Lost in Langevin's language

Tim88 · September 16, 2016

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? [...] 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 there differing POVs exists (of a single shared reality), not that different realities exist. That may be what you meant?

Best regards,

Celeritas

Good point.

It's very difficult to always avoid it, for "the car is moving relatively, and hence length contracted" is similarly suggestive of multiple realities, as it is equally true that "the ground is moving relatively, and hence length contracted". To avoid that impression I had added "according to the ground frame" to "the car is length contracted". I also did not copy here above a sentence with which I see issues, but that is subject of another discussion that we are already having in another thread (and out of scope here).

By chance I now came back here for just the same reason as you. I suddenly recalled that I had forgotten to explicitly clarify the "reality" part of Michel's sentence in my last post, but it's too late to edit that so I do the edit of post #24 in this post:

[after-edit of post #24:] It is also true that according to the "ground frame" the car "lives" in what one may call a "contracted reality", in the figurative sense (such as the saying "that person is living in his own reality"). To emphasize how it is a single reality in SR, I added the last picture.

Observers of both reference systems can determine (in principle, there are serious practical issues) that "10m" of the car corresponds to "15m" of the ground frame between those events; that is a single reality, with different interpretations, as elaborated.

[edit: typo, grammar, phrasing]

PS. funny enough, but maybe not surprising, the topic of "Langevin's phrasing" turned out to be not much different from the other parallel threads on clocks and rulers, and realities.

Edited September 16, 2016 by Tim88

michel123456 · September 16, 2016

______________________

| |

| |

| | ----->

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

__________________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.

So you seem to agree that 1.5m is the measurement as taken from the Earth.

What is the mathematical operation that the earthling must do with this number in order to find the proper length?

Is it 1,50m multiplied by ....

Or

Is it 1,50m divided by ....

??

VandD · September 16, 2016

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.

It would be fair to call 'reality' everything that exists 'now' in your surrounding 3D space: all events that simultaneously occur 'now' (f.ex when I say the word "Now").

An event that does NOT occur simultaneously with "Now", is an event of a past 3D world existence or of a future 3D world existence.

For me the car hitting the tree tomorrow is not an event of my present "now" 3D reality. Hence there is no damaged tree part of my present 3D space/world existenc.

Because different observers consider a different 3D space of simultaneous events, they consider different 3D realities. But they are all only 3D sections through one and the same 4D 'reality: 4D spacetime, aka block universe.

A few Einstein quotes:

<< Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >> (Albert Einstein, "Relativity", 1952).

<< From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >> (Albert Einstein. "Relativity: The Special and the General Theory." 1916. Appendix II Minkowski's Four-Dimensional Space ("World") (supplementary to section 17 - last section of part 1 - Minkowski's Four-Dimensional Space).

<<...for us convinced physicists the distinction between past, present, and future is only an illusion, although a persistent one." >> ( Letter to Michele Besso family, March 21, 1955. Einstein Archives 7-245).

Karl Popper about his encounter with Einstein:

<< The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. He agreed that this had been his view, and while discussing it I called him "Parmenides".... >> (Karl Popper, Unended Quest: An Intellectual Autobiography.Routledge Classics. Routledge. pp.148–150).

Tim88 · September 16, 2016

[..]

Because different observers consider a different 3D space of simultaneous events, they consider different 3D realities. But they are all only 3D sections through one and the same 4D 'reality: 4D spacetime, aka block universe.

[..]

There is no need for assuming different 3D realities, as most of us are trying to explain in the thread on clocks and rulers. In the paper that is discussed here, where do you see a suggestion of multiple 3D realities?

Further, you are advocating the block universe philosophy while you could equally well argue for the 3D ether.

Which reminds me of the Rules (by chance I just had a look at them, see "Converting the Heathens"): http://www.scienceforums.net/topic/7813-science-forums-etiquette/

Such discussions probably belong in the philosophy forum - and a related topic is actually being discussed there:

http://www.scienceforums.net/topic/97105-is-space-time-a-physical-entity-or-a-mathematical-model/

VandD · September 16, 2016

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?

I hope that you are fully aware of the fact that in above exercise the ground observer does not measure the car that's in ground frame of simultaneous events. The 'proper car events' are simultaneous only in the car's frame, hence they are not in the ground frame. (But at some time one event will be part of two frames: the event at the crossing of the two frames).

The car in the ground frame is made of different events than 'proper car events'. That's the reason why in the ground frame the car is shorter, and the 1 meter between holes in car will be less than 1 meter on ground ruler.

Why worry about the 1.5 meter measurement between events that are not simultaneous in ground frame?

Tim88 · September 16, 2016

So you seem to agree that 1.5m is the measurement as taken from the Earth.

What is the mathematical operation that the earthling must do with this number in order to find the proper length?

Is it 1,50m multiplied by ....

Or

Is it 1,50m divided by ....

??

In my adaptation of bvr's example, yes, that is the measurement as taken from the Earth.

But I see no need for a mathematical operation. The physical operation that the earthling does to determine proper length, is to simply put a ruler between the balls. As the balls are not moving relative to the ruler, no timing is needed. This proper length is the measured distance between the balls on the ground.

VandD · September 16, 2016

Which reminds me of the Rules (by chance I just had a look at them, see "Converting the Heathens"): http://www.scienceforums.net/topic/7813-science-forums-etiquette/

Sorry man, I only defend what Einstein says about his own theory!

Furthermore; this is a thread about Special Relativity. "For discussion of problems relating to special and general relativity."

No some pre-Einstein Lorentz theory that some try to keep alive.

Tim88 · September 16, 2016

I hope that you are fully aware of the fact that in above exercise the ground observer does not measure the car that's in ground frame of simultaneous events. The 'proper car events' are simultaneous only in the car's frame, hence they are not in the ground frame. (But at some time one event will be part of two frames: the event at the crossing of the two frames).

The car in the ground frame is made of different events than 'proper car events'. That's the reason why in the ground frame the car is shorter, and the 1 meter between holes in car will be less than 1 meter on ground ruler.

Why worry about the 1.5 meter measurement between events that are not simultaneous in ground frame?

Indeed, as I emphasized, it's the other way round: the events are simultaneous for the car's rest frame. However, there is no "crossing of frames"; such a concept is not SR.

1.5 m is the ruler reading of the moving ground frame according to the car frame. That is a measure of the length contraction of that moving ruler according to the car; there is nothing to worry about!

[edit:] Maybe you forgot to read to the end? Here it is again:

Full situation from the car's perspective:

__________________________

| |

| c1 c2 |

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

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

Sorry man, I only defend what Einstein says about his own theory!

Furthermore; this is a thread about Special Relativity. "For discussion of problems relating to special and general relativity."

No some pre-Einstein Lorentz theory that some try to keep alive.

That is philosophical interpretation by Einstein in the 1950's (your 1916 one did not fit in there), in complete disagreement with his own interpretation around 1920 and that of Langevin in 1911 (whose paper is discussed here) as well of that of Lorentz. Einstein did not possess SR, and he did not claim that either; and it was on purpose an operational theory, free of such ideas as ether or block universe. Once more, please discuss that in the appropriate thread; block universe philosophy is not the topic here.

[edit: little addition]

Edited September 16, 2016 by Tim88

VandD · September 16, 2016

Indeed, as I emphasized, it's the other way round: the events are simultaneous for the car's rest frame. However, there is no "crossing of frames"; such a concept is not SR.

1.5 m is the ruler reading of the moving ground frame according to the car frame. That is a measure of the length contraction of that moving ruler according to the car; there is nothing to worry about!

If you mesure a length of an object, you have to measure between simultaneous extents of that object in the ground frame.

If I ask you to measure a moving train you have to mark off the rear and front of the train simultaneously on your ruler on the ground. Ever thought about that? Or doesn't it make sense to you?

Similar for measurement of the ground ruler done with a car ruler: the car passenger has to measure between two simultaneous mark-off events of the ground ruler moving by.

Tim88 · September 16, 2016

If you mesure a length of an object, you have to measure between simultaneous extents of that object in the ground frame.

If I ask you to measure a moving train you have to mark off the rear and front of the train simultaneously on your ruler on the ground. Ever thought about that? Or doesn't it make sense to you?

Similar for measurement of the ground ruler done with a car ruler: the car passenger has to measure between two simultaneous mark-off events of the ground ruler moving by.

One last time, hopefully it will be clear to you one day:

__________________________

| |

| c1 c2 |

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

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

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them. This statement contains, as a particular case, what is called the Lorentz contraction

Edited September 16, 2016 by Tim88

VandD · September 16, 2016

That is philosophical interpretation by Einstein in the 1950's (your 1916 one did not fit in there), in complete disagreement with his own interpretation around 1920 and that of Langevin in 1911 (whose paper is discussed here) as well of that of Lorentz. Einstein did not possess SR, and he did not claim that either; and it was on purpose an operational theory, free of such ideas as ether or block universe. Once more, please discuss that in the appropriate thread; block universe philosophy is not the topic here.

[edit: little addition]

In that case please lock the thread "Relativity and shared realities".

One last time, hopefully it will be clear to you one day:

__________________________

| |

| |

| c1 c2 |

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

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

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them. This statement contains, as a particular case, what is called the Lorentz contraction

I like the "as a particular case" :rolleyes:

Tim88 · September 17, 2016

[..]
I like the "as a particular case"

Borrowing from bvr, I sketched for Michel that "particular case" of Langevin's general statement.

PS I had already done so in post #7, but apparently an illustration with a car and dropping balls on the ground is more appealing than two rulers with lasers. And balls are easier than lasers for illustrating the reciprocity thanks to relativity of simultaneity.

For Michel I'll refine the explanation by including the detailed view that I presented in post #7 to the last picture in the overview of post #24, and by adding the elements of the car and ground illustration in all applicable sentences by Langevin:

__________________________

| |

| c1 c2 |

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

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

<-- v (from the view that the car is in rest, the ground is moving to the left)

Here's a "zoom in" on those rulers, according to the car's inertial rest frame:

car [cm] 00 10 20 30 40 50 60 70 80 90 100

I I simultaneous positions "in" the car

ground ["cm"] 00 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

According to the determination with the "car frame", the ruler on the "moving" ground is length contracted by a factor 1.5.

A 150 cm long ruler on the ground is contracted to 100 cm, according to observers who take the view that the car is in rest.

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them:

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

Simultaneous means that the time between the events dt=0

=> dx² + dy² + dz² is maximal for the car observers in this example.

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

We have already seen that the length of the ground ruler for observers who see it passing by, is defined by the distance in space of two simultaneous positions (for those car observers) on both ends of the ground ruler. According to the preceding this distance will be shorter for those car observers than for all others, especially those attached to the ground ruler.

We also easily understand (from the illustrative pictures in post #24) 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.

[edit: added the space-time equation]

Edited September 17, 2016 by Tim88

Celeritas · September 17, 2016

It would be fair to call 'reality' everything that exists 'now' in your surrounding 3D space: all events that simultaneously occur 'now' (f.ex when I say the word "Now").

An event that does NOT occur simultaneously with "Now", is an event of a past 3D world existence or of a future 3D world existence.

For me the car hitting the tree tomorrow is not an event of my present "now" 3D reality. Hence there is no damaged tree part of my present 3D space/world existenc.

Because different observers consider a different 3D space of simultaneous events, they consider different 3D realities. But they are all only 3D sections through one and the same 4D 'reality: 4D spacetime, aka block universe.

Hi VandD,

Good point. We cannot predict the future, agreed. Also, we only ever experience the NOW. However, the required time-desynchronization of moving bodies cannot be explained unless all moments in time co-exist. To suggest such a continuum is not a part of reality, is IMO to suggest that relativistic effects are not a part of reality. The moving length-contracted train is comprised of different eras of the train's own proper-time, yet they must exist in the stationary observer's NOW per the theory. As you know, it is not optical effect. I do understand the desire to define reality based upon what is directly measurable in the current ever changing moment, because we only ever perceive an ever changing moment. However, the time-desynchronization of moving bodies suggests that NOW is not all that exists in the continuum, but rather that all NOWs co-exist. The fact that one never lives in anything but the present, does not negate the required existence of time-desynchronization of moving bodies ...

Consider a tree in your backyard. It was there for (say) 30 years, but yesterday it was cut down. Is that tree part of reality? You may say "not today it isn't", at least in the form it existed yesterday. Yet if I move in such a way and fast enough, your tree (just before) being cut down exists AND you yourself pondering its existence the next day in your own living room also exists, all in my own NOW. Does your tree in reality exist? I'm only suggesting that reality may be more than we might casually tend to figure.

We cannot measure the Planck length directly, yet its use in physics has been with astounding success. One may say that time-desynchronization of moving bodies is to relativity, as the Planck length is to quantum mechanics. Each are deducible and validated from repeatable test results that prove other aspects of the theory (or physics). Prove length-contraction and time dilation by consistent measurements of various repeatable experiments, then logical deduction leads that time-desynchronization of moving bodies "exists in nature".

I agree these things are best left to philosophical discussions. It comes down to what we accept as true. We define truth per consistent repeatable measurement and consistently predictable outcome. However if the math requires something to exist that is not directly measurable, then if the physics (or accepted theory) that involves that math is otherwise proven by repeatable measurements, then I would personally vote that that something be considered real and a part of single shared reality per many POVs of it.

Tomorrow, I may change my vote

Best regards,

Celeritas

Edited September 17, 2016 by Celeritas

Tim88 · September 17, 2016

[..] we only ever experience the NOW. However, the required time-desynchronization of moving bodies cannot be explained unless all moments in time co-exist. To suggest such a continuum is not a part of reality, is IMO to suggest that relativistic effects are not a part of reality.

[..]

I agree these things are best left to philosophical discussions. It comes down to what we accept as true. We define truth per consistent repeatable measurement and consistently predictable outcome. However if the math requires something to exist that is not directly measurable, then if the physics (or accepted theory) that involves that math is otherwise proven by repeatable measurements, then I would personally vote that that something be considered real and a part of single shared reality per many POVs of it.

Tomorrow, I may change my vote

Best regards,

Celeritas

I disagree with the first, and totally agree with the second statement above. It's an interesting topic. it will be fun to discuss such things as how our "conscious now" functions according to our interpretations of SR in the philosophy forum.

Edited September 17, 2016 by Tim88

michel123456 · September 17, 2016

Borrowing from bvr, I sketched for Michel that "particular case" of Langevin's general statement.

PS I had already done so in post #7, but apparently an illustration with a car and dropping balls on the ground is more appealing than two rulers with lasers. And balls are easier than lasers for illustrating the reciprocity thanks to relativity of simultaneity.

For Michel I'll refine the explanation by including the detailed view that I presented in post #7 to the last picture in the overview of post #24, and by adding the elements of the car and ground illustration in all applicable sentences by Langevin:

__________________________

| |

| |

| c1 c2 |

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

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

<-- v (from the view that the car is in rest, the ground is moving to the left)

Here's a "zoom in" on those rulers, according to the car's inertial rest frame:

car [cm] 00 10 20 30 40 50 60 70 80 90 100

I I simultaneous positions "in" the car

ground ["cm"] 00 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

According to the determination with the "car frame", the ruler on the "moving" ground is length contracted by a factor 1.5.

