# Length contraction in a block universe must be an illusion

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2 minutes ago, 34student said:

During the year 2050, was the train 1 meter or 100 meters?

Who was doing the measuring? The answer is frame-dependent

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6 hours ago, studiot said:

Modern derivations start with the idea that different observers will make different measurements on the same phenomenon.

Now how is that logically possible.  How can one length be two lengths?

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Just now, 34student said:

Now how is that logically possible.  How can one length be two lengths?

It's not one length.

Go back to the example of kinetic energy you have ignored.

In my frame, the train is at rest, and thus no kinetic energy. In your frame it is moving, and has a KE of, say 10^7 Joules. How can it have two values of kinetic energy?

(answer: because KE is a frame-dependent quantity)

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31 minutes ago, 34student said:

Now how is that logically possible.  How can one length be two lengths?

Does this mean you wish to follow / engage with my chaing of reasoning ?

It is logically possible because the circumstances of different observers are well,    .....  different.

It is necessary to find quantities that are not different for different observers and equate them.

This will enable you deduce the contribution made by the differing circumstances.

Markus' muon example is one good example of this because 'number' is invariant.

We can also look carefully at your depiction of Bob.

Do you realise what 'speed' he is going to achieve a 100 : 1  length contraction ?

It works out at 0.99995c to 5 decimal places.

This is why I asked you how you think Bob can 'see' this contraction.

Are you going to consider and answer this ?

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54 minutes ago, swansont said:

It's not one length.

Go back to the example of kinetic energy you have ignored.

In my frame, the train is at rest, and thus no kinetic energy. In your frame it is moving, and has a KE of, say 10^7 Joules. How can it have two values of kinetic energy?

(answer: because KE is a frame-dependent quantity)

Interesting.  So are you saying that the train does not have one absolute size in the past?

16 minutes ago, studiot said:

Does this mean you wish to follow / engage with my chaing of reasoning ?

It is logically possible because the circumstances of different observers are well,    .....  different.

It is necessary to find quantities that are not different for different observers and equate them.

This will enable you deduce the contribution made by the differing circumstances.

Markus' muon example is one good example of this because 'number' is invariant.

We can also look carefully at your depiction of Bob.

Do you realise what 'speed' he is going to achieve a 100 : 1  length contraction ?

It works out at 0.99995c to 5 decimal places.

This is why I asked you how you think Bob can 'see' this contraction.

Are you going to consider and answer this ?

It doesn't really matter if Bob can see the contraction or not.  My argument is against the idea that the train contracts for Bob whether he can see the train or not.

And as a thought experiment, if I go fast enough, the sun should turn into the shape of a dinner plate.  Wouldn't that cause a lot more fusion and other changes to its gravity and overall behavior?

Edited by 34student
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35 minutes ago, 34student said:

It doesn't really matter if Bob can see the contraction or not.  My argument is against the idea that the train contracts for Bob whether he can see the train or not.

And as a thought experiment, if I go fast enough, the sun should turn into the shape of a dinner plate.  Wouldn't that cause a lot more fusion and other changes to its gravity and overall behavior?

Either you want to understand or you want to argue.

Which is it ?

Just remember that if you want to argue you are arguing with one of the two most experimentally tested theories in history.

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48 minutes ago, studiot said:

Either you want to understand or you want to argue.

Which is it ?

Just remember that if you want to argue you are arguing with one of the two most experimentally tested theories in history.

I want to learn, always.

I always thought that the past was one 4 dimensional static block, literally a block of static particles that would look like like strings because of the dimension of time.  But now I am being told that there is a number of different pasts.  One past universe where the train is 1 meter long, another past universe where the train is 100 meters long, etc.

This is quite shocking.

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22 minutes ago, 34student said:

This is quite shocking.

It is indeed pretty shocking.
It shocked a lot of people all over the word when it was first proposed, including some very important thinkers.

22 minutes ago, 34student said:

But now I am being told that there is a number of different pasts.  One past universe where the train is 1 meter long, another past universe where the train is 100 meters long, etc.

I don't think this is the case.

There is only one train.

But there are many points of view and I am offering a slightly different route into them than usual.

Especially as I see you have actually been at SF for a long time so must have seen quite a few arguments about the subject.

It is a pity you don't address my questions about the example you specified since that example is impossible to conduct in practice.

This is why I would like to introduce and discuss a real world example, the first exerimental verification that the formulae actually  correspond to observation.

Edited by studiot
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12 minutes ago, studiot said:

It is indeed pretty shocking.
It shocked a lot of people all over the word when it was first proposed, including some very important thinkers.

I don't think this is the case.

There is only one train.

But there are many points of view and I am offering a slightly different route into them than usual.

Especially as I see you have actually been at SF for a long time so must have seen quite a few arguments about the subject.

It is a pity you don't address my questions about the example you specified since that example is impossible to conduct in practice.

This is why I would like to introduce and discuss a real world example, the first exerimental verification that the formulae actually  correspond to observation.

