Graphical Derivation of the CADO Equation

Discussion in 'Physics & Math' started by Mike_Fontenot, Sep 5, 2018.

  1. Mike_Fontenot Registered Senior Member

    Messages:
    622
    In my previous post, I gave the following excerpt from a recent addition to the end of Section 14 on my webpage (on a necessary and sufficient condition for the CADO equation to be usable when both twins sometimes instantaneously change their velocities wrt their mother):
    ___________________________

    "So you can easily spot potential regions where the (generalized) CADO equation might be usable by looking for portions of the home twin's world line that are horizontal. If those horizontal portions are long enough to include the entire base of the right triangle, then the generalized CADO equation can be used."
    ___________________________

    I have just added the following sentences to the above paragraph:
    ___________________________

    "If they are not, then the generalized CADO equation cannot be used. To be clear, if those horizontal portions are long enough to include the entire base of the right triangle, the generalized CADO equation can be used even if the home twin's world line changes its slope at the intersection of the base of the triangle with the vertical side of the triangle.
     
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  3. Neddy Bate Valued Senior Member

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    2,548
    Bold added by me.

    Okay, but what if time is only "calculated" to be flowing in reverse? Would that still be a violation of causality? And if not, then you should not need to invoke GR to "supersede" those calculations, right?

    Well, if I said "measured" anywhere, I apologize; I didn't mean to use that word. I have been thinking of this as more of a calculation that is made, rather than a direct measurement, per se.

    There can still be physical meaning to it, even if it is just a calculation. In the original NotEinstein scenario, Charlie's first acceleration (by himself without Bob) happened at at time when all three twins were standing together with their clocks all set to 0. Then, when Charlie's own clock displays 20, he calculates Alice's clock to display 10.

    The meaning behind this comes from the idea that his "traveling frame" can be considered to be populated with an abundance of helpers who are spread out throughout the traveling frame, all equipped with synchronised clocks. Think of it as a train with helpers and synchronised clocks on board, in all the train cars.

    The meaning behind Charlie calculating Alice's clock to display 10 can be considered to come from an unnamed helper in Charlie's traveling frame who is momentarily co-located with Alice, which allows him first-hand knowledge that her clock physically displays 10 at the time when the helper's own clock physically displays 20. The fact that the helper's and Charlie's sycnhronised clocks both display 20 meets the definition of simultaneous in their frame.

    So it is physically meaningful to say that, in the traveling frame, Alice's clock physically displays 10 simultaneously with Charlie's clock physically displaying 20. Note that this would hold true even if Charlie could not observe or measure Alice's clock in any way.

    Now imagine if Charlie were somehow able to jump off the train he is riding, stand on the ground momentarily, and then quickly jump back on the train again, all in a virtually infinitesimal amount of time. When he arrives back on the train, his own clock still displays 20, his helper's clocks all still display 20, and the helper who is co-located with Alice still concludes her clock displays 10. There is no way around that, because nothing on the train changes in a virtually infinitesimal amount of time.

    But note that, during the one moment Charlie was standing on the ground, he would have calculated Alice's clock as displaying 40. There is no way around that, because that is the time in the stay-home frame. And so, there is no way around Charlie calculating that Alice's clock changes from 10 to 40 to 10 again after he jumps back on the train. Her clock displaying 10 after he jumps on the train again has physical meaning supported by the helper, who never jumped off, and who therefore is passing Alice's clock as it displays 10.

    So there is no need for GR, because these are just calculations, even though they have the physical meanings described. That is how I see it, and I hope you will agree. I doubt that you will, and that is okay too. But could you please at least tell me what you think happens? Do you think when Charlie jumps back on the train he finds all the helpers' clocks displaying 80, and the helper co-located with a now-40-year-old Alice says, "Thanks a lot Charlie (sarcasm), you jumped off the train and it caused me to age 60 years, even though the train is in virtually the same spot it was before!" ??? Of course not. So what do you think happens? Please tell.
     
    Last edited: Sep 17, 2018
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  5. NotEinstein Valued Senior Member

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    Not in the space-like region of the light-cone, no.

    I never claimed that whenever time is calculated to flow backwards, one must use GR?

    Apology accepted, because you indeed used that wording in multiple posts.

    This meaning appears to be a different one: you don't mention any backwards ticking?

    This meaning appears to be a different one: you don't mention any backwards ticking?

    I never disputed that? Why are you arguing against a position I do not hold?

    Point out to me what information reached Charlie from Alice in that short moment that Alice's clock indicated 10 according to Charlie's calculation. Because if there isn't any, there's no physical impact/meaning to Alice's clock indicating 10 according to Charlie's calculation.

    Care to calculate the acceleration required for that jump "in a virtually infinitesimal amount of time", push it through the Einstein Equivalence Principle, express it as a gravitational field, and call that a "weak field"?

    That's begging the question; you can't assert the calculations to have physical meaning to prove they have physical meaning.

    I think you've described the situation quite well.

    Ah, a joke! A misrepresentation, of course, but I guess that's your form of humor?

    I think what you described happens (at least, I have no reason to doubt that). Alice's clock indicating 10 according to Charlie's calculation in that short moment just has no physical significance; it is of no consequence.

    But you would have known all this already, had you bothered to read post #153 better.
     
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  7. phyti Registered Senior Member

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    For any who are interested;
    The A perception on the left: B performs a gradual reversal from A4 to A6. This allows the red B axis of simultaneity to rotate forward in time. For the duration of the trip, B sends blue signals (not shown) to A to get clock readings that return to B.
    The B perception on the right: B records the (light gray) simultaneous B time, based on the SR clock synch method. The right drawing is also time dilated for a velocity of .6c out and back.
    B observes the A clock rate as slow until B2.5, then fast until B4. The results would be reversed for the 2nd half.
    The B perception is not a symmetrical image of the A perception, and shows the degree of distortion when applying the aos convention to an accelerating frame.
    For scientific correctness, B would experience a g-field from B3.2 to B4.8.
    Light emission is independent of the source, but light detection depends on detector speed. The acceleration of B affects the time of detection and therefore the calculated time of the A event.

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  8. Neddy Bate Valued Senior Member

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    2,548
    I was just pointing out what I thought was a physical meaning behind a calculation. Charlie's assistant is adjacent to Alice, and can verify that her clock displays 10 at the same time his own clock displays 20. And the assistant's clock displaying 20 is simultaneous (in the traveling frame) with Charlie's clock displaying 20. So the calculation has some meaning, that's all.

    I'm glad you do not dispute this, though.

    This seems to be in direct contradiction to you "never disputing" what I wrote above. I wrote that it is physically meaningful to say that, in the traveling frame, Alice's clock physically displays 10 simultaneously with Charlie's clock physically displaying 20.

    You are specifically "not disputing" the physical meaning I gave, while at the same time saying it is not physically meaningful. Giving you the benefit of the doubt, I suppose it is the "no physical impact" thing that I should be paying attention to?

    If so, then consider this: When Alice's clock displays 10, she calculates that Charlie's clock displays 5. But if she jumps on the train in that moment, she lands on the train right next to Charlie's assistant who's clock displays 20. She can then be informed by the assistant that Charlie's clock must also display 20, because the train clocks are all synchronised. So now her own clock displays 10, and she has been informed that Charlie's clock displays 20, not 5. You are okay with that because 20 > 5 so time did not go backwards.

    But the whole reason you did not like time going backward when Charlie jumped off and then back on the train was because you said his calculation of Alice's clock displaying 10 was physically meaningless. Yet here she is now, on the train, with her clock displaying 10!

    No thanks, but we can always go back to the two aliens who are 10 billion light years from earth, and use them instead. The acceleration in that case is only a bicycle being pedaled toward earth by Bikey, and yet his now-slice has the time on earth shifting 200 years to the future. Would you like to push that through the Equivalence principle and call it a "strong field"?

    Of course there is no need to do that, because all you have to do is call the 200-years-to-the-future earth time "not physically meaningful" and then Bikey can stop the bike, and the backward shift in time on earth is no problem. So why pretend we need GR?

    Okay, if that is your main gripe, then we certainly don't need GR. If you just don't care about certain times on certain clocks, then call them "not physically meaningful" and be done with it. At least stop pushing the GR angle.
     
    Last edited: Sep 18, 2018
  9. NotEinstein Valued Senior Member

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    1,986
    So all of your three attempts to give meaning were off-topic, because you've failed to even come close to mentioning the relevant point.

    Or is this another attempt at humor?

    There is no contradiction: being able to do a calculation does not give one information from some other place that one didn't have before.

    Which was not the topic of discussion, and thus off-topic.

    First you put words in my mouth by misrepresenting what I said, and then you "call me out" on it. That's quite dishonest, so I hope that was just another joke.

    If you read my posts more carefully in the first place, you wouldn't have needed to ask this question.

    (Note that the assistant's claim carries no information about Charlie's clock at that time; information of Charlie's clock hasn't reached them yet. For example, let's say a bomb exploded at Charlie's location at 19.99. Charlie's clock could thus never have said 20, but the assistant has no way to know that (yet), proving the assistant isn't actually giving information about Charlie's clock to Alice.)

    As I just pointed out, no, I'm not "OK" with this, because you don't seem to know that information can't travel faster than the speed of light. Or perhaps you don't know what information is in a physics context?

    And yet, Charlie has no information about that at this stage, in the same way Alice has none about Charlie, and thus your argument amounts to nothing.

    Once again, you are picking a preferred frame. It's equally valid to say the entire earth is accelerating (from the point of view of Bikey).

    A gravitational field accelerating the entire earth at typical bike-accelerations? Yes, that does sound quite strong to me!

    Another attempt at humor, no doubt. You're misrepresenting my arguments by mixing two completely different ones together, as I pointed out to you earlier.

    I invite you to consider the scenario where Bikey doesn't stop the bike; demonstrate that Bikey at any moment measures (not calculates, measures) earth going backwards in time.

    So Einstein was "pretending" in his 1918 paper?

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    We never needed GR to resolve that part; it's perfectly handled by SR, and I don't think I ever claimed otherwise?

    That's terribly ambiguous wording. What "certain times" on what "certain clocks", and according to whom?

    And it's not that I don't care: reality doesn't care. (Well fine, not reality, but the physics model in SR.) Again: demonstrate that it's possible to measure (not calculate, measure) that clock ticking backwards.

    You haven't demonstrated they are of any consequence. In fact, you've avoided addressing that exact point three times in your previous post.

    Tell that to Einstein in 1918.

    ---

    I'm sure you're having a big old laugh, seeing the amount of humor in your posts, but any chance of you actually addressing my arguments?
     
  10. Neddy Bate Valued Senior Member

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    2,548
    NotEinstein,

    I am honestly trying to make sense of your ideas. I'm sure they make perfect sense to you, but that does not mean that others (or at least that I) can necessarily understand them.

    Let me try to boil it down to one thing I can hopefully grasp correctly, then we can try to move on from there.

    After I described the scenario where Charlie jumped off, then back on the train, I wrote this:

    "That is how I see it, and I hope you will agree. I doubt that you will, and that is okay too. But could you please at least tell me what you think happens?"

    And you wrote:

    "I think you've described the situation quite well."

    So, now let's study this single statement from your previous post:

    Underline added by me. You are saying we never needed GR to resolve "that part" because it is perfectly handled by SR. I honestly do not know "which parts" you think need GR and "which parts" do not.

    Let me show you why I am genuinely confused. In my description of the scenario, I had Charlie calculating that Alice's clock displays 10 before he jumps off the train, and that Alice's clock displays 40 after he jumps off the train, and that Alice's clock displays 10 again after he jumps back on the train. Therefore I had Charlie's calculations including time going backward, when Alice's clock went from 40 to 10. I thought "the part" that you thought needed GR was "the part" where Charlie's calculations included time going backward.

    But you did not reply to that by saying, "But you need GR for that part (or some other part)." You replied by saying that you thought I described the situation quite well. Yet I did not use GR for any part, so can you at least see why I would be confused now?

    So please tell me, which specific part of my description actually was not done "quite well" because it needed GR? And why didn't you say so in the first place instead of saying I had done "quite well?"

    Thank you in advance.
     
  11. NotEinstein Valued Senior Member

    Messages:
    1,986
    Then perhaps you should read my posts more carefully, and stick with what I wrote, because at many occasions you've put word in my mouth.

    So why don't you ask for more explanations then, instead of ending up repeatedly misrepresenting my position?

    Follow the quotes back, and you'll know what "that part" refers to. Specifically, it refers not to the whole Einstein Equivalency Principle business, as the clocks ticking backwards and those are two separate issues. We were talking about clocks ticking backwards, and you keep assuming that I think that has something to do with GR.

    Nope. The part where GR comes in is if the accelerations are such that, when translated into a gravitational field, that field would be considered strong. That however is not relevant for the physical insignificance of the clocks ticking backwards.

    In your entire description, you haven't brought up any specific point which needs GR. If you however said that the accelerations involved, when translated into a gravitational field, would result in a strong field, then GR would need to be involved.

    None, if I read your scenario as-is. I see no reason to assume that the accelerations involved, when translated into a gravitational field, would result in a strong field.

    Because it was "done quite well"; your scenario was carefully crafted to avoid the need to involve GR, so I didn't.
     
  12. Neddy Bate Valued Senior Member

    Messages:
    2,548
    Okay then, maybe this is "the part" where we need GR?

    Okay, so please tell me what you think happens under GR. First Amy and Bikey are standing still relative to earth, and they both agree the current time on earth includes our friend at the gas station. Now Bikey gets on his bike and pushes off toward earth. What now?

    SR says that Bikey's now slice includes the earth as it would be 200 years in the future. Is that not the case under GR? What does GR say?

    SR says that when Bikey stops the bike, his now slice shifts from 200 years in the future back to the time when our friend was at the gas station. Is that not the case under GR? What does GR say?

    You're the one claiming GR is necessary here. I just want to know what you think it says. Please.
     
  13. NotEinstein Valued Senior Member

    Messages:
    1,986
    Yep. Notice the mentioning of a strong gravitational field? That's kind of a give-away.

    I have no clue; I haven't taken the effort of calculating it, because it's not relevant to the discussion about the clocks ticking backwards. In fact, due to the fact GR needs to be used in such cases, and (as I said before) the entire concept of simultaneity between space-like events being different in GR, it probably wouldn't even shed light on the issue of the clocks ticking backwards in SR.

    I don't know; you came up with the scenario, you calculate the strength of the gravitational field.

    I don't know; you came up with the scenario, you calculate the effects of GR.

    I don't know; you came up with the scenario, you calculate the strength of the gravitational field.

    I don't know; you came up with the scenario, you calculate the effects of GR.

    False; I only pointed out that if the gravitational fields are strong enough, GR needs to be used, similar in the way Einstein did. You are right now the one claiming GR is necessary here.

    I haven't the foggiest.
     
  14. Neddy Bate Valued Senior Member

    Messages:
    2,548
    So the scenario where you said, "I see no reason to assume that the accelerations involved, when translated into a gravitational field, would result in a strong field," does not even require GR. And we are left with the same result of Charlie calculating that Alice's clock goes from 10 to 40 to 10. Wow...

    Anyhow, we're finally down to this:

    I bid you good day, Sir.

    I said, good day Sir!
     
  15. NotEinstein Valued Senior Member

    Messages:
    1,986
    Yep. I think I've probably said this about a dozen times now, so I'm not sure why you only now seem to be surprised by this?

    Another joke? Or are you really going to insist that somebody goes through a calculation that is not relevant to the discussion at hand? Look, when I originally brought up GR, I did indeed think it might resolve this situation (and it still might). Contrary to popular believe in this thread at the time, I indeed was right that a seemingly only-SR scenario might require GR to handle. However, I later realized that even in SR, there is no big problem with the clocks ticking backwards, because they only ever do so in a calculation, not in any observation or measurement. It seems you're still stuck on the "GR must be used"-angle.

    Also interesting how you seem to expect me to calculate something irrelevant, while I've asked you multiple times to show how any information about backwards ticking of clocks can reach Charlie (a quite important point in this discussion), and you've flat-out ignored that. It seems that you are not interested in a discussion of the topic at hand. Combined with your constant "humor"-attempts that were little more than misrepresentation of my position (which I strongly suspect are making nobody laugh, including you), your departure from this discussion is no loss to me.

    When you finally un-confuse yourself, you know where to find me.
     
  16. Neddy Bate Valued Senior Member

    Messages:
    2,548
    No, I do not want anybody to go through any calculation which is not relevant to the discussion. I thought you were claiming the GR calculations were relevant.

    No, I don't think "GR must be used."

    I am glad to hear that somewhere along the thread you realized that even in SR, there is no big problem with the clocks ticking backwards, "because they only ever do so in a calculation". I am not sure exactly what that last phrase means to you, though. I suspect you might think the times that Charlie calculates are not correct, or not real in some way.

    Of course Charlie's calculations assume that Alice's clock is always in proper functioning order, and has not been blown up. Other than those types of exceptions, Charlie's calculations can be shown to be correct and real.

    I never claimed Charlie could obtain any information faster than the speed of light. I don't think I need to show anything like that, in order to demonstrate that the times that Charlie calculates are correct and real.

    Consider that when Charlie is on the train and his own clock and his helper's clock both display 20, Charlie calculates Alice's clock to be displaying 10. Now, if she jumps on the train at that time, her clock will display 10 while she is standing right next to Charlie's assistant whose clock is displaying 20. I think that shows that Charlie's calculation was correct, and that Alice's clock really did display 10 at that time. Of course I am assuming that Alice's clock is properly functioning and has not been blown up some time earlier. Charlie would be too far away to verify that directly at that time. But that is beside the point, and his calculation was still correct and real.

    Likewise, consider that when Charlie jumps off the train,charlie calculates Alice's clock to be displaying 40, which is the current time in the stay-home frame. I think that shows that Charlie's calculation was correct, and that Alice's clock really did display 40 at that time. Of course Charlie is too far away from Alice to verify that first hand at that moment. He would have to wait another 34.64 years before the light from her clock would reach him. But, again, assuming that Alice's clock is properly functioning and has not been blown up some time earlier, it is what her clock should really display.

    Likewise, consider that when Charlie jumps back on the train and his own clock and his helper's clock both still display 20, Charlie once again calculates Alice's clock to be displaying 10. And once again, if she jumps on the train at that time, her clock will display 10 while she is standing right next to Charlie's assistant whose clock is displaying 20. I think that shows that Charlie's calculation was correct, and that Alice's clock really did display 10 at that time.

    So, information still travels at a maximum speed of c, but the times that Charlie calculates are real, assuming that Alice's clock is properly functioning and has not been blown up some time earlier.

    So when you say that there is no big problem with the clocks ticking backwards in SR, "because they only ever do so in a calculation" I think you might be trivializing the fact that the calculated times can be shown to be real. Perhaps you agree with all of this, and you only wanted to point out that Charlie does not directly measure her clock ticking backwards, which would be fine by me.
     
  17. NotEinstein Valued Senior Member

    Messages:
    1,986
    As I said many, many, many times now: no, not to the discussion about the clocks ticking backwards.

    Great! Then let's drop that irrelevancy from the discussion about the clocks ticking backwards.

    Yes, I'm glad too that I realized that. It's a pity nobody pointed it out to me though, and I had to figure it out all by myself.

    Re-read my post where I describe the difference between my usage of the words "calculate", "measure", and "observer".

    Of course they are correct (as in: consistent with SR's prediction).

    The backward ticking ones have no physical impact/consequence; you may consider that "not real in some way", I don't know.

    Exactly, it's an assumption. Charlie has no information at the time he makes his statement; he knows that he may be wrong. In other words, what Charlie has is his calculation, not a measurement.

    Agreed; I never claim otherwise.

    Again, this depends on your definition of "real". It certainly isn't measurable, so many people probably wouldn't consider it "real".

    True, and I never claimed that.

    Again, this depends on your definition of "real".

    Yep, and I never claimed that it wasn't.

    I don't disagree there: if Alice jumps on the train when her clock displays 10, it displays 10.

    Agreed, and I never argued otherwise.

    Again, that depends on your definition of "real".

    Exactly; he doesn't know, he predicts.

    Sure.

    Sure.

    Again, that depends on your definition of "real".

