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Offline old guy

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Evidence for large scale length contraction?
« on: 31/08/2012 17:51:10 »
Special relativity (SR) theory has a part called length contraction. There is evidence for it on subatomic scale in particle accelerators, but none that I know of on large scale.
(Incoming muons having a longer than expected "lifespan" does not make Earth's atmosphere thinner (contracted), except "for a muon," not actually thinner)

SR theorists claim that from a frame of reference flying by Earth at near light speed ('c') Earth's diameter as measured in the direction of the fly-by would be contracted, making Earth a very oblate spheroid rather than the near sphere established by Earth science. Further, they claim that the effect is not just a distortion (appearance only) but that Earth is in fact flattened (like the subatomic particles) "for that frame of reference" and that "there is no preferred frame of reference," so that measurement (flattened shape of earth) is "equally valid."

What say you SR experts here?


 

Offline imatfaal

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Re: Evidence for large scale length contraction?
« Reply #1 on: 31/08/2012 18:09:37 »
From the rest frame of the ship it is completely valid.  From the lab on earth we can predict with accuracy what the ship would observe and it matches what the ship's crew do observe.

Welcome to the forum by the way. 


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Offline lightarrow

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Re: Evidence for large scale length contraction?
« Reply #2 on: 31/08/2012 19:28:33 »
Special relativity (SR) theory has a part called length contraction. There is evidence for it on subatomic scale in particle accelerators, but none that I know of on large scale.
(Incoming muons having a longer than expected "lifespan" does not make Earth's atmosphere thinner (contracted), except "for a muon," not actually thinner)

SR theorists claim that from a frame of reference flying by Earth at near light speed ('c') Earth's diameter as measured in the direction of the fly-by would be contracted, making Earth a very oblate spheroid rather than the near sphere established by Earth science. Further, they claim that the effect is not just a distortion (appearance only) but that Earth is in fact flattened (like the subatomic particles) "for that frame of reference" and that "there is no preferred frame of reference," so that measurement (flattened shape of earth) is "equally valid."

What say you SR experts here?
1. Definition of lenght between two points of an object: measure the simultaneous position of the two points and make the difference.

2. In SR simultaneity is frame-dependent (= if two spatially separated events are simultaneous in a frame, they are not in a frame which is moving with respect to the first).

1. + 2. = ...
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #3 on: 31/08/2012 20:29:20 »
Thanks folks for the replies and welcome. What I am still confused about however is the difference between the phrases "for a muon" (the atmosphere is contracted) or "for the ship flying by Earth (its diameter is contracted) and the actual depth of earth's atmosphere and its actual diameter(s) (polar a bit shorter than equatorial) as well known and documented by Earth science.

Surely SR's length contraction theory is not claiming that earth's atmosphere depth and diameter varies with how they are measured or how they would be observed from a muon's or the high speed ship's frames of reference. Right?
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #4 on: 31/08/2012 22:36:41 »
It actually is defined just the way you doubt :) As belonging to your 'local clock and ruler'. And that local clock will always fit that local ruler just the same, and the definer of how that come to be is 'c', lights speed in a vacuum. The question then becomes if this contraction is a illusion or a reality. As far as I can see it's a reality, not a illusion. The contraction logically follows from the stipulation that lights speed in a vacuum constantly is the same, no matter from where you measure it. If you assume that 'time' is a illusion you then will have to do the same with 'distance' as they are complementary phenomena, meaning that what A measure for B may be a slower 'time', but from B:s side his time will be the same as always although the distance measured is 'contracted'. So to invalidate it one will have to redefine 'c' as a variable instead of a constant.
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #5 on: 01/09/2012 03:01:29 »
It actually is defined just the way you doubt :) As belonging to your 'local clock and ruler'. And that local clock will always fit that local ruler just the same, and the definer of how that come to be is 'c', lights speed in a vacuum. The question then becomes if this contraction is a illusion or a reality. As far as I can see it's a reality, not a illusion. The contraction logically follows from the stipulation that lights speed in a vacuum constantly is the same, no matter from where you measure it. If you assume that 'time' is a illusion you then will have to do the same with 'distance' as they are complementary phenomena, meaning that what A measure for B may be a slower 'time', but from B:s side his time will be the same as always although the distance measured is 'contracted'. So to invalidate it one will have to redefine 'c' as a variable instead of a constant.
Thanks, yor_on. This leaves me wondering if SR recognizes a "natural world" or whether measurement from different frames, measuring the same object differently creates many different versions of reality for the same natural object. Or does SR deny that there are "natural objects" which exist independent of measurement?

Also I wonder how the well documented slowing down of clocks' rates of timekeeping at increased velocities makes distances shrink (like earth's diameter), I mean, other than the concept that time dilation and length contraction are reciprocal functions in the math.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #6 on: 01/09/2012 03:44:43 »
That's a question I'm working with too. Assuming, as I do, that a contraction/time dilation in some terms become a 'symmetry', you want the experimental proofs for it. the only proofs we have so far, that I know, ius those done high energy experiments in where we observe particles 'live' longer that they should, and in those astronomical as observing muons. But it is a very tricky thing to proof , if you think of it. Assume that you are on a very fast rocket measuring a distance. You may swear to that it is 'contracted', but that neigboor measuring you will swear to that it is your 'clock/time' that has slowed down. Time dilations, as gravitational are so much easier to proof experimentally and has been done by NIST at decimeters though. And if you think of it

Assume you are on that rocket.
Do you expect your time to slow down?
Read Tolstoy collected works in a second?

Or do you expect yourself (uniformly moving, just to make it more precise) to find your time to behave as usual?

If you expect that, how will you explain that you cover a distance in so much less time than what is possible from that definition of 'time as usual'?

To assume that 'time' really slows down is wrong, as proven by NIST in their gravitaiona time dilation experiments.

What is left?

Contractions.

As for a object being the 'same' relativity, loosely speaking now and depending on how strict you want to be, states that 'locally', using your own ruler and your own wrist watch, nothing change for you. Doesn't matter how fast you go as measured relative something else, or what mass you are on. Everything stays the same for you.

But, yeah, it's weird.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #7 on: 01/09/2012 04:06:53 »
The problem we meet is one of 'rigidity' relative 'plasticity'. Relativity is about 'plasticity' from a global perspective, comparing 'frames of reference', as you measuring some other position in (your local) time using that local ruler of yours. Locally 'rigidity' exist though, and object will behave the same when measured (uniform motion again:) as proved in our 'repeatable experiments'.

Accelerations are something that does not need another frame of reference to be defined. At least not macroscopically, we all know in our body when we accelerate. Although you can still question what make us know, do we need a 'universe' filled with mass for experiencing a acceleration or will it exist in a 'empty space' too?
=

The 'empty space' here must then be defined relative the smallest constituent making sense in physics, and as our definitions stops making sense somewhere around Plank scale you can then assume that for this question to have a validity you will have to refer it to that scale. What that also, and all as I see it, state is that as long as we keep above Planck scale you must have several frames of reference acting relative each other, meaning that ones body indeed consists of both time dilations as well as LorentzFitzGerald contractions. Now, if we use this definition then what creates the feeling must be a result of those different 'frames' acting relative each other and so we have defined a acceleration as a result of frames of reference acting relative each other, bound to matter (particles).

And if this solution lose its coherence around Plank scale, what we will find instead will be Quantum mechanics.
=

what such a definition fail to to account for is how matter at rest with each other can experience 'gravity' though. We all experience it, no matter if we move relative Earth or not. And in Einsteins terms a uniform acceleration becomes the equivalent of a 'gravity'. So, using frames of reference, how do I define Earths gravity?
« Last Edit: 01/09/2012 04:52:17 by yor_on »
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #8 on: 01/09/2012 18:40:00 »
So far no replies seem to have addressed my title question. I will reply to each point and ask more specifically in each case, in order of replies.
Imatfaal said:
Quote
From the rest frame of the ship it is completely valid.  From the lab on earth we can predict with accuracy what the ship would observe and it matches what the ship's crew do observe.

Does “valid” for the ship mean that ‘flattened’ ( having a contracted diameter) is a valid description of Earth? How can that be? You didn’t address my confusion in this regard in reply to your answer above:

Quote
What I am still confused about however is the difference between the phrases (edited)... "for the ship flying by Earth” (its diameter is contracted)  and its actual diameter(s) (polar a bit shorter than equatorial) as well known and documented by Earth science.

I do understand that the Lorentz formula can accurately translate or transform the observation of a contracted diameter (from the near ‘c’ frame) back to the nearly spherical shape as measured from at rest with Earth’s frame.

Lightarrow said:
Quote
1. Definition of lenght between two points of an object: measure the simultaneous position of the two points and make the difference.

2. In SR simultaneity is frame-dependent (= if two spatially separated events are simultaneous in a frame, they are not in a frame which is moving with respect to the first).

Regarding #1: This would seem to yield the familiar earth science measurements of earth, say as from in orbit, at rest with Earth.
Regarding #2: I don’t understand how the relativity of simultaneity addresses the question of a length contracted Earth diameter or my statement, “Surely SR's length contraction theory is not claiming that earth's atmosphere depth and diameter varies with how they are measured.”

Yor_on’s first reply included the statement, “...If you assume that 'time' is a illusion you then will have to do the same with 'distance'...”

As my reply indicated, I’m ok with “time” being the concept required for all movement, and that clocks ‘tick’ more slowly the faster they move, called “time dilation” though “time” need not be an entity of any kind which “dilates.”
Further, I still don’t see how a slower ‘ticking” clock, say onboard a ship flying at near ‘c’ makes the distance it travels or the objects it measures shorter.

I will give an illustration. The Alpha Centauri complex (AC) is 4.37 light years away from Earth. It takes  light from AC 4.37 (edit) years to reach Earth, and no “thing” with mass can travel that fast.
A ship traveling at near ‘c’ velocity from here to there must, therefore take longer than 4.37 years to get there, even though the onboard clock will have slowed down and recorded much less than 4 years passing. Yet Earth will have orbited the Sun (the "year" standard) much more than 4 times during the ship’s journey to AC. So, even though “for the ship” much less than 4 years will have passed, the distance between Earth and AC will not have contracted to way less than 4 light years. The ship’s journey obviously will not make Earth and AC move closer together.

I hope these more specific questions and the above example will be addressed, or this thread will be buried in more questions than answers to the title subject. Thanks.
« Last Edit: 01/09/2012 18:52:17 by old guy »
 

Offline David Cooper

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Re: Evidence for large scale length contraction?
« Reply #9 on: 01/09/2012 20:49:38 »
Does “valid” for the ship mean that ‘flattened’ ( having a contracted diameter) is a valid description of Earth? How can that be?

