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Homework Help: Twin Paradox

  1. May 17, 2005 #1
    I believe I understand the basics concerning the twin paradox. One twin leaves Earth on a near light speed round trip to a local star. On his return, he is younger than his twin due to the fact that time progresses more slowly the faster an object moves.

    Here are my points of confusion: If movement is relative and there is no absolute frame of reference for motion, why would time proceed at different rates for the twins. I read that it is due to the force of acceleration that the twin on the spacecraft feels. However, the twin on Earth would also feel acceleration in the form of gravity. I am sure there is more to this. Would someone mind filling in my gaps of understanding on this issue? Thanks.
  2. jcsd
  3. May 17, 2005 #2
    The paradox is not that one twin ends up younger/older than the other, but in SR alone BOTH twines should be younger and older than the other. The resolution of this paradox lies in GR, and is due to the periods of acceleration the travelling twin undergoes, and the difference between the time it takes to reach the point at which the twin turns back between the two observers. I don't think the gravitational effect of the Earth is really the issue in this thought-experiment, as you could just as well choose the 'at rest' twin to be in space too.
  4. May 17, 2005 #3
    Twines? No, that's string theory! ;o)
  5. May 17, 2005 #4
    GR isn't necessary to resolve the twins paradox. There've been many threads on the twins paradox and many webpages on the twins paradox and many books that treat it. I'd suggest reading one of them.

  6. May 18, 2005 #5
    Such as...?
  7. May 18, 2005 #6


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    The short answer

    Constant motion is relative, accelerated motion is not. The twin that made the trip had to accelerate at least twice. Relativity only applies to inertial frames of reference, it does not apply when accelerating. This is what distinguishes the travelling twin from the stationary twin.
  8. May 18, 2005 #7
    Thanks for all the replies. I thought I read somewhere that Einstein thought that gravity was accelleration. If this was the case, if the twin on the spaceship never exceeded 1g, then the Earth bound twin would "feel" more accelleration than the one on the trip. I am still trying to come to grips with this, and I will continue to do more reading on the subject.

    One thing that is really bugging me is why is one spot in space more special than another when it comes to relativity. There seems like there should be an absolute reference point for space/time. What if the 1971 J.C. Hafele and R.E. Keating experiment used a plane that traveled at 1000mph and traveled at the equater in the opposite direction of the Earth's rotation. Would the atomic clock on the plane lose time in reference to the Earth bound (stationary) atomic clock? The plane would accellerate (apply force) while the Earth bound clock would just sit there. However, if an outside observer (say the Sun) looked at the events, the Earth bound clock rotated around the diameter of the Earth while the plane stayed in the same position (not factoring the Earth's movement of orbit around the Sun).

    Since everything is moving (the Earth rotates, the Earth orbits the Sun, the Sun orbit the galaxy, our galaxy is moving, etc.), I am having a hard time understanding why with all these points of references moving in respect to one another, why there is not an absolute point in space/time to base all measurements of relativity against. I guess it is time for more reading on the subject. Thanks to all that replied.
  9. May 19, 2005 #8
    He did, but that was the general theory of relativity. The classical twins paradox only cares about the special theory of relativity.

    There are two main points Einstein's special theory of relativity makes: 1) Velocity affects the rate of time. 2) Velocity affects the order of time. The second point is very important to the twins paradox, but often overlooked.

    To help in understanding the order of time part, Einstein gives the example of a train moving very quickly over a track. An observer standing next to the track sees two lightning bolts hit the train at the exact same time. Each lightning bolt is the same distance from the observer, in opposite directions, thus the observer states "The lightning bolts hit simultaneously." However, if the train is moving fast enough, and if the observer could "watch" the propogation of the light beam, he would see the passenger sitting at the midpoint of the two strikes move toward the lightning bolt at the front of the train and away from the one at the back of the train. We know based on this observation that the passenger at the midpoint will see the front lightning strike first, but according to the law of propogation of light in vacuo, we know light always travels at the same speed. Thus, the passenger will claim the lightning bolt toward the front of the train actually happened first, because he is at the midpoint and that's the one he saw first. And in fact, both of these observers are valid in their claim; they're just experiencing time in a different order.
    Last edited: May 19, 2005
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