Train Stopping Time: Different Observers, Different Times?

[adelmakram]

Lets have a train moving to the right side in the shown direction of the arrow.

From a point near the end A, 2 rays of light are emitted toward both ends A and B so as they appear to reach both ends at the same time for an external observer.

At the moment the 2 rays reaches the 2 ends at the same time relative to the external observer, the train stops moving.

For the train observer, both rays reach both ends at different times. More precisely, the light ray reaches A before B.

The question, will the 2 observers record 2 different times of the train stopping? This is given that they both know that their train is moving.

Alternatively, if they interpret that the platform is moving in the opposite direction to the indicated arrow in the figure, will they record 2 different times of platform stopping?

 

[xox]

Lets have a train moving to the right side in the shown direction of the arrow.

From a point near the end A, 2 rays of light are emitted toward both ends A and B so as they appear to reach both ends at the same time for an external observer.

At the moment the 2 rays reaches the 2 ends at the same time relative to the external observer, the train stops moving.

For the train observer, both rays reach both ends at different times. More precisely, the light ray reaches A before B.

The question, will the 2 observers record 2 different times of the train stopping? This is given that they both know that their train is moving.

Alternatively, if they interpret that the platform is moving in the opposite direction to the indicated arrow in the figure, will they record 2 different times of platform stopping?

What kind of work did you do to solve the problem? Can you post your equations?

 

[ghwellsjr]

Lets have a train moving to the right side in the shown direction of the arrow.

From a point near the end A, 2 rays of light are emitted toward both ends A and B so as they appear to reach both ends at the same time for an external observer.

At the moment the 2 rays reaches the 2 ends at the same time relative to the external observer, the train stops moving.

For the train observer, both rays reach both ends at different times. More precisely, the light ray reaches A before B.

The question, will the 2 observers record 2 different times of the train stopping? This is given that they both know that their train is moving.

Alternatively, if they interpret that the platform is moving in the opposite direction to the indicated arrow in the figure, will they record 2 different times of platform stopping?

I think you have already answered your own question. You said that the train stops when the light reaches both ends. And then you said for the external observer, this happens at one time and for the train observer, this happens at two different times, so how can it be that the train could stop at the same time for both observers?

More precisely, when you talk about the external observer, you mean according to the Inertial Reference Frame in which the external observer, or the platform, is at rest and when you talk about the train observer, you mean the IRF in which the train is moving prior to it stopping.

To make things clear, it helps to draw spacetime diagrams and use a specific scenario. Let's consider the train to be moving at 0.6c and to have a length in the platform IRF while it is moving of 5000 feet and we'll put the light source at 4000 feet from the rear of the train. In this spacetime diagram, I've shown the ends of the platform in green, the front of the train in black, the rear of the train in red, and the light source in blue:

As you can see, in the platform IRF, all parts of the train stop at the same time, at the Coordinate Time of 10 microseconds.

Now we can transform the above IRF to the train IRF by applying the Lorentz Transformation to all the Coordinates and we get this spacetime diagram:

Now you can see that the different parts of the train stop at different Coordinate Times ranging from 4.25 to 8 microseconds.

Does that clarify everything for you?

 

[adelmakram]

Thank you for your clarification.

As the different parts of the train stop at different times, then the part B is still moving while the part A comes to a stop.

Now, suppose, there is a coil stretched between A and B. So the B -end of the coil moves toward the A-end of the string when A comes to a stop. That creates an extra tension on the coil to exceed its breaking point and then it breaks down. However, for the platform observer both ends come to a stop at the same time and then no extra-tension or breaking down of the coil should occur.

I drew a diagram quoted from yours. I fainted out the line colours but leaving the black and the red to represent A and B time-lines, respectively. The first one shows the time when A stops but B is still moving. The second one, I rotate the diagram clock-wise to bring the A at the origin with its time-line is vertical.

 

[dauto]

It is a mistake to think there is no extra tension from the point of view of the platform. In fact, due to Lorentz contraction the observer at the platform would expect the train to change length when it stops. That 's not happening which means the train must be under stress either before it stops or after it stops (or both).

 

[ghwellsjr]

Thank you for your clarification.

You're welcome.

As the different parts of the train stop at different times, then the part B is still moving while the part A comes to a stop.

This isn't a good way to say what is happening. In the IRF in which the train is originally at rest as depicted in my second drawing above, part B is still at rest while part A suddenly moves towards it. So the train is getting compressed, starting from the front and extending toward the rear. This is kind of like a train parked on the track experiencing a head on collision from another train coming toward it and then dragging it down the track.

Now, suppose, there is a coil stretched between A and B. So the B -end of the coil moves toward the A-end of the string when A comes to a stop. That creates an extra tension on the coil to exceed its breaking point and then it breaks down.

Again, this is backwards: the B-end remains stationary while the A-end suddenly moves toward it. This would create a compression, not a tension. Whether or not anything breaks is not an issue that Special Relativity can resolve. However, if it breaks in one frame, it breaks in all frames.

However, for the platform observer both ends come to a stop at the same time and then no extra-tension or breaking down of the coil should occur.

In the platform frame, the train, because it originally is moving, is Length Contracted. It's Proper Length is longer and can be determined by a platform observer. When all parts of the train are brought to a stop simultaneously, the Proper Length instantly changes from its original longer length to a shorter length. This will create compression but Special Relativity can't say whether or not anything breaks. That has to be answered by a materials or structural analysis.

I drew a diagram quoted from yours. I fainted out the line colours but leaving the black and the red to represent A and B time-lines, respectively. The first one shows the time when A stops but B is still moving. The second one, I rotate the diagram clock-wise to bring the A at the origin with its time-line is vertical.

I have no idea what your drawings are trying to depict which means you probably have no idea what mine are trying to depict.

 

[adelmakram]

The length of the train, which is equal to the length of the coil, as seen by the external observer is shorter when the train is moving than when the train comes to a stop. So when the train stops, the length contraction effect disappears and the coil length increases suddenly as seen by the external observer. This should cause tension on the coil not compression.

 

[ghwellsjr]

The length of the train, which is equal to the length of the coil, as seen by the external observer is shorter when the train is moving than when the train comes to a stop. So when the train stops, the length contraction effect disappears and the coil length increases suddenly as seen by the external observer. This should cause tension on the coil not compression.

What you are describing could happen when a train stops in an unrestrained way which would result in different parts of the train stopping at different times in the platform IRF but you said the different parts of the train stop simultaneously in the platform IRF. You can look at my first drawing to see that the length of the train under that specification does not change in the platform IRF.

In order for all parts of a train (or any object) to stop simultaneously according to a frame, there must be something like clamps set up all along the track which simultaneously stop the train (by preprogrammed timers) that bring all the parts to a halt (in that frame) and keep the different parts from expanding back to their natural length.

I can't help it if you specified an unrealistic scenario but that's what you did and Special Relativity doesn't address the realisticness of a scenario. That's why we can specify instantaneous accelerations (or decelerations) and see what happens in different IRF's even if they can't actually happen in reality.

 

[ghwellsjr]

Can I offer you a different scenario that may bring out the effects you want?

Consider two separate trains traveling down a track at the same speed with a coil stretched between them and then both trains suddenly stop simultaneously in the platform frame.

Is that really what you're asking about?

 

[adelmakram]

Can you tell the difference between the 2 scenario?

Here is my prepared reply on your previous post (No. 8) which I just about to post before you came with the new scenario...

"The set up aims to investigate the possible discrepancy between the observation of the coil length and the force exerted on it in the two spaces. Whether there is a mechanical constraint during stopping the train or not, this is not the issue. Because the same will be still held if we only use the coil alone as our set up and its two ends come to stop simultaneously relative to the external observer.

So the matter of force exerted on coil which appears to be tension for the external observer and compression for the train observer is still not clear for me."

 

[ghwellsjr]

Can you tell the difference between the 2 scenario?

Here is my prepared reply on your previous post (No. 8) which I just about to post before you came with the new scenario...

"The set up aims to investigate the possible discrepancy between the observation of the coil length and the force exerted on it in the two spaces. Whether there is a mechanical constraint during stopping the train or not, this is not the issue. Because the same will be still held if we only use the coil alone as our set up and its two ends come to stop simultaneously relative to the external observer.

So the matter of force exerted on coil which appears to be tension for the external observer and compression for the train observer is still not clear for me."

OK, then we'll go with my new scenario.

Here is a spacetime diagram showing a coil stretched between two locomotives. We assume that the coil can both get shorter (because it has been previously stretched) or get longer (because it can stretch even more) without breaking. And because we have an unrealistic scenario (instantaneous deceleration), we have to assume that the forces propagate at the speed of light (otherwise, the coil will break as soon as one part of it decelerates instantly). Some of the midparts of the coil are shown in grey. They start out separated by a thousand feet in the platform frame:

As you can see, all the midpoints continue to travel at 0.6c when the two endpoints suddenly stop. The coil is compressed starting from the front (black end on the right) until the forces, propagating at the speed of light as shown by the thin black line, cause the midpoints to respond. Special Relativity cannot provide an answer to how they respond. We have to make that up in our unrealistic scenario. To make things reasonably simple, I have assumed that the various midpoints respond by changing direction until the forces from the trailing end of the coil (red end on the left) have a chance to further constrain the coil.

In a realistic scenario, a coil (or any other structure that is not destroyed) will oscillate or vibrate until the motions all dampen out and they all come to rest with respect to one another as constrained by their two endpoints. This would take a very long time, not just a few microseconds. But I just want to give you the flavor of what happens. Although I show a large tension or stretching on the left side, in a realistic scenario, this tension would be intertwined with compression resulting in the oscillations or vibrations.

But the final result will be a more or less equal compression (actually, less stretching) of all parts of the coil from one end to the other. It will not end up all in tension. Again, these are issues that Special Relativity cannot address, they have to be determined by materials analysis. But once we decide how the midpoints oscillate during the dampening process, we can transform to any other frame, including the frame in which the train was at rest in the beginning of the scenario:

 

[PeterDonis]

The length of the train, which is equal to the length of the coil, as seen by the external observer is shorter when the train is moving than when the train comes to a stop.

Not according to the way you specified the problem. The way you specified the problem, both ends of the train stop at the same time in the frame in which the train is originally moving (call this frame O). So in frame O, then length of the train does *not* change. In the frame in which the train starts out at rest (call this frame T), the two ends of the train change their state of motion (note that they *start* moving in this frame, so you mis-stated this part earlier) at different times, so the length of the train changes in frame T. (Btw, you should think carefully about *how* the train's length will change in frame T; you should conclude that it will get *shorter*.)

In this version of the problem, the coil will be under tension at the start of the experiment (while the train is moving in frame O), and the tension will be completely removed at the end of the experiment (when the train is at rest in frame O).

So when the train stops, the length contraction effect disappears and the coil length increases suddenly as seen by the external observer.

This is a *different* problem specification than your original one. In this version of the problem, the train's length changes in frame O, which means the two ends must stop moving (with respect to frame O) at different times, in just the right way to undo the length contraction effect of the train's motion, in frame O. So in this version of the problem, the coil will be slack (no tension or compression) the whole time; its apparent length will increase in frame O, but this is purely due to the change in relative motion and does not correspond to any change in the internal forces in the coil.

 

[adelmakram]

This explanation of how the coil does react when the train stops will still hold even if there is no effect of special relativity. In other words, the compression wave which starts from A end and the tension wave which starts from B end will still be observed for the ground observer as well as for the train observer because it is related to the inertia of the coil even if there no SR effect and both ends of the train stop at the same time relative to the train observer. But my query is another compression force which is initiated by the relative motion of A relative to B when A stops first for the train observer which is not paralleled relative to the ground one. Because for the ground observer, there is no relative motion of A toward B when the train comes to a stop.

 

[adelmakram]

Not according to the way you specified the problem. The way you specified the problem, both ends of the train stop at the same time in the frame in which the train is originally moving (call this frame O). So in frame O, then length of the train does *not* change. In the frame in which the train starts out at rest (call this frame T), the two ends of the train change their state of motion (note that they *start* moving in this frame, so you mis-stated this part earlier) at different times, so the length of the train changes in frame T. (Btw, you should think carefully about *how* the train's length will change in frame T; you should conclude that it will get *shorter*.)

In this version of the problem, the coil will be under tension at the start of the experiment (while the train is moving in frame O), and the tension will be completely removed at the end of the experiment (when the train is at rest in frame O).
This is a *different* problem specification than your original one. In this version of the problem, the train's length changes in frame O, which means the two ends must stop moving (with respect to frame O) at different times, in just the right way to undo the length contraction effect of the train's motion, in frame O. So in this version of the problem, the coil will be slack (no tension or compression) the whole time; its apparent length will increase in frame O, but this is purely due to the change in relative motion and does not correspond to any change in the internal forces in the coil.

what are the 2 versions of the problem? In my version, I did not speak about the time the train starts moving. I started from an originally moving train relative to a ground observer. Would you please correct me here: The length of a moving train relative to a ground observer is shorter compared with the length when the train comes to a stop relative to the same observer! Was this the meaning of length contraction?

 

[ghwellsjr]

This explanation of how the coil does react when the train stops will still hold even if there is no effect of special relativity. In other words, the compression wave which starts from A end and the tension wave which starts from B end will still be observed for the ground observer as well as for the train observer because it is related to the inertia of the coil even if there no SR effect and both ends of the train stop at the same time relative to the train observer. But my query is another compression force which is initiated by the relative motion of A relative to B when A stops first for the train observer which is not paralleled relative to the ground one. Because for the ground observer, there is no relative motion of A toward B when the train comes to a stop.

You're making progress because in post #4 you said there was extra tension in the final state of the coil:

Now, suppose, there is a coil stretched between A and B. So the B -end of the coil moves toward the A-end of the string when A comes to a stop. That creates an extra tension on the coil to exceed its breaking point and then it breaks down. However, for the platform observer both ends come to a stop at the same time and then no extra-tension or breaking down of the coil should occur.

...and now you are realizing that the coil ends up in compression.

So if there was no compression or tension in the coil at the start of the scenario, in other words the two locomotives cannot measure any force on their attachments to the coil, then they will both measure a compression or a force trying to push the locomotives apart at the end of the scenario. It doesn't matter what frame we desire to describe the scenario in, they all will come to the same conclusion, although the "explanations" may be different.

So in the "train" frame, that is, the frame in which the train is originally at rest, we might want to "explain" by saying that the A end starts moving toward the B end before the B end starts moving and so the two ends come closer together and in the "platform" frame, we might "explain" by saying that the Proper Length of the coil is 6250 feet originally and 5000 feet finally so it experiences compression.

 

[ghwellsjr]

Would you please correct me here: The length of a moving train relative to a ground observer is shorter compared with the length when the train comes to a stop relative to the same observer! Was this the meaning of length contraction?

No, that is not the meaning of Length Contraction. Length Contraction is the ratio of the length of an object in a frame in which the object is moving compared (or divided by) the length of the object in a frame in which the object is not moving. It has nothing to do with the length of an object before and after it experiences an acceleration. As I said before, Special Relativity cannot address that issue, that is a materials or structural issue. It's no different than asking the question of how the length of a nail changes when you hit it with a hammer. Special Relativity cannot answer that question.

If you look at my diagrams, there are several examples of Length Contraction:

The length of the platform (depicted by the two green lines) in its rest frame is 7000 feet. In the frame in which it is moving at 0.6c, its length is 5600 feet.

The original length of the train in its rest frame is 6250 feet. In the frame in which it is orginally moving, its length is 5000 feet.

The final length of the train in the frame in which it is not moving is 5000 feet. In the frame in which it ends up moving, its length is 4000 feet.

In all these cases, we can calculate the ratio of the contracted length to the Proper Length (the length in its rest frame) as the inverse of gamma. But this only works for an object during periods of time when it is inertial, that is, not experiencing any acceleration or vibration anywhere along its length. In other words, in the frame in which the train is originally at rest, the length contraction formula does not work during the time that one end of the train starts moving until the other end starts moving.

 

[PeterDonis]

what are the 2 versions of the problem?

Version #1 is the one you explicitly described; the key thing about it, as I note below, is that you explicitly specified that both ends of the train stop moving at the same time, relative to the ground observer.

Version #2 is the one you implicitly switched to when you said that when the train stops, "length contraction disappears", i.e., the length of the train, as measured by the ground observer, changes when it stops. For this to happen, the two ends of the train *cannot* stop moving at the same time, relative to the ground observer; if they do, as I note below, the length of the train, relative to the ground observer, cannot change.

In my version, I did not speak about the time the train starts moving.

Yes, you did, although you didn't realize you were doing so. In the frame of the ground observer, the train is moving, and then stops moving. But in the frame of the train observer (the one who starts out moving along with the train), the train is at rest, and then starts moving.

Would you please correct me here: The length of a moving train relative to a ground observer is shorter compared with the length when the train comes to a stop relative to the same observer!

It depends on how the train stops moving. In your scenario, you explicitly *specified* that both ends of the train stop moving at the same time, relative to the ground observer. That means the length of the train, as measured by the ground observer, *cannot change*.

Was this the meaning of length contraction?

