Length contraction of space with multiple reference frames

[coktail]

My understanding is that as I move, from my FoR all objects and space itself (according to Einstein) contract along the direction of my movement. This length contraction occurs for all space and objects in front of me for an infinite distance. Furthermore, relative motion is relative, and the severity of a thing's contraction depends on its speed relative to me, or my speed relative to it. Same difference.

Given all this, imagine that there are three objects in space: Me, Ball A, and Ball B. I am moving at .8c relative to Ball A, and .9c relative to Ball B. I understand that from my FoR, Ball B would be contracted than Ball A. What I'm having trouble understanding is how it would be possible for space to contract to two varying degrees from my FoR. I'm sure there's some error in my thought process here.

Any help getting this straightened out is appreciated. Thank you!

 

[PAllen]

Distance contraction is all about simultaneity. I'll get to that in a moment. From your point of view, there is no distance contraction. Any object is contracted in your rest frame according its motion in your rest frame. Distance contraction, from your rest frame, can be described as follows:

- It two objects are moving at the same speed towards (or away) from you, but separated, you will measure the separation between them as less than either of them would measure it (and they would each measure it the same, since they are comoving).

Now you introduce two objects moving at different speeds towards you (for example). Well, you measure some distance between them, at some time (simultaneity!) - their position at some same time per you. Each of them considers that you have compared their positions at different times; however they disagree with each other about how large the difference is. Let's say you make your measurement at the moment one of the passes you. Then you measure e.g. 1 meter between them. If each of them measures the distance between them at the simultaneity they ascribe to the passing event, then B will say it is longer than one meter, and A will say it is even longer. Each sees a different degree of 'error' in the way you have made the measurement.

 

[coktail]

I see! I think... Here's the point I'm still confused about:

From your point of view, there is no distance contraction.

I thought that distance contracted along with objects from my FoR. I'd love some more clarity on this, if possible.

As for simultaneity, I think I have a good grasp on how it can lead to different observations from different reference frames without them contradicting each other. I think what I'm confused about here is the apparent contradiction from my own, single frame of reference. Or do I have multiple reference frames depending on what I measure my motion as relative to (e.g. I'm stationary relative to my desk right now, but moving at hundreds of miles per hour relative to a plane flying by)?

I've attached little diagram that hopefully explains where my thinking is at. What I'm wondering in my picture if if the dotted lines representing the distance from the balls would be the same from my reference frame even though I am moving at different speeds relative to the different balls.

Thanks for all of your help, PAllen.

 

[PAllen]

I see! I think... Here's the point I'm still confused about:



I thought that distance contracted along with objects from my FoR. I'd love some more clarity on this, if possible.

As for simultaneity, I think I have a good grasp on how it can lead to different observations from different reference frames without them contradicting each other. I think what I'm confused about here is the apparent contradiction from my own, single frame of reference. Or do I have multiple reference frames depending on what I measure my motion as relative to (e.g. I'm stationary relative to my desk right now, but moving at hundreds of miles per hour relative to a plane flying by)?

I've attached little diagram that hopefully explains where my thinking is at. What I'm wondering in my picture if if the dotted lines representing the distance from the balls would be the same from my reference frame even though I am moving at different speeds relative to the different balls.

Thanks for all of your help, PAllen.

You need to explain what 'contradiction' is bothering you. I have no idea what it is. FYI, in your diagram:

- if you imagine that A and B are as pictured, then A, B, and you, all agree on the distance between A and B. (We can fix a common simultaneity for part of the scenario be imagining that A and B pass through a tissue wall at rest with respect to you, orthogonal to A and B motion; then you, A and B are all in agreement that the A crossing tissue is simultaneous with B crossing tissue. This is because this surface orthogonal to all relative motion).

- As for distance between you and A, and you and B, all three disagree on both distances and simultaneity. The discrepancy in distance measures is coupled to the disagreement on simultaneity. Specifically, the point in your history you consider simultaneous with A/B tissue crossing is a completely different time in your history than what A thinks is simultaneous with tissue crossing; and both are different from the point in your history B thinks is simultaneous. Because of relative motion, this disagreement in which point in your history to measure to/from directly leads to 3 different measures of distance.

 

[coktail]

The "contradiction" that I see is that the space/distance in front of me can be contracted to two different degrees simultaneously from my perspective, even though I only have one reference frame. Or can I have as many reference frames as objects that I am moving relative to, and they can all exist without contradiction because of the relativity of simultaneity? I think that's where I'm stuck.

 

[PAllen]

The "contradiction" that I see is that the space/distance in front of me can be contracted to two different degrees simultaneously from my perspective, even though I only have one reference frame. Or can I have as many reference frames as objects that I am moving relative to, and they can all exist without contradiction because of the relativity of simultaneity? I think that's where I'm stuck.

Let go of thinking about space being contracted. Instead think about distance between comoving points contracted (an object being a collection of comoving points; a pair of stars also fits). Then, one collection of comoving points is contracted differently that a different collection, comoving differently. Nothing happens to space. And note that the contraction is relative to what the comoving points would measure as their mutual distances.

You can use the muon example here, but let's make you a neutrino. Earth goes by at (c-ε), and its atmosphere seem 1" thick to you. Then, some other Earth clone planet goes by at (c-4ε), and its atmosphere seems 2" thick to you.

 

[jartsa]

Is the following correct:

Object A is at rest relative to me.

If I accelerate towards A, distance between me and A contracts.

If A accelerates towards me, distance between me and A does not contract.

 

[coktail]

So the distance between comoving point contracts, which could be a single "object" composed of comoving particles, or an entire galaxy (which is, in a sense, one big object). Correct?

I'm feeling pretty good about this, but I'm still wondering about whether or not I can have more than one reference frame. For example, in my attached diagram, would I have two reference frames (one for each ball) because I'm referencing two different objects with two different velocities? I hope I'm not getting lost in the weeds here.

 

[coktail]

Is the following correct:

Object A is at rest relative to me.

If I accelerate towards A, distance between me and A contracts.

If A accelerates towards me, distance between me and A does not contract.

I'm going to take a shot at this and say "no." Wether you consider A as accelerating towards you vs you as accelerating towards A is an arbitrary distinction since motion is, by its nature, relative. That's why we call it "relative motion."

 

[HallsofIvy]

I'm going to take a shot at this and say "no." Wether you consider A as accelerating towards you vs you as accelerating towards A is an arbitrary distinction since motion is, by its nature, relative. That's why we call it "relative motion."

This is not correct. The error is hidden by saying that "motion" is relative. What is true is that speed is relative. Acceleration is not. The accelerating object will feel a 'force' while the non-accelerating object will not.

 

[coktail]

Thanks for the correction. I'll get it one day :)

So is jartsa correct?

I'm also pasting my previous comment here so it doesn't get lost in the shuffle:

So the distance between comoving point contracts, which could be a single "object" composed of comoving particles, or an entire galaxy (which is, in a sense, one big object). Correct?

I'm feeling pretty good about this, but I'm still wondering about whether or not I can have more than one reference frame. For example, in my attached diagram, would I have two reference frames (one for each ball) because I'm referencing two different objects with two different velocities? I hope I'm not getting lost in the weeds here.

 

[PAllen]

Is the following correct:

Object A is at rest relative to me.

If I accelerate towards A, distance between me and A contracts.

If A accelerates towards me, distance between me and A does not contract.

This is basically correct, and (depending on how you define your coordinates) can show up as A approaching you at faster than c in coordinate speed in the case where you accelerate towards A. No such thing would happen if A accelerates towards you. Note, that coordinate speed faster than c does not mean faster than light - in such an accelerated coordinate system light from A would travel towards towards you faster than c by a greater amount than A does.

 

[PAllen]

So the distance between comoving point contracts, which could be a single "object" composed of comoving particles, or an entire galaxy (which is, in a sense, one big object). Correct?

Yes.

I'm feeling pretty good about this, but I'm still wondering about whether or not I can have more than one reference frame. For example, in my attached diagram, would I have two reference frames (one for each ball) because I'm referencing two different objects with two different velocities? I hope I'm not getting lost in the weeds here.

You have only one rest frame. You 'exist' or are represented in all reference frames. In your diagram, you have one rest frame, A has one rest frame, and B has one rest frame. A and B are also 'in' your rest frame; you and A are also in B's rest frame; etc.

 

[phyti]

My understanding is that as I move, from my FoR all objects and space itself (according to Einstein) contract along the direction of my movement. This length contraction occurs for all space and objects in front of me for an infinite distance. Furthermore, relative motion is relative, and the severity of a thing's contraction depends on its speed relative to me, or my speed relative to it. Same difference.

Given all this, imagine that there are three objects in space: Me, Ball A, and Ball B. I am moving at .8c relative to Ball A, and .9c relative to Ball B. I understand that from my FoR, Ball B would be contracted than Ball A. What I'm having trouble understanding is how it would be possible for space to contract to two varying degrees from my FoR. I'm sure there's some error in my thought process here.

Any help getting this straightened out is appreciated. Thank you!

The degree of contraction of the world outside your frame (of reference) depends on the effect of your time dilation, which depends on your speed relative to the universal fixed frame.[1] It’s your speed compared to light speed, v/c, that term that appears in the Lorentz factor, aka gamma, that modifies so many equations of physics for relativistic effects.
Your speed is by choice, so there is no causative effect from the differences in your speed and that for A or B, otherwise you could manipulate the world. Your motion only affects your frame, so any change in the outside world is a change in your perception.
Imagine the fixed frame has distance markers every light second (ls) and you are moving at .9c. On Earth using t=x/v, you calculate a destination of 90 ls should take 100 sec. At arrival your clock reads 44 sec (time dilation). Since you cannot find fault with your clock or any time dependent devices, including your biological sense of time, you solve the issue by assuming your measured speed v is true, leaving distance x in your direction of motion, which must be contracted. The result is not a physical change in the outside world, but a physical change in the mind of the observer altering their perception.

[1] Lack of detection does not imply non-existence.
The MM experiment only demonstrated that the ‘ether of 1900” did not affect the results. If the particles of quantum physics had not existed, they could not have been discovered.
New varieties of sea creatures have been discovered in the ocean depths.

 

[Mentz114]

The degree of contraction of the world outside your frame (of reference) depends on the effect of your time dilation, which depends on your speed relative to the universal fixed frame.[1] It’s your speed compared to light speed, v/c, that term that appears in the Lorentz factor, aka gamma, that modifies so many equations of physics for relativistic effects.
Your speed is by choice, so there is no causative effect from the differences in your speed and that for A or B, otherwise you could manipulate the world. Your motion only affects your frame, so any change in the outside world is a change in your perception.
Imagine the fixed frame has distance markers every light second (ls) and you are moving at .9c. On Earth using t=x/v, you calculate a destination of 90 ls should take 100 sec. At arrival your clock reads 44 sec (time dilation). Since you cannot find fault with your clock or any time dependent devices, including your biological sense of time, you solve the issue by assuming your measured speed v is true, leaving distance x in your direction of motion, which must be contracted. The result is not a physical change in the outside world, but a physical change in the mind of the observer altering their perception.

[1] Lack of detection does not imply non-existence.
The MM experiment only demonstrated that the ‘ether of 1900” did not affect the results. If the particles of quantum physics had not existed, they could not have been discovered.
New varieties of sea creatures have been discovered in the ocean depths.

I don't have the time to refute this point by point, but it is nearly all wrong and contrary to SR.

Relativistic effects are caused by relative velocity between two FoR. It has nothing to do with 'physical changes in the mind of the observer'.

There is no absolute velocity as you seem to be saying.

 

[phyti]

I don't have the time to refute this point by point, but it is nearly all wrong and contrary to SR.

Relativistic effects are caused by relative velocity between two FoR. It has nothing to do with 'physical changes in the mind of the observer'.
There is no absolute velocity as you seem to be saying.

Relativistic effects are caused by absolute velocities. The speed of light is
an absolute value, therefore object speed relative to light speed is absolute.
The problem is, an observer can't measure their speed relative to light,
because they would have to move faster than light to detect the same
signal they sent, so they do the next best thing, measure the round trip
time. This relates to the note, that lack of detection does not imply
non-existence. What an observer measures for relative motion is the relative
differences in speed, time, length, etc. All measurements are made against a
standard, but standards are defined.

If the mind of the observer is not affected, why isn't he aware of his slow
clock rate?

If his perception isn't altered, what physical cause instantly produces a
contracted world outside his frame?

If a local experiment accelerated some particles to near light speed, literally
contracting the universe, how could anyone outside the local frame perform
an isolated experiment?

One can always describe the physics for two moving observers A & B
relative to a fixed frame, then remove the absolute variables, and describe
the physics of A relative to B.

SR is a theory of measurement, therefore it doesn't give explanations in
terms of physical phenomena.

 

[Mentz114]

Relativistic effects are caused by absolute velocities.

SR does not say that. In fact absolute velocity is a redundant concept. We only need *relative* velocity.

If the mind of the observer is not affected, why isn't he aware of his slow
clock rate?

Because it isn't slow. Relativistic effects happen in *other* frames.

If his perception isn't altered, what physical cause instantly produces a
contracted world outside his frame?

When measurements are translated between frames, moving rods have shorter lengths measured in stationary coordinates.

If a local experiment accelerated some particles to near light speed, literally
contracting the universe, how could anyone outside the local frame perform
an isolated experiment?

I don't understand this.

SR is a theory of measurement, therefore it doesn't give explanations in
terms of physical phenomena.

Only measurements show relativistic effects. There are no physical phenomena associated with them.

 

[coktail]

Part of what you're implying is that measurements that involve relativistic effects are not *true* measurements, but that true measurements could be arrived at by subtracting or otherwise removing relativistic effects from the equation, so to speak.

This is contrary to everything I've learned since joining these forums, and while I don't have the chops to dispute in a more technically, I believe it's erroneous.

 

[Mentz114]

Part of what you're implying is that measurements that involve relativistic effects are not *true* measurements, but that true measurements could be arrived at by subtracting or otherwise removing relativistic effects from the equation, so to speak.

This is not what I said. Do you understand 'change of coordinates' ?

(I'm assuming this post is addressed to me.)

 

[coktail]

Hi Mentz144. My post was addressed to phyti. Sorry for the mixup!

 

[1977ub]

If I find the relativistic effect that your clock is moving slower than mine, and you find the relativistic effect that my clock is moving slower than yours, then these are not "true" effects in the sense of being coordinate-independent or frame-independent. They are not like rest-mass is, the same for all observers.

 

[coktail]

Right. I think for once I'm actually clear on all of this. I was just disputing phyti's post and trying to process what he was saying. Thanks for clarifying, though.

 

[Mentz114]

Hi Mentz144. My post was addressed to phyti. Sorry for the mixup!

No worries.

 

[Sugdub]

My understanding is that as I move, from my FoR all objects and space itself (according to Einstein) contract along the direction of my movement.

I share your concerns. I think SR presentations are incorrect insofar there is no such thing as physical objects shrinking down just because they are in motion in respect to yourself. The same for space in general: you cannot be in motion in respect to space unless one comes back to the concept of an absolute space.
All this should be geared back to measurement protocols using a measuring device which propagates through open space at a finite speed in respect to you. Should you wish to measure the distance to a remote object, you won't have any choice but measuring the two-way travel time of – let's say – an electron. All things equal, the outcome of your measurement will vary depending on the relative speed of the object in respect to you, because the measurement protocol will take some time, thus the object will move (away from or toward you) during the process itself, so the distance to be covered by the electron will vary accordingly. However converting the measured duration into a distance to the object is not obvious and will require several hypotheses or postulates. This tricky problem vanishes thanks to the postulate of invariance of the speed of propagation of light: whatever the context, one can always assume this speed is equal to c. Thus, assuming one uses light rays instead of electrons, one can convert the measured duration into a distance.
If a remote object is at rest in respect to you, you will be able to measure its length by difference of two distance measurements targeting each end of the object, assuming you use two-way light rays as a medium. But if the same object is in motion in respect to you, the same measurement protocol (all things equal) will deliver a different value, because light rays will have to cover a longer or shorter distance as compared to the static case: the outcome of the process in the dynamic case will be different from the length of the object. One may state that the object appears to have a different length, but this is misleading because the key events of both light rays (emission, reflection, reception) cannot be all synchronous: this cannot account for a measurement of the length of the object.
The only possible rationale view is to admit that the measurement protocol based on two-way light rays will deliver the correct value for the length of the remote object in the static case, whereas it will deliver a biased value in the dynamic case. Nothing is shrinking down, neither physical objects nor space.

 

[PAllen]

I share your concerns. I think SR presentations are incorrect insofar there is no such thing as physical objects shrinking down just because they are in motion in respect to yourself. The same for space in general: you cannot be in motion in respect to space unless one comes back to the concept of an absolute space.
All this should be geared back to measurement protocols using a measuring device which propagates through open space at a finite speed in respect to you. Should you wish to measure the distance to a remote object, you won't have any choice but measuring the two-way travel time of – let's say – an electron. All things equal, the outcome of your measurement will vary depending on the relative speed of the object in respect to you, because the measurement protocol will take some time, thus the object will move (away from or toward you) during the process itself, so the distance to be covered by the electron will vary accordingly. However converting the measured duration into a distance to the object is not obvious and will require several hypotheses or postulates. This tricky problem vanishes thanks to the postulate of invariance of the speed of propagation of light: whatever the context, one can always assume this speed is equal to c. Thus, assuming one uses light rays instead of electrons, one can convert the measured duration into a distance.
If a remote object is at rest in respect to you, you will be able to measure its length by difference of two distance measurements targeting each end of the object, assuming you use two-way light rays as a medium. But if the same object is in motion in respect to you, the same measurement protocol (all things equal) will deliver a different value, because light rays will have to cover a longer or shorter distance as compared to the static case: the outcome of the process in the dynamic case will be different from the length of the object. One may state that the object appears to have a different length, but this is misleading because the key events of both light rays (emission, reflection, reception) cannot be all synchronous: this cannot account for a measurement of the length of the object.
The only possible rationale view is to admit that the measurement protocol based on two-way light rays will deliver the correct value for the length of the remote object in the static case, whereas it will deliver a biased value in the dynamic case. Nothing is shrinking down, neither physical objects nor space.

Then, imagine traveling along with muon created in the upper atmosphere by a cosmic ray. Its life time is two microseconds. The Earth's atmosphere and the Earth appear to be going by at near c. Only 600 meters of air can go by before the muon is likely to decay. Yet almost all muons make it to the ground. From a frame comoving with the muon, what explanation is there besides that the atmosphere is extremely compressed (many miles into 600 meters or so)?

 

[coktail]

Part of what you're implying is that measurements that involve relativistic effects are not *true* measurements, but that true measurements could be arrived at by subtracting or otherwise removing relativistic effects from the equation, so to speak.

This IS false, right? In length contraction, for example, if you were you measured an object while moving relative to it, then removed the relativistic effects from the equation, you wouldn't get the object's "absolute length," but rather just its length as measured from a resting frame, correct?

I imagine the same is true of measurements of speed. If I am moving at .9c relative to an object and then increase my velocity towards it by a certain amount, it is understood that relativistic addition of velocity comes into play, but if I remove that from the equation (or counteract it or whatever), I wouldn't get an "absolute speed," I'd just get...actually, I'm not sure what I would get, but I'm pretty darn confident this line of thinking is not correct.

 

[PAllen]

This IS false, right? In length contraction, for example, if you were you measured an object while moving relative to it, then removed the relativistic effects from the equation, you wouldn't get the object's "absolute length," but rather just its length as measured from a resting frame, correct?

Some physicists give meaning to rest length similar to (rest) mass. Absolute length is perhaps too strong a word, but if you want a 'default' interpretation of length (similar to what you mean when you just say mass), the only special speeds to measure it would at rest or at c - which is impossible.

I imagine the same is true of measurements of speed. If I am moving at .9c relative to an object and then increase my velocity towards it by a certain amount, it is understood that relativistic addition of velocity comes into play, but if I remove that from the equation (or counteract it or whatever), I wouldn't get an "absolute speed," I'd just get...actually, I'm not sure what I would get, but I'm pretty darn confident this line of thinking is not correct.

In this case you get nonsense. You get 'how fast would it be going if SR were false'. That is different from measuring length or clock rate of a moving object and computing from that (knowing motion) what you would measure if the object were not moving relative to you.

 

[nitsuj]

This is not correct. The error is hidden by saying that "motion" is relative. What is true is that speed is relative. Acceleration is not. The accelerating object will feel a 'force' while the non-accelerating object will not.

It's not an error, IT IS CALLED relative motion, you interpreting it to mean/include acceleration is the error.

The subtext inertial relative motion is hardly necessary.

Especially considering the emphasis Coktail placed on the "arbitrary" distinction of who is moving.

The error specifically I'd call an over sight, naming the comparative motion between the bodies as acceleration.

My point is that the error isn't calling speed; motion.

In case it wasn't an over sight,

Coktail there is two terms important to comparative motion. Inertial and non inertial, and as HallsofIvy pointed out it is merely a difference between (proper) acceleration or not (in this context).

 

[Sugdub]

Part of what you're implying is that measurements that involve relativistic effects are not *true* measurements, but that true measurements could be arrived at by subtracting or otherwise removing relativistic effects from the equation, so to speak.

The notion of "true measurement" is weird. You may run any measurement protocol as you wish but then you must decide whether the outcome of your experiment is representative of a physical quantity or not. And of course this depends on the protocol. As explained earlier, you may use two-way light rays to measure the propagation time between you and a remote object. Then, if the object is at rest in respect to you and assuming the propagation speed of light is the same in both directions (this is part of Einstein's postulate), you may convert half of the directly measured propagation time into an indirect measurement of the distance between you and the object. By difference of two such measurement processes respectively targeting each end of the object, you will then obtain an indirect measurement of its length.
However, should the object be in relative motion wrt you, then the notion of a "distance between you and the object" does not exist as such: it changes continuously across time. It would only make sense if associated with a reference to a given date. Since we have no choice but using a measurement tool (whatever light, electrons, etc...) with a finite speed, the measurement protocol is not instantaneous and therefore there is no way to decide objectively on a date at which the outcome of the propagation time measurement process, once converted into a distance, could account for the distance between you and the object at that date: the object has moved during the process.
In conclusion there is no way to measure the distance between you and a remote object in relative motion because an exact measurement is out of reach of any possible realistic measurement protocol. Hence the measurement of the length of the object cannot be derived either. Hope this explains why the notion of "true measurement" is misleading and why statements by physicists whereby the length of the moving object shrinks down cannot be reconciled with any realistic measurement protocol.

 

[ghwellsjr]

The notion of "true measurement" is weird. You may run any measurement protocol as you wish but then you must decide whether the outcome of your experiment is representative of a physical quantity or not. And of course this depends on the protocol. As explained earlier, you may use two-way light rays to measure the propagation time between you and a remote object. Then, if the object is at rest in respect to you and assuming the propagation speed of light is the same in both directions (this is part of Einstein's postulate), you may convert half of the directly measured propagation time into an indirect measurement of the distance between you and the object. By difference of two such measurement processes respectively targeting each end of the object, you will then obtain an indirect measurement of its length.

You have described the Radar Method of distance measurement.

However, should the object be in relative motion wrt you, then the notion of a "distance between you and the object" does not exist as such: it changes continuously across time. It would only make sense if associated with a reference to a given date. Since we have no choice but using a measurement tool (whatever light, electrons, etc...) with a finite speed, the measurement protocol is not instantaneous and therefore there is no way to decide objectively on a date at which the outcome of the propagation time measurement process, once converted into a distance, could account for the distance between you and the object at that date: the object has moved during the process.

Part of the Radar Method of distance measurement is to apply the time of the measurement to the midpoint of the measurement interval. This gives results that are in agreement with Einstein's conclusion regarding Length Contraction which he gives in his 1905 paper introducing Special Relativity where he states in section 4 entitled "Physical Meaning of the Equations Obtained in Respect to Moving Rigid Bodies and Moving Clocks":

the X dimension appears shortened in the ratio

, i.e. the greater the value of v, the greater the shortening...

It is clear that the same results hold good of bodies at rest in the “stationary” system, viewed from a system in uniform motion.

In conclusion there is no way to measure the distance between you and a remote object in relative motion because an exact measurement is out of reach of any possible realistic measurement protocol. Hence the measurement of the length of the object cannot be derived either. Hope this explains why the notion of "true measurement" is misleading and why statements by physicists whereby the length of the moving object shrinks down cannot be reconciled with any realistic measurement protocol.

Apparently you have not tried the Radar Method of distance measurement to see that it is consistent with Einstein's conclusion of Length Contraction. It is just as legitimate in the dynamic case as it is in the static case. Try it--you'll like it.

 

[1977ub]

In conclusion there is no way to measure the distance between you and a remote object in relative motion because an exact measurement is out of reach of any possible realistic measurement protocol. Hence the measurement of the length of the object cannot be derived either. Hope this explains why the notion of "true measurement" is misleading and why statements by physicists whereby the length of the moving object shrinks down cannot be reconciled with any realistic measurement protocol.

You can always presume to have prepared for this situation by using the Einstein protocol to set up a gridwork of confederates or sensors to make observations at or near to every point in question in the situation. Each sensor is synchronized with your clock, sits at known and established space coordinates, and sends information at light speed to your central location, where your CPU can put the information together to generate coordinates for the times and places of travelers with regard to your inertial rest frame.

 

[Sugdub]

You have described the Radar Method of distance measurement.

Correct and this should not come as a surprise since the experiment described by Coktail is precisely the same.

Part of the Radar Method of distance measurement is to apply the time of the measurement to the midpoint of the measurement interval. This gives results that are in agreement with Einstein's conclusion regarding Length Contraction...

Applying the time of measurement to the midpoint of the time interval is an approximation if the target object is in relative motion to the observer. This approximation is only valid for small values of the relative motion in respect to c. For large values the Radar Method of distance measurement is invalid.
However there is a significant difference between the radar measurement theory and SR: in the same way as it concludes to a blue shift or a red shift for time periods depending on whether the object moves toward or away from the observer, the radar measurement theory will lead to either a contraction or a dilation of the distance.

Apparently you have not tried the Radar Method of distance measurement to see that it is consistent with Einstein's conclusion of Length Contraction. It is just as legitimate in the dynamic case as it is in the static case. Try it--you'll like it.

As you can read above, the consistency of both theories is at stake.

 

[PAllen]

Applying the time of measurement to the midpoint of the time interval is an approximation if the target object is in relative motion to the observer. This approximation is only valid for small values of the relative motion in respect to c. For large values the Radar Method of distance measurement is invalid.
However there is a significant difference between the radar measurement theory and SR: in the same way as it concludes to a blue shift or a red shift for time periods depending on whether the object moves toward or away from the observer, the radar measurement theory will lead to either a contraction or a dilation of the distance.

Nonsense. SR provides no fixed recipe for non-inertial observer's coordinates. As for non-inertial observer's measurements, they can be computed correctly in any coordinates or any inertial frame. For non-inertial coordinates, radar coordinates are as valid as any other infinite number of coordinates, and have some nice properties.

There is no consistency problem due to choice of coordinates, as long as you use the appropriate metric for it.

Of course your following statement:

"As you can read above, the consistency of both theories is at stake. " is absurd.

 

[ghwellsjr]

Nonsense. SR provides no fixed recipe for non-inertial observer's coordinates. As for non-inertial observer's measurements, they can be computed correctly in any coordinates or any inertial frame. For non-inertial coordinates, radar coordinates are as valid as any other infinite number of coordinates, and have some nice properties.

There is no consistency problem due to choice of coordinates, as long as you use the appropriate metric for it.

Of course your following statement:

"As you can read above, the consistency of both theories is at stake. " is absurd.

I agree with everything you said here but Sugdub wasn't even talking about any observer being non-inertial, was he? He thinks there is a difference between SR and the Radar Method for high inertial speeds.

He needs to try it out.

Sugdub--please try it out before you continue to make such blatantly wrong statements.

 

[Sugdub]

I agree with everything you said here but Sugdub wasn't even talking about any observer being non-inertial, was he? He thinks there is a difference between SR and the Radar Method for high inertial speeds.

I did not address in any event "non-inertial" coordinates, frames, observers,... only relative motion at constant velocity. The previous input was simply irrelevant.
Two simple questions which attract simple answers:
Is it true that the radar method of distance measurement, when comparing the dynamic case where the target object is moving in respect to the observer with the static case where it is at rest, leads to either a shorter or a larger result (as compared to the static case) for the two-way wave propagation time, on the ground that the distance to be covered varies during the propagation itself (as opposed to the static case), the result being shorter if the object moves toward the observer and larger if the object moves away from him/her (Y/N)?
Is it true that SR always predicts a contraction of lengths and distances in the dynamic case as compared to the static case, irrespective of whether the target object moves toward or away from the observer (Y/N)?
Then we might understand where the discrepancy comes from.