It's best to think of things only being fully valid when viewed from their own frame of reference - any other frame will appear distorted.

Quote
A ship traveling at near ‘c’ velocity from here to there must, therefore take longer than 4.37 years to get there, even though the onboard clock will have slowed down and recorded much less than 4 years passing. Yet Earth will have orbited the Sun (the "year" standard) much more than 4 times during the ship’s journey to AC. So, even though “for the ship” much less than 4 years will have passed, the distance between Earth and AC will not have contracted to way less than 4 light years. The ship’s journey obviously will not make Earth and AC move closer together.

If we imagine the ship travelling at 86.6% the speed of light, that speed conveniently slows its clocks to half the normal speed and contracts the ship to half its normal length. From the point of view of the ship though, its length is normal and so is the rate its clocks are running at, but it sees everything else around it as having slowed clocks and being contracted in the direction in which it all appears to be moving in relative to the ship, and that includes the distance between AC and the Earth. Visually, the Earth will actually appear to be further away, but once an adjustment is made for the light received from it being emitted when the Earth was much further away (bear in mind that the Earth appears to be rushing towards the ship at 86.6% the speed of light), it will be calculated that the Earth is only about two lightyears away from AC. Once the ship has reached the Earth and stops there, the distance to AC will suddenly appear to be over four lightyears again. The distance measured depends entirely on the speed you're travelling at at the time.
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #10 on: 01/09/2012 21:55:41 »
David Cooper:
"The distance measured depends entirely on the speed you're travelling at at the time."
This must be a quick 'pass by' in reply to your last sentence.(More when I find "time.") I think I understand your post clearly. It seems to insist that AC and Earth are, in reality, closer together than 4+ light years as the ship travels between them.
(Ref: My statement above: "The ship’s journey obviously will not make Earth and AC move closer together."
Do you think there are no natural objects with intrinsic properties or distances between established by gravity as they were formed in space? All the cosmos depends on how it is observed? Is this not  relativity's version of classical subjective idealism, with 'frame of reference' as the abstract, virtual "subject."
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #11 on: 02/09/2012 04:31:45 »
Heh :)

What you are asking about is if your senses can lie to you, right?
As relativity state that the geometry we see can change with relative motion, not only accelerations.
And there is one more question hidden in that, if it is so that the geometry change it must do so relative some 'constant' if it have a proportionality, and it has, if it didn't no Lorentz transformations should be applicable.

The first question is very easy to answer. Sure, your senses lie to you, your brain does the same evaluating them and interpreting the environment you live in. People like to think that they know what happens, but the brain has all kind of strategies for simplifying and choosing a preference of interpretation.

Then we come to a experiment, does that lie too? Well, assuming you have made it very strict and simple to follow that question will be answered if it is 'repeatable' or not. Relativity has been tested over a hundred years, and the beautiful experiments NIST has made is to my eyes very convincing.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #12 on: 02/09/2012 04:47:24 »
You could choose to interpret it as 'time' doesn't exist, but then you will have to explain how a gravitational 'time dilation' measured on Earth at decimeters can exist, and they do, everywhere. Why the discrepancies if it isn't something that differs? And if something differs there, what is it you would like to call it :) I call it time, or the arrow.

But choosing that you can now, possibly, assume that any 'contraction' should be a illusion. But, what you've just done is to invalidate all measurements you make yourself, on that ship. What science builds on is 'repeatable experiments' done 'locally' in a similar, or 'same', environment. You've just invalidated them making this conclusion.
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #13 on: 02/09/2012 18:02:56 »
I really need answers to my reply #6 before proceeding, or at least to my last question:
Quote
Do you think there are no natural objects with intrinsic properties or distances between established by gravity as they were formed in space? All the cosmos depends on how it is observed? Is this not  relativity's version of classical subjective idealism, with 'frame of reference' as the abstract, virtual "subject."

I understand that "for the ship" time (its clock) has slowed down. I understand that what the ship observes appears shortened. The question is, does Earth's diameter or the distance to AC (in the "real world") vary with how it is observed? I think the answer is "no." And the question still remains, Is there any empirical evidence for large scale length contraction?
Saying that length contraction is the mathematical reciprocal of time dilation does no constitute empirical evidence for large scale length contraction.
David Cooper said:
Quote
If we imagine the ship travelling at 86.6% the speed of light, that speed conveniently slows its clocks to half the normal speed and contracts the ship to half its normal length
.

I get that its clock will have slowed to half speed. This slowing of clocks at higher velocities has been empirically verified. However, the ship is a solid object. It would require a tremendous force to squeeze the ship to half its normal length, (and it would be crushed in the process), and I am sure that "length contraction" is not claiming to be such a force. Likewise for the astronomically verified distance between Earth and AC. A ship observing the distance to have contracted is not the same as the two bodies actually becoming closer together in space.
The rest will remain "window dressing" until these questions are answered.
« Last Edit: 02/09/2012 18:07:12 by old guy »
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #14 on: 02/09/2012 19:15:58 »
It's geometry, not a force. The same way you can assume a gravitational wave to deform you, without yourself ever noticing. and there are no 'window dressing' implied in it. The muon will reach Earth although according to Newtonian values it should be unable to do so. And there are two frames of reference involved, the Earthly observer and the muons own frame of reference. Earths is a time dilation, the muon's 'clock' is slower than our local, The muon's perspective will be that its clock is as always, and that can only leave a contraction to be considered from its perspective. And as we have gravitational time dilations proofed, without us ever noticing them, you may assume that we have length contractions too.

To get away from a contraction you will have to introduce a 'slower time' existing at both frames of reference, 'Earths and the muon. And that is just not true, as proofed by NIST:s experiments on Earth. We all walk through time dilations without noticing, and even when being at rest relative Earth.

Einstein called 'time' a dimension, and that seems to catch it pretty well. The time and the room is entwined into one expression becoming SpaceTime.
=

Why it not can be a 'force' seems pretty simple to me, assuming length contractions, what would the force be for 'moving' a universe one light year closer? Than it was when you where at rest with Earth before accelerating. And is that force reciprocal to the energy you expended accelerating?
« Last Edit: 02/09/2012 19:35:09 by yor_on »
 

Offline lightarrow

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Re: Evidence for large scale length contraction?
« Reply #15 on: 02/09/2012 20:10:45 »

Lightarrow said:
Quote
1. Definition of lenght between two points of an object: measure the simultaneous position of the two points and make the difference.

2. In SR simultaneity is frame-dependent (= if two spatially separated events are simultaneous in a frame, they are not in a frame which is moving with respect to the first).

Regarding #1: This would seem to yield the familiar earth science measurements of earth, say as from in orbit, at rest with Earth.
Regarding #2: I don’t understand how the relativity of simultaneity addresses the question of a length contracted Earth diameter or my statement, “Surely SR's length contraction theory is not claiming that earth's atmosphere depth and diameter varies with how they are measured.”
In my post you had the answer: simultaneity is frame-dependent. If you are still with respect to Earth, every Earth's dimension has a value, in your frame of reference; if you are moving with resperct to Earth, those dimensions are different, as measured in that new frame of reference (because of #).
In relativity you *cannot* say: "the lenght is 2 metres", you have to say: "the lenght is 2 metres as measured in this specific frame of reference".
« Last Edit: 02/09/2012 20:17:41 by lightarrow »
 

Offline David Cooper

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Re: Evidence for large scale length contraction?
« Reply #16 on: 02/09/2012 22:57:05 »
Do you think there are no natural objects with intrinsic properties or distances between established by gravity as they were formed in space? All the cosmos depends on how it is observed? Is this not  relativity's version of classical subjective idealism, with 'frame of reference' as the abstract, virtual "subject."

You can look at things in two different ways. If there is a preferred frame of reference, then anything stationary in that frame will be undistorted in shape and everything moving through that frame would genuinely be contracted in the direction of travel, but you wouldn't be able to identify the preferred frame. If there is no preferred frame, then you can look at things differently and decide that everything is fundamentally undistorted, but that anything moving relative to it will appear to be contracted in its direction of travel. With a preferred frame, all the distances would be fixed, but they'd be impossible to pin actual values on as you wouldn't know if they've been contracted or not. Without a preferred frame, the distances between things can be measured accurately within the frame in which they're stationary, and that would arguably be the truest measurement. Einstein appears to offer a spacetime in which the contractions are not real - things appear to contract, but they're really just reorienting themselves in spacetime such that they appear shortened from other frames because part of their length is taking up part of the time dimension.

Is there any empirical evidence for large scale length contraction?

I don't know if anything with more than one component that can be seen clearly enough has ever moved fast enough to detect the contraction, but I don't think there's any reason to doubt that it does happen. The reason for this is the Michelson Morley experiment. We know that we can move the MM apparatus in any direction and at any speed without affecting how long it takes light to complete the journey along both arms. The arm which is pointing in the direction of travel would need to contract to make this possible if there is a preferred frame of reference, whereas with Einstein's theory there is no real contraction at all, but just the appearance of contraction when viewed from other frames.

Quote
...the ship is a solid object. It would require a tremendous force to squeeze the ship to half its normal length, (and it would be crushed in the process), and I am sure that "length contraction" is not claiming to be such a force.

With Einstein there is no real contraction, and therefore no issue. With Lorentz (and a preferred frame of reference) there is, but there's no crushing force involved. Imagine a room inside a space ship with a lamp in the middle of it. When the ship moves fast, the light has further to go to catch up with the leading wall of the room after it's been emitted from the lamp, and less far to go to reach the trailing wall which is rushing towards the lamp. This would lead you to expect the light to spread out more by the time it has reached the leading wall and to illuminate it less brightly as a result, whereas the rear wall would be brighter than it should be. That doesn't happen though, because the lamp will throw more light forwards than backwards. You can imagine why this happens if you work out what would happen to light being emitted sideways from the lamp and then reflected forwards or backwards by a flat mirror - because the light will take longer to reach some parts of the mirror than others, the mirror will actually act as if it is curved, concentrating light forwards or spreading it out more behind. The same kind of thing happens with lenses, so your eyes act as telephoto lenses when looking backwards and as wide-angle lenses when looking forwards. The end result of all this is that the front and rear walls remain equally well lit and appear to be the same distance from the lamp as they would if the ship wasn't moving, though only if the ship is contracted in the direction of travel.

If you now imagine forces being emitted and received in the same way as light, you can imagine them being concentrated forwards and spread out behind, and that means the atoms in molecules will attempt to maintain their separations and naturally sit closer together in the direction of travel at the points where the forces balance out for them, so there is no crushing force involved - just adjustments to maintain balance. Einstein simply sidesteps all of this by rotating the object in spacetime to get rid of all the distortions.

Quote
Likewise for the astronomically verified distance between Earth and AC. A ship observing the distance to have contracted is not the same as the two bodies actually becoming closer together in space.

They aren't becoming closer in space, just measured as appearing to be closer together in some frames of reference than others. It's exactly the same as judging a space ship flying past as being contracted in length - you will judge it's true length by eliminating the contraction. Whether it is actually contracted while moving fast relative to you isn't particularly important to you, and the same will apply to any stars racing past - you'll see them squashed too and automatically think that to get their real measurements you need to remove the contraction, so you'd automatically do the same with the distance between those stars as well and remove the contraction from it which your movement is introducing.

But you're still going to wonder how all the different distances between two stars depending on the speed you move past them can be valid at the same time, and the answer is in the way that things rotate into the time dimension. When you travel fast, you and the stars are at a different angles in spacetime, so while the distance between the stars appears to reduce, it's compensating by taking up more space in the time dimension (and reducing the amount of time dimension left over to control the passage of time for it, or for you, depending on which frame is taking up more of the time dimension than the other, though of course that leads into more difficult questions).
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #17 on: 03/09/2012 21:12:53 »
David Cooper:
Quote
Einstein appears to offer a spacetime in which the contractions are not real - things appear to contract, but they're really just reorienting themselves in spacetime such that they appear shortened from other frames because part of their length is taking up part of the time dimension.
... with Einstein's theory there is no real contraction at all, but just the appearance of contraction when viewed from other frames.
(Yes, I got your PM.)

This makes sense to me and fits with the idea that length contraction is an appearance of shortened objects and distances due to extremely high velocivity. This goes back to my question in reply #3 about the difference between ..."for the ship flying by Earth (its diameter is contracted) and... its actual diameter(s)...as well known and documented by Earth science.

I agree with your statement:
Quote
Without a preferred frame, the distances between things can be measured accurately within the frame in which they're stationary, and that would arguably be the truest measurement.

But lightarrow said:
Quote
If you are still with respect to Earth, every Earth's dimension has a value, in your frame of reference; if you are moving with resperct to Earth, those dimensions are different, as measured in that new frame of reference (because of #).
In relativity you *cannot* say: "the lenght is 2 metres", you have to say: "the lenght is 2 metres as measured in this specific frame of reference".

Does this claim that Earth changes shape as it is measured from different frames of reference? This, of course, is impossible and has never been empirically observed. So the claim seems to be that there are no actual objects with intrinsic properties (or distances between them) independent of how they are observed/measured?

Here is another “reality check” against large scale length contraction:
Say an alien probe is discovered heading toward Earth at a significant fraction of light speed. From Earth’s frame it is measured to be ten meters long, length contracted because of its velocity relative to earth. It is decided to go out and intercept/capture it in one of our very high speed space shuttles (of the future.)
Our shuttle has a ten meter cargo bay. Will the probe fit into the bay?... A very practical test of “actual length” vs “contracted length.”
The answer is “no” because the probe’s “actual length” must be longer than its “contracted length” for it to appear as ten meters long from earth’s frame in this case.
Lightarrow, please address this challenge and the “earth changing shapes (diameters)" challenge.  Thanks.

 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #18 on: 03/09/2012 21:37:22 »
David, "Einstein appears to offer a spacetime in which the contractions are not real - things appear to contract, but they're really just reorienting themselves in spacetime such that they appear shortened from other frames because part of their length is taking up part of the time dimension."

To speak of it as wandering of in 'time' isn't that clarifying to me David :) And to say that Einstein saw contractions as a illusion needs at least a citation from him. There are two views, some accept that time dilations exist, it's hard avoiding that, but adhere to that a LorentzFitzGerald contraction is a 'optical illusion', others as me expect it to be real, meaning true from the frame of reference finding/measuring it.

Let us assume that it really would be time ticking 'slow' in the muon-Spaceships 'inertial frame' (being in uniform motion), that will naturally include all decay and all 'change', including all 'force carriers' aka photons and 'virtual photons', add infinitum. What we now do is to introduce a 'variable speed of light' in where the constant 'c' has to adapt to the local frame of reference. That as it indeed will present a 'clock' for all natural processes assuming 'virtual photons', although we don't even need to do that. Just assume that the muon-ship has a device for measuring the speed of the light before leaving it, to then cross space to be received on Earth, aka a two way mirror sending that light of.

So let us assume that you ('inertial frame' Earth) really can see that other, uniformly moving frames light , and that you too let that light 'bounce' between two mirrors, just to measure that lights speed. Can you expect the measurement to come out as 'c'? And if assuming that light to have changed its speed somewhere? To 'fit' our notion of 'c' on Earth, where would that be? And what would should I call that light changing its speed, a acceleration/deceleration maybe? And a variable?

The whole idea of contractions relative time dilations is that it is a symmetry as I see it, that's also why you can use a 'light clock' to illustrate it. If it was a illusion those light clock examples in where you have a contraction 'compensating' the time dilation by necessity would have to be wrong.

"Suppose we observe a body A to rest in space relative to our reference system. Let another body impinge on it, causing it to deform slightly as the force of impact is transmitted throughout, also setting A in motion relative to us. Such motion and deformations involve physical causes, yet they may well be described kinematically. Next consider a body B resting in free space relative to us,and now let us just begin to glide sideways away from it until we achieve a constant inertial speed. In Newton’s framework we say that B now seems to move away from us, and we call that a kinematical effect. In Einstein’s framework, we say that the body moves away relative to us and that its length is shortened relative to us, and we call those effects kinematical.