No. As ghwellsjr pointed out, length contraction means the length of a moving train as measured by a ground observer while it is moving relative to the ground observer, is shorter than the length of the same train as measured by an observer at rest on the train, *while it is moving* relative to the ground observer. In other words, the two measurements must measure the train while it is in the same state of motion. Stopping the train changes its state of motion.

 

[adelmakram]

and in the "platform" frame, we might "explain" by saying that the Proper Length of the coil is 6250 feet originally and 5000 feet finally so it experiences compression.

You said that for the ground observer, he will not see any change in the train length of 5000 feet before and after stop according to your first diagram. So the compression wave which starts at A for the train observer should not be seen by the ground observer! at the time the train stops. If the ground observer has to see any compression, he should observe this force when the train starts moving. In this case the proper length of 6250 drops down to 5000 feet during moving.

 

[adelmakram]

In this version of the problem, the coil will be under tension at the start of the experiment (while the train is moving in frame O), and the tension will be completely removed at the end of the experiment (when the train is at rest in frame O).

Why will the coil be under tension at the start?

This is a *different* problem specification than your original one. In this version of the problem, the train's length changes in frame O, which means the two ends must stop moving (with respect to frame O) at different times, in just the right way to undo the length contraction effect of the train's motion, in frame O. So in this version of the problem, the coil will be slack (no tension or compression) the whole time; its apparent length will increase in frame O, but this is purely due to the change in relative motion and does not correspond to any change in the internal forces in the coil.

Why does not the change in the relative motion cause any change in the internal force of the coil?

 

[adelmakram]

In this version of the problem, the coil will be under tension at the start of the experiment (while the train is moving in frame O), and the tension will be completely removed at the end of the experiment (when the train is at rest in frame O).

Why will be a tension at the start?

the coil will be slack (no tension or compression) the whole time; its apparent length will increase in frame O, but this is purely due to the change in relative motion and does not correspond to any change in the internal forces in the coil.

How the change of the relative motion between the 2 ends of the coil will not affect the internal force of the coil?

 

[adelmakram]

In other words, the two measurements must measure the train while it is in the same state of motion. Stopping the train changes its state of motion.

So how does the length contraction work from the beginning of the motion as long as both train ends can not be in the same state of motion relative to all observers?

 

[adelmakram]

In other words, the two measurements must measure the train while it is in the same state of motion. Stopping the train changes its state of motion.

So how does the length contraction work in the beginning of train motion as long as both train ends can not be in the same state of motion relative to all observers?

 

[adelmakram]

And one important question: How does the length contraction work from the beginning of the motion as long as both train ends can not be in the same states of motion relative to all observers?

 

[PeterDonis]

Why will the coil be under tension at the start?

I should have said, it will be under tension at the start, if we *assume* that it is under zero tension at the end, when the train is at rest in the observer's frame. But you could alternately assume that the coil was under zero tension at the start, which would lead to it being under *compression* at the end, when the train is at rest in the observer's frame. The key point is that the internal stress has to go from more to less tension (or less to greater compression) in this version of the scenario.

To see why, consider what is happening in this scenario when the train stops. As the scenario is specified in this version, both ends of the train stop at the same time in the observer frame. Suppose that after the ends stop, the train is under zero internal stress. The stopping operation does not change the length of the train (in this version) as measured in the observer's frame; call this length L. But that means that the "natural", or unstressed, length of the train before it stopped must have been smaller than L, with respect to the observer, because of length contraction. So in order for the actual length of the train to be L when the train is moving, relative to the observer, the train must be under tension, because its actual length is greater than its unstressed length.

Why does not the change in the relative motion cause any change in the internal force of the coil?

Because in this version, the two ends of the train stop at *different* times in the observer frame, and the times are related in just the right way to keep the train at zero internal stress, by changing the length of the train, as measured in the observer frame, so that it is always equal to its unstressed length, in that frame, taking length contraction into account.

 

[PeterDonis]

So how does the length contraction work from the beginning of the motion as long as both train ends can not be in the same state of motion relative to all observers?

Length contraction is not well-defined relative to a given observer if the two ends of the train are in relative motion, with respect to that observer. That's because the train's "length" itself is not well-defined, relative to that observer, if the two ends of the train are in relative motion.

Bear in mind that length contraction, like "length" itself, is a derived phenomenon in relativity; it's not fundamental. The fundamental objects are the worldlines of the parts of the train, which are invariant curves in spacetime and can be described without even choosing a reference frame. Length contraction, time dilation, relativity of simultaneity, etc., are not necessary to describe the physics; the only reason we talk about them is that our minds are evolved to perceive things in these terms.

 

[xox]

Bear in mind that length contraction, like "length" itself, is a derived phenomenon in relativity; it's not fundamental. Length contraction, time dilation, relativity of simultaneity, etc., are not necessary to describe the physics; the only reason we talk about them is that our minds are evolved to perceive things in these terms.

Very true. In a better world, the books and the physics journals like American Journal of Physics and The European Journal of Physics would stop generated the never ending slew of meaningless exercises on length contraction and relativity of simultaneity. I ignore such "papers" out of principle, they contribute absolutely nothing.

 

[adelmakram]

The stopping operation does not change the length of the train (in this version) as measured in the observer's frame; call this length L. But that means that the "natural", or unstressed, length of the train before it stopped must have been smaller than L, with respect to the observer, because of length contraction. So in order for the actual length of the train to be L when the train is moving, relative to the observer, the train must be under tension, because its actual length is greater than its unstressed length.

I have some concerns here:
1) If the stopping operation does not change the length of the train when it stops then the compression should not been observed in the frame O at the moment the stop. while in the frame T it is supposed to be observed when the end A stops first. So if the coil has to fail and break, it should do so when the A-end stops relative to the T-observer but not relative to O-observer.
For example, if L=5000 feet according to the numerical example shown above, and 5000 feet after stop, where is the compression at the moment the train stops?
2) I don`t understand why you said that the natural or unstressed train length is smaller than L despite in the earlier discussion the length will not change after stop according to my specific scenario? again for the shown diagram, the train measures 5000 feet while moving and 5000 feet after stop. By the way what d you mean by natural length?

