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Jul 28, 2022, 6:07:50 AMJul 28

to

In my animation

https://www.geogebra.org/m/veezhbrr

there is a train moving at relativistic speed and there are two photons

leaving at the same time from the center of the wagon towards points A

and B where the ends of the dilated spring are fixed.

When the photons reach A and B, they release the mechanism that holds

the ends in place, so that the spring (no longer fixed) can contract.

However, in the reference of the train, the two photons arrive at their

destination at the same time and the (released) spring compresses

symmetrically, remaining in the center of the wagon.

But, in the ground reference, one photon arrives before the other and

the spring contracts asymmetrically, so that it does not stay in the

center of the wagon but moves to the side.

Since the spring cannot contract in two different ways, one of the two

contractions must be wrong: which of the two is correct and which is

wrong?

https://www.geogebra.org/m/veezhbrr

there is a train moving at relativistic speed and there are two photons

leaving at the same time from the center of the wagon towards points A

and B where the ends of the dilated spring are fixed.

When the photons reach A and B, they release the mechanism that holds

the ends in place, so that the spring (no longer fixed) can contract.

However, in the reference of the train, the two photons arrive at their

destination at the same time and the (released) spring compresses

symmetrically, remaining in the center of the wagon.

But, in the ground reference, one photon arrives before the other and

the spring contracts asymmetrically, so that it does not stay in the

center of the wagon but moves to the side.

Since the spring cannot contract in two different ways, one of the two

contractions must be wrong: which of the two is correct and which is

wrong?

Jul 28, 2022, 10:53:43 AMJul 28

to

understand the issue with simultaneity in special relativity.

BTW, one aspect of your simulation that is incorrect is that you are

showing the two springs contracting uniformly (i.e. the same

simultaneously along their length). What would really happen is

a wave of compression that starts at the end that is released, and

propagates at a speed much slower than the speed of light towards

the anchored end. But the fact that the springs are released at

different "times" in different frames is absolutely correct and

pretty well proven by experiment.

RIch L.

Jul 29, 2022, 7:14:57 AMJul 29

to

Richard Livingston alle ore 16:53:38 di giovedì 28/07/2022 ha scritto:

> On Thursday, July 28, 2022 at 5:07:50 AM UTC-5, Luigi Fortunati wrote:

>> In my animation https://www.geogebra.org/m/veezhbrr there is a train moving at relativistic speed and there are two photons leaving at the same time from the center of the wagon towards points A and B where the ends of the dilated spring are fixed.

>> When the photons reach A and B, they release the mechanism that holds the ends in place, so that the spring (no longer fixed) can contract.

>> However, in the reference of the train, the two photons arrive at their destination at the same time and the (released) spring compresses symmetrically, remaining in the center of the wagon.

>> But, in the ground reference, one photon arrives before the other and the spring contracts asymmetrically, so that it does not stay in the center of the wagon but moves to the side.

>> Since the spring cannot contract in two different ways, one of the two contractions must be wrong: which of the two is correct and which is wrong?

>

> They are both correct...
> On Thursday, July 28, 2022 at 5:07:50 AM UTC-5, Luigi Fortunati wrote:

>> In my animation https://www.geogebra.org/m/veezhbrr there is a train moving at relativistic speed and there are two photons leaving at the same time from the center of the wagon towards points A and B where the ends of the dilated spring are fixed.

>> When the photons reach A and B, they release the mechanism that holds the ends in place, so that the spring (no longer fixed) can contract.

>> However, in the reference of the train, the two photons arrive at their destination at the same time and the (released) spring compresses symmetrically, remaining in the center of the wagon.

>> But, in the ground reference, one photon arrives before the other and the spring contracts asymmetrically, so that it does not stay in the center of the wagon but moves to the side.

>> Since the spring cannot contract in two different ways, one of the two contractions must be wrong: which of the two is correct and which is wrong?

>

Impossible!

If the spring remains in the center of the wagon it does not move to the

left and if it moves to the left it does not remain in the center of the

wagon: one condition excludes the other.

Luigi Fortunati

Jul 30, 2022, 7:13:27 AM (13 days ago) Jul 30

to

is not what happens: but indeed the problem to begin with is that you

keep drawing "(incorrect) animations", not space-time diagrams...