Einstein expected that the effect is identical to what would transpire if instead B were moving away from us at the same rate. For example, if a rocket accelerates near the Earth, and we are inside that rocket, then relative to us the Earth now should have a narrower length. Yet nobody will claim that thus something happened to all the molecules that constitute the Earth. The way we describe their cohesion relative to our rocket may change, but we would not say that there is any material change in the Earthly molecules partly because no such change happened relative to all other systems. By contrast, certain common changes in the configuration of molecules on a body are material changes, which might be observed from any system. Special relativity takes the effects of relative motion as fully reciprocal regardless of which system is regarded as oving. Thus observers on Earth would judge, instead, that the rocket is moving and contracted. Neither contraction is more real than the other, and neither is an optical illusion. Relativity of length means just that any two points on a given body are separated not by one universal length but by indefinitely many lengths."

A.A. Martinez / Studies in History and Philosophy of Modern Physics 38 (2007) 209–215 213

As for Einstein I found this citation.

In 1911 Vladimir Varićak asserted that length contraction is "real" according to Lorentz, while it is "apparent or subjective" according to Einstein. Einstein replied:

    The author unjustifiably stated a difference of Lorentz's view and that of mine concerning the physical facts. The question as to whether the Lorentz contraction really exists or not is misleading. It doesn't "really" exist, in so far as it doesn't exist for a comoving observer; though it "really" exists, i.e. in such a way that it could be demonstrated in principle by physical means by a non-comoving observer.[15]
    —Albert Einstein, 1911
 

Offline David Cooper

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Re: Evidence for large scale length contraction?
« Reply #19 on: 03/09/2012 21:56:30 »
But lightarrow said:
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If you are still with respect to Earth, every Earth's dimension has a value, in your frame of reference; if you are moving with resperct to Earth, those dimensions are different, as measured in that new frame of reference (because of #).
In relativity you *cannot* say: "the lenght is 2 metres", you have to say: "the lenght is 2 metres as measured in this specific frame of reference".

Does this claim that Earth changes shape as it is measured from different frames of reference? This, of course, is impossible and has never been empirically observed. So the claim seems to be that there are no actual objects with intrinsic properties (or distances between them) independent of how they are observed/measured?

I suspect you're talking at cross purposes here. I don't think Lightarrow was suggesting that the size of anything changes as it's viewed from different frames, but that it is measured as having different sizes from within different frames. X has a constant size, but the measurements vary according to how you're moving relative to X when you measure X. Those measurements are only relevant in the frame in which they are made.

Quote
Here is another “reality check” against large scale length contraction:
Say an alien probe is discovered heading toward Earth at a significant fraction of light speed. From Earth’s frame it is measured to be ten meters long, length contracted because of its velocity relative to earth. It is decided to go out and intercept/capture it in one of our very high speed space shuttles (of the future.)
Our shuttle has a ten meter cargo bay. Will the probe fit into the bay?... A very practical test of “actual length” vs “contracted length.”
The answer is “no” because the probe’s “actual length” must be longer than its “contracted length” for it to appear as ten meters long from earth’s frame in this case.

If the 10.1m cargo bay is open at both ends such that the alien ship can fly through it, it will fit completely inside it for a moment, so in that sense it really is only ten metres long, but if you accelerate the shuttle up to the same speed as the alien ship and then try to capture it, it will then stick out of the cargo bay at both ends.
 

Offline David Cooper

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Re: Evidence for large scale length contraction?
« Reply #20 on: 03/09/2012 23:28:45 »
David, "Einstein appears to offer a spacetime in which the contractions are not real - things appear to contract, but they're really just reorienting themselves in spacetime such that they appear shortened from other frames because part of their length is taking up part of the time dimension."

To speak of it as wandering of in 'time' isn't that clarifying to me David :)

It isn't just wandering - it's a very precise rotation which enables objects to maintain their shape while appearing to be contracted when judged from other frames.

Quote
And to say that Einstein saw contractions as a illusion needs at least a citation from him. There are two views, some accept that time dilations exist, it's hard avoiding that, but adhere to that a LorentzFitzGerald contraction is a 'optical illusion', others as me expect it to be real, meaning true from the frame of reference finding/measuring it.

I wouldn't cite anything from Einstein - almost everything he says is worded in such a way that it's easy to misinterpret. It's better to go by other people's explanations of SR as a guide to what Einstein meant, but that opens you up to stating things about what Einstein thought that may not match up to what he actually thought, so it's easy to get it wrong. My judgement is that he saw contraction as both real and an illusion. It's real within the frame you're observing as moving, contracted object from, but it's only when you're in the same frame as the object that its real shape is revealed. The contraction is real in observational terms, but the object is not really contracted.

Quote
Let us assume that it really would be time ticking 'slow' in the muon-Spaceships 'inertial frame' (being in uniform motion), that will naturally include all decay and all 'change', including all 'force carriers' aka photons and 'virtual photons', add infinitum. What we now do is to introduce a 'variable speed of light' in where the constant 'c' has to adapt to the local frame of reference. That as it indeed will present a 'clock' for all natural processes assuming 'virtual photons', although we don't even need to do that. Just assume that the muon-ship has a device for measuring the speed of the light before leaving it, to then cross space to be received on Earth, aka a two way mirror sending that light of.