 

[PeterDonis]

1) If the stopping operation does not change the length of the train when it stops

Please bear in mind that this is your specification of the scenario. If you don't like its implications then you should re-think your specification of the scenario. But given that specification, the implications are what I have said.

then the compression should not been observed in the frame O at the moment the stop.

What compression? Do you mean the fact that the unstressed length of the train is smaller, relative to frame O, while the train is moving, relative to frame O? If so, that is not "compression" in any physical sense; it's tension--the coil is under tension because its actual length is longer than its unstressed length.

while in the frame T it is supposed to be observed when the end A stops first.

As I've already pointed out a couple of times, in frame T, the ends of the train do not stop moving. They start moving. As it starts moving, its unstressed length, relative to frame T, gets shorter; but its actual length, relative to frame T, also gets shorter (because the end that starts moving first moves *towards* the other end in frame T); and if you work out the details, it turns out that the actual length gets shorter *more*, in frame T, than the unstressed length. So in frame T, just as in frame O, the train starts out with an actual length longer than its unstressed length, so that it's under tension; and it ends up with its actual length equal to its unstressed length, so that it's under no stress at all.

Again, all this is according to your specification of the scenario. Perhaps you need to take a step back and think through what your specification actually implies.

So if the coil has to fail and break, it should do so when the A-end stops relative to the T-observer but not relative to O-observer.

Incorrect; see above. If the coil were going to fail and break, it would have done so before the start of the problem, since the train starts out under the maximum amount of tension it will have in this scenario.

what d you mean by natural length?

The length that the train would have if it were under zero stress. Normally in scenarios like this we just stipulate what this length is, but if we wanted to, we could construct a microphysical model of the train in which this length was derived based on the internal forces between its parts.

 

[adelmakram]

Can you please take a look at the figures. May be I can use drawing better than words. Look at the length of coils in every frame and please tell me whether you are also seeing that the coil gets shorter in frame T and not in frame O.

More precisely, for the frame T at the moment A-starts to move toward B, there is a shortening of the length and there must be accompanied with compression ( or release of the tension). This event is not appreciated in frame O

 

[PeterDonis]

Look at the length of coils in every frame and please tell me whether you are also seeing that the coil gets shorter in frame T and not in frame O.

These drawings look fine. The length gets shorter in frame T, but does not change in frame O.

for the frame T at the moment A-starts to move toward B, there is a shortening of the length and there must be accompanied with compression ( or release of the tension).

I have been assuming the latter, and will continue to do so here. (But, as I noted in a previous post, the former is also consistent with your original specification.)

This event is not appreciated in frame O

What "event" do you refer to? If you mean that the tension is not released in frame O, you are incorrect; it is. This release of tension is not accompanied by a change in actual length in frame O, but it *is* accompanied by a change in the train's state of motion. The change in the train's state of motion causes a release of the tension in both frames.

 

[adelmakram]

This release of tension is not accompanied by a change in actual length in frame O, but it *is* accompanied by a change in the train's state of motion. The change in the train's state of motion causes a release of the tension in both frames.

Would you explain why? how does the change in the train state of motion release the coil tension despite there is no change in the length between its 2 ends?

 

[PeterDonis]

how does the change in the train state of motion release the coil tension despite there is no change in the length between its 2 ends?

There is no change in *actual* length relative to frame O. The change in the train's state of motion changes its unstressed length, relative to frame O. As I noted in a previous post, the unstressed length of the train depends on the forces between its atoms; and the change in the train's state of motion, relative to frame O, changes those forces. Inter-atomic forces are electromagnetic in nature, and EM forces change when the state of motion changes. In other words, forces are frame-dependent.

 

[adelmakram]

In the specific version, the actual length at the time of the train stop =unstressed length ( given the length contraction into the account). Numerically = 5000 feet in the example shown. As I understood, at the stop, the train unstressed length will not change too. Did I go wrong?

 

[PeterDonis]

the actual length at the time of the train stop =unstressed length ( given the length contraction into the account).

The actual length *after* the train stops equals the unstressed length. The actual length *before* the train stops is larger than the unstressed length. I'm not sure which one you mean by "the actual length at the time of the train stop".

As I understood, at the stop, the train unstressed length will not change too. Did I go wrong?

Yes. The train's unstressed length changes when the train stops.

 

[ghwellsjr]

and in the "platform" frame, we might "explain" by saying that the Proper Length of the coil is 6250 feet originally and 5000 feet finally so it experiences compression.

You said that for the ground observer, he will not see any change in the train length of 5000 feet before and after stop according to your first diagram. So the compression wave which starts at A for the train observer should not be seen by the ground observer! at the time the train stops. If the ground observer has to see any compression, he should observe this force when the train starts moving. In this case the proper length of 6250 drops down to 5000 feet during moving.

It doesn't matter which frame we use to analyze or depict a scenario. They all will show that everything that any observer can actually see or measure is the same.

What I want to do now is show you how an observer riding at the rear (red) end of the train will measure the Proper Length of the train as it drops from 6250 feet at the beginning to 5000 feet at the end.

In order to do this, he will use a laser range finder which shoots a pulse of light to a reflector at the front (black) end of the train and how it measures the round-trip time for light to get there and back and divides that time interval by two and calculates how far light travels in that amount of time. For our scenario, I have defined the speed of light to be 1 foot per nanosecond (or 1000 feet per microsecond). Actually, the laser range finder also keeps track of when each light pulse was sent and when its reflection was received and it applies the calculation of the distance to the average of those two times. This holds to Einstein's second postulate that the light takes the same amount of time to get to a target as it takes for the reflection to get back, although Einstein only applied it to an inertial observer and I'm applying both during intervals when the observer is inertial and non-inertial.

We go back to the original spacetime diagram in post #3 except that I'm only showing the two ends of the train and the laser signals going between them. I had to extend the diagram at the beginning and the end to accommodate the laser signals and their echoes.

Here's the first diagram. The thin red lines going upwards to the right are some of the laser signals that the observer at the rear (red) end of the train sends out. I'm only showing the ones that hit the black dots representing 1-microsecond intervals of time at the front of the train. The thin black lines going upwards to the left are the echoes. I have marked in the times on the observer's clock when the signals were sent and received:

The observer compiles a list of the sent and received times and averages them to determine the "time" for which the measurement applies. He also calculates one-half of the difference between those same two times (in microseconds) and puts down the distance (or length) in thousands of feet to the front of the train. Here is his list of measurements in the first two columns and his calculations in the last two columns:

Sent	Rcvd	Avg	1/2 of
Time	Time	Time	diff

0	12.5	6.25	6.25
1	13.5	7.25	6.25
2	14.25	8.25	6.25
3	15.25	9.25	6.25
4	17	10.5	6.5
5	19	12	7
6	21	13.5	7.5
8	22	15	7
10	23	16.5	6.5
12	24	18	6
14	25	19.5	5.5
16	26	21	5
17	27	22	5
18	28	23	5

And from that, he makes a spacetime diagram showing how the length of the train gets from 6250 feet to 5000 feet. Along the way, it gets longer and then gets shorter. Keep in mind that when the two ends of the train stop at the same time, the front of the train initially experiences compression and the rear of the train experiences tension as shown in the diagrams in post #11 but after all the vibrations dampen out, the Proper Length of the train ends up at 5000 feet:

Now I want to show you that the observer makes exactly the same measurements and therefor makes exactly the same determination of the length of the train in the platform frame:

Can you see that even though the time for the light to traverse to the target and the time for the echo to get back to the observer are completely different in this diagram, the observer still makes the same calculations because he is assuming that those times are always equal?

Now what about the platform observer? Well, he can apply exactly the same technique to determine his distances to the two ends of the train and then make the same space time diagram shown at the beginning of this post. Then he can do the same thing that we did and determine the Proper Length of the train as it comes to a stop.

 

[adelmakram]

For the platform observer, he will calculate the train length before and after stop to be equal to 5000 feet. In his case, he does not have to wait for the echo to calculate the distance because he is not attached to any end of the train. He can only multiply the time of the light takes to travel from one end to another by the difference between the speed of the light and the speed of the train in case the light rays goes in direction of the train and by the sum of both speeds in case the ray goes in the opposite direction of the train.

So again the question is: as long as there is no change on the length of the train before and after stop as seen by the platform observer, then there is no compression after the train stop. The phenomena that is seen by the train observer after the train stop is not seen by the platform one after the train stop. In other words, for the platform observer (the contracted length before the train stop - the true length after the train stop)=0

 

[PeterDonis]

as long as there is no change on the length of the train before and after stop as seen by the platform observer, then there is no compression after the train stop.

Wrong. You are incorrectly equating "no change in actual length" with "no compression". That is not correct. It would be correct if reality were not relativistic; in non-relativistic, Newtonian mechanics, yes, for there to be "compression" (more precisely, release of tension), there has to be a change in actual length, because the unstressed length of an object in Newtonian mechanics does not change when the object changes its state of motion. But in relativity, that's no longer the case; an object's unstressed length *does* change when it changes its state of motion. So in relativity, there can be "compression" (or release of tension) when an object changes its state of motion, even if its actual length, with respect to a particular frame, does not change.

All this has been pointed out to you several times now, so there's no point in your continuing to contradict it and our continuing to point it out again. If you have no new arguments to present, then there's not much point in further discussion.

 

[ghwellsjr]

For the platform observer, he will calculate the train length before and after stop to be equal to 5000 feet. In his case, he does not have to wait for the echo to calculate the distance because he is not attached to any end of the train. He can only multiply the time of the light takes to travel from one end to another by the difference between the speed of the light and the speed of the train in case the light rays goes in direction of the train and by the sum of both speeds in case the ray goes in the opposite direction of the train.

There are, of course, numerous ways to make measurements, but that is a side issue to the main one having to do with compression. I don't understand your method for the platform observer to measure the length of the train and I would be interested in your further clarifying it. But since we all agree on what the two observers determine the length of the train to be before and after stopping according to the two reference frames, let's focus on the compression issue right now.

So again the question is: as long as there is no change on the length of the train before and after stop as seen by the platform observer, then there is no compression after the train stop. The phenomena that is seen by the train observer after the train stop is not seen by the platform one after the train stop. In other words, for the platform observer (the contracted length before the train stop - the true length after the train stop)=0

I think maybe you are now using a definition of "compression" to be "a shortening of length". If so, then you are correct, for the platform IRF, the train does not change length and so there is no compression.

However, throughout this thread, you have used the terms "tension" and "compression" to refer to a force. Let me quote what I said in post #8:

In order for all parts of a train (or any object) to stop simultaneously according to a frame, there must be something like clamps set up all along the track which simultaneously stop the train (by preprogrammed timers) that bring all the parts to a halt (in that frame) and keep the different parts from expanding back to their natural length.

After the train stops, the train observer can get out of the train and look at the track to see if there are any clamps firmly constraining the train to its length of 5000 feet. The platform observer can hop off the platform and do the same thing.

Do you agree that there must be some sort of restraint on the train to keep its length at 5000 feet after stopping or are you maintaining that if the brakes were applied at the front and rear of the train simultaneously in the platform frame, then after all the vibrations settle down, there will be no lateral force between the wheels and the rails?

 

[adelmakram]

So in relativity, there can be "compression" (or release of tension) when an object changes its state of motion, even if its actual length, with respect to a particular frame, does not change.

