# The Naked Scientists Forum

### Author Topic: What is a Lorentz contraction?  (Read 12051 times)

#### bardman

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##### What is a Lorentz contraction?
« Reply #25 on: 18/02/2011 04:26:30 »
The problem really is about defining what an illusion is. I understand why you see it as an illusion, it appears to be one. But you need to ask this question:

Should we accept the observer's recording of the two events or the runner's record?

It would seem we should accept the runner's, after all he is recording in the same frame as the ladder is moving.

But the barn is moving in the observer's frame.

And can we say that anyone observation is better than another?

According to physicists, and I'm inclined to agree, we should not make it a matter of preference. What we should do is realize what the motions are and take them into account. This is what the Lorentz is, and why we have defined dilations and contractions.

We take observations, describe the events, and if desired; we can change to another frame and describe the events in that frame.

This is exactly what we do with muon. We observe a very short lifetime and everything that stems from that works with the other observations we can make. If desirable, we can adjust the lifetime into that of the muons frame. In this case, the lifetime is longer and other observations made in that frame are adjusted too.

However, the laws of physics are the same in every frame. The measurements change, but the laws remain the same.

#### bardman

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##### What is a Lorentz contraction?
« Reply #26 on: 18/02/2011 04:29:10 »
It should also be noted that the clock is not actually ticking slower, the point in space where the tick takes place is farther away than the last one, thus it takes longer for us to realize it ticked, causing us to record more than one tick for every tick in the movers frame

#### yor_on

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##### What is a Lorentz contraction?
« Reply #27 on: 18/02/2011 06:02:21 »
Even using a sliding system you find yourself ending up in a universe where, alternatively.

1. Time dilation & Lorentz contraction is 'real'.

2. Time dilation is 'real'. Lorentz contraction is not.
3. Lorentz contraction is 'real'. Time dilation is not.

4. Neither is real, including your own arrow.

5. Neither are real, only our common Newtonian 'arrow of time' is

6.'Time dilation' is real, but your own arrow is an illusion? That one, as it starts from defining a 'time dilation' relative that same illusionary arrow :) Well, let's just say it begs for my imagination. Contradiction of terms that need some real brain gymnastics

7. Only your own arrow of time is 'real'. All other definitions are made from comparisons with other frames, containing their own unique 'arrows' relative you, when comparing.

What more possibility's?
« Last Edit: 18/02/2011 12:45:00 by yor_on »

#### yor_on

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##### What is a Lorentz contraction?
« Reply #28 on: 18/02/2011 06:44:25 »
We'll need to dissect this Bardman :)

But as for arguing that no positional SpaceTime system, or 'frame of reference' are more 'true' than any other? Conceptually I agree, but looked at from my reality I find it very easy to define what 'system' that 'ticks' for me.

That I also can compare it to a Black hole, or a speeding rocket, and see a sliding correlation between them does not invalidate that. A easy argument is how we define a experiment to be true. We need to be in the same frame of reference for finding it true, as for example Einstein did with a constantly uniformly accelerating rocket at one G. Proving all experiments to deliver the same results, ignoring tidal forces, as for a planet of the same gravity.

I believe that definition to hold for all uniformly moving frames too, that you will get the same results from a experiment in that black box, their speed relative each other making no difference.

But it all comes back to how we define it. I believe the arrow I know to be real, and to me it will be the same no matter what I do. That when comparing different 'frames' introduce a difference is no hindrance to that concept. Neither is the returning twin being younger that the one staying at home. As long as we define 'the arrow' you perceive as 'the same' as any other arrow, the difference being how it will express itself relative lights speed, distance defined, and gravity.

Doing so I can isolate the changes to the frame involved, in it expending energy adjusting 'distance' and/or gravity, as it had to do so at some time of its existence to change those relations, presenting you the same 'Time/Lorentz sliding relation' to other frames as we normally expect, but being restricted by its own energy expenditure and gravity.

