The case for True Length = Rest Length

[GrayGhost]

What any observer measures of space and time has no relation to any frame of reference. They don't need to be thinking about a frame of reference or have defined a frame of reference to make a measurement or to make an observation. How many times does this need to be repeated?

My understanding is that a frame-of-reference is equivalent to a spacetime coordinate system. Twin B assigns the origin of a coordinate system to himself, and that represents his own frame-of-reference. I've generally referred to this as a point-of-view (POV). I have always assumed that the coordinate system assigned to oneself, is in essence equivalent (in part) to one's own ruler. I'm presuming you will disagree with this, yes? I mean, in thought experiment, the cooridinate system could be considered of rigid axes. In that case, what then would the difference between the coordinate system and the measuring ruler?

GrayGhost

 

[Mike_Fontenot]

I think one of the "disconnects" in these recent exchanges, is this:

1) Any object or person exists in ALL reference frames, and

2) Anyone can choose to ANALYZE the motion of any other person, by using ANY reference frame,

BUT

3) When GrayGhost refers to "the reference frame OF a particular person", I think he intends that it be a reference frame in which that person is permanently at rest at the spatial origin. Such a frame describes what the given observer concludes about the world surrounding him. [ADDENDUM #2: In particular, the reference frame OF a particular person DIRECTLY tells him how far away all objects are from him, AND what the current age of each of those objects is (and how the age of each of those objects is currently changing).]

[ADDENDUM: I hadn't seen GrayGhost's above post when I wrote the above comments (we were typing at the same time!) ... his above post says essentially what I was trying to get at above.]

Mike Fontenot

 

[GrayGhost]

Unless twin A knows the whole story--twin B's acceleration and deceleration profiles and how far or how long twin B is going to travel--any discussion of a frame of reference for twin A is pointless. If they are aware of the whole scenario, like we are, then they could use the LTs to see what things look like in other frames. But if they want to see what things look like to themselves, they don't need frames or LTs, they just look.

Hmmm. I would think that twin A assigns a coordinate system to himself (as origin) regardless as to whether twin B exists, or whether he's considering twin B motion vs any other observer's motion. So I do not see it as pointless. Also, I don't see why either twin has to wait until the roundtrip is completed to use the LTs. They work no matter when they are applied by either twin, including for times after twin B departure and before twin B return.

Wrt your last sentence, I cannot understand why you are telling me this. I doubt anyone would disagree that "to see, you look".

Anybody can use LTs to convert the values of time/distance events in one inertial frame to any other inertial frame, if they are aware of the entire scenario. They won't help an observer take what he is seeing and measuring and somehow extrapolate to things that he cannot see and measure. The problem is not that twin B is non-inertial, it's that you are linking twin B to a non-inertial frame and trying to use the LT for some purpose that it cannot serve.

Wrt the 1st sentence here, please see my prior response above. Wrt the 2nd sentence here, SR is a local theory and no one can foresee the future. One might try to predict the future flight profile of another fellow, but it's nothing more than an educated guess. Wrt your 3rd sentence here, I am considering the twin B frame-of-reference equivalent to the collection of corresponding momentary-contiguous-inertial-frames-of-reference, and I figure the LTs work just fine as they always do. The only difference is that the LTs must be applied to infitesimal segments and summed (as you go), whereas in all-inertial scenarios you just run the LTs once because the motion never changes.

Although the definition and analysis of the Twin Paradox is easiest and most convenient in a frame in which they both start out at rest ... it's not a matter of it being a "preferred" frame. Even though I personally would choose that frame, I would not call it that because "preferred" has a distinct meaning in SR which is that a particular frame is more "true" or "right" or "matches reality better" or something along those lines as opposed to my personal arbitrary first choice.

I have never even remotely suggested that any frame is preferred. In fact, I've stated many times over that the non-inertial POV is less convenient but no less preferred.

If I knew what "solns" meant, maybe I could respond, but I doubt that whatever you said is obvious.

Easy ... LT solns = the results of LT calculations.

What's this about LT predictions? LTs cannot predict anything, they only convert values from one inertial frame to another.

Indeed the LTs transform, but they are run using the best known track data currently at hand. All systems have resolution limits, which thereby affects the precision of calculations. Observers attain LT results based upon imperfect input data. As the error in the input data approaches zero, the observer's prediction via Lorentz transformation approaches reality.

If there's one thing that anyone should learn after studying SR for any length of time, it is that time is relative.

I agree.

Why are you trying to predict or correlate the times on twin B's clock with the times on twin A's clock? The only times you can correlate are the start and end times. All other times can vary all over the place depending on the frame of reference used.

Indeed, observers of relative v >0 are allowed (and expected) to disagree on simultaneity, which means the relativity of simultaneity allows observers to agree on their disagreements...

At some twin A hypothetical moment, let's say twin A determines the twin B clock should read 1.275 hr-B (post twin-B departure), and that twin B should then hold twin A currently at 2.115 lh-B downrange, with current velocity at 0.922c. If twin A later asks twin B ... hey, when you were at 1.275 hr by your own clock, how far downrange was I and what was my speed? ... if twin B does not say 2.115 lh at 0.992c, then twin A either screwed up his LT calculation of B, or the input data he used was inaccurate. Every observer, and no matter what his frame of reference, must agree that B will say "2.115 lh at 0.992c", assuming that all data used for predictions are precise and accurate.

GrayGhost

 

[GrayGhost]

Mike_Fontenot,

I'd say you summed it up well there, thanx.

GrayGhost

 

[ghwellsjr]

ghWells,

Wrt "frame-of-reference", here's what was stated prior ...

OK, so you agree there is no natural sense in which changing velocities causes you to "transition" from one inertial frame to another, that it is just a matter of it being more convenient?

Well, I'd say it this way ... in a natural sense, whenever 2 observers are OF the same frame of reference, even if momentarily, they measure space and time the very same.

All observers are of all frames OF reference all the time

Here, you are debating the meaning of the word OF.

I'm debating the meaning of the word OF? You're the one that used it, I was just using your wording. I have no idea why you are saying that or why you bolded your first occurrence of the word OF and my second occurrence. I just don't know what this is all about.

I agree in that all bodies reside within the spacetime cooridinate system used by any observer.

It's not just that all bodies reside within the spacetime coordinate system used by any observer, it's that you should analyze the whole situation from one single spacetime coordinate system and it doesn't have to be one that is "used" by any observer.

I'll restate what I said prior as follows ...

in a natural sense, when the coordinate system of each of 2 observers overlay each other perfectly, even if momentarily, they measure space and time the very same.​

When they overlay "only momentarily", then they measure space and time the same "momentarily", far as the infitesimal segment is concerned and as the infitesimal duration considered approaches zero..

You seem to have this idea that there is a single natural coordinate system of any observer, but if natural is a meaningful term in this context, there are in fact many and I can think of several that might be considered "natural" but I have no idea if one of them is what you would consider "natural".

For example, in the Twin Paradox, it might be natural to define the X axis in the direction that twin B starts out and keep it that way for the whole trip, but it might also be natural to rotate it when twin B turns around so that he is always going in a positive X direction. Or maybe to be a little more like the Point Of View (POV) of an observer, we should place the positive X direction coming out of his nose, the Y direction going through his ears and the Z direction going out the top of his head. And, of course, after we get done doing this for twin B, we should do it all over again for twin A. (Why are you focusing so much on twin B?)

A while back, you made this statement:

I might add ... not only does twin A's distant luminal clock spin wildly during B's own rapid proper acceleration, eg the turnabout point, but twin A also flies wildly across the heavens (per B). Twin A doesn't do this on its own, but rather only because twin B's POV has changed while rapidly accelerating. The always inertial observer never experiences (or records) any such effect, because their sense-of-simultaneity never rotates since they never undergo proper acceleration.

But then you decided that twin B doesn't actually get to see twin A behaving this way but rather he has to determine that it is actually happening by calculation [I corrected a couple typos]:

First, a ways back, I made a misleading statement that suggested that the visual experience of twin B would witness the "time jump" (no time is ever missing though) during B's rapid turnabout. In fact, B will only observe the rapid doppler shift, not the A-time-jump. The A-time-jump exists, but must be determined because it cannot be seen visually. I believe I corrected that mis-statement in subsequent posts. The LTs reveal to twin B that the A clock advanced wildly during B's own rapid turnabout, even though the light signals show only a doppler shift ... and the doppler shift requires that twin A jumped wildly across space per B. In addition to the rapid doppler shift, if twin A was emitting pulses at periodic intervals, B (upon completion of his rapid turnabout) would note that the rate of receipt of said pulses increase by a factor of gamma.

Now this is really out of my realm because I don't like the torture of non-inertial frames but it seems to me that if you are using the first natural POV that I listed above, when twin A flies wildly across the heavens, that the heavens are also flying wildly and as soon as twin B's acceleration is finished, and he is back to his original speed (in the opposite direction), twin A has flown wildly back to his original position, along with the heavens. Since twin A has remained inertial during the entire trip and never experiences any acceleration, your frame of reference, POV, spacetime coordinate system, whichever term you like to use must calculate that twin A experienced no acceleration and therefore no wild flying or jumping. But I could be wrong. I don't like torture, especially self-inflicted.

But if you used the second POV, then twin A would not only fly across the heavens from far behind twin B but he would end up in front of him along with the heavens, because twin A never accelerates. But I could be wrong.

Now did you ever consider what would happen if twin A were using the third natural POV method going through the nose, ears and top of the head and he looked down at his feet? All of a sudden, wouldn't twin B go flying across the heavens (along with the heavens, of course)? I could be wrong. I hate torture.

I asked you before but never got a response: where did you come up with the factor of gamma in the last sentence of the previous quote?

It's not that I disagreed with JesseM. Indeed it is "more convenient" to do so, but I see something more there than JesseM does. Twin B always has his own frame-of-reference he uses. It just seems to me that there is no difference between twin B's own frame-of-reference, and the consideration of an inifinite number of contiguous-momentary-corresponding inertial-frames-of-reference (taken as a collective POV) that twin B resides in "at the origin of each" ... far as spacetime coordinate transformations are concerned. If said collective POV is equivalent, then the SPACE-JUMP and TIME-JUMP in my prior illustration exists ... even though doppler effects do not reveal it casually. I see no reason whatever, as to why the twin B frame-of-reference would differ from said collective inertial frames-of-reference. The LTs were designed for all-inertial motion, but IMO they inherently require the twin B frame-of-reference be equivalent to said collective inertial frames-of-reference. I assume here that twin B uses a euclidean coordinate system as inertial observers do.

GrayGhost

As stated earlier over and over again, you can use any frame of reference, even an arbitrary non-inertial one like "twin B's own frame-of-reference" (whatever that means) but twin B can't use it because he does not have the information that we have. He has to wait for the information concerning twin A to get to him (or rely on a prior agreement that twin A will not accelerate). And twin A can't use his "own frame-of-reference" (unless he relies on a prior agreement on when twin B will turn around and with what profile) because he certainly has no knowledge based on information available to him in real time of what twin B is doing. But whichever frame of reference you use, you must include both twins in it. After you figure out everything that is going on according to your favorite frame of reference, you can do it all over again with another one. If you had picked in inertial FOR, you could use the LT to convert from one FOR to another FOR, but that's even torture for me. I recommend picking the FOR that requires the least amount of calculation and that is one in which both twins start out at rest and end up at rest. You can use Relativistic Doppler to calculate what each twin sees and everything is soooo simple. I like that. Not that there is anything wrong with torture, I just don't like it.

 

[ghwellsjr]

I'm not aware of any time that I have been clearly wrong and someone brought it to my attention and I didn't express my appreciation. Maybe someone will do that now.

I've already said this thread is losing my interest but I'll post these simply so you can either clarify what you meant or thank me for pointing them out.

I said it is a relative speed over a period of time that leads to an age difference.

Could you please explain the effect of the orbiting twin aging more slowly than the twin experiencing no acceleration at the orbit's focus?

I already did in post #264.

If we say that the orbiting twin has a relative speed then the stationary one also has relative speed by definition, correct?

No, the orbiting twin has a relative speed and a non-relative or an absolute acceleration. That means we cannot consider the stationary one to be relative to the accelerating one, just like in the classic Twin Paradox when the traveling twin accelerates to take off, turn around or land at the end of the journey. It's only when the traveling twin is inertial that we can consider the stationary twin's speed to be relative to the traveling twin but the orbiting twin is never inertial so the stationary one cannot have a speed relative to him.

There is something that is different between the twins in this scenario, and it isn't speed...

That's right--it's acceleration.

It's not my claim that there is no absolute time in our real world. Clocks traveling at different relative speeds will tick at different rates. I didn't make that up. I learned it.

Your initial statement, plus the bolded word, imply that you believe in absolute time. However...

But, not all frames will agree that the twins' two clocks have kept time in any absolute sense.

Do you think absolute time exists or not??

No and I apologize for being so unclear. I thought when I said

"It's not my claim that there is no absolute time in our real world."​

you would understand that I was not claiming credit for the idea and that I didn't make it up but that I learned it. Oh, but I already said that.

I'm curious, what is the issue with your bolding my word "will"? How does that imply that I believe time is absolute?

Now how about you, rjbeery? Do you think absolute time exists or not??

It would surprise me if you did, but it also surprises me that you made the claim that the nature of the twins' age differential would vary depending upon the observing frame (even after their reunion??). This was another issue that you glossed over when I pointed it out.

This is like SR101. Clocks slow down in a frame where they have a speed. Please understand, this other frame will see the same time difference on their clocks that they see when the reunite, but since the two twins' clocks are running slow then their age difference will be smaller, according to this other frame. Do I need to defend this fact? I guess until I apologize for being "clearly wrong" on this point, you are going to continue claiming that I am dismissive and glossing over your points.

Speaking of being mistaken on a point, wasn't it you that said there is nothing contradictory in the sentence "I'm shorter than you and you're shorter than me"?

Maybe you should provide a link or a post # when you want to quote me instead of just making things up. Are you thinking of post #213?

OK enough with the petty stuff.

No, that's not enough. I have sincerely tried to address your claims of when you thought I clearly wrong on a point. I asked you a question here:

Now I'm beginning to wonder if you even understand such common terms as "acceleration", "co-moving", an "inertial". How else could you account for this posting of yours?

And you were dismissive, not even acknowledging the question. Please click on the link button (the little arrow to the right of your name) and tell me how you are not clearly wrong in the quotes following my question or thank me for pointing out your error. Otherwise, it is clear that you have no intention of playing by your own rules.

As for your general point...you're asking me whether or not I "really" think light is propagating at an angle between the mirrors or the distance between the mirrors is "really" contracted in a moving light clock (depending on the rotation of the clock), but your attempt to establish that this is the case is equivalent to denying the principle of equivalence. It's almost like you're arguing against my case for "true length" by attempting to assert a preferred frame of clock observation. There will always be a frame in which these things are not occurring.

It's truly a shame that you continue to misquote me. I have put in bold words that you claim I said that I didn't say. Why do you do that?

You have to pay careful attention to what I was saying. I was talking about an accelerating (not just moving) light clock. And there is no inertial frame in which the tick rate is not changing (I didn't say time dilation because that is a change in a particular direction) and then I said if the light clock were reoriented so that instead of the mirrors being parallel to the direction of acceleration, they were perpendicular to the direction of acceleration, then I said the distance between the mirrors must change while the light clock is accelerating in order to maintain the same changing tick rate as it would have had in the first orientation. I know this is somewhat complicated when expressed in words but I would appreciate it if you would ask for clarification instead of just arguing it down.

The basis of my stance is that to have two observers make what are apparently logically contradictory statements (i.e. "both observers claim the others' clock is narrower than their own") then we can assign no absolute truth to those statements. There is only a single circumstance in which both parties agree with the others' assessment of the width of their clock relative to their own, and that is when they are inertial to each other. I've said many times that this is nothing, really, but a semantic convention but I feel the logic contains some merit. Does that make sense?

It's not a matter of whether it makes sense, the issue should be does it comport with reality. Even though you are now claiming that the only time the width of objects carried by two observers can only be compared when they are at rest with each other, that's not how you started out this thread. You started out by saying that two rods in relative motion have the same true length equal to their rest length. Don't you see the difference between these two claims?

Here's what you should learn and understand: when you have two rods in relative motion and the rods are aligned along the direction of relative motion, each rod will see, measure, observe, conclude, etc. that the other one is shorter than itself. But this assessment is just like the assessment of the relative speed. Nobody claims that when you have two object/observers/rods/clocks in relative motion that both of them at the same time are traveling at the same speed but in opposite directions. You recognize what each of them observes but then you assign an inertial frame of reference. As soon as you do that, the lengths of the rods take on an absolute nature for that frame of reference. If you choose a frame in which one of the rods is at rest, it will have what is called its rest length, but the other one will be shorter, not just as an illusion but absolutely for that frame of reference. Then you can stop using that frame of reference and switch to a frame of reference in which the second rod is at rest. Now it will be what is called its rest length and the first one will be shorter, both according to the frame of reference that is in effect. Now you can pick another frame of reference, say one in which both rods are moving in opposite directions but at something more than half of their relative speed. Now both rods will be the same length but shorter than their rest lengths for that frame of reference.

I sincerely hope this helps you, rjbeery.

 

[ghwellsjr]

What any observer measures of space and time has no relation to any frame of reference. They don't need to be thinking about a frame of reference or have defined a frame of reference to make a measurement or to make an observation. How many times does this need to be repeated?

My understanding is that a frame-of-reference is equivalent to a spacetime coordinate system. Twin B assigns the origin of a coordinate system to himself, and that represents his own frame-of-reference. I've generally referred to this as a point-of-view (POV). I have always assumed that the coordinate system assigned to oneself, is in essence equivalent (in part) to one's own ruler. I'm presuming you will disagree with this, yes? I mean, in thought experiment, the cooridinate system could be considered of rigid axes. In that case, what then would the difference between the coordinate system and the measuring ruler?

GrayGhost

Yes, as I said in my reply to your previous post, a frame of reference is a spacetime coordinate system. It's used by those of us who are creating and using the thought experiments. It is sometimes thought of as a system of rigid rulers placed in all three dimensions with synchronized clocks at each junction. Or we could just draw it on a piece of paper or a screen in some approximation to 3-D. Or we could eliminate the Z-axis and illustrate just 2-axes of space and have it animated to provide the time axis in real time. Or we could eliminate one more axis of space and end up with a spacetime diagram. But do people do that in real-life? I've never seen it done. Astronomers probably have the closest thing to this streching out into space so that they can locate heavenly objects at any given time.

This kind of coordinate system is usually not too useful to the pretend observers in the thought experiment because we know everything that is going on from one end of the area of space under considertion to the other and our poor observers can only experience what they are aware of as it is made aware to them. I'm talking about the light travel time which is what these thought experiments are all about.

If you want to have your observers also have their own POV, or if you want to use their POV as the coordinate system for your scenario, then you are complicating matters immensely and for no good reason, in my opinion. But if you do, you should not think that because we can plop down information that we know into the coordinates system that that information is readily available to the observer. He can only know what comes to him by way of light signals when they reach him. This is the essence of the problem that Special Relativity addresses. You cannot sidestep that problem and allow the observer to know things ahead of time. It's important that we observe how that information is made available to the observer or otherwise, I don't see the point in doing the thought experiment.

I hope I have addressed your concerns. If not, please ask again.

 

[ghwellsjr]

I think one of the "disconnects" in these recent exchanges, is this:

1) Any object or person exists in ALL reference frames, and

2) Anyone can choose to ANALYZE the motion of any other person, by using ANY reference frame,

BUT

3) When GrayGhost refers to "the reference frame OF a particular person", I think he intends that it be a reference frame in which that person is permanently at rest at the spatial origin. Such a frame describes what the given observer concludes about the world surrounding him. [ADDENDUM #2: In particular, the reference frame OF a particular person DIRECTLY tells him how far away all objects are from him, AND what the current age of each of those objects is (and how the age of each of those objects is currently changing).]

[ADDENDUM: I hadn't seen GrayGhost's above post when I wrote the above comments (we were typing at the same time!) ... his above post says essentially what I was trying to get at above.]

Mike Fontenot

I think I have addressed most of these issues in my previous post to GrayGhost.

The real problem with what I see you attempting to do is promoting the idea that an observer can answer questions about things remote from him (times and distances) before information concerning those things has propagated to him via light signals. And the other problem is that you don't seem to grasp the concept of such things as the relativity of time. You seem to think that there are answers to questions like what time is on the one person's clock for every moment on the other person's clock. These questions do not have unique answers but are frame dependent and, of course, when you use a non-inertial frame or switch between frames, you can end up claiming that clocks go backwards or race forwards. It wouldn't be so bad if you would always bring up your first two points and just say, hey look what happens when you use my CADO technique, instead of claiming that it is the only correct way to do it (when there is no correct way).

 

[rjbeery]

ghwellsjr, the general demeanor of this thread has become, in my opinion, nonconstructive. I've made my case as best I can and we continue to have disagreements on many issues. Therefore I am bidding you adieu, thanks for your thoughts.

 

[ghwellsjr]

I would like to respond to some of your questions and concerns in this post but since you were composing it while I was composing post #275 where I addressed many of your concerns, I will not address them again here unless you call them to my attention again.

But if they want to see what things look like to themselves, they don't need frames or LTs, they just look.

Wrt your last sentence, I cannot understand why you are telling me this. I doubt anyone would disagree that "to see, you look".

Here's an example:

Your statement: The overall experience per twin B is twin A's wildly spinning distant clock during periods of twin B's own proper acceleration is not correct. Twin B (the traveling twin) never experiences twin A's clock wildly spinning and by that I mean twin B never observes twin A's clock wildly spinning.

In this you are mistaken, and so we disagree. The twins graphic rjbeery just threw up tells the story there. Not only can twin A's clock spin wildly per B "per the math" during B's own rapid proper accelerations (pos or neg), but twin B will also observe this similarly.

Now I realize that you have since changed your mind about this but I think it is a common mistake that many people make thinking that the information we have coming from a frame of reference is somehow available to an observer in our scenario if he is at the origin of the FOR.

Wrt the 2nd sentence here, SR is a local theory and no one can foresee the future. One might try to predict the future flight profile of another fellow, but it's nothing more than an educated guess.

I never liked the word "predict". I never thought it applied to LTs and never understood your use of the term.

Easy ... LT solns = the results of LT calculations.

So "solns" is an abbreviation for "solutions"?

Indeed the LTs transform, but they are run using the best known track data currently at hand. All systems have resolution limits, which thereby affects the precision of calculations. Observers attain LT results based upon imperfect input data. As the error in the input data approaches zero, the observer's prediction via Lorentz transformation approaches reality.

The Lorentz Transform is a mathematical process that perfectly converts events (space-time coordinates) from one frame of reference to another frame of reference. It does not make any kind of prediction. It does not give anybody any additional information that they did not already know. It only changes the four numbers that identify the X, Y, Z, and T values in one frame to their equivalent values in another frame.

At some twin A hypothetical moment, let's say twin A determines the twin B clock should read 1.275 hr-B (post twin-B departure), and that twin B should then hold twin A currently at 2.115 lh-B downrange, with current velocity at 0.922c. If twin A later asks twin B ... hey, when you were at 1.275 hr by your own clock, how far downrange was I and what was my speed? ... if twin B does not say 2.115 lh at 0.992c, then twin A either screwed up his LT calculation of B, or the input data he used was inaccurate. Every observer, and no matter what his frame of reference, must agree that B will say "2.115 lh at 0.992c", assuming that all data used for predictions are precise and accurate.

GrayGhost

I'm sure you have mistakes in this exercise. Could you please look it over and fix them, this doesn't make any sense at all. It appears that you have twin B traveling at faster than the speed of light.

But beyond that, the whole idea of trying to answer the question of how far twin B has traveled is frame dependent and you haven't specified the frame in which to answer the question. Or are you just assuming that it's obvious which frame should be used? Also, relative speeds are not frame dependent and yet you listed two different speeds but I suspect that is a typo.

 

[GrayGhost]

ghwellsjr,

To refocus this discussion, I previously posted this illustration ...

scenario description here ...

https://www.physicsforums.com/showpost.php?p=3182687&postcount=235"​

Illustration here ...

https://www.physicsforums.com/attachment.php?attachmentid=32982&d=1299884366"​

The question was never anything more than this ... How does twin A map into the twin B system IF everyone flew as such?

My goal was to show that twin A must wildly jump thru space (per B) when B undergoes virtually instant proper accelerations. Again, this is per B's POV as the result of B's own proper acceleration, not any energy expenditure on A's behalf. Hand in hand with that, the twin A clock-readout must jump wildly per B. Add, the doppler effect changes abruptly as well. Now, I assume here that either twin may apply the LTs as they usually do, even if non-inertial, except that they must apply it to infitesimally small segments and sum the solns (solutions) over the interval. Of course, none of that matters given I've reduced the duration of acceleration to a virtual zero, for sake of simplicity and point.

You've raised a number of issues that you seem to think others disagree with. Just to mention a few ...

(1) observers cannot know how others move until light signals arrive revealing such.
(2) observers in relative motion disagree on the measure of space and time.
(3) The LTs were designed for the all-inertial case.

No relativist disagrees with these statements, I included. Now I have addressed each of these issues, more than once, yet you continue to come back with statements time and time again suggesting I disagree. I'm not sure why you do that, but it does slow down the discussion.

A couple other matters you (or JesseM) raised ...

(4) non-inertial observers may use any convention-of-simultaneity that they feel is valid.
(5) the LTs do not apply to non-inertial POVs.

IMO, whether (4) is true depends on what convention you are talking about. In SR, one may use a different sense-of-simultaneity, but then the 1-way speed of light is not c, although the 2-way speed may be c. When non-inertial though, the convention JesseM raised whereby the observer assumes the reflection event (of emitted radar EM) occurred at the roundtrip's-center-point, is no good in my opinion. Far as (5) goes, I remain unconvinced that this is true, assuming the LTs are applied for infitesimal segments and summed over the interval. It seems to me that the LTs apply within any instant of time, even though they were designed under the simple all-inertial case.

Make no mistake, I understand that no one can predict with certainty where a moving vessel might be in the present moment. Yet, the body does indeed exist "at some specific location NOW", and time will tell the story. My discussion has never been about "guessing" where the moving vessel is at present, but rather how it must map into twin B own's system "wherever it really is". I merely provided one axample whereby twin A remains always inertial, and that was for sake of simplicity and point. My point was merely to show the related required SPACE-JUMP and TIME-JUMP that I mentioned prior here.

GrayGhost

 

[ghwellsjr]

JesseM already addressed your linked post and its diagram. I was never involved in that discussion. It's torture to me.

But I will address your persistent claim of twin B's POV. I asked you some questions about it in post #275 which you ignored. I sincerely don't know what you are talking about and I need you to address my concerns. Here is a repeat of what I said in that post:

You seem to have this idea that there is a single natural coordinate system of any observer, but if natural is a meaningful term in this context, there are in fact many and I can think of several that might be considered "natural" but I have no idea if one of them is what you would consider "natural".

For example, in the Twin Paradox, it might be natural to define the X axis in the direction that twin B starts out and keep it that way for the whole trip, but it might also be natural to rotate it when twin B turns around so that he is always going in a positive X direction. Or maybe to be a little more like the Point Of View (POV) of an observer, we should place the positive X direction coming out of his nose, the Y direction going through his ears and the Z direction going out the top of his head. And, of course, after we get done doing this for twin B, we should do it all over again for twin A. (Why are you focusing so much on twin B?)​

So when you say POV, do you mean one of the three that I described or something else?

 

[Mike_Fontenot]

Excellent summary of the situation, GrayGhost.

Mike Fontenot

 

[Dale]

In SR, one may use a different sense-of-simultaneity, but then the 1-way speed of light is not c, although the 2-way speed may be c.

The naieve simultaneity convention does not preserve the 1-way speed of light to be c.

 

[GrayGhost]

ghwellsjr, I know where you're going. You're wondering how I can consider time dilation to be absolute under acceleration but that I might still consider length contraction illusory. I must admit that bringing acceleration into the picture complicates things for me because I have little experience with the math involved (I'm neither a physicist nor a mathematician).

That being said, my response is that length contraction is always reciprocal; in other words, I'm not aware of any circumstance in which an inertial observer and an accelerating observer will both agree on which party is length contracted. Are you?

rjbeery,

Are you still out and about here? If so, I have a few comments that may help if you are interested.

GrayGhost

 

[rjbeery]

GrayGhost, I had planned on unsubscribing from this thread but hadn't done so yet. Anyway I'm all ears...

 

[GrayGhost]

JesseM already addressed your linked post and its diagram.

Indeed, but not to satisfaction.

I was never involved in that discussion. It's torture to me.

Could have fooled me. However, if you prefer not to discuss it then I'm OK with that. I'm happy to discuss whatever you wish, assuming time (and other factors) permit.

But I will address your persistent claim of twin B's POV. I asked you some questions about it in post #275 which you ignored. I sincerely don't know what you are talking about and I need you to address my concerns. Here is a repeat of what I said in that post:

You seem to have this idea that there is a single natural coordinate system of any observer, but if natural is a meaningful term in this context, there are in fact many and I can think of several that might be considered "natural" but I have no idea if one of them is what you would consider "natural".​

Well, it was JesseM (or DaleSpam) who injected the term "natural" in relation to frame-of-reference.

I realize that there are different types of reference-frames, eg inertial, rotating, and accelerating. In SR, we use euclidean coordinate systems, and it is "natural" IMO to assign yourself "the origin" of the system you elect to use. This is because all inertial observers are obliged to assume "the stationary", which is always the most convenient choice. I personally assign B a euclidean system, himself the origin, initially at rest in the twin A frame. As he accelerates, remote moving bodies relocate in space and their clocks tick uncharacteristically (wrt the all-inertial case) during periods of B's own proper acceleration. I realize there are other ways of doing it, but that's what I personally envisioned in my stated scenario and illustration.

For example, in the Twin Paradox, it might be natural to define the X axis in the direction that twin B starts out and keep it that way for the whole trip, but it might also be natural to rotate it when twin B turns around so that he is always going in a positive X direction. Or maybe to be a little more like the Point Of View (POV) of an observer, we should place the positive X direction coming out of his nose, the Y direction going through his ears and the Z direction going out the top of his head. And, of course, after we get done doing this for twin B, we should do it all over again for twin A. (Why are you focusing so much on twin B?)

So when you say POV, do you mean one of the three that I described or something else?

From a navigation standpoint, here's what would be natural IMO ...

The CAPT of the ship spins up his navigation-gyro. The onboard computer system would align the x, y, and z-axes per reference-markers built within the ship. The x-axis might be the centerline bow-to-stern, z-axis top-to-bottom, y-axis portside-to-starboard. Etc. Then it would not matter which way the CPTN turns his head within the ship.

GrayGhost

 

[GrayGhost]

ghwellsjr, I know where you're going. You're wondering how I can consider time dilation to be absolute under acceleration but that I might still consider length contraction illusory. I must admit that bringing acceleration into the picture complicates things for me because I have little experience with the math involved (I'm neither a physicist nor a mathematician).

That being said, my response is that length contraction is always reciprocal; in other words, I'm not aware of any circumstance in which an inertial observer and an accelerating observer will both agree on which party is length contracted. Are you?

rjbeery,

Here's my 2 cents ...

Relativistic effects exist when relative motion exists, and they are measurable. The only reason folks debate at length over whether they are real vs illusionary effect, is because ... although relative motion produces measurable length-contractions (and time-dilation), no body ever changes in-and-of-itself due to another's observational POV. This also suggests that no body ever changes in-and-of-itself due to its own proper acceleration. Relativists generally say that contractions "are real per the observer", because they are measurable. Others argue that the measured contractions are something-less-than-real, because the body never changed in-and-of-itself. IMO, it's just 2 ways of looking at the very same mechansim, and therefore comes down to semantics.

My view on "time being absolute in any way" is this ... Inherent in the special theory is that we all pass thru the 4d continuum at the rate of c. As an inertial observer, you experience this only as the passage of "proper time" while you sit there stationary, ie the rate at which time passes yourself per yourself. That may be considered an invariant IMO. However, the theory also requires that moving clocks tick slower, and each inertial observer is obliged to assume the stationary. The relative rate of clocks is about "relative time", which is something more than "proper time" alone, the result of "space and time" being interwoven into a single fused entity as the result of the 2 relativity postulates being true.

EDITED: Consider the 1st half of the classic twins sceanrio. The defined interval is A/B-to-A/X, X being planet X of the A frame (B's turnabout point). B begins and ends colocated with A, and of the A frame. Assume the proper separation between A and X to be length S, a length at rest in the A frame. Twin B must record this separation in motion, and thus length contracted at x = S/gamma. Assuming the rate of proper time is the same for all, then he who travels the shorter pathlength must accrue the least proper time, and thus age the least. That be twin B.