All this has been pointed out to you several times now, so there's no point in your continuing to contradict it and our continuing to point it out again. If you have no new arguments to present, then there's not much point in further discussion.

But frankly speaking, I don`t see any answer yet to my question in your comments so far. You mentioned many times that there would be a force even there is no change in the length but you didn't prove that. From where this force came? How did the change in the state of motion cause a force that is not length- dependent? How if the matter is not related to a compression but to a simple dynamical system where it depends on the distance between the 2 ends and that distance changes in one frame and not in another?

 

[adelmakram]

Do you agree that there must be some sort of restraint on the train to keep its length at 5000 feet after stopping or are you maintaining that if the brakes were applied at the front and rear of the train simultaneously in the platform frame, then after all the vibrations settle down, there will be no lateral force between the wheels and the rails?

Yes I agree about this restraint. But I don`t get the meaning of lateral force. Can you please elaborate more about your reasoning?

 

[ghwellsjr]

Do you agree that there must be some sort of restraint on the train to keep its length at 5000 feet after stopping or are you maintaining that if the brakes were applied at the front and rear of the train simultaneously in the platform frame, then after all the vibrations settle down, there will be no lateral force between the wheels and the rails?

Yes I agree about this restraint. But I don`t get the meaning of lateral force. Can you please elaborate more about your reasoning?

Since you agree that there must be a restraint, then you agree that there is a lateral force (along the direction of the rails) between the rails and the wheels to keep the train from expanding back to about 6250 feet. Or, to put it another way, if there were no clamps restraining the length of the train, then even though the brakes are applied and the wheels stop turning, the wheels are ineffective in stopping the train simultaneously in the platform frame and they just slide on the rails until the train gets back near to its original Proper Length of 6250 feet (in the platform frame).

 

[adelmakram]

Since you agree that there must be a restraint, then you agree that there is a lateral force (along the direction of the rails) between the rails and the wheels to keep the train from expanding back to about 6250 feet.

This reply makes me even more confused: however there 1 urgent issue and one cold issue:

The first urgent issue is: Does that mean that every object moving with an apparent contracted length relative to an observer is under natural compression that to be relieved when it comes to a rest. If so , this means that the train or the coil in this example was under compression from the very beginning relative to the platform observer. For if this would be the case, a compression force should have started from the beginning of the motion and stayed all the time not only at the end. It also raises an important question, is the length contraction a mechanical phenomena?The less urgent issue: you illustrated before that for the train observer attached to the rear end, he will measures that the train length gets longer and then gets shorter before all vibrations dampen down. Now, during the lengthen phase which the train length increases to 7500 feet, there would be no compression but rather tension for the train observer, but for the platform observer, the force, if any, has to be applied all the time to keep the train length equals to 5000 feet.

 

[PeterDonis]

I think maybe you are now using a definition of "compression" to be "a shortening of length".

Which is *not* a correct definition, because, as I noted in a previous post, you can have "compression" (or release of tension, i.e., a change in the internal stress of the object) without any shortening of length in a particular frame. And it's clear that the OP really intends "compression" to mean "change in internal stress"; he is just incorrectly assuming that you can only have a change in internal stress if there is a change in length.

 

[PeterDonis]

You mentioned many times that there would be a force even there is no change in the length

More precisely, I said many times that there is a change in *unstressed length* even though there is no change in *actual length*. The change in unstressed length comes from a change in *internal* forces inside the object--i.e., the forces between its atoms--when its state of motion changes.

you didn't prove that.

I did not give a detailed proof because that would require constructing a detailed model of the object, atom by atom, including the internal forces between the atoms. However, I did give a sketch of how such a proof would go: internal forces between atoms are electromagnetic, and it's obvious from Maxwell's Equations that electromagnetic forces change when an object's state of motion changes.

From where this force came?

If you mean, where do the internal forces between the atoms come from, surely that's obvious: if they weren't there, the train would not hold itself together as a single object. It would just be a cloud of atoms flying around. So obviously there must be *some* force holding the atoms together. It turns out, when you look at the details, that that force is electromagnetic.

If you mean, where do the forces that change the train's state of motion come from, well, you're the one that specified the problem as involving a change in the train's state of motion, so if you don't know, how am I supposed to know? I was assuming, as was ghwellsjr, that brakes were applied at each end of the train.

How did the change in the state of motion cause a force that is not length-dependent?

The force doesn't have to be length-dependent; it just has to be motion-dependent, i.e., it has to change as the state of motion of the atoms in the train changes. Electromagnetic forces have that property; see above.

How if the matter is not related to a compression but to a simple dynamical system where it depends on the distance between the 2 ends and that distance changes in one frame and not in another?

What kind of dynamical system would have this property? Remember that in relativity, "distance" is frame-dependent, and the laws of physics can't be frame-dependent. So any correct relativistic force law can't have a force that depends on "distance".

Note carefully, however, that this does not mean forces themselves can't be frame-dependent; what can't be frame-dependent are the *laws* that the forces satisfy. So, for example, the law for electromagnetic forces are Maxwell's Equations and the Lorentz force law, and these are not frame-dependent. But particular forces that obey these laws *are* frame-dependent; the forces transform along with the fields that appear in the laws so that the same laws--the same equations--hold in every frame, with respect to quantities in that frame.

 

[PeterDonis]

I agree about this restraint.

I don't; at least, not with the version of the scenario we have been using, where there is zero stress in the train (and the coil) after it stops (relative to the platform). In that scenario, the restraint, which keeps the train's length at something different than its unstressed length, would have to be present *before* the train stops (relative to the platform).

If there is a restraint present after the train stops (relative to the platform), then the train (and the coil) must be under compression after it stops, and must have been under zero tension before it stopped (relative to the platform). I said many posts ago that that is also a valid scenario; but I didn't think it was the version we were discussing.

 

[PeterDonis]

Does that mean that every object moving with an apparent contracted length relative to an observer is under natural compression that to be relieved when it comes to a rest.

