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I Twin paradox not including accelerations, it is wrong where?

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  1. Sep 27, 2017 #1
    Hi people!
    (Sorry for my poor english).

    I found everywhere that twin paradox need aceleration to explain it.

    Let me change the twin paradox a little:

    Suppouse that in Earth and before the traveller twin start his trip, you take two photos with old Polaroid camera, one to each twin. As we remember, those kinds of photos become older, degrading its colours, a little fast.

    Some day the traveller twin left his own photo pasted in one window of his house looking outward and takes the Polaroid camera and start a trip in his capable of reach near light speed spaceship. And he goes to the left respect to Earth. Goes to left far enough to be able to stop and start to travel to right in the direction to Earth in a way he could pass Earth going right at near speed of light without any aceleration.
    When pass Earth he points his Polaroid to his house and take a Polaroid photo of his Polaroid photo still on Earth and pasting it looking outwards in one of his capable of reach near speed of light spaceship window.

    Time later he pass another spaceship coming at the same absolute speed (in a frame at rest respect the non traveller twin) but with opposite direction, to left, and with no aceleration, and the captain of this spaceship take a Polaroid photo of his Polaroid photo and paste it looking outwards in one of his spaceship window.

    Time later this second spaceship pass Earth without change its speed, so no aceleration here neither, and the non traveller twin take a Polaroid photo of the Polaroid photo of the Polaroid photo of the Polaroid photo taked time ago in Earth.

    Then he compared his old Polaroid photo of him whith this new Polaroid Photo of his twin.

    Having both Polaroids photos images of them in younger ages, is this new Polaroid photo less degraded colours than the old Polaroid photo no matter none Polaroids photos suffered any aceleration never?

    If all motions are relative, why not the opposite?

    I hope I could explain my idea even with my poor english.

    Where I´m wrong if I say that yes, the new Polaroid have less degraded colours than the old one?

    Thanks!

    Luis.
     
  2. jcsd
  3. Sep 27, 2017 #2

    andrewkirk

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    Yes, the photo taken from the passing spaceship is less degraded than the one that stayed on Earth, even though it has undergone no acceleration.

    In the Twin Thought Experiment, acceleration is not the key explanation of why the travelling twin ages less than the non-travelling twin. Rather, it is the fact that the inertial frame of the travelling twin has changed. The inertial frame while going out is very different from the one while coming back. That reason applies whether we are considering the original thought experiment (in which there is acceleration) or the one you describe (in which there is no acceleration).

    In both cases the time measurement that is delivered back to the home twin (in your case the measurement is by the amount of fading of the photo) is based on two very different inertial frames, and hence is less than the measurement based on the home twin, which relates to only one inertial frame.

    In summary, it is the number of different inertial frames that contribute to each time measurement, rather than the acceleration, that allows to see why the travelled and the non-travelled time measurements differ.
     
  4. Sep 27, 2017 #3
    Thanks for your fast answer Andre!

    So.... why most answers (in fact yours is the first I saw that not) use aceleration to explain it? :-O

    Regards,

    Luis.
     
  5. Sep 27, 2017 #4

    Orodruin

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    They do not. At least not any serious ones.

    Acceleration explains why there is an asymmetry between the twins and why you cannot blindly apply the time dilation formula to the accelerating twin. A more accurate explanation would involve relativity of simultaneity rather than acceleration.
     
  6. Sep 27, 2017 #5
    Thanks! :-)
     
  7. Sep 27, 2017 #6

    Mister T

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    The twin that takes the shorter path through spacetime experiences the smaller amount of aging.

    To accomplish that a switch in inertial frames, which requires either acceleration or the clever way you've done it with photographs, is required.

    The usual way is with acceleration, though, so that's why you may have seen it mentioned that the twin who accelerates is the one who ages less. It's a very common misunderstanding. Another is that general relativity is required to explain things. Both mistakes can be found in many textbook presentations.
     
  8. Sep 29, 2017 #7
    How about this.
    When rocket A of speed v to the Earth passes by the Earth, rocket A pilot adjusts his clock to the observed Earth clock E.
    Then when rocket A and rocket B of speed -v to the Earth pass by, the rocket B pilot adjusts his clock to the observed A clock.
    Then when rocket B passes by the Earth, the Earth and pilot B observe commonly that clock B time < clock E time.
    No acceleration appear in it.
     
    Last edited: Sep 29, 2017
  9. Sep 29, 2017 #8
    Is exactly the same than mine and far more simply..... but without twins! :-D
     
  10. Sep 29, 2017 #9

    A.T.

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    In this context "acceleration" means that the world line along which you accumulate proper time is not inertial.
     
  11. Sep 29, 2017 #10
    Sorry, not sure if I understand you.... but seems to me there is none no inertial frames in mine or, most simplier, sweet spring´s example.
     
  12. Sep 29, 2017 #11
    Twin E stays in one inertia system but rocket twin transfer the inertia frames A to B. This makes difference.
     
  13. Sep 29, 2017 #12

    A.T.

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    There are no non-inertial physical objects, but the proper time is accumulated along an non-inertial world line.
     
  14. Sep 29, 2017 #13
    Sorry, I do not know what is a non inertial world line....
    Are not non straight world lines?
    Here I think all world lines are straights.
     
  15. Sep 29, 2017 #14

    vanhees71

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    Of course, not all world lines are straight lines. E.g., an electron moves on a curved world line in a magnetic field.
     
  16. Sep 29, 2017 #15
    OK, but in our example* I think all world lines are straights. I´m wrong?

    *:To be more precise, we could change the Earth by a Saint Exupery little prince´s asteroid.
     
  17. Sep 29, 2017 #16

    Orodruin

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    This is like saying that the sides of a triangle are straight when arguing for the triangle inequality. While it is true that each individual side is straight, one of the lengths involved is the length along a curve that has a kink (the length that involve two sides of the triangle).
     
  18. Sep 29, 2017 #17

    Demystifier

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  19. Sep 29, 2017 #18
    Han Solo´s line of world is not Mr. Spock´s line of world bended in a kick, is Han Solo´s straight line of world:

    tp03.jpg
     

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    Last edited: Sep 29, 2017
  20. Sep 29, 2017 #19

    Orodruin

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    I am sorry, but this makes no sense. Please use the appropriate nomenclature. Neither the world line of Han Solo or that of Mr Spock is the world line along which your total proper time is accumulated.
     
  21. Sep 29, 2017 #20

    Ibix

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    In the standard twin paradox Minkowski diagram there is a triangle formed of a vertical straight line (the stay-at-home worldline) and a > shape (the traveller worldline). In your variant you have a vertical line (the stay-at-home) a line slanted to the right (the outbound traveller) and a line slanted to the left (the inbound traveller). These three lines form the exact same triangle on the Minkowski diagram as is formed in the standard setup. So the outcome must be the same.
     
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