EDITED: In the all-inertial scenario, B travels inertially from A to X. The same mechanism applies there. All observers agree that B experiences less proper time than A (or X) wrt the defined interval A/B-to-X/B. However A & B do NOT agree on who aged the least. Each claims the other to have aged less, because of relative simultaneity and the fact that moving clocks tick slower. In the classic twins scenario, B ages less because his acceleration brings him back into the A frame, and since relativistic effects vanish at v=0, then disagreements (including who aged less) due to relative-simultaneity vanishes as well. So while you are correct in that acceleration forces "agreement" between the observers (which you refer to as absolute), the extent of aging over the interval depends entirely on the time spent in each inertial frame of reference (LTs depend on v, not dv/dt). Because the LTs use v and not acceleration, the twins must run the LTs for infitesimals and sum the results as they go.

GrayGhost

 

[Dale]

(4) non-inertial observers may use any convention-of-simultaneity that they feel is valid.
...

IMO, whether (4) is true depends on what convention you are talking about. In SR, one may use a different sense-of-simultaneity, but then the 1-way speed of light is not c, although the 2-way speed may be c. When non-inertial though, the convention JesseM raised whereby the observer assumes the reflection event (of emitted radar EM) occurred at the roundtrip's-center-point, is no good in my opinion.

Hi GrayGhost, I mentioned this briefly today, but now I have time to make a more complete response. Special relativity is based on two postulates, the first being that the laws of physics are the same in all inertial frames, and the second being that the (one way) speed of light is the same in all inertial frames. Now, unless you express things in terms of tensors, in non-inertial frames the first postulate is violated.

However, it is possible to make non-inertial coordinate systems which do not violate the second postulate (in flat spacetime). That is what the Dolby and Gull simultaneity convention does, it enforces the one-way speed of light postulate at all points. The usual naive simultaneity convention (aka Mike Fontenot's CADO convention) does not preserve the second postulate. This doesn't mean that it cannot be used, just that if maintaining the second postulate is one of your decision factors then you would probably opt for Dolby and Gull instead.

 

[ghwellsjr]

I realize that there are different types of reference-frames, eg inertial, rotating, and accelerating. In SR, we use euclidean coordinate systems, and it is "natural" IMO to assign yourself "the origin" of the system you elect to use. This is because all inertial observers are obliged to assume "the stationary", which is always the most convenient choice. I personally assign B a euclidean system, himself the origin, initially at rest in the twin A frame. As he accelerates, remote moving bodies relocate in space and their clocks tick uncharacteristically (wrt the all-inertial case) during periods of B's own proper acceleration. I realize there are other ways of doing it, but that's what I personally envisioned in my stated scenario and illustration.

What do you mean "obliged"? Is this something you think is a forced requirement of SR? Or is it just because you think it "is always the most convenient choice"? Or something else?

And what if there are two inertial observers in relative motion--how do they both assume "the stationary"?

From a navigation standpoint, here's what would be natural IMO ...

The CAPT of the ship spins up his navigation-gyro. The onboard computer system would align the x, y, and z-axes per reference-markers built within the ship. The x-axis might be the centerline bow-to-stern, z-axis top-to-bottom, y-axis portside-to-starboard. Etc. Then it would not matter which way the CPTN turns his head within the ship.

GrayGhost

But what about when the ship gets to its final position and starts coming back home--does the direction of the X-axis remain the same as it was before so that the ship is now progressing in the opposite direction or does the entire coordinate system flip around so that the ship continues in a positive X direction throughout the whole trip? I'm trying to figure out which of the first two options I previously mentioned you are describing.

 

[GrayGhost]

Hi GrayGhost, I mentioned this briefly today, but now I have time to make a more complete response. Special relativity is based on two postulates, the first being that the laws of physics are the same in all inertial frames, and the second being that the (one way) speed of light is the same in all inertial frames. Now, unless you express things in terms of tensors, in non-inertial frames the first postulate is violated.

Thanx DaleSpam. My reasoning has been conceptual, and based upon an extrapolation of the special theory to the non-inertial case. In my reasoning, I do not see how either postulate would be violated. It seems to me that the same laws of nature should be upheld. However, my line of reasoning requires that twin A advance (or digress) along his worldline more (or less) than the usual case when inertial, and (only) per twin B as he undergoes proper acceleration. In my view, such an interpretation should allow the laws of nature to be unscaved. I'm not using any math model of my own, just an extrapolation of the special theory to the accelerational case. I realize that when B undergoes proper acceleration, that the heavens do not expend energy to move just for him and his POV. I envision the heavens moving as B accelerates similarly as the stars move at nightime, except that relative velocity plays an equal role in the POV's relative orientation change.

I have found that when you make a statement online here, one can generally take it to the bank, so I suppose studying the general theory would help immensely. Just so I know, when you get a chance, can you state in a short brief para how the 1st postulate is violated w/o the use of tensors? I'm just curious as to whether what I'm envisioning can stand up against the violations. If not, I would then have to reconsider it.

However, it is possible to make non-inertial coordinate systems which do not violate the second postulate (in flat spacetime). That is what the Doby and Gull simultaneity convention does, it enforces the one-way speed of light postulate at all points. The usual naive simultaneity convention (aka Mike Fontenot's CADO convention) does not preserve the second postulate. This doesn't mean that it cannot be used, just that if maintaining the second postulate is one of your decision factors then you would probably opt for Doby and Gull instead.

Can you please post me a reference for that Doby and Gull convention? I can't recall if you posted that link in the past, and I'd like to take a look at it. Thanx.

GrayGhost

 

[Dale]

Just so I know, when you get a chance, can you state in a short brief para how the 1st postulate is violated w/o the use of tensors? I'm just curious as to whether what I'm envisioning can stand up against the violations. If not, I would then have to reconsider it.

It is actually essentially a tautology. The first postulate says that the laws of physics have the same mathematical form in all inertial frames. This is essentially a definition of an inertial frame, i.e. an inertial frame is one in which the laws of physics takes their standard form. Therefore, if you find a coordinate system where the first postulate holds, then that coordinate system is inertial. So conversely, in all non-inertial coordinate systems, the laws of physics do not take their standard form, by definition.

The reason I mention tensors is simply that in tensor notation all laws take their standard form regardless of the coordinate system. So there is no distinction between inertial and non-inertial coordinate systems that way.

Can you please post me a reference for that Doby and Gull convention? I can't recall if you posted that link in the past, and I'd like to take a look at it.

Certainly, here is the arxiv article by Dolby and Gull:
http://arxiv.org/abs/gr-qc/0104077

 

[bobc2]

GrayGhost, I had planned on unsubscribing from this thread but hadn't done so yet. Anyway I'm all ears...

Glad to see you are still all ears, rjbeery. I still cannot reconcile your idea that acceleration of the round-trip twin causes the difference in aging.

I've plagarized DrGreg's original graphics illustrating the 4-dimensional motion of three observers. I don't think you ever satisfactorily responded to DrGreg's post. However, I'd like to break the analysis down into two steps:

1) Do you maintain, or can you demonstrate that the changing instantaneous 3-D cross-section views of observer A below actually changes the rate of aging of observer C (remaining at rest in the black coordinate system)? Because you can see that manifestly C ages at a uniform rate as he moves along the tC (C's time axis) at the speed of light. Yes, A's view of C's clock is not consistent with the actual aging of C along C's world line. But, just because A views the C clock does not physically cause it to tick slower or faster. If A closed his eyes and decided not to observe C's clock, would C age faster during A's acceleration period? Of course not. A observing C has no effect on C's aging process.

Thus, the aging rate of C is fixed.

2) Now, what about the aging rate of A? Does A's rate of aging change while he is accelerating? As measured in the black coordinates, you can see clearly the variation in proper time increments along the world line of A. The rate of aging is very slow during the outgoing and return world line segments corresponding to constant relativistic speeds. During the turnaround negative acceleration portion of A's worldline, A's aging actually speeds up (with respect to black coordinates)--you see the proper time increments are much more closely spaced over that curved part of A's worldline.

Thus, even in the black coordinate system, C is actually aging much faster than A during A's constant speed (straight line) portion of A's worldline. And of course, A is always moving at the speed of light along his world line--thus, it is the fact that A is following a much shorter path through 4-dimensional space that accounts for the spaced out proper time increments.

Having said all of that, I return to my original analysis of many posts back that if you are going to compare aging processes, you should be comparing proper distance lengths of the world lines (remembering each observer moves along his worldline at the speed of light). And in this analysis the observer taking the shorter worldline path is the younger of the two observers when they meet again.

p.s. I included the worldline for observer B in case you wished to comment on DrGreg's original observations. If A and B both undergo the same acceleration, why is it that the age differences were not the same for A compared to C and then B compared to C? And did C's aging change two times, once for A and then again for B?

 

[GrayGhost]

What do you mean "obliged"? Is this something you think is a forced requirement of SR? Or is it just because you think it "is always the most convenient choice"? Or something else?

Obliged = is allowed to, if one desires to. The stationary POV is not a forced requirement in SR, however it is almost always the case that one desires to assume such, because of the convenience that results. It should also be pointed out that the LTs are designed for an observer who assumes the stationary POV. They transform spacetime coordinates in your own stationary system into coordinates of the moving observer's system (also deemed as stationary per he).

And what if there are two inertial observers in relative motion--how do they both assume "the stationary"?

Each assigns himself a euclidean coordinate system with himself assigned "always the origin".

But what about when the ship gets to its final position and starts coming back home--does the direction of the X-axis remain the same as it was before so that the ship is now progressing in the opposite direction or does the entire coordinate system flip around so that the ship continues in a positive X direction throughout the whole trip? I'm trying to figure out which of the first two options I previously mentioned you are describing.

Well, I suppose it depends on what you might be calling the x,y,z axes. There are a number of sets-of-axes involved aboard a ship. The gyro spin-axis is related to the gyro system's axes. The navigation system will relate the gyro system's axes to both "ship's-axes and spatial-axes". The ship's axes are fixed wrt the ship-hull itself. The spatial-axes are fixed wrt some unchanging reference in space ... true north for military vessels, but some fixed star(s) assuming deep space travel. So to cut to the chase, the navigation system will present a dynamic ship-coordinate-system (which rotates as the ship turns) within a fixed star-coordinate-system (which never rotates).

GrayGhost

 

[ghwellsjr]

Obliged = is allowed to, if one desires to.

What dictionary did you get that out of? Can you point me to an online reference that provides that definition, please?

Each assigns himself a euclidean coordinate system with himself assigned "always the origin".

According to the wikipedia article on Lorentz Transformation, "the origins of both coordinate systems are the same" (in order to be in the Standard Configuration which is what you used in your diagram). What do you think that means?

Well, I suppose it depends on what you might be calling the x,y,z axes. There are a number of sets-of-axes involved aboard a ship. The gyro spin-axis is related to the gyro system's axes. The navigation system will relate the gyro system's axes to both "ship's-axes and spatial-axes". The ship's axes are fixed wrt the ship-hull itself. The spatial-axes are fixed wrt some unchanging reference in space ... true north for military vessels, but some fixed star(s) assuming deep space travel. So to cut to the chase, the navigation system will present a dynamic ship-coordinate-system (which rotates as the ship turns) within a fixed star-coordinate-system (which never rotates).

GrayGhost

I'm asking you what you mean by POV and now you are suggesting that there are at least two coordinate systems in use by the traveling twin at the same time. Can you understand how I still don't know what you mean by POV?

 

[GrayGhost]

rjbeery,

I noticed that I butchered the last (long) paragraph in my prior post to you, as I made it in haste. I've redited that final para, which is now 2 paras. If you already read it before I edited it, please see it again. Sorry about that. The hyperlink is here ...

hyperlink above

When you say "acceleration makes time absolute", I take it that you mean all observers agree as to how much twin A and B aged, and who aged the least. I agree with you on that matter. However, I'd state it this way ... the age disgreement vanishes because they begin and end in the same frame of reference, because relative simultaneity (the cause of disagreement) vanishes when v=0. I suppose that's to say the same thing, but just in a different way.

GrayGhost

 

[GrayGhost]

What dictionary did you get that out of? Can you point me to an online reference that provides that definition, please?

Well, you got me there, lol. Call it the dictionary of GrayGhost ...

http://www.thefreedictionary.com/obliged"​

Each "does himself a favor" by assuming the stationary because it is convenient :)

According to the wikipedia article on Lorentz Transformation, "the origins of both coordinate systems are the same" (in order to be in the Standard Configuration which is what you used in your diagram). What do you think that means?

Man, you're picky :) OK then ... Of course the system origins are the same. However, "each possesses a vertical worldline" that intersects x=0 at t=0, and their worldlines are not colinear.

I'm asking you what you mean by POV and now you are suggesting that there are at least two coordinate systems in use by the traveling twin at the same time. Can you understand how I still don't know what you mean by POV?

OK, prior you said ...

For example, in the Twin Paradox, it might be natural to define the X axis in the direction that twin B starts out and keep it that way for the whole trip, but it might also be natural to rotate it when twin B turns around so that he is always going in a positive X direction. Or maybe to be a little more like the Point Of View (POV) of an observer, we should place the positive X direction coming out of his nose, the Y direction going through his ears and the Z direction going out the top of his head. And, of course, after we get done doing this for twin B, we should do it all over again for twin A. (Why are you focusing so much on twin B?)

Well, there are multiple sets of axes involved in any navigation system. However if I picked a coordinate system to act as the ship's POV, I figure the POV highlighted here could apply to the ship. That is, the positive X direction coming out of the bow, the +Y direction coming out of the starboard side, and the +Z direction coming out the roof of the ship. These are standard fixed-ship-axes. If the ship turns or accelerates, the heavens appear to move while it maintains itself the stationary.

The reason I've been focused on twin B, is because his POV is more complex than twin A's. Twin A's is far easier. The heavens do not move wildly within the A-system when twin B accelerates. So applying the LTs to infitesimals is a little more complex for B than A. Both have to account for the fact that velocity can change during any infitesimal. However, twin B also has to account for the heavens moving during the infitesimal segments considered, which affects his LT solns (when transforming to the twin A system) because his sense of simultaneity rotates. The smaller the infitesimal, the less the error.

GrayGhost

 

[ghwellsjr]

Well, you got me there, lol. Call it the dictionary of GrayGhost ...

http://www.thefreedictionary.com/obliged"​

Each "does himself a favor" by assuming the stationary because it is convenient :)

Chalk it up to "semantics".

Man, you're picky :) OK then ... Of course the system origins are the same. However, "each possesses a vertical worldline" that intersects x=0 at t=0, and their worldlines are not colinear

GrayGhost, have you seen any description or explanation of the Lorentz Transform that says that the "observer" for each frame is even in the frame, let alone forever fixed at the origin? I believe the intent is that the observer for each frame is as a super-observer like you or me looking at all parts of the frame at the same time. We are not bound by the speed of light or energy or anything else. We can pop down clocks, spaceships, rulers, and even people any where we want at any fraction of the speed of light.

OK, prior you said ...



Well, there are multiple sets of axes involved in any navigation system. However if I picked a coordinate system to act as the ship's POV, I figure the POV highlighted here could apply to the ship. That is, the positive X direction coming out of the bow, the +Y direction coming out of the starboard side, and the +Z direction coming out the roof of the ship. These are standard fixed-ship-axes. If the ship turns or accelerates, the heavens appear to move while it maintains itself the stationary.

The reason I've been focused on twin B, is because his POV is more complex than twin A's. Twin A's is far easier. The heavens do not move wildly within the A-system when twin B accelerates. So applying the LTs to infitesimals is a little more complex for B than A. Both have to account for the fact that velocity can change during any infitesimal. However, twin B also has to account for the heavens moving during the infitesimal segments considered, which affects his LT solns (when transforming to the twin A system) because his sense of simultaneity rotates. The smaller the infitesimal, the less the error.

GrayGhost

What would happen if twin B decelerated when he got to his maximum distance away from twin A until he was stopped with respect to twin A and came to rest? What would his POV be and what would he see?

 

[GrayGhost]

GrayGhost, have you seen any description or explanation of the Lorentz Transform that says that the "observer" for each frame is even in the frame, let alone forever fixed at the origin?

The LTs are designed with a starting reference event, ie the intersection of the 2 system origins. That's where the 2 worldlines intersect. There may be an observer anywhere at rest in either of the 2 frames. However, the cooridinate system itself is a POV whether any observer resides at its origin "to behold of it" or not. It's natural for an observer to assign himself the origin of the system he assigns to himself, and so it's also natural (in thought) to assume an observer located at the origin of a system.

I believe the intent is that the observer for each frame is as a super-observer like you or me looking at all parts of the frame at the same time. We are not bound by the speed of light or energy or anything else. We can pop down clocks, spaceships, rulers, and even people any where we want at any fraction of the speed of light.

I figure that the intent of the LTs is to map coordinates in spacetime between 2 inertial systems that move relatively. I don't see the need for "super", myself. It's just the way it is. Indeed, any material thing may be anywhere and of v<c, and in thought we might imagine anything that "could exist". Far as not being bound by c, all is possible in the imagination. That is, we can imagine the guy is "over there right now", even though light signals have not yet proven such. Time tells the story :)

What would happen if twin B decelerated when he got to his maximum distance away from twin A until he was stopped with respect to twin A and came to rest?

He'd be at rest with twin A, but remotely located.

What would his POV be and what would he see?

Depends. Do you define POV as what he sees per his own eyes, or the vessel he's in that got him there? I mean, let's say the CAPT turns his head for a moment while the ship's in transit. Should the POV change here? From an SR standpoint, coordinate systems are inertial and do not rotate.

In real life, granted, it's another matter and becomes much more complex. But from an SR standpoint, the origin of a coordinate system has a POV. If an inertial observer is at said origin, he beholds the same POV. Twin B's POV at the turnabout point, when back in the A frame, is defined by the LTs (amongst other things).

GrayGhost

 

[ghwellsjr]

Back in post #271, you defined your use of POV to be a specific FOR in which the observer is at the origin:

What any observer measures of space and time has no relation to any frame of reference. They don't need to be thinking about a frame of reference or have defined a frame of reference to make a measurement or to make an observation. How many times does this need to be repeated?

My understanding is that a frame-of-reference is equivalent to a spacetime coordinate system. Twin B assigns the origin of a coordinate system to himself, and that represents his own frame-of-reference. I've generally referred to this as a point-of-view (POV).

Now you are saying that it's still a POV even if there is no observer located at the origin:

The LTs are designed with a starting reference event, ie the intersection of the 2 system origins. That's where the 2 worldlines intersect. There may be an observer anywhere at rest in either of the 2 frames. However, the cooridinate system itself is a POV whether any observer resides at its origin "to behold of it" or not. It's natural for an observer to assign himself the origin of the system he assigns to himself, and so it's also natural (in thought) to assume an observer located at the origin of a system.

And then you ask me how I define POV:

Do you define POV as what he sees per his own eyes, or the vessel he's in that got him there? I mean, let's say the CAPT turns his head for a moment while the ship's in transit. Should the POV change here? From an SR standpoint, coordinate systems are inertial and do not rotate.

In real life, granted, it's another matter and becomes much more complex. But from an SR standpoint, the origin of a coordinate system has a POV. If an inertial observer is at said origin, he beholds the same POV. Twin B's POV at the turnabout point, when back in the A frame, is defined by the LTs (amongst other things).

GrayGhost

I never use POV except in this thread when trying to figure out what you mean by it. You will note in my first quote above, I said:

What any observer measures of space and time has no relation to any frame of reference. They don't need to be thinking about a frame of reference or have defined a frame of reference to make a measurement or to make an observation.​

You are the one who insists on putting a non-inertial observer at the origin of some kind of non-inertial FOR or at the origins of a series of inertial FOR's or whatever it is you have in mind that you don't seem to be able to communicate precisely. So that is why I'm asking you to consider the observer (twin B) after he has come to rest with respect to his twin at the halfway point of his trip. Please describe his POV that you think is natural for him and decribe what he sees of his twin, the twin's clock, the heavens, and all the other descriptions that you gave earlier when the traveling twin did not stop but instead reversed direction.

Please understand, I am not debating you, I'm trying to understand you, but you have to be precise and clear and not turn my questions back on me.