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Help me understand the twin rocket experiment

  1. Jun 13, 2009 #1
    One of the thought experiments Einstein proposed involved one twin staying on earth, while the second twin entered a shuttle. The twin in the shuttle went on a trip (a physical one), traveling near the speed of light. The twin in the shuttle would have time pass by slower than the one on the Earth, resulting in him being younger than his Earth twin when he returned.

    The Question:
    Since relativity is relative, can't we assume a reference frame where the shuttle is stationary and the earth moves away and comes back near the speed of light? This would result in the opposite outcome, a contradiction. I know that this was originally proposed by physicists as proof that Einstein's relativity was flawed, but later debunked. Can anyone explain to me, in laymen terms, why it doesn't work?

    If you can't explain it simply, you don't understand it well enough.
    -- Albert Einstein
  2. jcsd
  3. Jun 13, 2009 #2
    >>> If you can't explain it simply, you don't understand it well enough.
    Yes, indeed. The crux of the problem lies in how to compare time with a moving clock? How many stationary clocks do you need to compare time with a moving clock? That's the key. If you figure this out, you will see why you have been fooled by this paradox.
    Last edited: Jun 13, 2009
  4. Jun 13, 2009 #3


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    It doesn't work because the time dilation equation is only intended to be used in inertial reference frames, meaning a spacetime coordinate system where an object at rest in those coordinates would be moving inertially forever, never accelerating. The shuttle is not moving inertially when it turns around (any change in speed or direction is an acceleration, and will be felt as G-forces so it's an objective thing whether or not you accelerated in special relativity), so it can't just take the Earth's relative velocity to it at all times and then plug that into the time dilation equation to predict how much the Earth twin will age.

    For way more info on the twin paradox and various ways of understanding it, see this page:

  5. Jun 14, 2009 #4
    Hey there! I asked a similar question not too long ago and after hours of searching for an answer, I found this really helpful site - http://www.pbs.org/wgbh/nova/einstein/hotsciencetwin/. I’m so glad I bookmarked it! You’ve got to check out the applet! It’s so funny! It always makes me laugh when I see it. ;)
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