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What does the travelling twin actually see?

  1. Aug 31, 2009 #1
    I have finally understood the resolution of twin paradox. If you choose the earths frame of reference you will see that the travelling twin's time runs slowly on his both outward and inward journey. So he ends up younger.
    In the frame of reference that runs with the travelling twin in the outward journey; the travelling twins time is the proper time and the earths time runs slowly but after the turn-around the travelling twin's time start to run much more slowly than the earths time so in the end the travelling twin ends up younger.
    Same result will be arrived if viewed from the frame of reference that moves with the travelling twins inward journey.

    Now my question is because the travelling twin isn't in inertial frame of reference I can't use SR to see what things will look like from his perspective?

    My attempt answer: I can however split up what he sees. During the outward trip he observes the other twins clock to run slowly. Also during his return trip also he sees other twins clock run slowly. But since he is to find the other twin much older on the meeting, the resolution should be that --> At the turn-around he should find the other twin instantly grow much older so that even if he continuously grow younger he would anyway finally end-up older.

    If all my reasonings are correct then please let me know.
  2. jcsd
  3. Aug 31, 2009 #2


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  4. Sep 1, 2009 #3

    George Jones

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    No, it doesn't work like this. As well as the links that gave, see

  5. Sep 1, 2009 #4
    This is exactly true, if I read it correctly. If the turnaround is considered instantaneous, the traveling twin would "see" earth's clock "jump" ahead during the turnaround. This is the standard resolution for an instantaneous turnaround.

    Of course the traveling twin will not actually "see" this, but it will match his later observations when he subtracts light travel time.

    You might even take a look at Einstein's resolution. He doesn't use an instantaneous turnaround, and attributes the earth clock running very fast in the ship's frame during the turnaround to gravitational time dilation. But this is really just the same thing as breaking up the turnaround into many comoving inertial frames and taking account of the shift in simultaneity for each "segment".
  6. Sep 2, 2009 #5
    I agree with this and the OP statement. There ference "https://www.physicsforums.com/showthr...96#post2186296" [Broken] is an error.
    Don't sweat it. I've made my share of errors too.
    Last edited by a moderator: May 4, 2017
  7. Sep 2, 2009 #6
    I just want to add that no one reading this thread should walk away thinking that time actually jumps, for anybody. Its a physically imposible solution because its a physically impossible problem.
  8. Sep 2, 2009 #7


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    It appears to me that your strategy in attempting to understand the twin scenario is an unfruitful one.

    What the travelling/staying twin sees from instant to instant is difficult to parse. For instance, if the travelling twin relies on timing pulses that he receives from the stay-at-home twin, then some of the effect is the same effect as classical Doppler shift. Should the traveling twin first subtract the classical Doppler shift, and only then interpret the observations?
    You seem to be trying to form an understanding on the basis of tracking what the traveller sees from instant to instant. That is doing it the impossibly hard way, because then you have to grapple with several layers of different effects, among them the same effect as classical Doppler shift.

    A better stategy, in my opinion, is to avoid the complications of transmission delays altogether. Transmission delays play no part in special relativity. They need to be taken into account of course, but the substance of special relativity only begins _after_ correcting for transmission delays.

    No doubt the extensive Usenet physics FAQ twin scenario discussion is among your sources. The spacetime diagram representation of the twin scenario is a mapping that offers a global picture. The whole duration of the twin scenario is represented in a single "frozen" picture. You can draw that picture without any signal transmissions, and it will be the twin scenario just the same.

    In the spacetime diagram representation everything that is not necessary to elicit the twin scenario is left away. What remains is the following: over the duration of the proceedings the travelling twin travels a longer spatial distance than the stay-at-home twin. That is what is necessary and sufficient to elicit the twin scenario.

    You may yearn for explanation on a deeper level than that. That is not to be had. Relativistic physics _asserts_ the twin scenario, it does not explain the twin scenario on a deeper level.

    This is how it has always been in the history of physics. For instance, Newton had no explanation for gravity, he asserted the existence of universal gravity, and the justification of that assertion lay in the effectiveness of the theory. Some of Newton's contemporaries argued that if gravity could not be explained exhaustively then it should be disqualified from consideration. Obviously that's a dead end approach. In science you must not let yourself get bogged down by rigidly insisting on _exhaustive_ explanation before you proceed. If a theory is very effective then that is justification in itself.

    The same applies in the case of relativistic physics. Relativistic physics does not explain the relativistic effects, such as time dilation, relativistic physics _asserts_ these effects. The justification of the theory comes from its effectiveness.

    So why do many commentators claim to "explain" the twin scenario?
    Invariably the purported "explanation" is some demonstration that the twin scenario indeed follows logically from the postulates. But that is not what puzzled people _yearn_ for. Puzzled people are not wanting proof of self-consistency, they yearn for explanation on a deeper level, which is unavailable.

  9. Sep 2, 2009 #8
    Cleonis thanks for long but useful explanation. But for now, I would like to walk away telling that the travelling twin finds the age of the other-twin to abruptly grow old, but bearing in mind that he doesn't actually receive any such information due to Doppler Effect.
  10. Sep 3, 2009 #9
    From the American Mathematical Monthly-Volume 66 Number 1.

    This edition devotes 18 pages to the twin "paradox" and examines it from many perspectives. It is one of the best treatments I have come across. This small extract, treating what the twins actually see, is given here almost exactly as in the original text.This short extract, however, is not very representative of the detailed description of the rest of the article.

    ---We see that neither twin sees his brother ageing abruptly or discontinuouly. Each twin only sees a jump in the rate of ageing, a transition from a red shift to a violet shift.

    The discontinuity of the rate at which clocks go is due to the fact that we have
    simplified our problem and put in a sharp corner ( in the spacetime diagram of the traveling twin )at the turnaround: that is we have assumed that the travelling twin suffers an infinite acceleration there. Even the discontinuity in rates can be avoided by rounding out all corners of the space-time diagram.

    At the start the travelling twin gradually accelerates to his final velocity, and then gradually decelerates to rest and then accelerates until his velocity is reversed and, finally he gradually decelerates to rest. This rounding out of corners can be done so smoothly that there is no discontinuity at all.

    In direct observation each twin first sees his brother ageing at the same rate, then sees the rate gradually slowing down as a red shift develops, and at a later time sees this red shift gradually changing to a violet shift, and finally near the end of the journey sees the violet shift gradually disappearing until the two brothers again age at the same rate.Of course when the brothers are reunited their ages will still be different.------

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