A 150 cm long ruler on the ground is contracted to 100 cm, according to observers who take the view that the car is in rest.

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them:

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

Simultaneous means that the time between the events dt=0

=> dx² + dy² + dz² is maximal for the car observers in this example.

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

We have already seen that the length of the ground ruler for observers who see it passing by, is defined by the distance in space of two simultaneous positions (for those car observers) on both ends of the ground ruler. According to the preceding this distance will be shorter for those car observers than for all others, especially those attached to the ground ruler.

We also easily understand (from the illustrative pictures in post #24) 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.

[edit: added the space-time equation]

OK I understand that from the car's frame, the ground would appear contracted but

please clarify.

From the car's frame, what is the measurement?

Is it 1m or 1,50m?

i mean, the input in the Lorentz equation that will determine the proper length of the moving object ( the ground), is it 1m or 1,50m?

I ask this because I really don't understand how one takes the measurement and multiplies it.

Tim88 · September 17, 2016

OK I understand that from the car's frame, the ground would appear contracted but

please clarify.

From the car's frame, what is the measurement?

Is it 1m or 1,50m?

i mean, the input in the Lorentz equation that will determine the proper length of the moving object ( the ground), is it 1m or 1,50m?

I ask this because I really don't understand how one takes the measurement and multiplies it.

I see that the fonts are different, so that in your post on my screen the length contraction factor in the sketch is slightly different. :huh:

Anyway, the car's frame measurement is as follows: The car's observers measure distances on the ground. The "car's" measurement is that the distance between the two balls on the ground is 1m.

Celeritas elaborated on the Lorentz transformations in post #14 and I think that he explained it nicely. Be aware that L_p there corresponds to the proper length of the "rest system" S - which is this moving length measurement the car.

But after loosing time on this, I'm afraid that something went wrong :eek: and he messed up the equations, as

x’_sep = γ(L_p-v(t'₂-t'₁))

is, I suppose, short for:

_{x’₂- x’₁ = γ ((x₂- x₁) -}_{v (t'₂ - t'₁))}

_{which corresponds to the subtraction of two transformations of the type:}

_{x’ = γ (x -}_vt')

But the Lorentz transformation in the direction of motion is:

_{x’ = γ (x -}_vt)

Thus, concretely:

x₂- x_{1 = 1 m}

x’₂- x’₁ = 1.5 m

γ = 1.5

1.5 = 1.5*1 - v*<time difference between the events according to clocks in the car>

Celeritas · September 17, 2016

Celeritas elaborated on the Lorentz transformations in post #14 and I think that he explained it nicely. Be aware that L_p there corresponds to the proper length of the "rest system" S - which is this moving length measurement the car.

But after loosing time on this, I'm afraid that something went wrong and he messed up the equations, as

x’_sep = γ(L_p-v(t'₂-t'₁))

is, I suppose, short for:

_{x’₂- x’₁ = γ ((x₂- x₁) -}_{v (t'₂ - t'₁))}

Indeed, that was quite the blunder on my behalf. I had started with improper convention, with primes on the right and unprimed on the left. Had just started to make the changes, got sidetracked, forgot to complete it and posted. Thanx for pointing that out. It's such a mess, I'll repost it for michel123456 below ...

Best regards,

Celeritas

EDIT: Below is a re-post of my Post 14 for michel123456, with corrections as per Tim88

Primed variables should only have existed on the left side of the LTs shown (S' system).

******************************************************************************************************

[ 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.

This thread is about Langevin's The Evolution of Space and Time

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 (L_p) of a stationary ruler corresponds to a contracted-length (L_p/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' = L_p/γ) must traverse a distance (x’_sep = 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) + (L_p/γ). And it is always the case that x'_sep > L_p for v > 0 , as follows ...

x’ = γ(x-vt)

x’_sep = γ(L_p-v(t₂-t₁))

for endpoint measurements taken simultaneously (t₂= t₁, and so t₂-t₁ = 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(L_p-v*0)

x’_sep = γL_p

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

L_p ... 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 ruler.

