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

Relativity paradox!

  1. Dec 28, 2007 #1
    Could somebody please clear up this paradox for me.

    Two objects, A and B, are moving past each other at close to the speed of light relative to each other.

    So far so good.

    Object A looks at object B and sees object B fly by at close to the speed of light. Object A figures time dilatation takes effect and concludes the clocks on B are moving more slowly than on A.

    Object B looks at object A and concludes the same thing.

    A while later, the two objects meet up with each other and compare clocks. Are their clocks indicating the same time or different times? Which clock is ahead? If none of the clocks are ahead, then what happened to relativity?

    This ongoing paradox seems to undermine the entire theory. Could somebody clear up this paradox. Thank you.
    Last edited: Dec 28, 2007
  2. jcsd
  3. Dec 28, 2007 #2


    User Avatar
    Science Advisor
    Gold Member

    There's no paradox, whilst it is counterintutive that both should see each other's clocks running slower than their own, it's not inconsistant.

    When they meet up for a second time their clocks can show either the same or different amount of times elapsed. This depends on how they meet up (i.e. the details of the accelartion, path, etc). Generally speaking if they maintain symmetry between each other they will show the same amount of time elapsed, if they break the symmetry then they will show a different amount of time elapsed.
  4. Dec 28, 2007 #3

    Doc Al

    User Avatar

    Staff: Mentor

    The Twin Paradox

    In order for the two "objects" to meet up and compare clocks, at least one of them must accelerate. That breaks the symmetry.

    This situation is usually called the "Twin Paradox" and has been discussed here many times in numerous threads. Rather than start yet another thread on this same old thing, I recommend that you search the archives. Also read this: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html" [Broken]
    Last edited by a moderator: May 3, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook