No definite viewpoint for the accelerating traveler?by Alain2.7183 Tags: accelerating, definite, traveler, viewpoint 

#1
Feb1413, 11:19 AM

P: 24

If an accelerating traveler, at some given instant in his travels, is told that his question "How old is my home twin right now?" has no unique, definite answer, then wouldn't he have to regard that statement as implying that she must not really EXIST at that instant at all? If she DOES exist right then, wouldn't he have to believe that she must be DOING something definite right then?




#2
Feb1413, 11:36 AM

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But when the accelerated traveler speaks of what she's doing "right now", we have to ask him what "right now" means. Which time on her wristwatch is the accelerated traveler talking about when he says "right now"? That question has no unique definite answer; so "right now" has no unique definite meaning, and therefore the question "What is she doing right now?" also has no unique definite answer. 



#3
Feb1413, 12:07 PM

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But Special Relativity provides a way to deal with the problem by defining an Inertial Reference Frame (IRF). You can pick any IRF you want and then you can talk about "right now" in a meaningful way. But you can pick another IRF and have a different meaning to "right now" that is just as valid as the first one. 



#4
Feb1413, 03:33 PM

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No definite viewpoint for the accelerating traveler?Now, once you specify a coordinate system then the questions become uniquely defined. Frame variant quantities are perfectly legitimate things to ask questions about, you just have to be clear what frame they refer to. 



#5
Feb1813, 04:29 PM

P: 297

Once we're in GR, and we're comparing a "Far out in space" twin with a "surface of the earth" twin, do we still have these same ambiguities?




#6
Feb1813, 04:32 PM

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#7
Feb1813, 07:20 PM

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But they can do so without having a definite opinion about which tick of a satellite is simultaneous with a particular time EST, I take it. 



#8
Feb1813, 07:29 PM

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#9
Feb1813, 08:19 PM

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If I want to compare the rate at which someone else's clock is ticking relative to my own, I don't need to worry about what time the other guy thinks it is, nor how much time he thinks has passed since some event in his past. All I need to do is count how many times my clock ticks in a given interval, count how many times his clock ticks in the same interval, and compare. The trick, and the place where the simultaneity convention comes into play, is in deciding what "the same interval" means (and I hope that alarm bells went off in your mind when you saw those words). 



#10
Feb1813, 08:25 PM

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Does the GR earthsurface observer experience the same ambiguity deciding the distance to an orbiting satellite that the AO SR observer does an RF source? 



#11
Feb1813, 10:11 PM

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#12
Feb1913, 10:05 AM

P: 24

In the several descriptions I've seen that use a fictitious gravitational field to resolve the twin paradox from the traveler's perspective, there was no ambiguity anywhere ... the procedure always gave a specific (unique) answer to the question of how much the home twin ages during the traveler's turnaround (according to the traveler). And that answer was always the same answer that is given by the SR analysis that uses the momentarily comoving inertial reference frames. See, for example, the Wikipedia page on the twin paradox, and in particular, their section on the traveler's perspective. 



#13
Feb1913, 12:39 PM

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The idea that 'one method is presented' implies there is a unique preferred answer, is your (invalid) inference. It is not stated in explanations using this simplest approach to pseudogravity. 



#14
Feb2013, 11:56 AM

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#15
Feb2013, 01:39 PM

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Both at 3pm on my watch and at 4pm on my watch, an earth home clock read 7pm ? (in between, it advanced to 8pm). That is, can you describe any way at all to correlate this statement with any observation you could make? (You cannot; not only that, all direct observation contradicts such a description  the earth clock is seen to move monotonically forward, throughout any traveler journey). Given that no observation correlates to this, why should a sane person believe it? It NOT a required implication of SR; in fact I recently researched that Einstein, for example, never used such lines of simultaneity in any of his SR or GR work. So you would posit that Einstein never understood SR? 



#16
Feb2013, 02:20 PM

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#17
Feb2113, 08:12 PM

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So it seems to me that if lines of simultaneity intersect, that is not a "hole" in space time that is unmapped by the traveller's coordinate system. Rather it is an area of space time that is multiply mapped; assigned more than one time coordinate. 



#18
Feb2113, 08:18 PM

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(It would be different if there was some observation consistent with multiple simultaneity. But there is none.) 


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