No, of course not. The only reason that happens in this scenario (and it's natural *tension*, not compression, if the stress is present before the train stops; I've pointed this out several times already) is because of the way you specified the scenario. You specified that, in the platform frame, the train's length does not change when it stops. *That* is what forces the change in stress in the train (either from tension to zero stress, or zero stress to compression).

is the length contraction a mechanical phenomena?

No. Changing an object's state of motion is a mechanical phenomenon. How the object's length changes, or doesn't change, relative to a particular frame, and how the internal stresses in the object change, or don't change, depends on how you change its state of motion.

The less urgent issue: you illustrated before that for the train observer attached to the rear end, he will measures that the train length gets longer and then gets shorter before all vibrations dampen down. Now, during the lengthen phase which the train length increases to 7500 feet, there would be no compression but rather tension for the train observer, but for the platform observer, the force, if any, has to be applied all the time to keep the train length equals to 5000 feet.

These are two different versions of the scenario again. If the train's total length is constrained, in the platform frame, to always be 5000 feet, then it can't lengthen and shorten in the train frame; it can only shorten. The vibrations ghwellsjr was describing cannot happen the way he described them in this case.

 

[ghwellsjr]

I think maybe you are now using a definition of "compression" to be "a shortening of length". If so, then you are correct, for the platform IRF, the train does not change length and so there is no compression.

Which is *not* a correct definition,

It certainly is one of the many correct definitions of compression according to dictionary.com.

because, as I noted in a previous post, you can have "compression" (or release of tension, i.e., a change in the internal stress of the object) without any shortening of length in a particular frame. And it's clear that the OP really intends "compression" to mean "change in internal stress"; he is just incorrectly assuming that you can only have a change in internal stress if there is a change in length.

And that is the point I made in post #38. But if you think I was unclear, then thanks for the added clarity.

 

[ghwellsjr]

I agree about this restraint.

I don't; at least, not with the version of the scenario we have been using, where there is zero stress in the train (and the coil) after it stops (relative to the platform). In that scenario, the restraint, which keeps the train's length at something different than its unstressed length, would have to be present *before* the train stops (relative to the platform).

If there is a restraint present after the train stops (relative to the platform), then the train (and the coil) must be under compression after it stops, and must have been under zero tension before it stopped (relative to the platform). I said many posts ago that that is also a valid scenario; but I didn't think it was the version we were discussing.

In the opening post, there was no tension, no compression, no coil. It was a question about timing as indicated by the title of this thread and for which I gave an answer in post #3. After that, the OP introduced the coil and an "extra tension" as a result of the train stopping that he thought would result in the coil breaking. I pointed out that this was wrong but that there was a compression and that SR was inadequate to determine if the coil would break.

I also pointed out that it was not enough to have just the two ends of the train stop simultaneously in the platform frame but that there needed to be clamps all along the length of the train programmed by timers to make all points of the train stop simultaneously.

So to avoid these problems, I suggested two separated trains with the coil stretched between them to avoid the issue of having any stress in a single train while it was moving. I still don't why you think that "version" of the scenario is the one that we were discussing. I also don't understand how you think such a tension could be enacted in a train moving on the tracks.

 

[PeterDonis]

In the opening post, there was no tension, no compression, no coil.

No coil, yes. No tension or compression, no. The initial specification of the scenario *requires* that there is either tension in the train before it stops, or compression after. There is *no* way to physically realize the scenario as the OP specified it without one of those two being present. So even though the OP did not say so, the OP scenario *does* have either tension or compression.

It was a question about timing as indicated by the title of this thread and for which I gave an answer in post #3.

Yes, but as the subsequent discussion has made clear, the OP was really interested in more than just the timing. He was interested in the physical implications of "length contraction" (and part of the problem is that he's confused about the different possible meanings that term can have). Your spacetime diagrams implicitly contain the answers to those questions as well.

After that, the OP introduced the coil and an "extra tension" as a result of the train stopping that he thought would result in the coil breaking.

He thought he was introducing "extra tension"; but that's only because he didn't realize that his original specification of the problem already included tension (or compression), as above.

I pointed out that this was wrong but that there was a compression

That depends on whether we assume that the train is under zero stress after it stops (in which case it's under tension before it stops), or before it stops (in which case it's under compression after it stops).

SR was inadequate to determine if the coil would break.

Correct, you also need a material model of the coil.

I also pointed out that it was not enough to have just the two ends of the train stop simultaneously in the platform frame but that there needed to be clamps all along the length of the train programmed by timers to make all points of the train stop simultaneously.

This is true if you want every part of the train to stop simultaneously, yes, which is what the OP seems to have intended.

So to avoid these problems, I suggested two separated trains with the coil stretched between them to avoid the issue of having any stress in a single train while it was moving. I still don't why you think that "version" of the scenario is the one that we were discussing.

I don't. I think the one we were discussing is the OP's original scenario, with the assumption that the train is under zero stress after it stops. The OP never explicitly stated this, but I think that's what he originally had in mind (not realizing the full implications, of course). If the OP wants to specify a different assumption, that's fine. The real point is that the OP needs to understand *all* the implications of whatever specification he makes of the scenario.

I also don't understand how you think such a tension could be enacted in a train moving on the tracks.

It certainly wouldn't be easy to do; but I'm not the one that made up the scenario. One possible way would be to have an engine at each end of the train, with the front engine pulling forward and the rear engine pulling backward, and a frictionless track so that the center of mass of the train is moving inertially. Of course, in a real train, this wouldn't work at relativistic velocities because the train would be torn apart; we don't make trains out of materials that can sustain relativistic stresses. But in principle you could do it that way.

 

[PeterDonis]

It certainly is one of the many correct definitions of compression according to dictionary.com.

You're right, the statement I made was too strong. Your point in post #38 (with my clarification) is really what I was trying to say, and I should have just stuck to that.