Best regards,

Celeritas

Edited September 17, 2016 by Celeritas

VandD · September 18, 2016

About events, proper length, length contration, Lorentz Transformation, ...

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

Tim88 · September 24, 2016

Michael, it's unclear if you stopped replying to answers on your own question because:

1. you now understand it and simply forgot to tell us

2. you still don't understand it and gave up on it

3. you need more time to ponder over some of the answers, before you'll be back to this thread

Which is it?

michel123456 · September 24, 2016

3

And 2.

The whole thread slipped away.

It has gone into explaining Relativity again.

The aim of this thread was to show how much words can be confusing.

To speak frankly, I think that 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."

is not compatible with your explanations.

Which means either

1. it is a small mistake from Langevin (that can happen especially if it is a speech)

2. all your explanations are inaccurate (that is highly unlikely)

3. I don't interpret correctly the wording.

I have chosen 3 and opened this thread.

Edited September 24, 2016 by michel123456

Tim88 · September 24, 2016

3

And 2.

The whole thread slipped away.

It has gone into explaining Relativity again.

The aim of this thread was to show how much words can be confusing.

To speak frankly, I think that 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."

is not compatible with your explanations.

Which means either

1. it is a small mistake from Langevin (that can happen especially if it is a speech)

2. all your explanations are inaccurate (that is highly unlikely)

3. I don't interpret correctly the wording.

I have chosen 3 and opened this thread.

In post #37 I applied that sentence to the well illustrated example with the car.

In that example, the car observers assume to be in rest (as observers in SR commonly do) so that the ground is moving fast under them. They drop balls on the ground, and the distance between the balls is constant from the time that they both hit the ground.

In my variant of that example, the car observers measure the distance between the two balls on the ground as 1 m; and they think that these events are simultaneous.

In contrast, the ground observers think that the balls land one after the other, and they measure the distance between the balls on the ground as 1.5m.

As 1m is shorter than 1.5m, I agreed with Langevin:

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them.

michel123456 · September 25, 2016

In post #37 I applied that sentence to the well illustrated example with the car.

In that example, the car observers assume to be in rest (as observers in SR commonly do) so that the ground is moving fast under them. They drop balls on the ground, and the distance between the balls is constant from the time that they both hit the ground.

In my variant of that example, the car observers measure the distance between the two balls on the ground as 1 m; and they think that these events are simultaneous.

In contrast, the ground observers think that the balls land one after the other, and they measure the distance between the balls on the ground as 1.5m.

As 1m is shorter than 1.5m, I agreed with Langevin:

The spatial distance of two events that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them.

Obviously your goal is to drive me crazy.

If the ground observer measures the distance 1,50m, it means that he observes 1,50m.

IOW he does not observe contraction. As in some previous post (from VanDD if i remember well) if the ground observer could catch the ball in his hands, he would have to extend his hands to 1,50m: IOW the measurement from the ground is LARGER than the proper length. And the calculation consists into taking a large measurement (1,50m), apply the Lorentz contraction formula and find the contracted result=1m=proper length.

IOW the observer does NOT observe the object contracted. He is observing the object longer.

And then I agree with Langevin's statement.

Or tell me again that I understand nothing.

Edited September 25, 2016 by michel123456

VandD · September 25, 2016

As in some previous post (from VanDD if i remember well) if the ground observer could catch the ball in his hands, he would have to extend his hands to 1,50m

Correct, but the ground observer does not feel the two balls simultaneously dropping from the car.

One hand will feel one ball falling. Then he has to wait some time, and then he will feel the other ball falling.

For the length contraction of the car the scenario is different (see my previous sketches). The green observer will feel simultaneously front and rear of shorter car. Measuring a shorter car is between different events than Langevin's scenario.

Below I add a sketch for the Langevin scenario.

IOW the observer does NOT observe the object contracted. He is observing the object longer.

In Langevin's scenario the ground observer doesn't observe the car as it is at one moment in time in his ground frame.

Langevin's ground observer does a wrong measurement to measure/observe the car present in his ground frame of simultaneously occuring events.

Tim88 · September 25, 2016

Obviously your goal is to drive me crazy.

If the ground observer measures the distance 1,50m, it means that he observes 1,50m.

IOW he does not observe contraction. As in some previous post (from VanDD if i remember well) if the ground observer could catch the ball in his hands, he would have to extend his hands to 1,50m: IOW the measurement from the ground is LARGER than the proper length. And the calculation consists into taking a large measurement (1,50m), apply the Lorentz contraction formula and find the contracted result=1m=proper length.

IOW the observer does NOT observe the object contracted. He is observing the object longer.

And then I agree with Langevin's statement.

Or tell me again that I understand nothing.

Proper length is not a label that is reserved for one observer, excluding other observers; each observer has his own proper lengths. Possibly you misunderstood "proper" - that would explain a lot of the confusion!

And probably additional confusion occurred because the example in the other thread about observing the length contraction of a moving car involves the inverse measurement of the example using Langevin's car with dropping balls, since the corresponding length contraction measurement is here the contraction of the distance on the ground.

I'll retake your above statements and add precisions in bold; if you agree with the result, then you probably understand the meaning of that sentence now:

If the ground observer measures the distance 1,50m, it means that he observes 1,50m; that's his proper distance.

IOW he does not observe contraction of his own ruler that is in rest. As in some previous post (from VanDD if i remember well) if the ground observer could catch the balls in his hands, he would have to extend his hands to 1,50m: OW the measurement from the ground (the proper distance on the ground) is LARGER than ~~the proper length~~ that distance according to the car, as determined with the ruler in the car. And the calculation consists into taking a large measurement (1,50m), apply the ~~Lorentz contraction~~ space-time formula and find the contracted result=1m=proper length of the ruler in the car.

IOW the observer on the ground does NOT observe the distance on the ground contracted. He is observing that distance longer than according to the car.

And then I agree with Langevin's statement.

post-19758-0-16717300-1473953897_thumb.j

I will drive you all crazy, I know that.

Screen Shot 09-15-16 at 06.33 PM.JPG

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?

I forgot to reply on this one, and I think nobody did. As I mentioned earlier, in this drawing is a glitch, but it's not essential for your question.

Yes indeed:

* Dground is the measurement of the ground taken in the FOR of the ground.

* Dcar is the measurement of the moving ground taken in the FOR of the car.

* Dground is longer than Dcar.

* Dcar is shorter than Dground

Edited September 25, 2016 by Tim88

michel123456 · September 25, 2016

Dear VandD & Tim, your answers do not coincide.

If I have to follow VandD then it goes like this:

* Dground is the measurement of the ground taken in the FOR of the ~~ground~~.car

* Dcar is the measurement of the ~~moving ground~~ car taken in the FOR of the car.

* Dground is longer than Dcar.

* Dcar is shorter than Dground

Always considering that a measurement must include simultaneity, otherwise it gives a false result.

IOW the car observer measures that 1 meter in its own frame corresponds to 1,50m on the ground. It means that his measurement of a 1,50m placed on the ground is 1 meter (the measurement is 1 meter). He is observing an object of 1,50m contracted to 1 meter.

And then Langevin's statement gets comprehensible to me with the following addition (sorry for that)

The spatial distance of two events [that happen on the ground]* that are simultaneous for the car observers, is shorter for them than for ground observers in motion relative to them.

* [that happen on the ground] added by me

Please confirm.

Edited September 25, 2016 by michel123456

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