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

Time and speed

  1. Sep 20, 2014 #1
    Einsteins special theory of relativity states that time passes differently for different observers. Basically the faster you travel the slower you aged. So I assumed this means to aged the fastest, you don't move.

    We also know earth is moving around the sun. The solar system is moving upward out of the Milky Way. The Milky way is also moving in the general direction of the Great Attractor and other galaxy clusters. I believe the Local Group is also moving within the Local Supercluster. The Supercluster should also be moving, etc...

    So my question is how do you aged the fastest? Do you move super fast to counter the movement of earth, solar system, Milky way, Local Group, Supercluster, and etc?
  2. jcsd
  3. Sep 20, 2014 #2


    User Avatar
    Science Advisor

    For which observer?
  4. Sep 20, 2014 #3
    Time passes different for different observers, depending on the observers' motion. The common example is the two hypothetical twins: One of them stays at home, on Earth. The other journeys into space in an ultra-fast rocket, nearly as fast as the speed of light, before returning home.
  5. Sep 20, 2014 #4


    User Avatar
    Science Advisor

    So you are asking how to age most between two meetings with another guy? Stay in freefall and away from gravity sources.
  6. Sep 20, 2014 #5
    My question is if I get into an ultra-fast rocket and go into the direction that counter the movement of earth, solar system, Milky way, Local Group, Supercluster, and etc, would I aged faster or slower than if I stay on earth?
  7. Sep 20, 2014 #6


    Staff: Mentor

    Are you coming back to earth?

    If yes, then the answer is you will age slower than if you had stayed.

    If no, then the answer depends on the reference frame you choose. If you choose the earth's reference frame then you will age slower, if you choose the CMB reference frame then you will age faster.
  8. Sep 20, 2014 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Well, I would interpret your remark about wanting "to age the fastest" as wanting to maximize the proper time on your wristwatch.

    But some more detail is needed. You need to specify a pair of points that you want to maximize the proper time between. Assuming that's what you want, perhaps I didn't interpret your question properly.

    So lets say you want some location on Earth, and some location at the same point 1 second later. Well, you need to define what you mean by "the same point". Also, what you mean by one second later. Lets assume you mean "the same point on Earth, using the International Terrestrial Reference Frame (ITRF), and use the ITRF to define what you mean by 1 second later as well. Then the path that will maximize proper time is if your object moves upwards away from the earth at some velocity v, in such a manner that it falls back to the specified ending point.

    In general, once you specify your starting and ending point, (you can use any means, but a coordinate system is the usual means), then a geodesic path will extremize the proper time. If there are more than one geodesics between those two points, one of these extremized paths will be the one that maximizes the proper time.

    I probably should explain what a geodesic is here, but I don't quite have the time.

    If we assume you mean "the same point on the Earth", and consider longer and longer timescales, the procedure remains the same, but the details vary considerably.

    For short times, you want to get the object out of the Earth's gravity well. For longer time scales, the path will whirl around the Earth on the way out, but the main effect on proper time will be the suns gravity well. Longer than that, and you worry about the galactic gravity well, and the supercluster gravity well. Of course, the Earth might not be around to get back to , so you might pick a different pair of points, like the solar system barycenter if you want galactic timescales.
  9. Sep 21, 2014 #8

    Inertial frame A and Inertial frame B have same rights. For A, time of B is slow. For B, time of A is slow.
    It does not contradict because they do not meet again. They do not have time to check answers.
    To meet again for checking answers, one of them, say B, has to leave his inertia frame and get on another frame of inertia for return. Whem they meet A who has been still in his frame, is younger than B who moved by go-return journey.
  10. Sep 21, 2014 #9


    User Avatar

    Staff: Mentor

    Older, you mean?
  11. Sep 21, 2014 #10
    Yes. Thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook