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Length contraction in a block universe must be an illusion


34student
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The year is 2050.  Bob goes close to light speed in his ship.  He sees a 100 meter train contract to 1 meter.  He gets back from his trip in the year 2100.

Aliens from another dimension are looking at our block universe from 2050 to 2100.  Will they see a 100 meter train or a 1 meter train or something else?

This is meant to support the argument that length contraction is an illusion and not actually happening.

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30 minutes ago, 34student said:

Aliens from another dimension are looking at our block universe from 2050 to 2100.  Will they see a 100 meter train or a 1 meter train or something else?

Additional dimensions are not part of established relativity theories. AFAIK the models does not try to answer your question. For an observer within our universe the relative velocity between the observer and the train together with Lorentz transformation will get you the answer.

30 minutes ago, 34student said:

This is meant to support the argument that length contraction is an illusion and not actually happening.

Relativistic effects are confirmed in experiments, I don not think illusion is a proper label. Here is an example of time dilation / length contraction:

Quote

When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions are created. These decay within a relatively short distance (meters) into muons (their preferred decay product), and muon neutrinos. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 meters ( 2.197 µs × ln(2) × 0.9997 × c ) at most (as seen from Earth) the time dilation effect of special relativity (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer half-life due to their velocity. From the viewpoint (inertial frame) of the muon, on the other hand, it is the length contraction effect of special relativity which allows this penetration, since 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. Both effects are equally valid ways of explaining the fast muon's unusual survival over distances.

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

Edited by Ghideon
clarifying
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38 minutes ago, 34student said:

Will they see a 100 meter train or a 1 meter train or something else?

This is meant to support the argument that length contraction is an illusion and not actually happening.

In your aliens' view of the universe, is Bob's ship moving? Is the train moving? If so, how fast? Does this question support the argument that movement is an illusion and not actually happening?

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1 hour ago, 34student said:

The year is 2050.  Bob goes close to light speed in his ship.  He sees a 100 meter train contract to 1 meter.  He gets back from his trip in the year 2100.

Aliens from another dimension are looking at our block universe from 2050 to 2100.  Will they see a 100 meter train or a 1 meter train or something else?

It depends on their speed relative to the train.

1 hour ago, 34student said:

This is meant to support the argument that length contraction is an illusion and not actually happening.

False dichotomy. It is not an illusion, and it's not a physical effect that happens to the object. If an object is length-contracted to 1 meter, then everything measured in that frame will be consistent with the object being 1 meter long. In that frame, the length is 1 meter. Length is relative.

Let's change the experiment to the train's kinetic energy, which is another relative quantity. If you are standing next to the train, or are otherwise at rest with respect to it, it has no kinetic energy. In Bob's frame, the train has a gamma factor of 100, so the train's kinetic energy is 99m0c2

Is that an illusion or actually happening? Are you willing to be Bob and collide with the train because it's just an illusion?

The underlying issue is the assumption that there is a preferred frame of reference, which tells us the "truth" when there is no such thing. These measurements are frame-dependent. We are familiar with kinetic energy being frame-dependent, as it is Newtonian and obvious even for slow speeds, but much less so with time and length.

 

 

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In a 'block' universe ( 3D+T ) nothing is moving.
Events trace out worldlines within the 'block'.

So why would you even expect to see length contraction in that model.

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2 hours ago, swansont said:

These measurements are frame-dependent.

That seems to be the relevent point to be taken with this question. Each set of measurements or measure of time is as real as each other in any frame, and of course the relationship and interchangeability between space and time.

 

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3 hours ago, md65536 said:

In your aliens' view of the universe, is Bob's ship moving? Is the train moving? If so, how fast? Does this question support the argument that movement is an illusion and not actually happening?

Nothing moves in a block universe.

 

2 hours ago, MigL said:

In a 'block' universe ( 3D+T ) nothing is moving.
Events trace out worldlines within the 'block'.

So why would you even expect to see length contraction in that model.

Exactly.  My point is that I do not believe length is relative.

 

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

Exactly.  My point is that I do not believe length is relative.

The fact that space and time are interchangeable and the evidence is conclusive that time dilation is real, and relative, means that space [length] also is relative.

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10 minutes ago, swansont said:

So how is Bob moving?

He isn't.  Things do not really move in special relativity either.

8 minutes ago, beecee said:

The fact that space and time are interchangeable and the evidence is conclusive that time dilation is real, and relative, means that space [length] also is relative.

My op explains the contradiction with the way we think about special relativity today.

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28 minutes ago, 34student said:

He isn't. 

“Bob goes close to light speed in his ship” sounds like Bob is moving.

28 minutes ago, 34student said:

Things do not really move in special relativity either.

That’s an interesting delusion.

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The 'Block' Universe model does not allow for motion because it encodes past, present, and future.
Worldlines are the paths that events take through space AND time.
Similarly the BU model does not allow for 'external' observers.

As the BU model is based on GR, it is self-consistent with its rules, including length contraction, and the impossibility of an external frame of observation which encompasses the whole universe.
IOW, the extra-dimensional observers of the OP are non-sensical.

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27 minutes ago, MigL said:


Similarly the BU model does not allow for 'external' observers.

  

Why not?

45 minutes ago, swansont said:

That’s an interesting delusion.

Please read up on the block universe.

 

Edited by 34student
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6 hours ago, 34student said:

Bob goes close to light speed in his ship.

This has no meaning in (special) relativity.

All motion is 'relative', and you have not specified this part.

Such a distinction is not necessary for light itself as it has the same speed relative to all bodies.
But this is the only case where this happens.
Even zero speed is relative to something.

 

6 hours ago, 34student said:

He sees a 100 meter train contract to 1 meter.

Really ?

How

Again this statement is woefully short on detail.

 

6 hours ago, 34student said:

Aliens from another dimension are looking at our block universe from 2050 to 2100.  Will they see a 100 meter train or a 1 meter train or something else?

Where did the 'block universe' creep into your theory of relativity ?

I will grant you the aliens bit with the proviso that what you are describing is what said aliens could calculate if they existed.
Of course they do not, but with suitable maths (do you have any ?) the appropriate relationships can be modelled and calculated.
But again the calculations have to be done in somebody's terms and again your have left the issue of whose terms unaddressed in your question.

2 hours ago, 34student said:

Exactly.  My point is that I do not believe length is relative.

Not 'exactly'. Again I'm looking for some maths here, not belief.

Ask (less than) half a question and you are getting more than half an answer since I am pointing to what needs to be done to complete your question or statement.

Edited by studiot
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1 hour ago, 34student said:

My op explains the contradiction with the way we think about special relativity today.

I don't see any contradiction, particularly in light of the irrefutable evidence for the interchangebality of space [length]  and time, and as per the link I gave and the explantion in the first post answering your predicament. 

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6 hours ago, 34student said:
6 hours ago, MigL said:


Similarly the BU model does not allow for 'external' observers.

 

Why not?

Because the 'Block' Universe is a product of GR, and, as such, does not allow a universal frame of reference.
A 'God's eye' view of the universe, past, present, and future, would be such a frame, and not allowed by GR.

Edited by MigL
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13 hours ago, 34student said:

Exactly.  My point is that I do not believe length is relative.

In SR, when a frame is said to be ‘in motion’ with respect to another frame, that means it is rotated about a hyperbolic angle in Minkowski spacetime, again with respect to the coordinate system of the reference frame. This is why speed can be expressed as an angle, called rapidity. This is just what Lorentz transformations do - they deliver a rotation, plus a boost (which is irrelevant here).

Measurements of length are the projection of the world line onto the spatial axis of the associated coordinate system; measurements of duration are projections onto the time axis. Now, if you rotate the coordinate system about some angle (=relative motion!), then this will quite naturally alter the magnitude of these projections. This is why we see kinematic time dilation and length contraction. Note that the geometric length of the world line itself does not change - all observers agree on it. They are just looking at the same thing from a different angle in spacetime - quite literally so. Note also that the rotation is a hyperbolic one in Minkowski spacetime, which is not quite the same as a Cartesian rotation in Euclidean space.

None of this is in any way in conflict with the ‘block universe’ picture, nor are there any contradictions anywhere. It’s just elementary hyperbolic geometry.

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1 hour ago, Markus Hanke said:

In SR, when a frame is said to be ‘in motion’ with respect to another frame, that means it is rotated about a hyperbolic angle in Minkowski spacetime, again with respect to the coordinate system of the reference frame. This is why speed can be expressed as an angle, called rapidity. This is just what Lorentz transformations do - they deliver a rotation, plus a boost (which is irrelevant here).

Measurements of length are the projection of the world line onto the spatial axis of the associated coordinate system; measurements of duration are projections onto the time axis. Now, if you rotate the coordinate system about some angle (=relative motion!), then this will quite naturally alter the magnitude of these projections. This is why we see kinematic time dilation and length contraction. Note that the geometric length of the world line itself does not change - all observers agree on it. They are just looking at the same thing from a different angle in spacetime - quite literally so. Note also that the rotation is a hyperbolic one in Minkowski spacetime, which is not quite the same as a Cartesian rotation in Euclidean space.

None of this is in any way in conflict with the ‘block universe’ picture, nor are there any contradictions anywhere. It’s just elementary hyperbolic geometry.

Markus has mentioned 'projections' several times recently.

It is worth noting that there is a difference between a projection (called resolved parts in my attachment) and a component.

Unfortunately distinction is blurred for many because they are the same thing in orthogonal coordinates, but become different in other geometries and the use of tensors.

This arises because of interaction between the variables in non orthoganal systems.

 

resolvedparts1.jpg.8573ed7e613ca67177a1bf5fff8ebdab.jpg

 

Edited by studiot
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12 hours ago, MigL said:

Because the 'Block' Universe is a product of GR, and, as such, does not allow a universal frame of reference.
A 'God's eye' view of the universe, past, present, and future, would be such a frame, and not allowed by GR.

You didn't explain why; please explain why.  Note that a God's eye view of the block universe would have no effect on it. 

6 hours ago, Markus Hanke said:

Note that the geometric length of the world line itself does not change - all observers agree on it. They are just looking at the same thing from a different angle in spacetime - quite literally so.

This statement supports my argument that length contraction is not actually happening and is just an illusion.  

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52 minutes ago, 34student said:

This statement supports my argument that length contraction is not actually happening and is just an illusion.  

It is only an "illusion" if you consider something else to be "real". Which particular angle do you consider to be "real"?

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On 10/26/2021 at 10:59 AM, MigL said:

In a 'block' universe ( 3D+T ) nothing is moving.
Events trace out worldlines within the 'block'.

So why would you even expect to see length contraction in that model.

 

Don't you have to choose your coordinates for such a block universe? Then you can measure the distance between points with the same T value.

For example, choose coordinates for the block universe where T corresponds with a clock that is stationary relative to the train. Take a pair of events with the same T value, one at the front of the train and one at the back. You can find the proper length of this train, say 100 m.

Or choose coordinates where T' corresponds with Bob's proper time and find 2 different events with the same T' and find that the distance between those 2 events is 1 m.

The aliens would also have to choose a set of coordinates, and could measure the length contraction of the train relative to its proper length.

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3 hours ago, 34student said:

This statement supports my argument that length contraction is not actually happening and is just an illusion.  

The length contraction is real in those circumstances, as there would also be a related dilation with the aspect of time.

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