It's the way I look at it for the moment :) from a unchanging internal 'arrow of time' :) But with distance and so also 'speed' becoming questionable as I expect the Lorentz contraction to be a real thing. And the 'time dilation' being defined as a relation to that distance, as expressed in lights invariant speed in a vacuum and gravity.

So three things I trust in :)

Lights speed in a vacuum.
Gravity permeating all of SpaceTime, measurable or not, as defined by Inertia.
Your own 'unchanging' arrow of time.

For the moment, that is:)
==

It's not that I can't see your point. But one can alternatively see it as more of a conceptual exercise, describing how different positional systems, as defined from lights invariant speed in a vacuum, versus 'relative motion' & gravity introduces differences when comparing 'frames of reference'. To me it's about 'reality', and the question, if this is real, what does it say about our universe?

And I'm sure you agree that if it is real we have a very interesting universe :) In it you will be able to shrink the length of a distance by speeding up, as defined relative your place of origin. In it there are no defined speeds as all uniformly moving objects can be defined as inertial frames, actually stating that your speed relative A can be any speed, no matter what you define it as relative A. Yes, I think one can use the CBR as a speedometer? Or lights blue-shift, but both of those involves outside 'relations' which you won't have in a black box scenario.

Somehow reality seems to come down to 'relations'.

« Last Edit: 18/02/2011 07:39:48 by yor_on »

#### simplified

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##### What is a Lorentz contraction?
« Reply #29 on: 18/02/2011 14:38:00 »
On the other hand, shouldn't we all be 'time dilated' constantly, relative all other 'frames of reference'? If we should, where is the Lorentz contractions? Against that one might argue, as I did before, that it's only a acceleration that creates it? possibly also that in the twin experiment neither twin notice any time dilation, and only the moving twin noticing the Lorentz contraction. but then we come to the question of how 'time' differ between uniformly moving frames, knowing who should age slower?

If we have two relatively moving frames (constant and one being 0 or not), then as I understand it, the time is always being dilated for both and both are being length contracted. Here is what I understand of the twin paradox.

If you end up with two equally aged people moving relative to each other, accelerating to the relative motion is a problem but suppose we ended up with equally aged people with motion relative each other, and they both die at the same age (their life is the same amount of time in their respective frames of reference after the position and time coordinates are synced) Each will have appeared to died after the other in the frame they are in. To a person who also synced up the time and position of the twins and had them moving at equal speed in opposite directions relative to that person, they appeared to die simultaneously, but after the time they did in their frames.

The problem with saying who actually aged more is that relative to each other, they both aged more and the only way to compare clocks is at the proper time, which means you read the clocks at the same point in space. This is impossible unless they turn (or one person turns) around and come back. Then acceleration takes effect. This destroys the formulas for constant motion. When the clocks are brought back to the same point in space, the times read the same and so the twins must have died at the same time.
Speed = 259627884 m/s. Their lives move to the future with one speed.They have grown old for twenty years. Then the first has visited the second. The visitor is younger than the owner for ten years.And so acceleration of the first  is process of rejuvenation then, and in general you have opened travel to the past.BillS was asking about it.

#### bardman

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##### What is a Lorentz contraction?
« Reply #30 on: 18/02/2011 15:42:28 »
I think we agree and disagree on somethings yor_on.

I understand and agree that your time (and space coordinates) are the golden measure for you, as well as the indisputable speed of light.

However, I think were we part paths is on the reality of the dilation and/or contractions. I simply state it as the variance in the 4-coordinates for different observers. I think that in certain parts of your arguments you try to present it as an actual change in the evolution of time. But, other times I feel like you're agreeing with what I'm saying, just in a roundabout way.

But as for arguing that no positional SpaceTime system, or 'frame of reference' are more 'true' than any other? Conceptually I agree, but looked at from my reality I find it very easy to define what 'system' that 'ticks' for me.

That I also can compare it to a Black hole, or a speeding rocket, and see a sliding correlation between them does not invalidate that. A easy argument is how we define a experiment to be true. We need to be in the same frame of reference for finding it true, as for example Einstein did with a constantly uniformly accelerating rocket at one G. Proving all experiments to deliver the same results, ignoring tidal forces, as for a planet of the same gravity.

I believe that definition to hold for all uniformly moving frames too, that you will get the same results from a experiment in that black box, their speed relative each other making no difference.

Somehow reality seems to come down to 'relations'.

You should define things in they the system "ticks" for you, I agree. So, when you measure different values than a uniformly moving frame, your results work for you. But, you have to realize those values work for the other frame too, realizing based on the Lorentz formulas, the values hold in that frame too.

The second paragraph there has no change in reference as far as I see (although if the non-rocket observer is on earth, they have no acceleration from g because the normal force balances the equation). But if two observers are accelerating with the same g, their frames are identical, the coordinates they measure will be the same if they choose the same zero coordinates (this is not the case in uniformly moving frames with nonzero relative motion, even if they zero their coordinates, they get different values). I said that all experiments must yield the same result.

The Lorentz transforms were created to maintain the laws of physics in moving frames. If you do an experiment on a single event in two relativistically moving frames, then the results will not provide the same laws of physics unless the Lorentz transformed distances and times are applied. Just for clarity: the same classical laws apply for each inertial frame, in that frame. If you step outside a relativistic inertial frame, you then must go relativistic.

I agree that reality is relations in regard to the fact that you must apply a transformation to understand why two people experience different things. But I hold that the reality is the same no matter what frame you are in. Our clocks might tick differently, but they tick differently in the same way for both of us. The rate at which my clock ticks faster than yours is the same rate faster your clock ticks than mine. And I realize that if I had the same perspective, they would be ticking the same, thus they are the same clock, just in different frames of reference. Its all a matter of perspective.

#### yor_on

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##### What is a Lorentz contraction?
« Reply #31 on: 18/02/2011 22:31:54 »
Would the second paragraph be the one about 1G at Earth being identical to a uniform (constant) acceleration at the same G? When it comes to Earth some like to turn it around and say that Earth is 'accelerating' at one G. Although if we look at it from a time dilation it won't be true, it will still be impossible to differ it, inside a black box, tidal forces ignored that is.

"the coordinates they measure will be the same if they choose the same zero coordinates (this is not the case in uniformly moving frames with nonzero relative motion, even if they zero their coordinates, they get different values)"

What coordinates are we speaking of here, and how would you measure them inside a black box? I think we are thinking of two different definitions here. you are using positional systems, meaning that you expect them to have a way of defining those, whereas I'm talking about what you experience locked inside that 'black box'. Do you agree?

The Lorentz transformations, as I see it, is a way to define how two frames of reference differ from each other, translating them into each others frame. It's a extremely smart mathematical device, but also a  mathematical 'tool', as I see it, You use it to define those frames in time and space relative each other to get a conceptual overview.

As I understand it Lorentz used them first to explain the Michelson-Morley experiment "In order to explain this absence of any effect of the Earth's translation, I have ventured the hypothesis, that the dimensions of a solid body undergo slight change, of the order of v2/c2, when it moves through the ether.

From this point of view it is natural to suppose that, just like the electromagnetic forces, the molecular attractions and repulsions are somewhat modified by a translation imparted to the body, and this may very well result in a change of dimensions. The electrons themselves become flattened ellipsoids.

This would enable us to predict that no experiment made with a terrestrial source of light will ever show us an influence of the Earth's motion."

But to apply them you will need to have a defined 'position' relative something else, or a 'origin' common to both as a reference point, and that's not what I was referring too. I discussed it from a 'black box scenario' when i called them 'equal'.

And no, it's not really true, although in fact it is one of the major headaches you lift up when you write "But I hold that the reality is the same no matter what frame you are in. Our clocks might tick differently, but they tick differently in the same way for both of us. The rate at which my clock ticks faster than yours is the same rate faster your clock ticks than mine. "

That you know that the 'time dilation' will belong to one twin, not both, is why there have been a recent proposition where they offer valid proofs for how you can solve that problem relative very far so called 'fixed stars'. It is in fact so that you in two moving frames of reference, like Earth versus a uniformly moving rocket near light speed, are free to define either Earth or the rocket as being a 'inertial frame' having a 'zero motion' relative the other, although some might want to differ here.

But as it turns out, in the twin experiment only one of our twins can be said to be 'time dilated' relative the other, if we accept that they otherwise originally should show the same biological age (Earth). And it's to get around that fact this proposal has been lifted up as a answer (Fixed stars) as I understands it.
==

This is in fact one of my main reasons for wondering if the universe have a 'gold standard' defining motion, hidden or not. It may seem a solution, but it's ad hoc to me. On the other hand there might be no definite 'truths' when it comes to motion? It may be us having given it a definition that is wrong, using 'distance' and 'time' as our 'yard stick'. If relativity is right those two are plastic and ultimately reduces all matter to point particles, although always 'existing' (taking up 'place' in a prolonged manner, time wise.) as a proof for invariant mass/fermions relative the opposite, bosons/light.
===

And yes I agree. That is if you ask if I see both the time dilation and Lorentz contraction as being very real, like that the pole actually being inside the barn? I'm afraid I do, and it makes it a very weird universe doing so? But so interesting to me :)

To invalidate the effects, we will need to invalidate the twin experiment first I think.
« Last Edit: 19/02/2011 15:01:05 by yor_on »

#### yor_on

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##### What is a Lorentz contraction?
« Reply #32 on: 18/02/2011 23:04:07 »
In a way I'm contradicting my former statement here, that I see us all as time dilated and Lorentz contracted, referring to how only one twin can be said to be time dilated. But it's a way to conceptualize the fact that we have all those 'frames' moving at different 'speeds' relative us, even though we don't share the same origin.

The shared origin is the ultimate proof for a 'time dilation' and that's why the twin needs to be back on Earth comparing their 'biological age'. When I call it a 'time dilation' relative a frame not originating from Earth, I can't really prove that, not without first bringing it to a same 'origin' finding a way to define the 'clock ticks' valid for both , then sending it of and returning to see if they now differ.

But if the twin experiment is correct, then I can extrapolate it to those other 'unknown' frames too, as they should experience the exact same, if I did the experiment. And then there must be a constant 'time dilation taking place between both accelerating objects, as well as all uniformly moving. and the Lorentz contraction will be there if compared against moving 'frames', and then placing one self at rest with what one first compared to see the difference.

#### yor_on

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##### What is a Lorentz contraction?
« Reply #33 on: 19/02/2011 12:12:56 »
"The problem really is about defining what an illusion is."

I totally agree there.

It's a factor of the utmost importance. We are so incredibly clever in creating our tools that we sometimes forget the pre/assumptions making us construct them. So what I'm also wondering about is where the limit goes for defining what is real. In relativity we have two definitions, they are not the same but we act as if they are. One is 'my black box' in where we absolutely goes only from what one observer can and will observe in his own 'frame of reference', and then act on what we expect him to observe. Even though this too is a conceptual exercise in that we're not 'there' it's as close as we can get to that 'reality'.

The other is the purely conceptual. In that realm we use Lorentz transformations and juggle with all kinds of concepts, mathematical or not, comparing them and drawing conclusions that, to their nature, is very hard to experimentally verify. As long as we want our ideas to be experimentally testable we need to be as close as possible, as I see it, to what a 'black box' will tell us. That is, going from what we believe us to be able to prove from a minimalistic viewpoint. But they constantly go into each other, don't they :)
« Last Edit: 19/02/2011 14:15:30 by yor_on »

#### bardman

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##### What is a Lorentz contraction?
« Reply #34 on: 19/02/2011 16:02:40 »
Would the second paragraph be the one about 1G at Earth being identical to a uniform (constant) acceleration at the same G?

I'm sorry, that got bound up. In order for us to talk about this, the frames need have the same acceleration, therefore there is a zero acceleration between the two.

As I understand it Lorentz used them first to explain the Michelson-Morley experiment "In order to explain this absence of any effect of the Earth's translation, I have ventured the hypothesis, that the dimensions of a solid body undergo slight change, of the order of v2/c2, when it moves through the ether.

From this point of view it is natural to suppose that, just like the electromagnetic forces, the molecular attractions and repulsions are somewhat modified by a translation imparted to the body, and this may very well result in a change of dimensions. The electrons themselves become flattened ellipsoids.

This would enable us to predict that no experiment made with a terrestrial source of light will ever show us an influence of the Earth's motion."

Yes, this is correct. They decided ether isn't a real medium of motion though. Also, the dimensions do change (in the direction of motion) for the observer. However, were you to be in frame of reference of the moving object, you would experience no change in dimensionality. You would see the earth change in the same way it saw you change. It is mutual.

But to apply them you will need to have a defined 'position' relative something else, or a 'origin' common to both as a reference point, and that's not what I was referring too. I discussed it from a 'black box scenario' when i called them 'equal'.

If I am understanding correctly, the origin you reference would be the 2, 4-coordinate axes of space-time (one for each observer) that at some point in space-time were aligned (synced or zeroed as I called it before) and as time passed, the time and a number of the space axes (the ones that are in relative motion) become dilated (time) and contracted (space).

And no, it's not really true, although in fact it is one of the major headaches you lift up when you write "But I hold that the reality is the same no matter what frame you are in. Our clocks might tick differently, but they tick differently in the same way for both of us. The rate at which my clock ticks faster than yours is the same rate faster your clock ticks than mine. "

That you know that the 'time dilation' will belong to one twin, not both, is why there have been a recent proposition where they offer valid proofs for how you can solve that problem relative very far so called 'fixed stars'. It is in fact so that you in two moving frames of reference, like Earth versus a uniformly moving rocket near light speed, are free to define either Earth or the rocket as being a 'inertial frame' having a 'zero motion' relative the other, although some might want to differ here.

But as it turns out, in the twin experiment only one of our twins can be said to be 'time dilated' relative the other, if we accept that they otherwise originally should show the same biological age (Earth). And it's to get around that fact this proposal has been lifted up as a answer (Fixed stars) as I understands it.

I think this is where we really diverge and we need to converge in order to make progress. Unfortunately, I read about the twin paradox in a textbook and I don't have it right here to reference specifically at the present time.

However, this is what I know to make sense. You say that the person on Earth is the person at rest. For the time being, in order to remove any bias as to what "rest" is, let us say instead that we have two people floating through space, in different directions, person 1 and person 2. Now let us reexamine the twin paradox. Suppose they are "twins" in our sense and their motion relative can be 100% uniform (I say this now because I will introduce acceleration later).

Let us assume that we observe from person 1's perspective, that is person 1 is our rest frame. Person 2 moves uniformly through space relativistically for some time. During this time, person 2's clock appears to read that person 2 is getting progressively younger than person 1 (not aging as fast-time dilation). Suppose person 2 instantaneously turns around, not losing any factor of this time dilation, thus as person 2 returns relativistically, it still appears that they are aging more slowly. Thus, person 2 is younger when they return to person 1.

Thus far you agree with me, it would seem person 2, being that they traveled, is younger. Now I think we diverge.

Suppose we reverse the roles now, we observe from person 2's perspective. This is an equally valid perspective, all perspectives MUST be otherwise physicists wouldn't bother proving anything. According to these observations, the space-time coordinate axes are at rest throughout this whole process. The motion of person 1 appears to be the same to person 2 as the motion of person 2 appeared to be to person 1 (except directionally). Thus, person 2 sees the identical effect. Person 1 must be younger than person 2.

This is why we call it the twin paradox. Both persons observe identical effects, meaning that each appears younger to the other. If you disagree with me here, I don't know how we can continue this discussion. The reason being, you must see that both perspectives are valid, the picking of which one to use is arbitrary because physics says the laws must hold for all perspectives.

