Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Not banal question on twins

  1. Jul 7, 2006 #1
    Imagine an empty universe, just two planets exists.

    If they move with relative speed, it is impossible to decide which is moving right?

    But when the people on the moving planet go to meet their twins on the other planet, they are younger.

    So in this way is possible to decide which is moving???? thx.
  2. jcsd
  3. Jul 7, 2006 #2


    User Avatar

    Staff: Mentor

    Which twin travels to which planet?
  4. Jul 7, 2006 #3

    Doc Al

    User Avatar

    Staff: Mentor

    The issue is not who is moving, but who is accelerating.
  5. Jul 7, 2006 #4
    isn't that indifferent?
    If i call Planets A and B, and when after some journey peoples meet together, if all the people who where in B are younger than their twins in A, doesn't it mean that B moved?
  6. Jul 7, 2006 #5
    i know that this type of question is like doing 1+1 for you but still i don't understand.
    What about if the two planets are alredy in motion, and instead to stop and chech the age of their twins, they see it with a telescope.
    Please is fundamental.
  7. Jul 7, 2006 #6
    If they check with a telescope the people on the other planet always appear younger. :smile:
  8. Jul 7, 2006 #7


    User Avatar

    Staff: Mentor

    In order to come together, one twin has to accelerate and move to where the other twin is. That breaks the symetry.
  9. Jul 7, 2006 #8
    so... if we erase all Doppler effects, the twin who is travelling believes that his brother is younger, but when he returns home, he magically sees that his brother is older then himself??!!
  10. Jul 7, 2006 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No. While he is traveling outward he sees his brother aging slowly.Just before he starts his return trip, he will determine that his brother is younger, As he decelerates to a stop and then accelerates for the return trip he will see his brother aging much faster. After this he will determine that his brother is older. On the return trip, he will see his brother aging more slowly again. When he returns to his brother he will find that the period where the brother aged faster dominated and his brother will be older.
  11. Jul 7, 2006 #10
    so if the brother travels for 1E100000000000000 years and then returns home, the acceleration/decelaration time dominate ALWAYS his trip?
  12. Jul 7, 2006 #11
    Well this is confusing to say the least, as it reeks like "this can be all explained due to the space-time effects of the acceleration stages only".

    Twin paradoxes are about shorter or longer paths traveled in space-time and not particularly about the space-time effects during the short acceleration and decelleration stages.

    When the planets are in relative motion with each other each will encounter a slower progression of time on the other planet. When one of the twins decides to travel then his space-time interval will be shorter with a result that the elapsed time on his local clock is less than of the twin who does not travel. Thus when the traveler reaches the planet he is in fact younger than his twin brother. The space-time effects during the short acceleration and deceleration stages of his voyage do have a small effect on the calculations but are most certainly not an completel explanation for the difference in age between the twins.

    Furthermore no one aged any faster or slower.
    The younger brother simply traveled a different path in space-time which had an effect that less time elapsed on his clock than on the one that stayed behind.
    Last edited: Jul 7, 2006
  13. Jul 7, 2006 #12
    Thus we return to my first question:"so... if we erase all Doppler effects, the twin who is travelling believes that his brother is younger, but when he returns home, he magically sees that his brother is older then himself??!!"
  14. Jul 7, 2006 #13
    There is no magic involved in this.
  15. Jul 7, 2006 #14
    Yes, sure! My question is only a provocation. Nothing is magic in physics
  16. Jul 7, 2006 #15

    Why doon't you try reading this simple and CORRECT explanation before making broad and incorrect statements:

    http://sheol.org/throopw/sr-ticks-n-bricks.html [Broken]
    Last edited by a moderator: May 2, 2017
  17. Jul 7, 2006 #16


    User Avatar
    Staff Emeritus
    Science Advisor


    To answer this question, I have to understand what you mean by "erasing all Doppler effects".

    I don't understand what you mean by this question. In fact, it is the only part of your problem description that seems "magic" to me, because Doppler shifts are a part of physical law, and erasing them would take some sort of "magic" that I don't understand.

    Offhand, it sounds like you're probablly mired with an incorrect notion of simultaneity - i.e. that you think "at the same time" has an unambiguous meaning. Unfortunately, "at the same time" means different things to different people according to relativity.

    This can be seen with a space-time diagram.

    \unitlength 1mm
    Above is a space-time diagram. Arbitrary coordinates have been assigned to the set of events that comprise space-time. Every event in space-time is represented by one and only one location on the diagram.

    In this diagram the solid red line represents the path through space-time of observer #1.

    The solid blue line represents the path through space-time of observer #2.

    Observer #1 and observer #2 are moving with respect to each other. In fact, as you can see, we have adopted the coordinates of observer #1 to label the points on this diagram. This was an arbitrary choice on our part, and has no physical significance - coordinates never have any physical significance, they are the map, not the territory,.

    For our convenience, however, we need to use some unique labels at every point to distinguish different points in space-time from each other.

    We can see that on this diagram, for obsever #1, the "time coordinate" is the vertical position on this diagram, and the "space coordinate" is the horizontal position on this diagram.

    The dotted red line represents the notion of simultaneity of obsever #1 - all points on this line are regarded as simultaneous by observer #1.

    The dotted blue line represents the notion of simultaneity of observer #2.

    Note that the two set of events that are regarded as simultaneous are not the same - they are different sets!
    Last edited: Jul 7, 2006
  18. Jul 7, 2006 #17


    User Avatar
    Gold Member

    Hey that second graph makes a lot of sense.

    How can two people, each receding relativistically from each other, see the other as aging slower? Isn't this a pardox?

    No. No more than two people driving trucks, each receding from each other at a 45 degree angle, see the other's truck as being shorter (because, realtive to themselves, the other truck is foreshortend due to the angle).

    And that's not just an analogy. If you see time as simply another dimension.
    Last edited by a moderator: May 2, 2017
  19. Jul 7, 2006 #18
    Sigh, what can we do with the never ending generations of relativity deniers that hang onto the "paradox"
  20. Jul 7, 2006 #19
    The original poster was correct in his statement. Two syncronized clocks in the same frame will always stay in sync. But then give one a velocity so he can go meet the other and his proper time will slow. So yes, you can decide which one is moving. What's the controversy?
  21. Jul 7, 2006 #20
    A very good question and something perhaps that many educators who visit this forum should think about. :smile:

    Why is it that about a century after relativity theory we still teach students Newtonian concepts as true? I for one think it is an embarrassment!
    And the pompousness in which it is presented, "Newton's laws", laws, presented as eternal truths engraved in the stones of nature, "invisible forces that causes objects to attract each other" and more of that.
    What can you expect once that stuff is taught?

    I am not talking about applying Newtons' formulas, since they are sufficient most of the time, but why teach Newtonian concepts as truths, why call it laws while they are really approximations of a false model?
    Teach relativity as the correct model first, the concepts, I am not talking about GR formulas here, and then inform them about the incorrect but often practical Newtonian model and formulas.

    So are we really surprised that when most high school kids get taught and are examed on Newtonian concepts that they have difficulties in understanding relativity? I am not!

    Understanding relativity is not hard at all in my view. The problem is not understanding it, the problem is the difficulty in unlearning what people were taught at school as correct.

    So, to answer your question about the never ending generations, start with the high school kids.
    Last edited: Jul 7, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook