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Jul 28, 2022, 6:07:50 AM7/28/22

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 AM7/28/22

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 AM7/29/22

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 AM7/30/22

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 AM8/2/22

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 PM8/2/22

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 PM8/3/22

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 PM8/3/22

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 PM8/3/22

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 AM8/4/22

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.

Oct 31, 2022, 9:03:02 PM10/31/22

to

Op woensdag 3 augustus 2022 om 19:02:10 UTC+2 schreef Luigi Fortunati:

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

There are two definitions.

1) You can call this the global definition.

This definition depends on the question:

At any instant, in the evolution of the universe, are there

simultaneous events happening?

IMO the answer is Yes.

For example, at any instant, all the planets around the Sun have a

specific position. Each position can be considered as an event.

2) You can call this the local definition.

This definition is observer depended and is based on what an

observer sees.

For example: you can have three events A, B and C and three observers

1, 2 and 3.

Observer 1 can see A and B simultaneous; Observer 2 can see B and C

simultaneous and Observer 3 can see A and C simultaneous, but that

does not say anything about the order of the events A, B and C.

What makes all of this more complicated is that the observers also

can move relative of each other.

This problem sounds like the tower of Babel problem, where everyone

speaks a different language and nothing can be achieved.

(https://www.theatlantic.com/magazine/archive/2022/05/social-media-democracy-trust-babel/629369/

IMO the only way to solve this problem is, if all the three observers

agree to one reference frame and that all the clocks used are linked

to that frame.

> 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 some way we all must agree on something.

There only exists one universe at each instant.

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

> -dependent; there's no universal notion of tilt.

The physical reality (evolution) is not observer dependant.

What each of us observes is something different.

> 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.

In physics people should try to predict the future.

Whatever both observe, they should predict the same future.

https://www.nicvroom.be/

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

There are two definitions.

1) You can call this the global definition.

This definition depends on the question:

At any instant, in the evolution of the universe, are there

simultaneous events happening?

IMO the answer is Yes.

For example, at any instant, all the planets around the Sun have a

specific position. Each position can be considered as an event.

2) You can call this the local definition.

This definition is observer depended and is based on what an

observer sees.

For example: you can have three events A, B and C and three observers

1, 2 and 3.

Observer 1 can see A and B simultaneous; Observer 2 can see B and C

simultaneous and Observer 3 can see A and C simultaneous, but that

does not say anything about the order of the events A, B and C.

What makes all of this more complicated is that the observers also

can move relative of each other.

This problem sounds like the tower of Babel problem, where everyone

speaks a different language and nothing can be achieved.

(https://www.theatlantic.com/magazine/archive/2022/05/social-media-democracy-trust-babel/629369/

IMO the only way to solve this problem is, if all the three observers

agree to one reference frame and that all the clocks used are linked

to that frame.

> 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".

There only exists one universe at each instant.

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

The physical reality (evolution) is not observer dependant.

What each of us observes is something different.

> 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.

Whatever both observe, they should predict the same future.

https://www.nicvroom.be/

Nov 16, 2022, 1:44:30 PM11/16/22

to

the wagon , activate the detonator of a bomb ..IFF and ONLY IFF they

arrive simultaneously. The END RESULT should be good enough to show

clearly.."which of the two is correct", I think. :)

Regards, Laszlo Lemhenyi

Nov 17, 2022, 1:51:04 AM11/17/22

to

> Regards, Laszlo Lemhenyi

Luigi.

[[Mod. note -- I have several comments.

First, note that Luigi's animation shows the entire spring responding

*instantaneously* to the release of the endpoints. That's not possible

(in special relativity). Rather, when each endpoint is released, a

wave of rarefaction will propagate along the spring away from the

endpoint. That propagation can't happen any faster than the speed of

sound in the material making up the spring, and for a realistic spring

would be considerably slower. I won't try to analyze this in detail

here.

For the rest of what I'm going to write, let's set aside the

speed-of-sound issue, and consider instead some other interesting

physics questions posed by Luigi's animation.

First, it's useful to introduce a bit of terminology:

Let's call the release of the left end of the spring "event L".

And, let's call the release of the right end of the spring "event R".

Luigi's animation poses the following two questions:

(a) If event's L and R each send a photon (i.e., a signal which travels

at the speed of light) back to the wagon's central point, which

photon (left-end or right-end) arrives first?

(b) Which end of the spring is released first, i.e., which of events

L and R happens first?

Let's first consider question (a):

In relativity, the relative temporal ordering of different events

*along a single observer's worldline* is universal: all observers

agree on this ordering. (Since these events are all located along a

single observer's worldline, they are necessarily *timelike*-separated.)

That means that question (a) has a universal answer, i.e., the answer

to question (a) does *not* change from one reference frame to another.

This in turn means that we can compute the answer by using whatever

reference frame (RF) is most convenient. In this case, the wagon RF

is very convenient: the problem is fully symmetric, and it's clear

that in this frame both the left-end and right-end photons arrive

back at the wagon center at the *same* time. By the argument given

in the previous paragraph, that statement ("the left-end and right-end

photons arrive back at the wagon center at the *same* time") is

necessarily true in *any* RF.

Exercise for the reader: explicitly work out the photon propagation in

the ground RF and show that the two photons also arrive simultaneously

in this RF.

Now let's consider question (b):

In relativity, the relative temporal ordering of different

*spacelike-separated* events isn't universal: different observers (RFs)

will in general disagree on this ordering.

Events L and R are spacelike-separated, so they have *no* universal

temporal ordering. So, question (b) as I've written it is inherently

observer-dependent. As we've seen, in the wagon RF events L and R

are simultaneous. But in the ground RF, events L and R are *not*

simultaneous. That is:

(1) In the wagon RF, the time coordinate of event L is equal to the

time coordinate of event R. In other words, the two ends of the

spring are released at the same time, so the spring contracts

symmetrically.

(2) In the ground inertial reference frame, the time coordinate of

event L is *not* equal to the time coordinate of event R. In

other words, the two ends of the spring are released at different

times, so the spring contracts *asymmetrically).

Both statements (1) and (2) are correct.

So, the statement

> 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?

is ill-posed. The correct statement is that *both* pictures are correct;
> contractions must be wrong: which of the two is correct and which is

> wrong?

the notion of "symmetrical contraction" is observer-dependent, i.e.,

the answer to the question "is the spring contracting symmetrically"

varies from one RF to another.

-- jt]]

Nov 18, 2022, 3:28:24 AM11/18/22

to

On Thursday, November 17, 2022 at 1:51:04 AM UTC-5, Luigi Fortunati wrote:

> xray4abc mercoled=EC 16/11/2022 alle ore 19:44:27 ha scritto:

> > On Thursday, July 28, 2022 at 6:07:50 AM UTC-4, 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?

> >

> > Well, let us imagine... that those 2 photons arriving to the centre of

> > the wagon , activate the detonator of a bomb ..IFF and ONLY IFF they

> > arrive simultaneously.

> Only if they arrive at the same time "in which reference"?

Of the wagon of course!
> xray4abc mercoled=EC 16/11/2022 alle ore 19:44:27 ha scritto:

> > On Thursday, July 28, 2022 at 6:07:50 AM UTC-4, 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?

> >

> > Well, let us imagine... that those 2 photons arriving to the centre of

> > the wagon , activate the detonator of a bomb ..IFF and ONLY IFF they

> > arrive simultaneously.

> Only if they arrive at the same time "in which reference"?

Then, if an explosion takes place, it is clear that NO MATTER what exactly

the theoretical predictions regarding simultaneity in the other frames of reference

are..the photons must have been arrived simultaneously, and the observers in the other frames

may scratch their heads as long as they want, to figure out...why the explosion occurred !

Regards, LL

Nov 18, 2022, 2:00:46 PM11/18/22

to

connections between effective measurements made in 2 relatively moving

inertial reference frames, as we would like the case to be.

Max Born, in his book "Einstein's theory of relativity" says about relativistic effects,

on page 254 (Dover publicatons , New York)..."Thus the contraction is only a consequence of our way of

regarding things and is not a change of physical reality"

I say, same applies for any other measurements !

In my interpretation, this means:

We can not have/measure/possess the data from a moving reference frame! We can have only the data WE ATTRIBUTE

to that frame. The example with the explosion, I have given, shows though, that some things......

like simultaneity of a given event.....the signals reaching the detonator in this case, must apply in

in any reference frame.

Regards, LL

Nov 18, 2022, 9:34:08 PM11/18/22

to

Luigi Fortunati mercoled=EC 16/11/2022 alle ore 15:50:58 ha scritto:

> xray4abc mercoled=EC 16/11/2022 alle ore 19:44:27 ha scritto:

> ...
> xray4abc mercoled=EC 16/11/2022 alle ore 19:44:27 ha scritto:

> [[Mod. note -- I have several comments.

>

> First, note that Luigi's animation shows the entire spring responding

> *instantaneously* to the release of the endpoints. That's not possible

> (in special relativity). Rather, when each endpoint is released, a

> wave of rarefaction will propagate along the spring away from the

> endpoint.

Right observation.
>

> First, note that Luigi's animation shows the entire spring responding

> *instantaneously* to the release of the endpoints. That's not possible

> (in special relativity). Rather, when each endpoint is released, a

> wave of rarefaction will propagate along the spring away from the

> endpoint.

However, the speed of propagation of the rarefaction wave is not that

of light (absolute speed) but it is speed v<<c and, therefore, is not

influenced by the reference.

It is like the speed of the train which is the same in both references,

because what changes are the distances and times, not the speeds.

So, it doesn't affect the final result, as you can see from the updated

animation

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

where I added the blue wave of rarefaction that comes to the center of

the spring at the same instant in the train reference but not in the

wagon one.

[[Mod. note -- It's not true to say that the speed of the rarefaction

wave "is not influenced by the reference". In the reference frame of the

wagon the rarefaction wave propagates at a speed v. But to figure out

what it does in the ground reference frame, you have to use the

special-relativity velocity addition formula,

https://en.wikipedia.org/wiki/Velocity-addition_formula

-- jt]]

Nov 19, 2022, 4:25:57 AM11/19/22

to

Luigi Fortunati venerdě 18/11/2022 alle ore 11:34:05 ha scritto:

> However, the speed of propagation of the rarefaction wave is not that

> of light (absolute speed) but it is speed v<<c and, therefore, is not

> influenced by the reference.

>

> It is like the speed of the train which is the same in both references,

> because what changes are the distances and times, not the speeds.

>

> So, it doesn't affect the final result, as you can see from the updated

> animation

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

> where I added the blue wave of rarefaction that comes to the center of

> the spring at the same instant in the train reference but not in the

> wagon one.

>

> [[Mod. note -- It's not true to say that the speed of the rarefaction

> wave "is not influenced by the reference". In the reference frame of the

> wagon the rarefaction wave propagates at a speed v. But to figure out

> what it does in the ground reference frame, you have to use the

> special-relativity velocity addition formula,

> https://en.wikipedia.org/wiki/Velocity-addition_formula

> -- jt]]

Right, you're right, I was wrong
> However, the speed of propagation of the rarefaction wave is not that

> of light (absolute speed) but it is speed v<<c and, therefore, is not

> influenced by the reference.

>

> It is like the speed of the train which is the same in both references,

> because what changes are the distances and times, not the speeds.

>

> So, it doesn't affect the final result, as you can see from the updated

> animation

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

> where I added the blue wave of rarefaction that comes to the center of

> the spring at the same instant in the train reference but not in the

> wagon one.

>

> [[Mod. note -- It's not true to say that the speed of the rarefaction

> wave "is not influenced by the reference". In the reference frame of the

> wagon the rarefaction wave propagates at a speed v. But to figure out

> what it does in the ground reference frame, you have to use the

> special-relativity velocity addition formula,

> https://en.wikipedia.org/wiki/Velocity-addition_formula

> -- jt]]

Luigi

Jan 3, 2023, 8:11:55 PMJan 3

to

Op 29/07/2022 om 12:57 schreef Luigi Fortunati:

> Richard Livingston alle ore 16:53:38 di gioved=C3=AC 28/07/2022 ha scritto:

In the standpoint of the rest frame, the wagon's "back" end will receive

the light signal first, and release the spring earlier; but the latter

has to do a longer trip to reach its rest point, since it is heading for

a "receding" spring center. And vice versa in the "front" end, where the

signal arrives later, but the spring end does a shorter trip as the

spring center is heading towards it.

