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A question about reference system and time dilation

  1. Aug 12, 2010 #1
    Hi everyone,
    These days I've been reading about relativity (special) in a book of general physics, and I have a doubt about the reference system:
    If I stay on earth and I observe a rocket travelling away from the earth with a great velocity (let's be 0.5c), and I notice that time inside the rocket is slower than on earth, ok, that's allright, but, in the same way, the person who travels in the rocket will see that he's got velocity zero and the earth moves at 0.5c, thus a clock ticks slower on the earth.
    Which one of those people will get older faster? For each one will be himself, and no the other, whose seconds are loooong.

    Twin paradox confuses me...

    Thanks to all.
  2. jcsd
  3. Aug 12, 2010 #2


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    In a nutshell, if two travellers meet up and synchronise watches then go on separate journeys, when they meet up again to compare watches, the time shown for each will depend on the 4 dimensional path they took through space-time. Observing that each others clocks appear slow doesn't come into it - it's a different phenomenon.
  4. Aug 12, 2010 #3
    like, Mentz114, pointed out, you are confusing two different phenomenum

    Phenomenum first:
    I and you are moving away from each other. You 'see' my clock ticking slowly and I 'see' your clock ticking slowly. I see you are still young while I am old, and you see the same! (if we are able to maintain our 'seeing' for that long. :) ).
    This is time dilation.
    Both are right. This is in some respect same as:
    You and I are in opposite side of a bridge. I see you too small, you see me too small: Who is infact small? who is correct?
    [Ask if its not clear to you.]

    Phenomenum second:
    Ok, lets continue from the previous. I see you young, while I am old and ditto for you.
    NOW, if you make an effort (like fire your rocket engine) and make the turn over and return to me, then on our meeting you will be "infact younger". If instead I make the effort I will be yonger.
    This is the twin paradox.
    [Ask if its not clear]
  5. Aug 13, 2010 #4
    Thank you for the answer.

    So, If I'm not wrong, if you and I are moving (each one from the other ones reference system) and one of us changes his acceleration,(becomes a non inertial r.s.), for example, you, then when you came back to my rocket and we met up again, I'd be older than you, due to your velocity change. But, isn't his once again a problem of perspective? I mean, if you decelerate, you could say that it was me that slowed down my velocity.
    I'm afraid that the problem about acceleration/deceleration must be defined in relation to the space-time continuum, or smthg like that, and not to the other's perspective, cause if so, we both had the same age after your backing home travel. Is this correct?

    Thanks again,

  6. Aug 13, 2010 #5


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    Yes, you are correct. Acceleration is not relative. Someone in a spaceship can easily tell if they are accelerating or not, so it's not like uniform motion in that respect. In the case where one traveller accelerates and the other does not, there is no reciprocity, only the accelerating one feels the force.
  7. Aug 13, 2010 #6
    Yes it is. This is called the 'absolute' or 'proper' acceleration. The other type called the 'coordinate' acceleration is the relative acceleration. Relative acceleration only depends on our 'seeing' and not 'feeling'. For a relative acceleration to exist one or both of the entity must be in proper acceleration.

    It is in fact this proper acceleration that makes the distinction in the twin paradox. If you want to dig upon how much difference in time this 'proper' acceleration creates, then thats the subject of 'general relativity' at which I know nothing. Nevertheless, you can find-out the time difference in the twin paradox by using special relativity alone; provided that you make one assumption--> The turning over twin (the accelerating twin) turns over almost instantly, so that he remains in non-inertial frame for very short time, short enough to be ignored!

    Happy learning.
  8. Aug 13, 2010 #7


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    We really, really need a sticky FAQ on this topic. Nothing against the people asking it. It's a reasonable question to ask. But it gets asked and answered multiple times per week.
  9. Aug 13, 2010 #8


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    Great example, by the way! I hope you don't mind if I use that :wink:
  10. Aug 14, 2010 #9
    Thanks and yeah, use it freely. :)
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