Julio

Aug 2, 2022, 2:42:05 AM (11 days ago) Aug 2

to

need a spring, you could simply bounce light rays off the front and rear

walls (or even massive particles, with ideal bouncing), which is all 1-D

by disregarding transversal distances, and it is enough to see how the

light rays come back together, i.e. at the center of the wagon, whichever

the frame!

On that line, here is a little space-time diagram I have put together

with Desmos: <https://www.desmos.com/calculator/mngma52fol>

There are limitations to what can be done in Desmos: I had to use

coords of the form (x,t) and in most places t becomes y, plus I am

doing the inverse transformation, hence (-v) in some places: in fact,

to the point, **with Lorentz transformations I am going from what

happens in the frame of the wagon (represented by the 4 events

C,L,R,D), to what appears in the external frame** (which, if relativity

means what it means, is a/the valid procedure here).

It is then obvious by the diagram that, to the ground observer, the

bouncing of the light rays is (in general) not simultaneous, yet the

light rays must indeed rejoin at the center of the wagon whichever

the relative frame speed.

HTH,

Julio

Aug 2, 2022, 8:00:45 PM (10 days ago) Aug 2

to

Julio Di Egidio alle ore 08:42:01 di marted=EC 02/08/2022 ha scritto:

> I think you also tend to overcomplicate your setups: e.g. here you don'=

t

> need a spring, you could simply bounce light rays off the front and rea=

r

> walls (or even massive particles, with ideal bouncing), which is all 1-=

questions to ask.

But the theory must also be valid with springs and not only with light

rays.

I updated my animation and added the spring drop all the way to the

floor:

<https://www.geogebra.org/m/mejqfmrf>

In the reference of the train, the fall is without inclinations and

without lateral displacements, neither to the right nor to the left:

the spring always remains in the center of the wagon.

In the ground reference, the spring tilts and does not stay in the

center of the wagon.

One condition excludes the other and, therefore, one of the two must be

wrong: which of the two?

[[Mod. note -- As others have noted, both of these "conditions" are

correct; there is no contradiction between them.

To understand how they can both be correct, it's useful to ask how

one could distinguish one condition from the other *observationally*.

That is, how could you *measure* whether whether the spring is or isn't

tilted? Presumably you'd need to measure the heights of the spring's

two ends and compare them. But the spring is falling, so you need

to measure the heights of the two ends at the same time. And that's

where the problem appears -- what does the phrase "at the same time"

mean in special relativity? Your apparent paradox is due to the fact

that the phrase "at the same time" does *not* have the same meaning for

different observers.

Similarly, how could you *measure* whether one end of the spring

hits the floor before the other end of the spring hits the floor?

You could, for example, have an inertial observer measure the time

when each end of the spring hits the floor, then compare those times.

But this leaves open the question of *which* inertial observer should

make these measurements? Again, your apparent paradox reflects the

fact that different inertial observers will in general disagree about

the relative times of spatially-separated events.

These issues aren't straightforward, and benefit a lot from more

carefully-thought-out and lengthly presentations than are possible

in a newssgroup discussion. I highly recommend studying a good book

or two on special relativity. My two personal favorites are:

@book {

author = "Edwin F. Taylor and John Archibald Wheeler",

title = "Spacetime Physics",

edition = "2nd",

publisher = "W. H. Freeman",

year = 1992,

isbn = "0-7167-2326-3 (hardcover) 0-7167-2327-1 (paperback)",

note = "free download at https://www.eftaylor.com/spacetimephysics/"

}

@book {

author = "N. David Mermin",

title = "Space and Time in Special Relativity",

publisher = "Waveland Press",

X-publisher-original-edition = "McGraw-Hill (1968)",

address = "Prospect Heights, Illinois, USA",

year = "1968, 1989",

isbn = "0-88133-420-0 (paper)",

}

-- jt]]

> I think you also tend to overcomplicate your setups: e.g. here you don'=

t

> need a spring, you could simply bounce light rays off the front and rea=

r

> walls (or even massive particles, with ideal bouncing), which is all 1-=

D

> by disregarding transversal distances, and it is enough to see how the

> light rays come back together, i.e. at the center of the wagon, whichev=
> by disregarding transversal distances, and it is enough to see how the

er

> the frame!

>

> On that line, here is a little space-time diagram I have put together

> with Desmos: <https://www.desmos.com/calculator/mngma52fol>

> There are limitations to what can be done in Desmos: I had to use

> coords of the form (x,t) and in most places t becomes y, plus I am

> doing the inverse transformation, hence (-v) in some places: in fact,

> to the point, **with Lorentz transformations I am going from what

> happens in the frame of the wagon (represented by the 4 events

> C,L,R,D), to what appears in the external frame** (which, if relativity

> means what it means, is a/the valid procedure here).

>

> It is then obvious by the diagram that, to the ground observer, the

> bouncing of the light rays is (in general) not simultaneous, yet the

> light rays must indeed rejoin at the center of the wagon whichever

> the relative frame speed.

With the light everything is normal, linear and correct, so I have no
> the frame!

>

> On that line, here is a little space-time diagram I have put together

> with Desmos: <https://www.desmos.com/calculator/mngma52fol>

> There are limitations to what can be done in Desmos: I had to use

> coords of the form (x,t) and in most places t becomes y, plus I am

> doing the inverse transformation, hence (-v) in some places: in fact,

> to the point, **with Lorentz transformations I am going from what

> happens in the frame of the wagon (represented by the 4 events

> C,L,R,D), to what appears in the external frame** (which, if relativity

> means what it means, is a/the valid procedure here).

>

> It is then obvious by the diagram that, to the ground observer, the

> bouncing of the light rays is (in general) not simultaneous, yet the

> light rays must indeed rejoin at the center of the wagon whichever

> the relative frame speed.

questions to ask.

But the theory must also be valid with springs and not only with light

rays.

I updated my animation and added the spring drop all the way to the

floor:

<https://www.geogebra.org/m/mejqfmrf>

In the reference of the train, the fall is without inclinations and

without lateral displacements, neither to the right nor to the left:

the spring always remains in the center of the wagon.

In the ground reference, the spring tilts and does not stay in the

center of the wagon.

One condition excludes the other and, therefore, one of the two must be

wrong: which of the two?

[[Mod. note -- As others have noted, both of these "conditions" are

correct; there is no contradiction between them.

To understand how they can both be correct, it's useful to ask how

one could distinguish one condition from the other *observationally*.

That is, how could you *measure* whether whether the spring is or isn't

tilted? Presumably you'd need to measure the heights of the spring's

two ends and compare them. But the spring is falling, so you need

to measure the heights of the two ends at the same time. And that's

where the problem appears -- what does the phrase "at the same time"

mean in special relativity? Your apparent paradox is due to the fact

that the phrase "at the same time" does *not* have the same meaning for

different observers.

Similarly, how could you *measure* whether one end of the spring

hits the floor before the other end of the spring hits the floor?

You could, for example, have an inertial observer measure the time

when each end of the spring hits the floor, then compare those times.

But this leaves open the question of *which* inertial observer should

make these measurements? Again, your apparent paradox reflects the

fact that different inertial observers will in general disagree about

the relative times of spatially-separated events.

These issues aren't straightforward, and benefit a lot from more

carefully-thought-out and lengthly presentations than are possible

in a newssgroup discussion. I highly recommend studying a good book

or two on special relativity. My two personal favorites are:

@book {

author = "Edwin F. Taylor and John Archibald Wheeler",

title = "Spacetime Physics",

edition = "2nd",

publisher = "W. H. Freeman",

year = 1992,

isbn = "0-7167-2326-3 (hardcover) 0-7167-2327-1 (paperback)",

note = "free download at https://www.eftaylor.com/spacetimephysics/"

}

@book {

author = "N. David Mermin",

title = "Space and Time in Special Relativity",

publisher = "Waveland Press",

X-publisher-original-edition = "McGraw-Hill (1968)",

address = "Prospect Heights, Illinois, USA",

year = "1968, 1989",

isbn = "0-88133-420-0 (paper)",

}

-- jt]]

Aug 3, 2022, 1:02:10 PM (9 days ago) Aug 3

to

Luigi Fortunati alle ore 12:00:40 di martedě 02/08/2022 ha scritto:

> [[Mod. note --

> ...

If the theory tells me that the two extremities are released

simultaneously, I obviously deduce that (falling) it does not tilt.

If he tells me that one end is released before the other, I equally

obviously deduce that (falling) it tilts.

[[Mod. note -- What does the word "simultaneously" mean? In special

relativity simultaneity is observer-dependent, i.e., different observers

will in general not agree on whether two (spatially-separated) events

are simultaneous. There's no universal notion of "simultaneous".

In the same way, whether or not the spring tilts is observer-dependent;

there's no universal notion of tilt.

Your two "conditions" are each internally consistent and correct.

There's no contradiction between them; they're simply different ways

of describing the same events.

-- jt]]

> [[Mod. note --

> ...

> To understand how they can both be correct, it's useful to ask how

> one could distinguish one condition from the other *observationally*.

>

> That is, how could you *measure* whether whether the spring is or isn't

> tilted?

It is the theory itself that tells me if the spring tilts or not.
> one could distinguish one condition from the other *observationally*.

>

> That is, how could you *measure* whether whether the spring is or isn't

> tilted?

If the theory tells me that the two extremities are released

simultaneously, I obviously deduce that (falling) it does not tilt.

If he tells me that one end is released before the other, I equally

obviously deduce that (falling) it tilts.

[[Mod. note -- What does the word "simultaneously" mean? In special

relativity simultaneity is observer-dependent, i.e., different observers

will in general not agree on whether two (spatially-separated) events

are simultaneous. There's no universal notion of "simultaneous".

In the same way, whether or not the spring tilts is observer-dependent;

there's no universal notion of tilt.

Your two "conditions" are each internally consistent and correct.

There's no contradiction between them; they're simply different ways

of describing the same events.

-- jt]]

Aug 3, 2022, 5:44:27 PM (9 days ago) Aug 3

to

Luigi Fortunati alle ore 05:02:06 di mercoledě 03/08/2022 ha scritto:

> [[Mod. note -- What does the word "simultaneously" mean? In special

> relativity simultaneity is observer-dependent, i.e., different observers

> will in general not agree on whether two (spatially-separated) events

> are simultaneous. There's no universal notion of "simultaneous".

>

> In the same way, whether or not the spring tilts is observer-dependent;

> there's no universal notion of tilt.

The tilt with respect to the floor of the wagon does not vary as the
> [[Mod. note -- What does the word "simultaneously" mean? In special

> relativity simultaneity is observer-dependent, i.e., different observers

> will in general not agree on whether two (spatially-separated) events

> are simultaneous. There's no universal notion of "simultaneous".

>

> In the same way, whether or not the spring tilts is observer-dependent;

> there's no universal notion of tilt.

observer changes!

[[Mod. note -- The whole point is that there's no generic

observer-independent "tilt with respect to the floor of the wagon".

Rather, different observers measure different tilts with respect to the

floor of the wagon.

If you disagree, please describe a way to (correctly) measure the tilt

which doesn't give different answers for different observers.

[For example, suppose we mount a (level) protractor

on the wagon and try to read the spring's tilt on the

protractor scale. We immediately run into the problem

that the spring is falling, so we need to read the two

sides of the protractor at the same time.... but different

observers disagree about "the same time".]

The underlying logic of your apparent paradox (and the resolution that

"tilt" is observer-dependent) is very similar to that of the well-known

"stick and hole" apparent paradox, e.g., see sections 5 and 6 of

https://en.wikipedia.org/wiki/Ladder_paradox

or

http://www.relativitysimulation.com/Tutorials/TutorialMeterstickAndHole.html

https://physics.stackexchange.com/questions/83520/a-relativistic-meter-stick-and-a-thin-disk

https://www.physicsforums.com/threads/meter-stick-slides-over-a-meter-wide-hole-at-a-high-speed.945765/

-- jt]]

Aug 3, 2022, 5:48:59 PM (9 days ago) Aug 3

to

On Wednesday, 3 August 2022 at 02:00:45 UTC+2, Luigi Fortunati wrote:

> Julio Di Egidio alle ore 08:42:01 di marted=EC 02/08/2022 ha scritto:

<snip>
> Julio Di Egidio alle ore 08:42:01 di marted=EC 02/08/2022 ha scritto:

> > It is then obvious by the diagram that, to the ground observer, the

> > bouncing of the light rays is (in general) not simultaneous, yet the

> > light rays must indeed rejoin at the center of the wagon whichever

> > the relative frame speed.