Your example is not much different from the example of symmetrical light

clocks, exhibiting ROS together with time dilation and length

[[Mod. note -- I think by "ROS" the author means

"relativity of simultaneity" -- jt]]

contraction in light clock systems moving WRT each other.

See eg my video

https://youtu.be/AYpD9JRWjdU?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

Or also, from the rivet paradox case with the apparent incompatibility

of the rivet being stopped "head first" in its own system, and "tail

first" in the (stationary) hole system.

See my videos

https://youtu.be/v80fNhAhds4?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

and

https://youtu.be/3oEEE_-JslY?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

--

guido wugi

> Richard Livingston alle ore 16:53:38 di gioved=C3=AC 28/07/2022 ha scritto:

>> They are both correct...

>

> 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.

If the wagon moves, the spring center moves with it.
>

> 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.

In the standpoint of the rest frame, the wagon's "back" end will receive

the light signal first, and release the spring earlier; but the latter

has to do a longer trip to reach its rest point, since it is heading for

a "receding" spring center. And vice versa in the "front" end, where the

signal arrives later, but the spring end does a shorter trip as the

spring center is heading towards it.

Your example is not much different from the example of symmetrical light

clocks, exhibiting ROS together with time dilation and length

[[Mod. note -- I think by "ROS" the author means

"relativity of simultaneity" -- jt]]

contraction in light clock systems moving WRT each other.

See eg my video

https://youtu.be/AYpD9JRWjdU?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

Or also, from the rivet paradox case with the apparent incompatibility

of the rivet being stopped "head first" in its own system, and "tail

first" in the (stationary) hole system.

See my videos

https://youtu.be/v80fNhAhds4?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

and

https://youtu.be/3oEEE_-JslY?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

--

guido wugi

Jan 6, 2023, 4:21:37 AMJan 6

to

wugi martedě 03/01/2023 alle ore 10:11:51 ha scritto:

> Op 29/07/2022 om 12:57 schreef Luigi Fortunati:

>> Impossible!
> Op 29/07/2022 om 12:57 schreef Luigi Fortunati:

> If the wagon moves, the spring center moves with it.

> Your example is not much different from the example of symmetrical light

> clocks, exhibiting ROS together with time dilation and length

> [[Mod. note -- I think by "ROS" the author means

> "relativity of simultaneity" -- jt]]

> contraction in light clock systems moving WRT each other.

> See eg my video

> https://youtu.be/AYpD9JRWjdU?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

I had already admitted that I was wrong on November 19, 2022.
> clocks, exhibiting ROS together with time dilation and length

> [[Mod. note -- I think by "ROS" the author means

> "relativity of simultaneity" -- jt]]

> contraction in light clock systems moving WRT each other.

> See eg my video

> https://youtu.be/AYpD9JRWjdU?list=PL5xDSSE1qfb6zyVKJbe8POgj-8ijmh5o0

[Moderator's note: Much quoted text trimmed. -P.H.]

Jan 29, 2023, 1:39:22 AM (3 days ago) Jan 29

to

On Thursday, July 28, 2022 at 5:07:50 AM UTC-5, fortuna wrote:

> 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.

No it doesn't.
> 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.

All the parts of the spring between the ends stay put

and don't change at all,

until the "I was released" signal reaches it,

the signal being conveyed by the action of the spring, itself.

It doesn't get to that part of the spring any faster

than the speed of sound in the spring, whatever that may be.

Until that sound signal reaches that part of the spring,

it remains in the whatever state of compression it was in

as if nothing had happened to the ends.

Your intuition is wrong.

It is grounded in small objects, where you don't see the propagation.

You've never worked with huge objects, by which I mean objects

hundreds or thousands of meters in length.

Even large trees exhibit this delayed reaction and response -

as those of us who are out and about all the time know full well.

No spring acts as a cohesive unit at all; there is no such thing.

That's an illusion borne of being of size too small for you to see its fluidity.

You have to treat it as a fluid, for all intents and purposes.

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