I addressed your issue about Bob observing the contraction.  Why does it matter whether Bob sees the contraction or not.  I do not know what this has to do with my issue.

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1 hour ago, 34student said:

I want to learn, always.

3 hours ago, 34student said:

So are you saying that the train does not have one absolute size in the past?

From the viewpoint (inertial frame) of a muon traveling through the earth's atmosphere the length contraction effect of special relativity allows the muon to penetrate and reach earth. In the muon frame its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame.

The above is not an illusion; the distance through the atmosphere is, was, and will be different in different inertial frames and it has physical effects.

(above text is based on https://en.wikipedia.org/wiki/Muon)

Edited by Ghideon
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2 hours ago, studiot said:

Just remember that if you want to argue you are arguing with one of the two most experimentally tested theories in history.

And obviously if one wants to argue against some of the most tested theories in history, one should know all there is about such theories. I certainly don't and I doubt that our doubter in this thread does...why even some scientists fail to understand it properly. The point is that the experimentally shown interchangeability of space [length] and time, means that if one is affected then the other is also. My interval of time and space is your space...your space and time, is my time. In essence, they are the same thing...one cannot exist without the other.

6 minutes ago, Ghideon said:

Agreed, your first example was convincing as I did mention earlier...our "doubting Thomas" appears to have overlooked it though.

3 hours ago, 34student said:

Now how is that logically possible.  How can one length be two lengths?

Edited by beecee
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From any valid frame, inside the universe, length is frame dependent, as so many people have repeatedly told you.
There is NO valid frame outside the universe ( block or any other model; nor other dimensions in the way you describe ), from which to measure the train.

The OP is ill-posed, and cannot be answered, or even considered, as such.

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1 hour ago, 34student said:

I addressed your issue about Bob observing the contraction.  Why does it matter whether Bob sees the contraction or not.  I do not know what this has to do with my issue.

Really?

You said

4 hours ago, 34student said:

It doesn't really matter if Bob can see the contraction or not.

And yet you continue to describe your pet example to others as though it is of vital importance.

5 hours ago, 34student said:

Now how is that logically possible.  How can one length be two lengths?

and you received my explanation to which you have made a defeaningly silent response.

Yet you say cavalierly that you have dealt with my issue.

You have also not responsed to my earlier offers of other simple explanations like invariants and Bob's speed etc and my proposal to discuss a different (and better) example since Bob's was irrelevant to you and impossible for me.

In short you are just trolling.

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11 hours ago, 34student said:

I always thought that the past was one 4 dimensional static block, literally a block of static particles that would look like like strings because of the dimension of time.

Sure, that would be a block of events, and the "strings" are world lines of particles.

Imagine you have a block of say wood, and you draw a small line on it to represent a world line. You can rotate the block in your hands, but the shape and length of the line remains invariant. Say that you rotate it through a fixed grid of coordinates in a room. Using just 2 dimensions for simplicity, you can make the line align with the x-axis, or the y-axis, or anything in between. By rotating it, you can change how long the line is in the x-dimension and how long it is in the y-dimension, without changing its length.

But the block universe also has a time dimension, so instead of x and y, imagine it's t and x. The line could be a function x(t) representing how far x a particle moves in time t in a particular set of coordinates, and you get different x-lengths in different coordinates just by rotating the block.

In this analogy the rotation is a spherical rotation which keeps r^2 = x^2 + y^2 invariant as you rotate it. With spacetime, the rotation is hyperbolic, which instead keeps s^2 = x^2 - t^2 invariant as you rotate it. So you can't just turn it upside down and reverse time etc.

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18 hours ago, 34student said:

During the year 2050, was the train 1 meter or 100 meters?

It depends on the observer of course - there will exist an observer in whose frame it is 1m, and there will also exist an observer in whose frame it is 100m. All of these observers are right - but only in their own frames.

The length of the train is thus not an intrinsic property of it, any more than eg. its speed or kinetic energy. It is meaningful only in relation to a specific observer measuring it, so asking about its “real” length is meaningless.

There are indeed quantities that all observers in spacetime agree on (tensors and invariants), but length is not one of them; it’s strictly a quantity that describes a relationship to a given observer only.

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19 hours ago, 34student said:

Interesting.  So are you saying that the train does not have one absolute size in the past?

Yes. The length is relative (hence “relativity”). It depends on the observer.

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8 hours ago, md65536 said:

Sure, that would be a block of events, and the "strings" are world lines of particles.

That is what I thought, and this is why I am so confused.

Going back to the example in my OP, let's think about a particle sitting at one end of the train.  The train lies on the x axis.  The particle has one "String"/world line for Bob and another for Earth observers, (and possible an infinite more creating a plane).

Which world line is real, or do they both exist?  If both particle locations exist, why can't one of the two observers detect both particles?

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25 minutes ago, 34student said:

That is what I thought, and this is why I am so confused.

Going back to the example in my OP, let's think about a particle sitting at one end of the train.  The train lies on the x axis.  The particle has one "String"/world line for Bob and another for Earth observers, (and possible an infinite more creating a plane).