    Here's the thing: I already explained to you how to figure out what Charlie would consider real (as in: what really happened). See what happens when all the light from Alice finally reaches Charlie. What does Charlie observe? Does he observe Alice's clock ticking backwards at any moment?

    You have failed to do so, at least for any physically meaningful definition of "real". If Charlie cannot observe (or even measure) it happening, did it really happen?

    So... if only you had read my posts more carefully, we wouldn't have needed to go through this.

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  18. Neddy Bate Valued Senior Member

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    Great, so it seems we are in general agreement about most things.

    No, you have failed to understand.

    What a crock. YES, any event which happens, and which Charlie cannot observer or measure, still happens. I find it strange that you could possibly suggest otherwise.

    We can always change our scenario to allow enough time for Charlie to confirm that his calculations were correct, (assuming all clocks remain in proper functioning order at all times).

    For example, before Charlie jumps off the train, he calculates Alice's clock to display 10 and his assistant's clock displays 20. If, in that moment she jumps on the train, she ends up standing next to Charlie's assistant with her clock displaying 10 and his displaying 20. We would only have to allow for 17.32 more years to go by, and the light from both of those clocks will reach Charlie's eyes and he can see it for himself.

    Now you might complain that Charlie's eyesight is not good enough to see that far, or some other nonsense. That only proves my point further. Charlie's calculation was already shown to be correct when his helper verified Alice's clock displayed 10, even before she jumped on the train. I only added her jumping on the train in case you thought his helper might be seeing an illusion or some other NotEinsteinian nonsense.

    Good day, sir. Good day.
     
  19. NotEinstein Valued Senior Member

    Messages:
    1,986
    Then please give the definition of "real" you are using.

    It's funny: I've in the very post you partially quotes implicitly asked you that multiple times if you wanted to continue using that word. Guess you misread or misinterpreted that as well?

    That's not my claim; you are once again misrepresenting me. Of course an even that happens, happens.

    But any event that Charlie cannot observe or measure, did not occur according for Charlie. Charlie cannot (by construction) even obtain any information about it. How then can it be said to have happened in any meaningful way?

    And note that not a single observer, wherever they are, ever measure or observe any clocks ticking backwards. If all observers agree on that, how can it be said to have happened in any meaningful way?

    And I find it strange that you continue to avoid addressing the important parts of my posts, and only focus on bits you can, through misinterpretation of misrepresentation, make irrelevant comments about.

    (Something which I've obviously implicitly did.)

    Please demonstrate that this is possible, and that Charlie then indeed measures or observes Alice's clock ticking backwards.

    Sure, but what does he measures or observes Alice's clock to be right before that happens? If you're arguing he measures or observes a clock going backwards, then please demonstrate that he measures or observes a clock going backwards. Now you're mixing calculations and measurements!

    *sigh* Obviously not, so I have no idea why you'd bring something like that up?

    A point that never needed to be made, so it proves nothing further.

    But that was never in question: of course the information of what happened doesn't change when it travels from Alice to Charlie. To suggest that would be ludicrous.

    Again, something you dreamt up, not me.

    It's funny that it's you that's constantly coming up with the nonsense in this discussion. So I guess I can add misattribution to your list of misses.

    Yes, as it seems you are unable to rise to the challenge of properly and intellectually handling this discussion, not engaging in it might be better for you.
     
  20. Neddy Bate Valued Senior Member

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    2,548
    An event that happens, happens regardless of who observes it, or measures anything about it.

    Incorrect. An event that happens, happens regardless of who observes it, or measures anything about it.

    I suspect your confusion arises because you are probably calling the 'time ticking backward' effect an "event". It is not an event, it is a series of events, and the order of events can be frame-dependent in SR.

    Incorrect. Charlie can always obtain information about an event, provided the information can reach him.

    Again, I suspect your confusion arises because you are probably calling the 'time ticking backward' effect an "event".

    An event that happens, happens regardless of who observes it, or measures anything about it. The only meaningful question that anyone can ask about an event is whether it happened or not. If it did, then it did.

    Again, I suspect your confusion arises because you are probably calling the 'time ticking backward' effect an "event".

    The 'clock ticking backwards' that we are discussing is not one event, but a series of events:

    1. One event is when Charlie's assistant is passing Alice while her clock displays 10 and his displays 20. Because the train clocks are synchronised, by definition of simultaneity in the traveling frame, that event is simultaneous in the traveling frame with Charlie's clock displaying 20.

