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Two spaceships in opposite direction at near c 
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#37
Dec812, 08:38 AM

P: 126




#38
Dec812, 09:23 AM

P: 848




#39
Dec812, 11:03 AM

P: 126

I think we might have the same thoughts about this, but to me the whole point about the coordinate systems is that whatever coordinate system you use (apply to the outside observer independent events) then SR's 'relativity of simultaneity' shows you that reality out there is a 4D block spacetime/universe. But this is not allowed to be discussed here. 


#40
Dec912, 06:24 AM

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P: 4,696

I later drew two more diagrams in response to K^2's request in post #31. In each of these, the signal going from the red spaceship at his 1minute mark is received by the black spaceship at his 9minute mark, even though the light signal takes a varying amount of time in each IRF but still travels at c in each of them and not with respect to the speeds of the spaceships. This was the whole point of these diagrams in support of my comment to jartsa in post #29: And now to my main point: The blue spacestation cannot notice the black (or red) spaceship's time dilation. Time dilation is not observable by anyone anywhere anytime. It is a calculation related to the speed of an object in a given IRF. If it were ever observable, then the observer would know which arbitrary IRF we were using. Or, more significantly, if it were observable, then we could identify the an absolute ether rest state and all of Special Relativity would be out the window. Just because I drew three IRF's in which one of the observer's was a rest, you should not extapolate that observer's observations to what is assigned by the IRF, such as the time dilation related to the speed of the other objects. I could just as easily have drawn another diagram in which none of the observers was a rest, for example one in which the black spaceship and the blue spacestation are traveling at the same speed in opposite directions. Then how would you explain time dilation? Look, the reciprocal time dilation is very easily illustrated by looking at each of the IRF's for each observer and my text succinctly states what it is. For example, in the first IRF, blue's rest frame (post #30), I state that gamma for red and black is 1.6667 and I show their dots spaced by that amount with respect to the coordinates which also happens to be with respect to blue since blue is stationary in this IRF. Then if you go to the next IRF, black's rest frame (the first small IRF in post #32), I state in the text that the time dilation for the spacestation (incorrectly identified as the spaceship) is 1.6667 and you can see the exact same spacing of the blue dots in this IRF as you do for the black dots in the first IRF. You can do the same thing for each of the other pairs of space ships/station. 


#41
Dec912, 06:40 AM

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For example, in the first IRF (post #30), minute five for the blue spacestation is simultaneous with minute three for the black and red spaceships. However in the other two IRF's (post #32) these three events occur at different coordinate times and so are not simultaneous. But minute three for the blue spacestation is simultaneous with minute five for one or the other of the two spaceships in these other two IRF's. What are you guys concerned with? 


#42
Dec912, 08:06 AM

P: 126

It's far more correct to explain time dilation by means which clock indication (event) pops up in a selected frame (3D space), whether the dots are spaced or not is besides the point. (That's why a loedel diagram is beter to show time dilation.) The time dilation occurs because the worldlines take a different direction in 4D spacetime, and hence the lines of simultaneity take other directions.... 


#43
Dec912, 06:42 PM

P: 848

Vandam, I feel like your point was very well placed and fully appropriate to the discussion. Not to take away from the basic point ghwells was makingthat was a good response to the original post. But, you certainly brought additional valuable insight to the discussion. Your emphasis on relativity of simultaineity really needed to be presented. It's always good to put the discussion in the context of the foundational physical concepts available to us with special relativity theory.



#44
Dec1212, 09:54 AM

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#45
Dec1212, 09:14 PM

P: 848

Beyond that I thought perhaps you may have over emphasized the significance of the derived basis of the "measurement" of time dilation: "And now to my main point: The blue spacestation cannot notice the black (or red) spaceship's time dilation. Time dilation is not observable by anyone anywhere anytime. It is a calculation related to the speed of an object in a given IRF. If it were ever observable, then the observer would know which arbitrary IRF we were using. Or, more significantly, if it were observable, then we could identify the an absolute ether rest state and all of Special Relativity would be out the window." 


#46
Dec1312, 06:17 AM

P: 848

p.s. Vandam also made the very significant observation that for the red and black guys going in opposite directions with the same relativistic speed, the spacing of the minute marks on the two time axes are the same. So, you could not use the spacings to tell you anything at all about time dilation. His emphasis of the use of the hyperplanes of simultaneity was quite appropriate. The hyperplanes of simultaneity are always different in the 4dimensional universe for any two observers moving with respect to each other. And their spacetime diagram minute mark spacings may or may not be the same, depending on the choice of charts used in the diagram. Penrose highlights this even for two observers just walking past each other (his Andromeda Paradox).



#47
Dec1412, 04:02 AM

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But then it finally dawned on me. Apparently you and Vandam take the viewpoint that time dilation is calculated based on the relative speed between the clock and the observer, not between the clock and the coordinate time of the IRF. When I said in another thread: 


#48
Dec1412, 06:26 AM

P: 126

Ghwellsjr,
You show us 3 sketches but apparently have big problems of reading my sketch of post #35. No offence, but do you know how to read a 4D Minkowski spacetime diagram? I never said your IRF charts are wrong. But if you would be able to read your 3 charts all in one spacetime diagram only, then you would immediately see what's really going on, in 4D. 


#49
Dec1412, 09:44 AM

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#50
Dec1412, 10:24 AM

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Vandam, this is a 2D problem. There is no 4D. There is one time dimension and one spacial dimension. A single diagram shows everything that's going on.



#51
Dec1412, 01:29 PM

P: 126

Let me give you an analogy. A draftsman showes you a bunch of technical drawings. Lots of sheets of paper. Twodimensional drawings. You say: "Awesome! 2D drawings is what architecture is about. Nothing else." An architect has a glimp at it and says: "The 2D drawings are fine, but... they are all 'only" sections and elevations/façades of a building. The building is reality, your 2D drawings only observations. Let me quickly sketch you the 3D perspective of the house so that you can grasp what you are working on." Draftsman reaction: "All this 3D stuff is ridiculous.' ... Sigh. But what then happens is even more pathetic: the more the architect explains how it all works, the more the draftsman holds tight on his 2D drawings. But that's normal behavior. Draftsmen are very good technical experts, they protect what they are good at. But they are, or become very seldom good architects. Oh yes, sorry,... architects are probably philosophers, artistic lunatics. Isn't it? (By the way: our two eyes capture 2D images. 2D observations. Are you going to tell me there is no 3D building out there to be observed?) 


#52
Dec1412, 02:15 PM

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Let me give you an analogy: A movie maker hands you an old 2D movie (on film). Instead of watching the movie using a projector and a screen, you cut the film into the individual frames and then you stack them one next to the other and you claim that you have discovered the 3D reality of the movie and anyone who actually watches the movie is stuck in a 2D world.



#53
Dec1412, 03:27 PM

P: 126

The difference is that I give you an explanation where the 2D drawings come from. You do not tell me where de 2D frames come from.



#54
Dec1412, 04:38 PM

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P: 4,696

But your 4D block universe explanation of "reality" with regard to relativity is just a philosophical opinion. This has been explained to you and bobc2 so many times, I'm surprised you're still promoting it.



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