So let us assume that you ('inertial frame' Earth) really can see that other, uniformly moving frames light , and that you too let that light 'bounce' between two mirrors, just to measure that lights speed. Can you expect the measurement to come out as 'c'? And if assuming that light to have changed its speed somewhere? To 'fit' our notion of 'c' on Earth, where would that be? And what would should I call that light changing its speed, a acceleration/deceleration maybe? And a variable?

I don't know what point you're trying to make here. If you're analysing a moving thing, you will assume that light is travelling at the speed it does in your own frame, thereby measuring different values of the speed of light relative to that moving object depending on which way the light's going, but it will always be going at c in your frame. If you're measuring the speed of light while travelling with the moving object, you will then measure it as being c and regard it as being higher or lower than c relative to other frames. There appear to be contradictions there, but they may disappear when you account for the realignment of things in spacetime - it's hard to visualise so I don't know.

Quote
The whole idea of contractions relative time dilations is that it is a symmetry as I see it, that's also why you can use a 'light clock' to illustrate it. If it was a illusion those light clock examples in where you have a contraction 'compensating' the time dilation by necessity would have to be wrong.

A light clock is no different from either arm of the Michelson Morley experiment, so it will be contracted or appear to be contracted if it's aligned with the direction of travel. Lorentz would have said that it is actually contracted if it is aligned with the direction of travel, but Einstein would perhaps have said you can consider it to be contracted if it's in a frame different from the one you're viewing it from and not contracted if you're moving with it - both are correct views, but the contradiction can be dealt with by considering the uncontracted version to be the superior one. There are different kinds of validity involved in this - the measurements are all valid for the frame they're made from, but if you want the real shape of anything you would want to measure it from its own frame.

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"Suppose we observe a body A to rest in space relative to our reference system. Let another body impinge on it, causing it to deform slightly as the force of impact is transmitted throughout, also setting A in motion relative to us. Such motion and deformations involve physical causes, yet they may well be described kinematically. Next consider a body B resting in free space relative to us,and now let us just begin to glide sideways away from it until we achieve a constant inertial speed. In Newton’s framework we say that B now seems to move away from us, and we call that a kinematical effect. In Einstein’s framework, we say that the body moves away relative to us and that its length is shortened relative to us, and we call those effects kinematical.

[By the way, no one should be misled by deformations caused by impacts and decelerations - they are temporary and go away when the force is no longer being applied. The contractions caused by relative movement have nothing to do with that.]

Quote
Einstein expected that the effect is identical to what would transpire if instead B were moving away from us at the same rate. For example, if a rocket accelerates near the Earth, and we are inside that rocket, then relative to us the Earth now should have a narrower length. Yet nobody will claim that thus something happened to all the molecules that constitute the Earth. The way we describe their cohesion relative to our rocket may change, but we would not say that there is any material change in the Earthly molecules partly because no such change happened relative to all other systems. By contrast, certain common changes in the configuration of molecules on a body are material changes, which might be observed from any system. Special relativity takes the effects of relative motion as fully reciprocal regardless of which system is regarded as oving. Thus observers on Earth would judge, instead, that the rocket is moving and contracted. Neither contraction is more real than the other, and neither is an optical illusion. Relativity of length means just that any two points on a given body are separated not by one universal length but by indefinitely many lengths."

A.A. Martinez / Studies in History and Philosophy of Modern Physics 38 (2007) 209–215 213

What that appears to be saying is that all the measurements are valid and that there is no most correct measurement that can be made (e.g. by measuring from the same frame as the thing you're measuring). That would mean that there is an infinite range of equally valid measurements for the size of any object or the distance between any two points, and changing the frame from which you make the measurement simply gives you access to a different version of that length - you aren't changing the shape of it, but merely changing your viewpoint on it.

Even so, that appears to me to be just one interpretation of what Einstein thought. It's clear that there is a maximum measurement for the length of anything, and that measurement shows up when you measure the object from the same frame. I'd be surprised if Einstein didn't consider that to have greater validity.

Quote
As for Einstein I found this citation.

In 1911 Vladimir Varićak asserted that length contraction is "real" according to Lorentz, while it is "apparent or subjective" according to Einstein. Einstein replied:

    The author unjustifiably stated a difference of Lorentz's view and that of mine concerning the physical facts. The question as to whether the Lorentz contraction really exists or not is misleading. It doesn't "really" exist, in so far as it doesn't exist for a comoving observer; though it "really" exists, i.e. in such a way that it could be demonstrated in principle by physical means by a non-comoving observer.[15]
    —Albert Einstein, 1911

That's typical of Einstein - it leaves it open.
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #21 on: 04/09/2012 00:15:31 »
What it says to me is that it depends on what frame of reference you use. And so a contraction becomes a observer dependent fact, not illusion. And that's what Einstein himself say too.

As for my example I was referring to how you would like to see it from the muon's side if there was no contraction to be seen, as I discussed earlier too btw. Assuming no contraction it being a 'illusion' you only have time dilations left, and then assuming ( as they say:) that 'time' indeed goes slow 'objectively' not only as seen from Earth measuring, but really goes sloow in the muon frame locally, although unobservable from the muon's side, the rest I wrote follow, using two-way mirrors to measure a speed. And reasoning it out from such a premise becomes something of a logical fallacy to me.

Because there is never any change of your time/ruler locally, only between frames of reference.
==

But yes, in a way I agree David. I think he found himself rather uncomfortable acknowledging it as a fact. It, in a much more definite way introduce 'frames of reference' as something 'real'. The idea of 'time' and 'the arrow' as a 'illusion' is easier to handle than varying lengths depending on your relative motion mass energy etc, aka comparisons between 'frames of reference' (SpaceTime positions). He wanted one cosmos and one 'reality', and if this was a fact?