So, this is how the paradox is resolved. The paradox is acceptable, time is allowed to appear dilated if we synced the clocks at one point in space and then try to measure them both from one point in space after the clocks have distanced themselves. The amount of dilation depends on distance and speed traveled to that point. However, they should not be dilated when we bring the clocks back to the same point to measure them. This measurement is called the proper time interval. Meaning synchronized clocks should read the same when measured at the same point in space. The flaw is in the fact that we tried to do an instantaneous reversal of velocity.

The deceleration and re-acceleration, no matter how large, affect the clocks. The deceleration performed a sort of "de-synchronization", if you will. Then the acceleration "re-synchronized" the clocks, by this I mean that it changed them in a way such that the dilation of the return trip made it so that the clocks read the same when read at the same point in space.

Hopefully you see that this is a resolution to the paradox.

#### yor_on

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##### What is a Lorentz contraction?
« Reply #35 on: 19/02/2011 18:19:13 »
You're quite right, we diverge there :)

If your interpretation is right all clocks 'ticks' the same, and there we converge :) but your definition of why & how they 'tick the same' seem to differ from mine. In your twin experiment it seems as if both twins when meeting should be the same age, as you can interchange their frames. Assume that you have a position A<-0->B where 0 is a 'origin' for both A and B. Then assume both A and B accelerate as depicted in opposite directions from their 'origin' to then continue in a uniform motion.

Here we have three objects. 0=origin (Earth) and A&B are our rockets.

Assume identical trinns for this, not twins but three persons. One on Earth, one in A, and one in B. Also assume them making a circle in space meeting each other on their way back so that they can measure each other ships. They both measure each other as they move, each finding the other clocks to go equally slower and also their ships being equally Lorentz contracted. Assume that they both had a equal speed as measured against Earth. That should mean that they, according to how I understands it, both will be equally 'time dilated' relative Earth when returning, having done identical journeys, but starting in different directions relative each other.

Will the the sibling on Earth find the other two younger than him in your definition, after returning to 0 (Earth)?
Will the two travelers find each others age to be the same when compared?

Now assume that their journeys wasn't equal. A accelerated double the amount of B before 'coasting', moving uniformly. They meet up and measure each other again, both finding the others clock going 'slow', and the other ship being shorter than their own. But, will the others clock, as observed from either one 'tick' equally slow in the other ship, and will both clocks be equally slower than in our first example, as A accelerated double than B this time? And will their respective 'contraction' be equally larger, than measured the first time?

When they return to Earth (0) will the travelers age be the same, or will A now be biologically younger than B in your definition. How would you grade them in form of 'age', meeting up on Earth this time, after traveling?

Either you agree with me in that their age will differ. If you do so then we have the universe I expect. Or you have a different definition of 'time dilation' and 'Lorentz contraction'.
==

(Sorry, I'm slow today. Think I overslept here :)
Phieew, this took some time, and I'm not sure it's understandable now either..

==

(You might mean that a true 'time dilation' only comes to be in the acceleration though?
I see it as a effect from both acceleration and uniform motion.)
« Last Edit: 19/02/2011 19:01:53 by yor_on »

#### yor_on

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##### What is a Lorentz contraction?
« Reply #36 on: 19/02/2011 19:12:04 »
This is not a perfect description in that ideally we would like 0 Earth being equally 'still' relative both. It's much simpler when comparing only two uniformly moving frames, as Earth moves a third way relative A and B, as I think of it. But for this one we can assume 0-Earth to be a really, really, still world :) Our gold standard for zero movement sort of :)
==

We could alternatively let them go the exact same direction, but at different times (Greenwich Mean Time :) so that they would have to meet each other while journeying forth to some predefined position, and back. Not that this is perfect either, but slightly better maybe.
=

Never mind, I hope my question is clear anyway :)

==

Anyway, my view of those two situations is that, when measuring each other ships in space, both situations will show them equally contracted and their time ticking equally slow, although in the second version A and B will find each other clocks slower than the first time, also having a bigger contraction passing each other. And, finally back on Earth, in the first situation A & B will find themselves, assuming having the same time dilation relative Earth under their journeys, both to be younger than 0, but having the same age relative each other. In the second version I would expect A to be youngest, then B, with 0 becoming the biologically oldest.