    2. Another event is Charlie passing by a home-frame-clock synchronised to Alice's while that clock displays 40 and Charlie's displays 20. Because the home-frame clocks are synchronised, by definition of simultaneity in the home frame, that event is simultaneous in the home frame with Alice's clock displaying 40.

    Event #1 is the reason that if Alice jumps on the train when her clock displays 10, she will land on the train right next to the helper who's clock displays 20, and that event will be simultaneous in the traveling frame (by definition) with Charlie's clock displaying 20.

    Event #2 is the reason that if Charlie jumps off the train when his clock displays 20, he will land on the ground right next to the home-frame-synchronised clock which displays 40, and that event will be simultaneous in the home frame (by definition) with Alice's clock displaying 40.

    It follows logically that if Charlie immediately jumps back on the train, he will land on the train right next some train-helper's clock displaying 20, and that event will be simultaneous (by definition) with all train-helper's clocks displaying 20, and also simultaneous in the traveling frame with event #1 where Alice's clock displays 10.

    Thus Charlie concludes the time on Alice's clock goes from 10 to 40 and back to 10, because he changed reference frames.

    Charlie concludes that the time on Alice's clock goes from 10 to 40 and back to 10, because he changed reference frames, as demonstrated above. I am not claiming he observes or measures it. What he actually observes/measures is his own clock displaying 20, and the home-frame synchronised clock displaying 40. What his assistant actually observes/measures is his own clock displaying 20, and Alice's clock displaying 10. The rest follows from logic, and having consistent definitions of simultaneity in the two different frames.

    Because you brought up that a bomb might go off blowing up a clock, and preventing it from reaching its predicted time. Irrelevancies are your specialty.
     
  21. NotEinstein Valued Senior Member

    Messages:
    1,986
    Right, but we're not talking about an event here, we're talking about the calculated order of events, for which you've already stated this doesn't hold. (Alice never calculates, measures, or observes her clock ticking backwards).

    In SR, information about any event will, with enough time, become available to Charlie. One cannot outrun the speed of light. So you are wrong here.

    I have explicitly called it an ordering of events before, so you are once again wrong about my position.

    Which I've already pointed out earlier. Please read my posts more carefully.

    Yes, but we're not talking about an event here, so your statement is wrong.

    Again, it's you that's making it into a statement, not me. Please stop misattributing the confusion.

    True, and I've never claimed otherwise. In fact, I've explicitly stated the opposite earlier in the very post you are responding to.

    Again, it's your confusion, not mine.

    This is not in dispute, but you've missed the point. Every single possible observer never measures or observes Alice's clock ticking backwards. So stop dodging the question I asked you: "If all observers agree on that, how can it be said to have happened in any meaningful way?"

    We agree on this.

    So Charlie's calculations do not match his observations and measurements. Doesn't that mean Charlie's calculation that Alice's clock ticked backwards is wrong?

    That was to demonstrate that Charlie's calculation doesn't have any information about Alice's clock: it's just an educated guess.

    No, my Alice's-clock-stopping-right-before-10 was quite relevant: it demonstrated that Charlie's calculation doesn't actually prove Alice's clock reaching 10. Your eyesight thing however is completely irrelevant. Once again, you've misattributed something.

    And I see you've ignored some crucial points in my post. You do that a lot. It's not very intellectually honest. But how to add it to your list of misses? It's not misquoting, really... Oh, I know! It's "missing"!

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  22. Neddy Bate Valued Senior Member

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    The "meaningful way" that it happens is simply that when Charlie changes reference frames, his definition of simultaneity changes also, to become that of his new frame. That's what is happening.

    It is true that Alice's clock never runs backward in any one inertial frame. However, Charlie does not stay in one inertial frame. He jumps back and forth between two different inertial frames. There. Question answered.

