
#37
Dec1012, 01:15 PM

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So when I made the three separate diagrams in post #9, I used the same list of data from all observations and measurements with the aid of Einstein's definitions of what an Inertial Reference Frame (IRF) is. So the coordinates of each IRF are not part of the observations and measurements and the relationship between the coordinates and the measurements/observations are not recognizable to the observers. As I keep saying, how could they be? They change each time I use the Lorentz Transformation process to generate a new IRF and draw all the events with the new coordinate system. You are right, I'm not interested in your opinion of "what lays at the origin of the observations" because I don't believe you or anyone else knows. In fact, I get the impression that you are promoting an idea that claims that it is no longer relative, once you see the forest for the trees. In any case, I have tried to understand the block universe concept that bobc2 and you are so fond of, but I find it so complicated that my eyes glaze over every time I see another one of those posts like #6 in the thread you linked to earlier. My opinion is that my diagrams and explanations would be easier for a newby to understand than the diagrams and explanations promoting the block universe. But, like I said in that other thread, we will have to get some feedback from various newbies to settle that issue. In any case, I don't post comments about bobc2's or your diagrams saying that my way is "far more correct" or that there is "a danger of misinterpreting his diagrams" or implying that they are inherently incorrect or misleading. How would I know? I can't understand them. 



#38
Dec1012, 01:43 PM

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However, Darwin123 is right that except for the "corner", all motion involved is geodesic; so the "corner" is where you need to look if you want to find out why the "change in geodesics" occurred. Similar remarks apply to the other scenarios you give: you have to look at the entire path through spacetime that each twin takes to get a final answer on relative aging, but if you want to understand why the paths are "crooked", why they're composed of segments of different geodesics instead of just one geodesic all the way, you need to look at the corners since that's where the "change in geodesics" happens. 



#39
Dec1012, 05:10 PM

P: 126

Everything is said by that. Your time coordinates do not give any additional 'explanation' to it. But Block universe does. Of course if you are not interestand in explanations then you have not to worry about that. But I was worried about that when I came accros SR... I had to to understand the observations and maths. But again, if you are just happy putting numbers in Lorentz Tranformations... so be it. 



#40
Dec1012, 05:52 PM

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#41
Dec1012, 08:34 PM

P: 848

We could have a long discussion about examples of direct measurement of quantities in physics versus derived quanties. So, I think one has to be careful about minimizing the significance of derived "measurements" in physics. I think you make way too big a deal about the derived quanties leading to fundamental concepts in special relativity. I don't understand your difficulty in interpreting Vandam's Loedel sketches. Many of my undergraduate students had a little trouble at first but caught on after spending some time really thinking about it. (No, I did not push block universe on them, but we did have interesting class discussions about some of the implications) I'm really not trying to push the block universe concept here. However, I've documented in other threads the many notable physicists who embrace the concept (Paul Davies's book "About Time" is a good reference on the subject). You should not look upon it as a separate theory. It is a direct manifestation of Minkowski's geometric picture, i.e., SpaceTime. Some people reject it, thinking it is a philosophical outlook. It is not. On the contrary, folks who reject it are doing it after they themselves bring in the field of philosophythey are actually rejecting it on a philosophical basis, probably because they don't like some of the implications. The implicationsthose are the cause of my own struggle with the concept. On the one hand I can't deny what Minkowski's SpaceTime is showing us quite directly, but on the other hand I cannot quite make peace with it at a subjective level. I have too strong of a psychological sense of existing in a 3D world that evolves with time. And I must give the disclaimer that I am a serious Christian and think a lot about the theological implications of foundational physics in that context. So, my problem is that on the one hand I cannot refute the picture of the physical block universe that is manifest in special relativity, but on the other hand I can't handle it at the subjective level. Vandam seems to have overcome those kinds of concernshe just sticks to the facts and lets the chips fall where they may. So, for him the block universe is physical reality. (hope I haven't misrepresented Vandam). I always appreciate his comments and Loedel diagrams. 



#42
Dec1212, 05:07 AM

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But the unmeasurable oneway speed of light is defined in any Inertial Reference Frame to be equal to the measurable round trip speed of light. But there is no way to measure or observe the time dilation of any clock. If you think there is then please describe how you would propose doing it. For example, let's say you are moving in an IRF at some high speed. Your clock is time dilated but you can't tell, can you? Or let's say you are observing a distant moving clock. All you can observe is the Relativistic Doppler shift which is independent of any reference frame but the propagation time of the image of the clock coming to you is dependent on the reference frame as well as the time dilation. Different frames trade off these two factors in such a way that the observation remains the same so you cannot observe or measure this trade off without which you cannot determine the time dilation. And unless you actually know what the distant clock is doing at any particular time, you can't even say what its speed is until the image of it gets to you. Look, as I said before, I don't complain that what you and Vandam are doing is wrong when you are presenting your explanations. Sometimes I have asked for clarification and understanding but I let you carry on without any hindrance from me. You just shouldn't complain about my explanations or insinuate that they are not accurate or not complete (unless you think they are in which case it would be helpful to specify exactly what the problem is). 



#43
Dec1212, 07:49 AM

P: 126

I think we have to have a little converstion on what we both mean by 'observation'. You do not understand relativity of simultaneity; that's the origin of time dilation! 



#44
Dec1212, 08:08 AM

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Vandam, I'm going to let others respond to your post. You obviously aren't going to accept anything I say. Maybe bobc2 can straighten you out since you seem to hold him in high esteem.




#45
Dec1512, 02:44 AM

P: 126

SR talks about observers/observations. In order to understand SR we have to agree what observations mean, otherwise it is pointless to start dealing with observers and observations, or SR at all. Just to make sure we understand each other as far as 'observation' is concerned; You 'observe' (see) that an observer in the train sees a lightning hitting the front of the train because first there was lightning, then light travels to the observer, and then the light hits the observer's retinae. This is the meaning of 'to observe'. You see lightning hitting the front of the train because there is an observer independent event that's later observed by you (and other observers). There has/have to be event(s) to be observed and measured. Basic stuff. I guess you accept this. Why then are you not interested in "what lays at the origin of the observations", i.e. the observer independent event: lightning hitting the front of the train? You tell us about observation of events, but you refuse to tak about the events... Don't you contradict yourself? SR is about observer independent events, which means: there are events out there (not part of your mind, be it physical or mental, whatever) even before you observe them, otherwise there simply can not be an observation, nor observers. If you refute this, then what are observations and observers in SR? 



#46
Dec1512, 09:07 AM

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Vandam, I don't understand why you are asking me these questions when I have already specifically addressed these issues on this thread. Why don't my previous answers satisfy you?
The issue you and I are dealing with on this thread is whether or not time dilation can be observed and measured. I have said over and over again that it is dependent on the arbitrary frame of reference that you use to describe the scenario so how can it be measured by the observers in the scenario? Please go back and study my post #9 on this thread with regard to time dilation and tell me what observations or measurements the two observers can make that will enable them to determine the time dilations during each phase of the scenario and for each IRF. 



#47
Dec1512, 01:55 PM

P: 126

Really amazing, Ghwellsjr. Really.




#48
Dec1512, 05:03 PM

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#49
Dec1512, 06:07 PM

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Now what's important is that two (or more) events that are simultaneous in one IRF (because they have the same value for their time coordinate) may not be simultaneous in another IRF as can be seen if you look at the three different graphs. I never really stopped to think in terms of a volume of simultaneity, assuming that that is what you mean by a hyperplane of simultaneity, but it is obviously the case although I would say it is so obvious that it doesn't need to be said. Now if we wanted to show a twodimensional scenario where the observers were moving around in both the x and ydimensions, we'd have a hard time putting that on a piece of paper but what we could do with today's technology is make an animation and present it as a movie. Each frame of the movie marks out a plane of simultaneity but the assumption is that it extends out into the zdimension and so there really is a volume of simultaneity. Does that communicate? Does it make sense to you? Is it in agreement with your concept of the hyperplane of simultaneity? 