>

> With the light everything is normal, linear and correct, so I have no

> questions to ask.

>

> But the theory must also be valid with springs and not only with light

> rays.

Indeed it is, because the basic experiment I have reduced it to is
> > bouncing of the light rays is (in general) not simultaneous, yet the

> > light rays must indeed rejoin at the center of the wagon whichever

> > the relative frame speed.

>

> With the light everything is normal, linear and correct, so I have no

> questions to ask.

>

> But the theory must also be valid with springs and not only with light

> rays.

sufficient to see that, *whatever happens* (there is something magic

about light, e.g. it sets the boundary for any exchange of classical

information, but exactly the same outcome you'd have with massive

particles, or even springs and combinations thereof...), as long as

what happens on the left side is exactly symmetric to what happens

on the right side, you are guaranteed a rendez-vous at the center *in

any frame*. Which is about what is essential and what is unneeded

complication in an ideal/thought experiment.

But overall, you even seem to miss fundamental notions like *event*

and what it means, i.e. what are we actually modelling, so I'd second

the moderator's suggestion that you at least go through some good

introductory books, and try and follow *that* progression: including

how to rather draw space-time diagrams (only once you got those

you can confidently build simulations...).

Julio

Aug 4, 2022, 9:08:29 AM (8 days ago) Aug 4

to

Luigi Fortunati alle ore 09:44:22 di mercoledì 03/08/2022 ha scritto:

> [[Mod. note -- For example, suppose we mount a (level) protractor

> on the wagon...

Done.

In my animation

<https://www.geogebra.org/m/zyarm93v>

I added the protractor and also a stop and go to be able to stop the animation at any time.

> [Mod. note -- What does the word "simultaneously" mean? In special

>> relativity simultaneity is observer-dependent, i.e., different observers

>> will in general not agree on whether two (spatially-separated) events

>> are simultaneous. There's no universal notion of "simultaneous".

This is exactly what happens in my animation.

The arrival of the two photons (and the release of points A and B) is simultaneous in the wagon reference but not in the ground reference.

> [Mod. note -- The whole point is that there's no generic

> observer-independent "tilt with respect to the floor of the wagon".

> Rather, different observers measure different tilts with respect to the

> floor of the wagon.

> If you disagree, please describe a way to (correctly) measure the tilt

I agree, so much so that in my animation (which respects the criteria of

Relativity) the inclination in the reference of the wagon is different

from that in the reference of the terrain.

But what if there is an explosive bottle on the plane that explodes if

it slips (and bangs) but doesn't explode if it doesn't slip (and doesn't

slam)?

It happens that it does not explode for the observer for the observer on

the train (for which the plane does not tilt and the bottle does not

slip) but it explodes for the observer on the ground (for which the

plane tilts and the bottle slips).

And this is not acceptable.

> [[Mod. note -- For example, suppose we mount a (level) protractor

> on the wagon...

Done.

In my animation

<https://www.geogebra.org/m/zyarm93v>

I added the protractor and also a stop and go to be able to stop the animation at any time.

> [Mod. note -- What does the word "simultaneously" mean? In special

>> relativity simultaneity is observer-dependent, i.e., different observers

>> will in general not agree on whether two (spatially-separated) events

>> are simultaneous. There's no universal notion of "simultaneous".

The arrival of the two photons (and the release of points A and B) is simultaneous in the wagon reference but not in the ground reference.

> [Mod. note -- The whole point is that there's no generic

> observer-independent "tilt with respect to the floor of the wagon".

> Rather, different observers measure different tilts with respect to the

> floor of the wagon.

> If you disagree, please describe a way to (correctly) measure the tilt

Relativity) the inclination in the reference of the wagon is different

from that in the reference of the terrain.

But what if there is an explosive bottle on the plane that explodes if

it slips (and bangs) but doesn't explode if it doesn't slip (and doesn't

slam)?

It happens that it does not explode for the observer for the observer on

the train (for which the plane does not tilt and the bottle does not

slip) but it explodes for the observer on the ground (for which the

plane tilts and the bottle slips).

And this is not acceptable.

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