Which world line is real, or do they both exist?  If both particle locations exist, why can't one of the two observers detect both particles?

Did none of the rest of what I wrote make sense?

The particle has only one world line. The world line is the events (the x and t locations) that the particle passes through over its lifetime. The world line is a fixed set of events, but the x and t values of those events change if you rotate it into other coordinate systems representing other observers.

For example, put a stick on a grid with an x and t axis. If you align it with the t axis, it has the same x value at different t, representing the world line of a particle at rest. If you rotate the grid, the stick stays the same but now it has different x at different t, representing a different observer, for which the particle is moving.

This is an oversimplified analogy and it uses the wrong rotation, so don't draw too many conclusions from it. If you rotate the stick or grid, the stick's x and t components change, but the length of the stick stays the same. It's the hypotenuse in the Pythagorean theorem r^2 = x^2 + t^2. With the correct hyperbolic rotation, s^2 = x^2 - t^2 stays the same.

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2 hours ago, 34student said:

If both particle locations exist, why can't one of the two observers detect both particles?

A specific observer can't move relative themselves; one observer is always in one inertial frame*. So the observer will make observations, consistent with relativity theories, in that single frame. That means the observer will observer one and only one set of x and t coordinates (as described by @md65536).

*) Multiple observers may share a single frame and any observer may change frame but as far as I know no observer can be simultaneously in two different frames.

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2 hours ago, Ghideon said:

A specific observer can't move relative themselves; one observer is always in one inertial frame*. So the observer will make observations, consistent with relativity theories, in that single frame. That means the observer will observer one and only one set of x and t coordinates (as described by @md65536).

*) Multiple observers may share a single frame and any observer may change frame but as far as I know no observer can be simultaneously in two different frames.

Ok, but objectively speaking, does more than one world line exist for the particle?

3 hours ago, md65536 said:

Did none of the rest of what I wrote make sense?

The particle has only one world line. The world line is the events (the x and t locations) that the particle passes through over its lifetime. The world line is a fixed set of events, but the x and t values of those events change if you rotate it into other coordinate systems representing other observers.

For example, put a stick on a grid with an x and t axis. If you align it with the t axis, it has the same x value at different t, representing the world line of a particle at rest. If you rotate the grid, the stick stays the same but now it has different x at different t, representing a different observer, for which the particle is moving.

This is an oversimplified analogy and it uses the wrong rotation, so don't draw too many conclusions from it. If you rotate the stick or grid, the stick's x and t components change, but the length of the stick stays the same. It's the hypotenuse in the Pythagorean theorem r^2 = x^2 + t^2. With the correct hyperbolic rotation, s^2 = x^2 - t^2 stays the same.

I think that I understand what you are saying.

Now here is my question that will either put my issue to rest for me or keep me wondering.  If the particle is at x1 for Bob, and x2 for people on Earth, does the particle necessarily have to go through x1 and x2 (not necessarily in this sequence)?

7 hours ago, swansont said:

Yes. The length is relative (hence “relativity”). It depends on the observer.

Then that would seem to imply that the train does not have a single world line.

11 hours ago, Markus Hanke said:

It depends on the observer of course - there will exist an observer in whose frame it is 1m, and there will also exist an observer in whose frame it is 100m. All of these observers are right - but only in their own frames.

The length of the train is thus not an intrinsic property of it, any more than eg. its speed or kinetic energy. It is meaningful only in relation to a specific observer measuring it, so asking about its “real” length is meaningless.

There are indeed quantities that all observers in spacetime agree on (tensors and invariants), but length is not one of them; it’s strictly a quantity that describes a relationship to a given observer only.

With world lines in mind, this means that there are at least 2 trains in existence (and probably many more from other frames of reference).  Do you agree?

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1 hour ago, 34student said:

Then that would seem to imply that the train does not have a single world line.

I don’t see how. The train isn’t in two places at once.

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The system of the whole train is not. The individual cars are, in which case he’s mixing frames (pretending that the cars being in different places is equivalent to the train as a whole being in more than one place). He thinks he’s being clever.

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1 hour ago, 34student said:

With world lines in mind, this means that there are at least 2 trains in existence (and probably many more from other frames of reference).  Do you agree?

No, there is one train. If you had two, one could crash and the other not, but every observer has to agree events, like whether it crashed. The observers will disagree on what time and how far away the event occurred.

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48 minutes ago, iNow said:

The system of the whole train is not. The individual cars are, in which case he’s mixing frames (pretending that the cars being in different places is equivalent to the train as a whole being in more than one place). He thinks he’s being clever.

So if there are 10 cars to the train, are you saying that there actually are 20 cars?

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38 minutes ago, 34student said:

So if there are 10 cars to the train, are you saying that there actually are 20 cars?

Nobody said anything like that.

There’s nothing legitimate to be gained by making the problem more complex.

There is nothing about relativity that says anything about the number of cars being relative.

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