#50
Dec1512, 07:18 PM

P: 848

ghwellsjr: I have never used the term "hyperplanes of simultaneity" so now I guess I have to try to figure out what you mean by the term. If you go back to post #9 and look at the three graphs representing three different IRF's, each one of them is showing just one spatial dimension because, as is common in spacetime diagrams, we use the other dimension for time and we limit the activity in the scenario to just one dimension (usually referred to as the xdimension) and we assume that the audience is familiar enough with this type of diagram that they know that the y and zdimensions are not shown but since nothing is happening at locations other than y=0 and z=0, we mentally recognize that when the graph shows a horizontal grid line, that is a line of simultaneity for a particular value of time which you look up at the left side of the graph and it means that all events along that horizontal line are simultaneous meaning they happen at the same time in that IRF. (I can't believe I'm explaining all thisnevertheless, I carry on.) Now since we don't show the y and z dimensions, we mentally realize that all the events that are simultaneous along that line are extrapolated out in those two extra dimensions so it is really a volume of simultaneity which I suppose is identical to your term hyperplane of simultaneity.
Bobc2: Yes, we are on the same page here. Actually you do find the term “hyperplanes of simultaneity" in many places in the special relativity literature—and you have correctly figured out its meaning. I’m glad we have no problem reducing the analysis to the use of just two dimensions in our sketches. ghwellsjr: Now what's important is that two (or more) events that are simultaneous in one IRF (because they have the same value for their time coordinate) may not be simultaneous in another IRF as can be seen if you look at the three different graphs. I never really stopped to think in terms of a volume of simultaneity, assuming that that is what you mean by a hyperplane of simultaneity, but it is obviously the case although I would say it is so obvious that it doesn't need to be said. Bobc2: Yes, we are in perfect agreement on that. And when I use the term "hyperplanes of simultaneity" I also don't see a need to show all dimensions in the spacetime diagrams. ghwellsjr: Now if we wanted to show a twodimensional scenario where the observers were moving around in both the x and ydimensions, we'd have a hard time putting that on a piece of paper… Bobc2: But, that’s just what I’ve been trying to do with the spacetime diagrams that include the various X1 axes for the different observers as well as the X4 axes. These axes are of course all identified using the velocities of the moving observers along with the Lorentz transformation. (see my first sketch below) ghwellsjr: …but what we could do with today's technology is make an animation and present it as a movie. Each frame of the movie marks out a plane of simultaneity but the assumption is that it extends out into the zdimension and so there really is a volume of simultaneity. Does that communicate? Does it make sense to you? Is it in agreement with your concept of the hyperplane of simultaneity?. Bobc2: Yes, it certainly does. I have among my special relativity computer files examples of such an animation. And I’ve seen one posted on our forum here. So, the sketch below illustrates how I show two different hyperplanes of simultaneity, blue and red, where two different observers are moving at the same speed in opposite directions with respect to the black inertial reference frame (the perpendicular coordinates representing X1 and X4 axes). I have included the representation of a rod moving to the right with respect to the black frame, but the rod is at rest in the blue inertial frame. Thus, we see directly the length contraction aspect of special relativity. Blue sees the length of the rod as L0, whereas Red sees the rod length as L. And the reason I've used the symmetric spacetime diagram (first introduced by Loedel of Mexico who received Einstein's blessing during their visit), is that it avoids the need to worry about the meaning of the line distances when comparing Blue and Red coordinates (you don't really need to be concerned with the hyperbolic calibration curves). This scheme was introduced to me in my first grad school special relativity course. My prof was fond of this means of communicating special relativity. I used it also later on when I was a physics instructor for undergrad physics and engineering students. Of course it is easy to account for both X1 and X4 coordinates of Blue and Red using the Lorentz transformation hyperbolic calibration curves as shown below (the Red and Blue colors are reversed from the above sketch). I was just trying to see if we are on the same page about the significance of these two different 3D worlds (represented within the 4dimensional space with just two coordinates) that blue and red occupy at points along their respective worldlines. Finally, here is an interesting sketch, using the above concepts of hyperplanes of simultaneity to illustrate the motivation for the Block Universe model of special relativity. For now, I will spare you the pain of the addition of world lines of many different laser pulses (idealized in the diagrams as single photons). So, there is a scheme for deciphering the many laser light measurements that could be performed on signals transmitting back and forth and intersecting along the different world lines. To make the measurements more convincing you just add more observers at rest in the Blue inertial frame (collaborating results with any amount of data desired), and have matching Red observers participating in the experiment. Perhaps I have not communicated these concepts well, or perhaps you understand the concept quite well and simply reject it. I just wanted to make sure I understood your thinking on these hyperplanes of simultaneity (X2 and X3 coordinates suppressed for clarity) Maybe my basic questions are: 1) Do you accept the validity of the above sketches as correctly representing key aspects of special relativity (regardless of whether you attach any physical significance to it)? 2) Do you attach any physical significance to these hyperplanes of simultaneity? 3) What significance at all to the hyperplanes of simultaneity represented in the above spacetime diagrams. 



#51
Dec1512, 07:33 PM

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None of this makes absolutely any difference to anything ghwellsjr stated earlier. You don't need time in X' frame for every position x' to show time dilation. It is sufficient to show time along a single line of constant x_{13}. And world line of observer that is static in X' is just as good as any other. So proper time of observer static in X' is entirely sufficient to show time dilation. The fact that simultaneity lines are going to be different in X and X' is entirely irrelevant to this fact.




#52
Dec1512, 11:10 PM

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Your diagrams also show an X4 "direction", yes (which most people would call the "T" direction). But that's not a spatial direction. 



#53
Dec1612, 01:10 AM

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I can understand how these kinds of graphs would be important a hundred years ago but nowadays, we can let our computers take care of all the computations. I don't handdraw my plots. I use a computer and once I set up a scenario, the computer draws the first plot in the same IRF that I entered the scenario into. Then I enter a speed parameter that creates a new plot using the Lorentz Transformation. I repeat for the third plot. So I know that there is no more information in the second and third plots (or as many others as I want to make) than there is in the first one. My question to you is: would it be possible to have a computer take the scenario the way I set it up for the first IRF and then instead of transforming to an IRF at a different speed, could it generate one of your diagrams that combines the information from three simple IRF graphs? There is one piece of information that can be gleaned from watching the computer redraw the graphs for the different IRF's that you would not see from any one of them and that is it makes it obvious which characteristics are frame invariant and which are not but aside from that, no new insight or conclusions can be obtained simply by presenting the same information in different IRF's or in one of your (or Vandam's) diagrams that combine the information from multiple IRF's. Do you agree with this assessment? Do you accept the validity of graphs like the ones on page #9 as being exactly equivalent to your diagrams? Do you attach any physical significance to the origin of an IRF? All of Special Relativity, not just issues of simultaneity are very important in our understanding of the world. Without it, we would still be floundering around searching for that illusive ether. Without it, we would not have the simple and consistent means of interpreting the data from our measurements. One of the most important tenets of SR is that there is no preferred reference frame. It appears to me that you and Vandam want to get rid of all reference frames in favor of some super interpretation that incorporates several reference frames all at the same time. One of the other important tenets of SR is that you don't conflate coordinates from two or more reference frames which is what I see you and Vandam doing. One last question: what does any of this have to do with the issue of whether time dilation is observable or measurable by the observers in the scenario? 



#54
Dec1612, 04:38 AM

P: 126

Of course you can say: "I do not care what time coordinates are. They are figures, and that's all what I need...". Sigh. http://www.physicsforums.com/showpos...ount=45http:// 


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