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Why is a 3rd party important

  1. Aug 11, 2008 #1
    Hey guys,

    I have no background in physics but have been interested in time dilation for a while and have understood the concept without getting technical...

    My question is this... this is the atomic clock experiment...

    Suppose there are only 2 living beings in the universe ... one stands still on earth with an atomic clock in his hand... the other goes in an aeroplane at a high uniform speed also with an atomic clock in his hand..

    After 60 minutes(on the person standing on earth's clock) the plane returns and they compare their clocks..

    only 59.99999 or wtvr minutes have passed on the person in the planes clock right?

    Now here comes my question... Why do we look at this from the person standing on earth's frame of reference... according to the person on the plane, the person standing on earth was moving away from HIM at a high uniform speed... so why is the person standing on earth's clock not slower when they compare... this is my basic confusion...

  2. jcsd
  3. Aug 12, 2008 #2


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    Staff: Mentor

    The airplane must turn around in order to return to its starting point so the two persons can compare their clocks. The person on the airplane "feels" the turnaround. The person on the ground does not.

    To put it another way, the person on the ground remains at rest in a single inertial reference frame. The person on the airplane does not. His reference frame is non-inertial during the turnaround.
  4. Aug 12, 2008 #3
    Hey jtbell,

    Thanks for your reply. I appreciate your perspective on the solution and it certainly is a very good argument but i don't somehow feel satisfied.

    Plus even if the person never returns would his time not be dilated even though neither of them could never compare. What if they send each other radio messages telling the times on their clocks... do you see what im getting at.. There doesnt have to be a turnaround..

    You will then tell me that he feels the forces of acceleration while the person on the grounnd does not...

    Well my argument is that in this situation, only a human being would perceive this as motion.. This is becuase we have built planes and felt acceleration before... Some being that has never felt accelration before would not term himself as moving, rather would say that the earth and the person on earth are accelrating away. It seems to me to be irrelevant who created the machine, or that there is a machine at all.

    What if the 2 people were placed 2 planets moving away from each other. What of time dilation then?
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