If that is wrong I will get a headache :)
Not that I ever haven't had those before.
I have I mean ::))

This one might help define it, my point of view.
the relativity of simultaneity and time dilation,

But I agree, the idea is certainly hard to 'melt' as we say in Sweden (as in stomach:), But I think it is true, even though weird. I can definitely see why you would prefer yours, it makes sense, whilst my view make for some real weird statements. It is also this effect of really, truly, desynchronizing clocks by motion, meaning that my and yours time actually differs, for real, that creates the statement that a 'event' can be happening before X for A but after X for B, and both be true. Lorentz transformations is not only a transformation between frames, but also a way of linking those frames to each other in SpaceTime, treating time and space as one whole 'unit', but differentiating with motion and gravity. That's also why I wonder if each SpaceTime observed won't have to be unique for this to be true.
« Last Edit: 19/02/2011 21:20:12 by yor_on »

#### yor_on

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##### What is a Lorentz contraction?
« Reply #37 on: 20/02/2011 08:07:00 »
I think that Einsteins description of it being a 'whole SpaceTime' is to blame for this one. With motion and, as JP calls it, the stress–energy–momentum tensor defining gravity rebuilding that SpaceTime. To me it's very weird as it opens for so many questions.

1. How can we have a undifferentiated SpaceTime if both distance and time change, just by me moving?

2. How can we communicate?

3. Two Rindler observers synchronized without ever knowing it, do they have their own SpaceTime, I mean they should? Is then that SpaceTime the 'real one' for them, themselves unknowing.

4. And depending on what I compare myself against it seems I can get a 'new SpaceTime' just by changing my focus, or do I get that wrong?
==

I think I can guess about 1 and 2.

I blame those on lights speed in a vacuum always being the same. At the same time as that is what seem to introduce those effects it is also what binds them together. I can't see any other answer to that one? It has to be light.

And what the he* is 'reality'.

==

When it comes to the stress–energy–(momentum?) tensor i find that so much nicer as a description of gravity that the idea of it being invariant mass creating it. That we thought so earlier made sense as gravity is so strongly coupled to invariant mass but lately I've started to wonder about why it is. Before I just took it for granted that 'Gravity' must have invariant mass to exist. Now I'm not sure about that any more.

To see how I started to wonder let's assume that gravity is a expression, not a force. Also assume that SpaceTime is the only game in town, with its own set of gaming rules. Those rules only have to make sense to and in the 'game'. If you like, or don't like, the game will not matter as it is the only game you have. And the expression (gravity) will then express itself according to strict 'rules'. One of them seems to involve acceleration of your personal SpaceTime, another involves 'invariant mass'. I think this one is easier to stomach for those into RPG:s (Role playing games) :) and those used to see their 'screens' reality change with the site they get on :) And for us a little older more questionable as we remember such weird things as real trees, stretches of wet stuff that we called water, etc, etc :)
« Last Edit: 20/02/2011 08:25:06 by yor_on »

#### yor_on

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##### What is a Lorentz contraction?
« Reply #38 on: 20/02/2011 09:28:14 »
The pole in the barn makes sense if we remember that from the position of the man at rest relative the barn, both the barn and him will be 'un-contracted' relative the speeding pole. you might see it as a spear thrown really fast, contracting relative us watching it, standing still relative that spear, at rest on the sideline so to say, just as the barn is. If we could get a gnome to sit on the spear he would find it as long as it was before being thrown, but that is a result of him adapting to that contraction too.

So when the spear gets inside the barn, we looking at it from the side will find it to 'fit' momentarily inside that barn, before moving out the back door. but what did the gnome see? I mean it's not the spear that have shrunk to him, it's everything that he sees that is contracted instead, including the barn and us looking at him. That is a direct effect from him moving so fast, all distances shrinking. So if we ask him he would tell us that at no point was the spear inside that same barn, the barn being, in his own words, 'Just too da*ed small for my big spear' Well, he was actually looking, very meaningfully, at our female reporter as he said that, but I still think it was the spear he was talking about?

