Neutral simultaneity for two frames.

[studiot]

 

I feel this train is running further and further down the track of fancy, rather than fact and further and further from the OP original intention.

 

30 minutes ago, Mordred said:

I was showing the problems of Born rigidity. Either way there are no rigid objects under GR. 

 

This is another way of putting what I said earlier, though I didn't know it was called Born rigidity I can well believe that this line of thought was started by Max Born, he certainly discusses it in his book.

 

No one has yet replied to it.

[Lorentz Jr]

1 hour ago, Mordred said:

Ah so now we're changing goalposts. I was showing the problems of Born rigidity.

We're not changing anything, my friend. The OP said the engines don't interact. That requires a setup along the lines of what I described, to keep the engines synchronized without real-time signals between them. So I have no idea which goalposts you think we might be changing.

Also, the OP mentioned a long train, not long cars. Rigidity isn't a serious issue for short cars with flexible couplings.

 

56 minutes ago, studiot said:

I feel this train is running further and further down the track of fancy, rather than fact and further and further from the OP original intention.

What do you think the OP's original intention was? He presented a scheme for accelerating all the cars of a lengthy train together, and in another post he explicitly said it would need to be done without signals being sent between cars.

Quote

Born rigidity ... No one has yet replied to it.

I addressed that implicitly in my second post. In order to keep the cars all spaced properly as they get length-contracted, the rear cars have to initially accelerate faster than the forward cars, and then the forward cars have to speed up later as the rear cars catch up to them.

Edited December 10, 2022 by Lorentz Jr

[studiot]

22 minutes ago, Lorentz Jr said:

45 minutes ago, studiot said:

I feel this train is running further and further down the track of fancy, rather than fact and further and further from the OP original intention.

What do you think the OP's original intention was? He presented a scheme for accelerating all the cars of a lengthy train together, and in another post he explicitly said it would need to be done without signals being sent between cars.

Quote

Born rigidity ... No one has yet replied to it.

I addressed that implicitly in my second post. In order to keep the cars all spaced properly as they get length-contracted, the rear cars have to initially accelerate faster than the forward cars, and then the forward cars have to catch up later.

Actually he didn't.

What he actually said, in a rather strange style, was

On 12/9/2022 at 7:01 PM, DimaMazin said:

If we want to accelerate very long train,

instantly accelerating each railway carryage into a moving frame at v, then we should accelerate them simultaneously in neutral simultaneity.

t =t_o + (gamma -1)_*x/(v_*gamma)

If we want to accelerate a very long train.

which makes sense.

The next part was nonsense.

So we are left with wanting to accelerate a conceivable physical object to whit one very long train.

Perhaps that is what led you to make the next post which also seemed nonsense to me

On 12/9/2022 at 11:07 PM, Lorentz Jr said:

Once the cars start moving, their accelerations won't be simultaneous in their reference frame anymore. The front cars will be accelerating earlier than the rear cars, so the cars will be pulled away from each other.

 

The differential timing of the motions between adjacent parts or sections of the train in the limit tends zero as the initial distance between the sections of the train tends to zero.

This is the condition that allow the train to move as a whole. This will happen quite naturally in an orderly fashion without all the paraphnalia later described.

It is also the 'long rod' example as Mordred has pointed out.

[Mordred]

I still believe your missing the point. Even if every single coordinate was effectively it's own engine and you could contrive some means of synchronization. Once the train needs to turn you would end up with differing accelerations. 

 So once again we're stuck with needing communication.

 The reason I asked how much GR you understood is that I wanted an idea of your math skills. Are you familiar with the four momentum in mathematical form ? 

 

Edited December 10, 2022 by Mordred

[Lorentz Jr]

40 minutes ago, Mordred said:

I still believe your missing the point. Once the train needs to turn you would end up with differing accelerations.

The point of what? The OP didn't say a word about turning. No one except you said anything about turning. You talk about signals that nobody mentioned; you talk about turning, which nobody mentioned; every time someone proves you wrong, you change the subject and talk like that's what we were discussing all along.

PS: It's "you're", not "your". Your English is almost as bad as your physics.

Edited December 11, 2022 by Lorentz Jr

[Mordred]

No we're still talking about the same topic a train are we to assume the train never turns ? not that it matters in the Lorentz gauge acceleration is a rotation called rapidity. 

[Lorentz Jr]

2 minutes ago, Mordred said:

are we to assume the train never turns ?

I have no idea. I was just addressing the OP's idea of accelerating the cars together.

[studiot]

5 minutes ago, Mordred said:

No we're still talking about the same topic a train are we to assume the train never turns ? not that it matters in the Lorentz gauge acceleration is a rotation called rapidity. 

In fairness I don't see that allowing the train to turn makes any difference, except to add needless complications to the mix.

 

I don't know if anyone can remember th slow motion video I posted a while back of a slinky being dropped from a tallish building by first letting go the lowest part of the slinky so it started uncoiling, and then releasing the top from its mounting so the slinky as a whole aquired a downward motion, additional to the stretching of the coils.

 

Edited December 10, 2022 by studiot

[Lorentz Jr]

14 minutes ago, Mordred said:

So once again we're stuck with needing communication.

What we're stuck with is needing to stick to the subject of the thread and read other people's posts more carefully.

[Mordred]

3 minutes ago, studiot said:

In fairness I don't see that allowing the train to turn makes any difference, except to add needless complications to the mix.

 

I don't know if anyone can remember th slow motion video I posted a while back of a slinky being dropped from a tallish building by first letting go the lowest part of the slinky so it started uncoiling, and then releasing the top from its mounting so the slinky as a whole aquired a downward motion, additional to the stretching of the coils.

 

Sure I can go with that 

Here is the Born Rigidity examination

https://fnegari.github.io/files/notes/009.pdf

From this article it should be clear just how tricky any rigid rod examination can get

 

Edited December 10, 2022 by Mordred

[studiot]

50 minutes ago, Lorentz Jr said:

What do you think the OP's original intention was? He presented a scheme for accelerating all the cars of a lengthy train together, and in another post he explicitly said it would need to be done without signals being sent between cars.

Are you going to comment on my reply to this ?

[Mordred]

2 minutes ago, Lorentz Jr said:

What we're stuck with is needing to stick to the subject of the thread and read other people's posts more carefully.

This is mainstream physics my earlier statements stress that under GR there is no rigid rods thus applies to the train no matter how its being examined 

[Lorentz Jr]

36 minutes ago, studiot said:

What he actually said, in a rather strange style, was ... If we want to accelerate a very long train.... The next part was nonsense.

So he wasn't very coherent. This is the internet, not Princeton or Cal Tech. 😄

Quote

So we are left with wanting to accelerate a conceivable physical object to whit one very long train.

You mean a long chain of many smaller objects.

Quote

Perhaps that is what led you to make the next post which also seemed nonsense to me

That post was to a different person and about a different subject. I was commenting on MD's suggestion of instantaneous acceleration to the target speed.

Quote

The differential timing of the motions between adjacent parts or sections of the train in the limit tends zero as the initial distance between the sections of the train tends to zero.

I've been assuming the lengths of the cars are neither especially long nor especially short. In order for them to all accelerate simultaneously without getting pulled apart or crashing together, the information about their accelerations as a function of time has to be contained in each car from the start, and all the engines have to start accelerating at the same time.

Quote

This will happen quite naturally in an orderly fashion without all the paraphnalia later described.

I don't see how. The OP's goal was for the cars to all accelerate simultaneously, and Mordred pointed out the obvious fact that doing so isn't possible if the accelerations have to be controlled externally.

Quote

It is also the 'long rod' example as Mordred has pointed out.

After I had already addressed the issue, although not explicitly, because I thought the OP had made it clear that he was at least trying to deal with that problem.

18 minutes ago, studiot said:

Are you going to comment on my reply to this ?

Patience. We'll get there. 🙂

Edited December 11, 2022 by Lorentz Jr

[Mordred]

Have you looked at the article I posted yet Lorentz ?

[Lorentz Jr]

25 minutes ago, Mordred said:

This is mainstream physics my earlier statements stress that under GR there is no rigid rods thus applies to the train no matter how its being examined 

This entire thread is about the rigid-rod problem. That's the whole point of the OP's formula. The fact that you introduced the term "rigid rod" and name-dropped Born doesn't mean you said anything new. If I understood correctly, the OP's goal was to accelerate each section of the "rod" as needed to minimize the stresses between them.

Edited December 11, 2022 by Lorentz Jr

[Mordred]

Correct it is precisely about the rigid rod have you looked at the link I posted or did you miss it?

[studiot]

Thank you Mordred, I downloaded the pdf  though I have seen some of this before it is more modern than my knowledge.

 

[Lorentz Jr]

2 hours ago, Mordred said:

Each engine clock will be in coordinate time.

What in the world is this supposed to mean? It sounds like more nonsense.

[Mordred]

17 minutes ago, studiot said:

Thank you Mordred, I downloaded the pdf  though I have seen some of this before it is more modern than my knowledge.

 

your welcome.

 

the criteria to meet is given in the first equation that the distance does not change between any two particles.

\[ V(r,t)=V_0(t)+\Omega(t)r\] is the solution to the first equation \[\Omega(t)\] being an antisymmetric tensor

that's the criteria. the rest of the article deals with the examination

10 minutes ago, Lorentz Jr said:

What in the world is this supposed to mean? It sounds like more nonsense.

Ok your not familiar with GR terminology fair enough

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

please note that the following from the article.

"In other words, the existence of a non trivial (i.e. not in uniform motion) Born rigid body contradicts the causality principles. In fact, our initial assumption that led to the equations of motion, was not causal to begin with. Even for small distances, the assumption that the velocities are adjusted momentarily will lead to super-luminal signal"

 

Edited December 11, 2022 by Mordred

[Lorentz Jr]

3 minutes ago, Mordred said:

Ok your not familiar with GR terminology

I am familiar with GR terminology, and I'm also familiar with trolling tactics. Like intentionally misreading comments, for instance.

What in the world does "Each engine clock will be in coordinate time" mean?

What does it mean for a clock on a moving train car to "be in coordinate time"?

@studiot? Do you understand that? Because I'm not getting anything that makes sense to me from this other guy, except as ignorant trolling.

[studiot]

21 minutes ago, Lorentz Jr said:

So he wasn't very coherent. This is the internet, not Princeton or Cal Tech. 😄

I think Dima uses a translator.

His aim was stated very clearly and coherently, as I have already noted.

However the conditions he proposed to achieve this aim were not, in fact I agree that instanteity and simultaneity are both nonsense, quite apart from the 'acceleration into a moving frame a v' whatever that might mean.

So I described it as nonsense.

Your post was next and it could only have been reply to Dima's opening post as it followed on straight after th op (How I wish this forum would go back to its original format and number the posts). But it made no sense either.

The cars couldn't have a reference frame once they started moving, given your stated conditions.

33 minutes ago, Lorentz Jr said:

That post was to a different person and about a different subject. I was commenting on MD's suggestion of instantaneous acceleration to the target speed.

So if you were trying to say the same as I am now saying, viz that Dima's original aim meant his conditions were inappropriate that's fine,

But I don't see it put very well.  BTW who is MD please ?

 

37 minutes ago, Lorentz Jr said:

I've been assuming the lengths of the cars are neither especially long nor especially short. In order for them to all accelerate simultaneously without getting pulled apart or crashing together, the information about their accelerations as a function of time has to be contained in each car from the start, and all the engines have to start accelerating at the same time.

I think most participants is this thread know and agree that these conditions are neither necesary nor possible.

39 minutes ago, Lorentz Jr said:

I don't see how. The OP's goal was for the cars to all accelerate simultaneously, and Mordred pointed out the obvious fact that doing so isn't possible if the accelerations have to be controlled externally.

Ditto

40 minutes ago, Lorentz Jr said:

After I had already addressed the issue, although not explicitly, because I thought the OP had made it clear that he was at least trying to deal with that problem.

Forgive me, I don't see any explicit reference to such a rod, although rigidity is again not the issue.

8 minutes ago, Lorentz Jr said:

I am familiar with GR terminology, and I'm also familiar with trolling tactics. Like intentionally misreading comments, for instance.

What in the world does "Each engine clock will be in coordinate time" mean?

What does it mean for a clock on a moving train car to "be in coordinate time"?

@studiot? Do you understand that? Because I'm not getting anything that makes sense to me from this other guy, except as ignorant trolling.

I respectfully suggest that both of you are getting tangled up trying to square the circle and start accelerating all parts of a train both instaneously and simultaneously, with or without magic signalling.

If you want to accelerate a train, just tell tha man to put the engine at the front and pull or at the back and push.

No part of the train will ever be moving sufficiently fast, relative to any other part of the train, for either SR or GR calculations to be needed.

If you want to introduce an outside observer (which may be what Dima had in mind) then again say so and introduce relativity appropriately.

[Mordred]

48 minutes ago, Lorentz Jr said:

I am familiar with GR terminology, and I'm also familiar with trolling tactics. Like intentionally misreading comments, for instance.

What in the world does "Each engine clock will be in coordinate time" mean?

What does it mean for a clock on a moving train car to "be in coordinate time"?

@studiot? Do you understand that? Because I'm not getting anything that makes sense to me from this other guy, except as ignorant trolling.

trolling tactics you are aware I am a Resident Expert on this forum correct ? Any clock at a given reference frame is the coordinate time. Proper time is a clock that follows the Worldline given by the line element of the applicable geometry. usually denoted by the separation distance between two events. For Euclidean geometry ( I did provide links earlier to coordinate time and proper time )

\[ds^2=dx^2+dy^2+dz^2\]

The worldline is the null geodesic path which is represented by the line element or if you prefer the affine connections given by the Christoffel connections. The line element is typically easier to understand though lol. Though the geodesic equation includes the Christoffel

here is the Minkowskii line element

\[ds^2=-c^2dt^2+dx^2+dy^2+dz^2=\eta_{\mu\nu}dx^{\mu}dx^{\nu}\]

 

Edited December 11, 2022 by Mordred

[Lorentz Jr]

1 hour ago, studiot said:

Your post was next and it could only have been reply to Dima's opening post as it followed on straight after th op .... But it made no sense either.

I hadn't quite understood his plan at that point. I thought he wanted to accelerate the cars equally in the ground frame, and I was pointing out that they need to be closer together once they're moving. So their accelerations need to be different functions of time.

1 hour ago, studiot said:

The cars couldn't have a reference frame once they started moving, given your stated conditions.

Maybe not. I was just pointing out that, even taking only special relativity into account, if events along the length of a moving train are simultaneous in the ground frame, the ones at the front happen before the ones at the back in the train's frame, so the cars would be pulled apart if they accelerate equally in the ground frame.

1 hour ago, studiot said:

BTW who is MD please ?

md65536.

1 hour ago, Lorentz Jr said:

I've been assuming the lengths of the cars are neither especially long nor especially short. In order for them to all accelerate simultaneously without getting pulled apart or crashing together, the information about their accelerations as a function of time has to be contained in each car from the start, and all the engines have to start accelerating at the same time.

1 hour ago, studiot said:

I think most participants is this thread know and agree that these conditions are neither necesary nor possible.

Really? Based on what? How else would one accelerate train cars without stress between them, and what do you think makes synchronized self-propulsion impossible?

1 hour ago, studiot said:

Forgive me, I don't see any explicit reference to such a rod, although rigidity is again not the issue.

Then why did you say "This is another way of putting what I said earlier" in response to this comment:

3 hours ago, Mordred said:

showing the problems of Born rigidity. Either way there are no rigid objects under GR. 

.. and then point out that no one had explicitly answered the comment, if it's not at issue?

1 hour ago, studiot said:

No part of the train will ever be moving sufficiently fast, relative to any other part of the train, for either SR or GR calculations to be needed.

Ladders don't normally fly through barns at relativistic speeds either, but they make for good exercises. 

1 hour ago, studiot said:

If you want to introduce an outside observer (which may be what Dima had in mind) then again say so and introduce relativity appropriately.

I think Dima just wanted to accelerate the train quickly without introducing internal stresses. I don't think his post had anything to do in particular with any specific observers.

Edited December 11, 2022 by Lorentz Jr

[Mordred]

15 minutes ago, Lorentz Jr said:

I hadn't quite understood his plan at that point. I thought he wanted to accelerate the cars equally in the ground frame, and I was pointing out that they need to be closer together once they're moving. So their accelerations need to be different functions of time.

Maybe not. I was just pointing out that, even taking only special relativity into account, if events along the length of a moving train are simultaneous in the ground frame, the ones at the front happen before the ones at the back in the train's frame, so the cars would be pulled apart if they accelerate equally in the ground frame.

md65536.

Then why did you say "This is another way of putting what I said earlier" in response to this comment:

.. and then point out that no one had explicitly answered the comment, if it's not at issue?

Ladders don't normally fly through barns at relativistic speeds either, but they make for good exercises. 

I think Dima just wanted to accelerate the train quickly without introducing internal stresses. I don't think his post had anything to do in particular with any specific observers.

Ok let us get back on track here no pun intended.

I located an easier to understand SR treatment

"Rigid body motion in special relativity"

https://arxiv.org/pdf/1105.3899v3.pdf

hope this helps the dscussion, the equations are readily applicable to the train

Edited December 11, 2022 by Mordred

[Lorentz Jr]

1 hour ago, Mordred said:

trolling tactics you are aware I am a Resident Expert on this forum correct ?

Argument from authority

I've seen the label, but your comments still don't make sense to me, and I've met way too many trolls on the internet to trust labels.

Quote

Any clock at a given reference frame is the coordinate time.

Sorry, but that's more meaningless nonsense. When an object moves relative to an observer, "coordinate time" refers to time in the observer's frame and "proper time" refers to time in the object's frame. This thread is about accelerating a train relative to the ground, so time in the ground's frame is the coordinate time for the problem and time in each car's frame is the proper time for that car.

Quote

Proper time is a clock that follows the Worldline given by the line element of the applicable geometry.

Again, you're throwing around terms like "line element" without knowing what they mean. There are lots of line elements for lots of different objects. The line element of the car's motion is exactly what the clock in the car follows, so each engine clock in a car will measure that car's proper time, not the ground's coordinate time.

Quote

the affine connections given by the Christoffel connections

Blah blah blah blah blah blah blah blah blah blah blah blah.

You're using words to intimidate, not to communicate. You're using them incorrectly, and you're not saying anything with them.

"Each engine clock will be in coordinate time" is 100 percent, Grade-A baloney.

Edited December 11, 2022 by Lorentz Jr