If you consider a frame F where THOSE TWO events A and B are NOT simultaneous, then that frame measures a LONGER length between THOSE TWO events A and B.

But that's NOT what one does measuring a moving train. Measuring a moving train is measuring between two events of the train that are simultaneous in frame F .

Events A and B are NOT simultaneous in frame F. The shorter train in frame F 3D space of simultaneous events is NOT a train with simultaneously lightning hitting the rear and front hit by a bird.

That's what the different angles of 3D sections thorug 4D spacetime show visually.

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Exactly. Relativity of simultaneity is key to this (and why two events are not the same as two endpoints). This is kinda obvious when you remember that we are talking about a rotation between space and time.

If we consider the famous train with two lightning strikes that hit each end simultaneously (for the observer on the train), you may think that the observer on the platform sees the train foreshortened and therefore "the lightning strikes must be closer together".

But no, the lightning strikes are not simultaneous to the observer on the platform and so the lightning strikes are further apart than the length of the train. (For the platform observer, the rear of the train is truck first and then the train rushes forward to the place where it will be struck on the front.)

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[ 45 ] ... The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

This statement contains, as a particular case, what is called the Lorentz contraction, that is to say, the fact that the same ruler considered by different groups of observers, some resting, others in motion relative to it, is shorter for those who see it passing by as for those who are attached to it.

Here in English https://en.wikisource.org/wiki/Translation:The_Evolution_of_Space_and_Time
And in French https://fr.wikisource.org/wiki/L’Évolution_de_l’espace_et_du_temps

Quoted from page 45

(enhancing by me)
The first bold part is understood by me as the contrary of the second bold part.

(I checked the original text in French and it is not an error in translation)

Where is my error?

Hi michel123456,

I understand your question. It is a standard obstacle along the path to a better SR understanding.

wrt body length ... the length of a body (eg ruler) is obtained by measuring the spatial location of its endpoints within one's own system. The measurement of each endpoint marks an EVENT, the measurement occurring at a specific location in both space and time. As such, 2 simultaneous events define the length of a body, whether moving or not. While the length determination may be made from 2 non-simultaneous events, maybe with more rigor, the measurements are generally taken simultaneously, simply because its easy. For measurements taken simultaneously, one can simply subtract one event's spatial coordinate from the other event's spatial coordinate to attain the body's length in the measurer's own frame (moving or not). Consider the 2 cases ...

If the body is in motion ... and its endpoints measured at 2 different moments in time, the body moves between the 2 moments of measurement ... and one must then subtract the ruler's distance-traversed from the total-spatial-separation between those 2 events to determine the moving ruler's simultaneous contracted-length. But if one prefers doing things the simply way, one takes their endpoint measurements simultaneously, if possible.

If the body is stationary ... the measurer need NOT bother to take the measurements simultaneously (although usually one does). The reason, because no point of the ruler moves over any duration, ie all points of the ruler are INDEPENDENT OF TIME (in the rest system). As such, one may measure one endpoint of the ruler at any time, and the other endpoint at any (or any other) time, subtract the two x coordinates, and one attains the very same length every time. This length being the PROPER LENGTH of the ruler, an invariant length, and it's longest recordable length.

wrt the LTs ... Events are occurrences that exist in any and all frames, whose locations are defined in both space and time, ie by 0 dimensional points in 4 dimensional spacetime. Here's the important part ... The LT's of Relativity require observers to disagree as to what are simultaneous events if in relative motion. The moving contracted-length is a simultaneous consideration, and so those same 2 events CANNOT exist AS SIMULTANEOUS in the rest frame. Does that hurt anything? The LTs relate the 2 simultaneous events of the moving ruler (say separation 0.5), to 2 asynchronous events in the ruler's rest frame (say separation of 1, given v=0.866c). Since the location of any point of the stationary ruler is independent of time, they never change in their location irregardless of when said endpoints are considered in the rest frame. And so a simultaneous measurement of its endpoints always produces the very same length as any non-simultaneous consideration of its endpoints. As such, one may say that a proper-length (Lp) of a stationary ruler corresponds to a contracted-length (Lp/γ) of that same ruler viewed in relative motion at v, without bothering to mention all the details of how the LTs make that happen. Yet, to understand relativity, one must know all the details, whether its mentioned or not. Anyone who does not understand all the details, will forever be confused to some extent when discussing relativistic scenarios.

You may be wondering why I bother to mention all the above ...

wrt your error ... What you believe to be a contradiction (2 different measures of the same length), is instead 2 entirely different considerations. Realize that, then there is no conflict ...