    Charlie's calculation of what time Alice's clock will display when he jumps off the train with his own clock displaying 20 does match his observation and measurement of the home-frame-clock next to him which is synchronised to Alice's and displaying 40. The reason that calculation is successful is because of the synchronisation of the clocks in the home frame. Note that Charlie does not need to observe Alice's clock himself, because it is given that all of the home-clocks are displaying the same time simultaneously in his new frame, which is the stay-home frame after he jumps off the train. When you say a clock could have been blown up, you are just taking away what should have been a given at the outset. Synched clocks throughout an inertial reference frame are standard in SR.

    And Charlie's calculation of what time Alice's clock will display if she jumps onto the train when his own clock displays 20 does match the observation and measurement of Charlie's assistant who records Alice's clock displaying 10 when his own clock displays 20. Charlie can rely on his assistant for the same reason that he can use the local home-frame clock instead of Alice's in the paragraph above. The reason is because of the synchronisation of the clocks in the traveling frame. Once again, Charlie does not need to observe Alice's clock himself, because it is given that all of the traveling-frame-clocks are displaying the same time simultaneously in his new frame, which is the traveling-frame after he jumps on the train.

    Both calculations work on the same principles, and both are correct. Even though it is not necessary, Charlie can always wait until the information from Alice's clock reaches him, in both cases, and that way he can prove to himself that both of his calculations are correct, (unless you want to talk about vandalizing Alice's clock again).

    Good, then we are done. Thanks for a fun conversation. I will let you have the last post, probably telling me how I am wrong, how I failed to show this or that. Whatever. I know the principles I have described are sound.
     
    Last edited: Sep 20, 2018
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  23. NotEinstein Valued Senior Member

    Messages:
    1,986
    That's not even an answer. We were talking about clocks ticking backwards, not somebody changing reference frames.

    Yes, but you still haven't explained how the resulting calculation showing a clock ticking backwards is meaningful. In fact, you've avoided answering the exact question asked to you. That's very intellectually dishonest of you.

    Please, for the umpth-teen time, demonstrate that any observer can observe of measure Alice's clock running backwards. Your continued failure to do so speaks volumes.

    Did that clock tick backwards? No? Then that clock is entirely irrelevant, isn't it?

    Which I never questioned (for certain definitions of "successful").

    Except that he does, in order for the statement "Alice's clock ticked backwards" to be congruent with his observations and measurements.

    You've missed the point of the clock blowing up. In that case, Charlie's statement that Alice's clock reached 10 is false. No matter how much he stares at a local clock, no matter how well that clock is (was) synced up to Alice's, he is wrong.

    It's a clear demonstration that Charlie (at that time) has no information about Alice's clock around the time it would have been showing 10. So Charlie cannot make any definitive statements (yet) about the state of Alice's clock.

    Yes, and I've never disputed that. In fact, I've used that very same construction in my scenario.

    You've forgotten to describe a critical part of your scenario: the information traveling from Charlie's assistent to Charlie. Demonstrate that it does, and that when it reached Charlie, he'll still be of the opinion that Alice's clock ticked backwards.

    But Charlie does need to receive information about Alice's clock. You continually forget about that part. I wonder why?

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    Actually, as I've pointed out multiple times, that's vital. Otherwise, Charlie only has his calculation, based on assumptions.

    You really don't understand the point of the bomb-scenario, do you?

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    For the umpth-teen time: demonstrate that it shows his calculations to be correct. It's rather pathetic that you continue claiming that, and continue failing to provide evidence for your claim.

    You really don't understand the point of the bomb-scenario, do you?

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    Well, another contradiction. You are claiming that Charlie can measure/observe it; read your own post more carefully, and you'll notice that's all the helper is: your smokescreen to avoid having to confront this contradiction.

    You too. Although I would have appreciated a little more intellectual honesty; I think that would have made the conversation a lot more productive, too.

    Says the person that, for the last umpth-teen posts has been throwing around misattributions, misrepresentations, misinterpretations, has been missing the point, etc. I find it telling that not once you've managed to counter any of those "mis-"es claims. Heck, you barely even tried! Combined with all the times you conveniently didn't respond to critical points in my posts, I think we both know "how you are wrong, how you failed to show this or that."

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    Yeah, you don't care if you're wrong, I figured that out a long time ago.

    And I don't know if mine are. I guess that's the difference between you and me: I'm willing to investigate, learn, and admit my mistakes. And you... you are happy the way you are.
     

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