People still find this idea deeply uncomfortable I think.
« Last Edit: 04/09/2012 01:06:01 by yor_on »
 

Offline yor_on

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Re: Evidence for large scale length contraction?
« Reply #22 on: 04/09/2012 01:20:59 »
As for your comment on all lengths being 'the same' if measured from a 'same frame of reference' I totally agree :) That's what I call a 'principle of locality'. The same principle that allow us to have 'repeatable experiments'. And I think you are perfectly correct in deeming that as the most important cohesive principle for a 'unified SpaceTime', that is if I read you right?
 

Offline old guy

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Re: Evidence for large scale length contraction?
« Reply #23 on: 04/09/2012 19:36:25 »
David Cooper:
Quote
The contraction is real in observational terms, but the object is not really contracted.

In other worlds, just to be clear, large scale length contraction is an illusion, if “the object is not really contracted.”

That is what I’ve been saying (with no replies directly to my points) about Earth’s diameter, the distance to Alpha Centauri, and the length of the alien probe... all staying the same regardless of how they are variously observed/measured.

Obviously earth’s diameter does not shrink, as measured from a near ‘c’ fly by. Obviously Earth and AC do not move closer together as measured from a near ‘c’  ship flying between them (even though its clock will slow down.) Obviously an alien probe, measured as 10 meters long as it approaches Earth at near ‘c’ will not fit into a 10 meter cargo bay, because its “contracted length” is an illusion. As the shuttle pulls alongside the probe, in the same frame at rest with the probe, its actual length will be found to be much longer than its “contracted length” as seen from Earth.
Comments to the points above, the point of this thread, anyone?

Btw, David, I agree with you on the following, with one vital point of exception: (my bold)
Quote
That would mean that there is an infinite range of equally valid measurements for the size of any object or the distance between any two points, and changing the frame from which you make the measurement simply gives you access to a different version of that length - you aren't changing the shape of it, but merely changing your viewpoint on it.

... It's clear that there is a maximum measurement for the length of anything, and that measurement shows up when you measure the object from the same frame. I'd be surprised if Einstein didn't consider that to have greater validity.

It is clear that measuring something from the same frame, at rest with the object, yields the valid and correct measure. I think that the dictum "there is no preferred frame of reference" is in blatant denial of what science already knows for sure about Earth's diameter, the distance to AC, the distance to the Sun, etc. They don't change with how one looks at them.

If objects like Earth have intrinsic shape independent of “your viewpoint” then there is a “valid” shape (both polar and equatorial diameters are well documented to a high degree of precision), and other measurements from all different varieties of viewpoint are not “equally valid."

yor_on said:
And so a contraction becomes a observer dependent fact, not illusion. And that's what Einstein himself say too.

I think that length contraction as an “observer dependent fact” is an oxymoron. A very oblate spheroid might be "observer dependent" but it is not a factual description of Earth.
« Last Edit: 04/09/2012 19:40:08 by old guy »
 

Offline David Cooper

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Re: Evidence for large scale length contraction?
« Reply #24 on: 04/09/2012 20:06:11 »
David Cooper:
Quote
The contraction is real in observational terms, but the object is not really contracted.

In other worlds, just to be clear, large scale length contraction is an illusion, if “the object is not really contracted.”

That is what I’ve been saying (with no replies directly to my points) about Earth’s diameter, the distance to Alpha Centauri, and the length of the alien probe... all staying the same regardless of how they are variously observed/measured.

Yes - we're all pretty much agreed now (I think) that the objects don't change, but that the measurements do, and where there's always going to be room for disagreement is in how we describe the differences in measurements. I think it's valid to refer to the contracted lengths as being illusions, and it's also valid to refer to them as being real because they are real measurements, but also because they are, if Einstein's theory is correct, genuine lengths of the things being measured - lengths which only show up when viewed from other frames.

Quote
It is clear that measuring something from the same frame, at rest with the object, yields the valid and correct measure. I think that the dictum "there is no preferred frame of reference" is in blatant denial of what science already knows for sure about Earth's diameter, the distance to AC, the distance to the Sun, etc. They don't change with how one looks at them.

But a preferred frame of reference would not orbit around the sun with the Earth, nor with the sun around the Milky Way, so a preferred frame of reference would actually render just about everything genuinely contracted in it's direction of travel through that frame, and more so depending on its speed relative to that frame.

Quote
If objects like Earth have intrinsic shape independent of “your viewpoint” then there is a “valid” shape (both polar and equatorial diameters are well documented to a high degree of precision), and other measurements from all different varieties of viewpoint are not “equally valid."

There are different kinds of validity. If the alien space ship is flying through a shuttle's cargo bay it may fit inside it completely for an instant, so it's apparent length of ten metres is a valid measurement of it for that situation. If you then try to keep it in the cargo bay, it will then stick out at the ends and the measurement won't seem so valid after all. In one way it's valid, and in another way it isn't, but the apparent contradiction is in the flexible way that language is being used to describe the situation, so there is no actual contradiction in valid = not valid because they're not the same kind of valid - each one depends on its own conditions, and the conditions don't match up.
 

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Re: Evidence for large scale length contraction?
« Reply #24 on: 04/09/2012 20:06:11 »

 

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