Anyway, that is how I understands the pole in the barn, excluding gnomes and reporters that is :) The same way as the muon finds the distance to Earths surface being shorter than we do being 'at rest' with and on it, ah, Earth that is. And if that is the 'truth' then the gnome and I didn't share the same 'room time geometry' at that moment, according to how I see it, or 'frame of reference' as we also call it. And maybe not even the same 'SpaceTime' although that is a matter of definition. If we let 'communication' be the arbiter of that I expect us to have been able to 'communicate' as any radiation he sent out, like radio waves still would move relative both him and me at the speed of light in a vacuum, more or less :) Just being extremely blue-shifted for me. So, using light we share the same 'SpaceTime', but in all other manners we see two totally different 'reality's'.

In mine reality the spear was fitting inside the barn.
In his reality it never was.

==

If we get rid of the idea of 'speed' then both the gnome and me was at rest, relative our own 'reality'. Meaning that the 'room time geometry' you have always will be 'at rest' relative you, just as the clock-ticks never will change for you, no matter how fast you go, or what 'stress tension' are acting on you in form of gravity.
==

In the link I gave we can find this explanation to the pole experiment.

"Can we or can we not trap the rod inside the barn by closing the front door while the whole rod is inside according to a ground observer? When the front end of the rod hits the rear door, information about this impact will travel backwards along the rod in the form of a shock wave. The information cannot travel faster than c, so the rear end of the rod will continue to travel forward at its original speed until the wave reaches it. Even if the shock wave is traveling at the speed of light, it will not reach the rear end of the rod until after the rear end has passed through the front door even in the runner's frame. Therefore the whole rod (albeit quite scrunched up) will be inside the barn when the front door closes. If it is infinitely elastic, it will end up compressed and "spring loaded" against the inside of the closed barn."

This is one explanation but it does not explain how the whole of SpaceTime become contracted as you travel. And so it's not sufficient at all. Well, not as I can see at least? Space is not light, the only way I see to make sense of it, looking at space, would then be to assume that space do not contain 'distance' at all?

==

There are some other things too, that explanation builds on Lorentz first descriptions of how atoms gets 'squeezed' in the direction of motion. but if they did we could assume that the spear not only would get contracted but also thicker. If we assume that it doesn't and that it keep its 'normal' thickness as seen from all frames then there should be a state where the spear is so contracted/compressed in the direction of its motion that it could become a Black Hole. Also it assume that when you 'speed up' the contraction will have a 'starting point' on that spear the contraction traveling as a wave down, or possibly up, the shaft, as it seems to me? Now that might be right in rocket where I might assume that the force from the engine 'pushes' the contraction to greater and greater 'shrinkage' starting where the force is strongest, the engines to then spread through the ship, possibly? but with that spear?

Ah well, it's weird :)

I don't agree to that assumption, to me it's the whole of SpaceTime that gets contracted when you accelerate, not only a geometric illusion combined with 'atom squeezing'. That also seem to have been the way Einstein described it, as I understand it? If I would guess, reading that, I would say that we once more have tried to apply what we've seen working from our common frame of reference (Earth) and tried to apply that on Einsteins concept of SpaceTime? Hey, it's just a guess :) Stop throwing things..
« Last Edit: 20/02/2011 14:28:21 by yor_on »

#### butchmurray

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##### What is a Lorentz contraction?
« Reply #39 on: 26/09/2011 18:48:29 »
The answer in Einstein’s own words.

“Relativity The Special and General Theory” by Albert Einstein is great. Translated by Robert Lawson. It the 1920 translation. It’s a free book online.

XII.  The Behaviour of Measuring-Rods and Clocks in Motion
http://www.bartleby.com/173/12.html

#### JP

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##### What is a Lorentz contraction?
« Reply #40 on: 26/09/2011 19:22:36 »

#### The Naked Scientists Forum

##### What is a Lorentz contraction?
« Reply #40 on: 26/09/2011 19:22:36 »