EDIT: If you start from the rest frame (S) and take 2 simultaneous measurements of the stationary ruler's endpoints, those same 2 events cannot be simultaneous in the other frame (S') that holds the ruler in motion. The LTs, can only transform those 2 simultaneous points in S to a set of asynchronous points in S'. Since those points occur sequentially in S', the moving contracted length (L' = Lp/γ) must traverse a distance (xsep' = vt') between the occurrence of those events (t' = t'2-t'1). So the length between those 2 events in S', includes a traversal-distance of the moving ruler PLUS the contracted-length of the moving ruler ... x'sep = v(t'2-t'1) + (Lp/γ). And it is always the case that x'sep > Lp for v > 0 , as follows ...

x’ = γ(x-vt)

x’sep = γ(Lp-v(t'2-t'1))

for endpoint measurements taken simultaneously (t'2 = t'1, and so t'2 -t'1 = 0) in the ruler's rest frame (S), then the separation between those events in the frame that moves relatively (S') is then ...

x’sep = γ(Lp-v*0)

x’sep = γLp

and so it is always true for v>0 (hence γ>1) that ... x’sep > Lp

Lp ... the spatial separation between the 2 events in the rest frame (S), ie the rest length or proper length.

x’sep ... the spatial separation between the same 2 events in the frame that moves relatively (S').

That's the reason the S' observer records a greater spatial-length between the 2 events, than does the observer at rest with the ruler.

Best regards,

Celeritas

Edited by Celeritas
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Quoted from page 45

(enhancing by me)

The first bold part is understood by me as the contrary of the second bold part.

(I checked the original text in French and it is not an error in translation)

Where is my error?

With the minkowski metric, we get a pretty neat spacetime distance equation.

(spacetime distance)2 = (temporal distance in a frame)2 - (spatial distance in the same frame)2

A neat feature of the spacetime distance is that it's frame invariant. It's the same for all observers. So, for each observer (those for whom the events are simultaneous and those for whom it is not), the spacetime distance is the same.

So:

(temporal distance in frame 1)2 - (spatial distance in frame 1) = (temporal distance in frame 2)2 - (spatial distance in frame 2)2

In a frame where the events are truly simultaneous, their temporal distance is zero. So:

(spatial distance in simultaneous frame)2 = (spatial distance in a non-simultaneous frame)2 - (temporal distance in the same non-simultaneous frame)2

So, the further apart in time two events are in a frame, the further apart they are spatially in that frame. The simultaneous frames will have the minimum spatial distance between the two events.

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With the minkowski metric, we get a pretty neat spacetime distance equation.

[..]

(spatial distance in simultaneous frame)2 = (spatial distance in a non-simultaneous frame)2 - (temporal distance in the same non-simultaneous frame)2

So, the further apart in time two events are in a frame, the further apart they are spatially in that frame. The simultaneous frames will have the minimum spatial distance between the two events.

Indeed, I now think that he may have written the space-time equation:

ds2 = invariant = (dx2 + dy2 + dz2) - c2dt2

on the blackboard during his speech, when stating what is then immediately obvious:

"It is noteworthy that, while the spatial distance of two events can not be canceled, it reaches a minimum precisely for reference systems in which the two events are simultaneous."

I'm now considering to add such images as illustrations to my own translation of that paper.

Edited by Tim88
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This is how I understand Langevin:

I refer to Langevin's example at [40] of two objects dropped thru a hole in the floor of a car.

Let's now consider two holes at some distance.

The observer in the car carefully synchronized two clocks in order to drop the objects at the same time.

______________________

| |

| |

| c1 c2 |

/ \--- -------- ---/ \

\_/ o o \_/

If the car is moving, the observer on the ground notices that the clocks are not synchronized to him.

Therefore the object at c1 is dropped before the other, and the distance on the ground between the objects is longer than the distance between the holes in the car's floor.

______________________

| |

| |

| | ---->

/ \--- --------o---/ \

____________\_/___o____________\_/_____________________________________

______________________

| |

| |

| | ----->

/ \--- -------- ---/ \

__________________o__\_/____________o___\_/____________________________

But if the observer in the car defined the distance between the holes as 1m by mesuring the time for a light signal going on and back in 2/300000000 sec, the observer on the ground will say for him that's less than 1m. That's the ruler contraction.

That is OK for me.

What I conclude is this:

The measured distance on the ground is larger than the measured distance on the car.

The measured distance on the car is the proper distance.

So, in order to find the proper distance from the ground measurement, you take the ground measurement and divide it by a contraction factor. (because the result is less than what has been measured)

Or do I miss something again?

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That is OK for me.

What I conclude is this:

The measured distance on the ground is larger than the measured distance on the car.

The measured distance on the car is the proper distance [of the car].

So, in order to find the proper distance [of the car] from the ground measurement, you take the ground measurement and divide it by a [the] contraction factor. (because the result [proper distance] is less than what has been measured [from the moving frame])

Or do I miss something again?

I added some missing words for clarity in bold; I hope that that is what you meant. If so, I think that it is correct. That is good.

However the reason for the "so" is not clearly stated, nor the connection to the statements by Langevin. Moreover you did not comment on most of the foregoing explanations. Thus it is unclear to me if you may have "missed" something; but probably you did.

PS. A silly matter that always confused me, and which perhaps confuses you as well, is that if you use a ruler that is shrunk (and as a result it is shorter than before), you will measure longer distances with it, for example the distance between two walls appears longer. It's very logical but it's easy to mix up the "shorter" and the "longer" - I still sometimes go wrong there.

Edited by Tim88
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PS. A silly matter that always confused me, and which perhaps confuses you as well, is that if you use a ruler that is shrunk (and as a result it is shorter than before), you will measure longer distances with it, for example the distance between two walls appears longer. It's very logical but it's easy to mix up the "shorter" and the "longer" - I still sometimes go wrong there.

An observer measures with a ruler AT REST relative to him.

For the car passenger a ruler at rest in his car is never contracted for the car passenger (in his 3D frame of simultaneous events).

And for the street observer a ruler at rest on the floor is never contracted for the street guy (in his 3D frame of simultaneous events).

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I added some missing words for clarity in bold; I hope that that is what you meant. If so, I think that it is correct. That is good.

However the reason for the "so" is not clearly stated, nor the connection to the statements by Langevin. Moreover you did not comment on most of the foregoing explanations. Thus it is unclear to me if you may have "missed" something; but probably you did.

PS. A silly matter that always confused me, and which perhaps confuses you as well, is that if you use a ruler that is shrunk (and as a result it is shorter than before), you will measure longer distances with it, for example the distance between two walls appears longer. It's very logical but it's easy to mix up the "shorter" and the "longer" - I still sometimes go wrong there.

Sure I did miss something.

Here below the text with your corrections and my (mis)understanding

----------------------

| That is OK for me.

| What I conclude is this:

| The measured distance on the ground is larger than the measured distance on the car.

| The measured distance on the car is the proper distance [of the car].

| So, in order to find the proper distance [of the car] from the ground measurement, you take the ground measurement and divide it by a [the] | contraction factor. (because the result [proper distance] is less than what has been measured [from the ground frame])

| Or do I miss something again?

---------------------

I mean that the proper distance is less than the measurement from the ground frame, IOW that the measurement from the ground is more* than as measured on the car i.e. that the car "lives in a contracted reality"

*It is what has been described in the sketches.

Edited by michel123456
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---------------------

I mean that the proper distance is less than the measurement from the ground frame, IOW that the measurement from the ground is more* than as measured on the car i.e. that the car "lives in a contracted reality"

The car that is present in the ground frame is NOT the one that's in the car frame.

The set of events of the car in the car frame is NOT the set of car events in the the ground frame.

Hence the measurement of distance in the ground frame between two car events that are simultaneous in car frame is NOT what is in the ground frame. And you bet, because that length is LONGER than proper car length. The car in the ground frame is SHORTER.

The car events that ground frame measures -to find out what car is in the ground frame- are NOT the ones simultaneously in the car frame.

The ground that is present in the car frame is NOT the one that's in the ground frame.

The set of events of the ground in the ground frame is NOT the set of ground events in the car frame.

Hence the measurement of distance in the car frame between two ground events that are simultaneous in ground frame is NOT what is in the car frame. And you bet, because that length is LONGER than proper ground length.

And you bet, because that length is LONGER than proper car length. The ground in car frame is SHORTER.

The ground events that car frame measures -to find out what ground is in the car frame- are NOT the ones simultaneously in the ground frame.

There is no way to understand this without a spacetime diagram. Why don't you try to draw one showing the events, lengths, and different frames? Use my rocket/muon diagram and make the atmosphere length the street ruler, and the rocket the car ruler.

Edited by VandD
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An observer measures with a ruler AT REST relative to him.

[..]

My confusions have nothing to do with relativity; just with inversions.

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I will drive you all crazy, I know that.

Do you all agree that Dground is the measurement taken in the FOR of the ground?

Do you all agree that Dground is longer than Dcar?

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Sure I did miss something.

Here below the text with your corrections and my (mis)understanding

----------------------

| That is OK for me.

| What I conclude is this:

| The measured distance on the ground is larger than the measured distance on the car.

| The measured distance on the car is the proper distance [of the car].

| So, in order to find the proper distance [of the car] from the ground measurement, you take the ground measurement and divide it by a [the] | contraction factor. (because the result [proper distance] is less than what has been measured [from the ground frame])

| Or do I miss something again?

---------------------

I mean that the proper distance is less than the measurement from the ground frame, IOW that the measurement from the ground is more* than as measured on the car i.e. that the car "lives in a contracted reality"

*It is what has been described in the sketches.

I see that my completion was ambiguous, sorry for that. The example of the car here above corresponds to the ground being the frame that Langevin says to be "in relative motion", and which I therefore labeled as "moving", just as in my earlier post:

The spatial distance of two events that are simultaneous for a certain group of observers, is shorter for them than for all other observers in arbitrary motion relative to them.

Also, the (measured) spatial distance, is shorter for those who see it passing by as for those who are attached to it.

I suppose that you can now agree with that phrase as well.

It is also true that according to the "ground frame" the car lives in a contracted reality. But - and I insist - in fact that does not directly follow from what Langevin said; what directly follows is that according to the "car frame", rulers in the ground frame are length contracted.

I'll combine the pictures, maybe that will be clearest. I also correct a subtle drawing error that I now notice.

We assume, as usual, that the car is "Einstein synchronized"; in other words, the car is assumed to be in rest according to the "car observer".

According to the car observer:

__________________________

| |

| |

| c1 c2 |

/ \--- ----1m---- ---/ \

\__/ o o \__/

As you see, I added the distance measurement according to the car in red. According to the car, the balls were dropped simultaneously at 1 m distance, so that they also hit the ground at 1 m distance.

The ground is similarly synchronized with the assumption to be in rest (in perfect disagreement with the car observer).

Therefore, according to the ground, not only is the car contracted, but the balls are not dropped at the same time so that they arrive at different times:

______________________

| |

| |

| | ---->

/ \--- --------o---/ \

____________\_/___o____________\_/_____________________________________

______________________

| |

| |

| | ----->

/ \--- -------- ---/ \

__________________o__\_/___1.5m_____o___\_/____________________________

Distance between the corresponding events as measured with a ruler on Earth in blue.

The ground observers measure a distance of for example 1.5 m between the events. How is this difference of opinion explained with the different measurement systems?

According to the car observer the balls were dropped at the same time so that the distance between the balls is 1 m when they are dropped, and also 1 m when the touch the ground.

But he notices that the distance between the balls on the ground according to a ruler on the ground is 1.5 m.

He infers that the Earth ruler is length contracted, because from his perspective, a "1.5 m" Earth ruler is only 1 m long.

Full situation from the car's perspective:

__________________________

| |

| |

| c1 c2 |

/ \--- ----1m---- ---/ \

____________\__/___o__"1.5m"__o___\__/___________________

And as Langevin remarks, it's easy to understand how length contraction can be reciprocal so that each system "sees" the other as length contracted (and blames the other for giving a distorted account of reality):

According to the ground observers, the car is length contracted but the effect of the synchronization error by the car operator is greater, so that the distance between the events was more than 1 m.

[edit: added one more sketch and improved the "proper car sketches"]

Edited by Tim88
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...

I mean that the proper distance is less than the measurement from the ground frame, IOW that the measurement from the ground is more* than as measured on the car i.e. that the car "lives in a contracted reality"

*It is what has been described in the sketches.

micehl123456,

Nothing in SR or Langevin's work suggests a different reality. I think you might be good here, but one must be careful with their choice of words.

Here, you are considering the car per the ground observers ... the car is moving relatively, and hence length contracted. But do the car passengers live in a contracted reality? Per themselves, they never notice any change in their own length or their own clock rate over duration, accelerating or not. The reason ... nothing within their car moves (relativistically) relative to themselves. Given v=0, then no relativistic effects exist. Same holds for ground frame observers considering things that essentially co-move with them. Only things observed in relative motion (at relativistic rate) undergo measurable relativistic effects. This is only to say that differing POVs (of a single shared reality) exist, not that different realities exist. That may be what you meant?

On the other hand, Lorentz's LTs (which are the same as Einstein's) mean something much different. Lorentz built his LTs upon a foundation of an absolute ether frame, where light only moves at c in that frame. Lorentz's LTs require that moving observers physically contract in a stationary ether as they move thru it, and as such they would actually be length-contracted in-and-of-themselves even though their measuring apparatus are incapable of ever measuring it (contracted rulers measure equally contracted lengths as uncontracted). However per SR, we all live in a PROPER CONFIGURATION where oneself (and all that co-move with oneself) undergoes zero relativistic effects, ie they are not contracted in-and-of-themselves ever. In that particular respect, SR follows Galileo and Newton (no change). This is just one of many reasons SR was accepted over Lorentz's version.

So, relativistic effects only exist between PROPER CONFIGURATIONs, ie between POVs in relative motion (v>0) who each hold themselves stationary and the other in motion. Of course, it requires a relativistic rate (non everyday speeds) to measure the relativistic effects.

Best regards,

Celeritas

Edited by Celeritas

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