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Time in GR

  1. Apr 28, 2007 #1
    Hello everyone,
    I'm new to this sector of the Gould's Belt, so let me introduce myself :I go by the name of Santural,the resident of...ok I'll stop being such a dork:rolleyes: .
    Well, I recently read ("studied") The G theory of R, and one thing failed to penetrate the walls of stupidity into my tiny drop of intelligence.
    What I can't seem to understand, is the application of time as a fourth dimension. How exactly does one measure time? Is it all simply relative? Ex. You can measure time relative to earth, but is there a cosmological "time" that applies to everything?
    Do forgive me if I'm being a complete dolt, but try to understand my position...:approve:

    Also, as a random thing, isn't the scale of the solar system something amazing, what with the Oort cloud and stuff?

    Well, sorry for taking your time,
    Santural.
     
  2. jcsd
  3. Apr 28, 2007 #2

    HallsofIvy

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    This would probably get more responses in the "relativity" section rather than "cosmology". There is no "absolute" time and so no "cosmic" time. How time passes, at what time events occur, and whether two separated events occur at the same time all depend on your "frame of reference" and so specifically on your speed relative to another observer of the same events.
     
  4. Apr 28, 2007 #3
    Sorry!
    Thanks, I'll post it there A.S.A.P.
    And thanks for the info!
     
  5. Apr 28, 2007 #4
    Hello everyone,
    I'm new to this sector of the Gould's Belt, so let me introduce myself :I go by the name of Santural,the resident of...ok I'll stop being such a dork .
    Well, I recently read ("studied") The G theory of R, and one thing failed to penetrate the walls of stupidity into my tiny drop of intelligence.
    What I can't seem to understand, is the application of time as a fourth dimension. How exactly does one measure time? Is it all simply relative? Ex. You can measure time relative to earth, but is there a cosmological "time" that applies to everything?
    Do forgive me if I'm being a complete dolt, but I'm just trying to learn... but try to understand my position...

    Also, as a random thing, isn't the scale of the solar system something amazing, what with the Oort cloud and stuff?

    Well, sorry for taking your time,
    Santural.
     
  6. Apr 28, 2007 #5

    pervect

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    The most basic sort of time is a time interval. This is what you measure with a clock. You have to specify a path that the clock takes through space and time to measure the time with a clock, however.

    Two clocks, travelling along different paths, can read different times when they next meet. This is sometimes called the "twin paradox", though it's not really a paradox.

    The fact that time turns out to be path dependent in this manner implies that there is not any such thing as "universal time".

    People use other sorts of time, such as "coordinate time", but it's probably more important that you understand time intervals, as measured by a clock, first.
     
  7. Apr 28, 2007 #6
    Understood.
    Sorry about all those posts, don't know where they came from.
    Now, why is it that the two clocks in that example read differently?
    What exactly causes the time to change as is?
    It probably has something to do with the speed you're traveling at, I'm guessing.
    But how exactly does the time differ, or actually, why? Does gravity play a roll? Or some other universal force?
    And forgive me about the previous posts, I asked my brother to ask this question on this forum because I was busy, and apparently he made a fool of me.

    Regards,
    Santural.

    P.S.: I think those posts were because a moderator transfered the thread.
     
  8. Apr 29, 2007 #7

    pervect

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    I merged the thread, which combined all posts from both threads into this one into the best forum.

    We really prefer not to have duplicate threads, I probably should have mentioned what I was doing and mentioned that we don't like duplicate threads.

    Anyway, consider the fact that the shortest distance between two points is a straight line. What is the "reason" for this? I suppose one could say, succinctly, "geometry".

    This is exactly the same reason that elapsed time is different for SR along different paths - the geometry of the path itself. Straight paths through space-time are called geodesics, are characterized by the lack of any "felt" acceleration, and have the longest elapsed time, at least in special relativity.

    A lot of people get "stuck" on the idea that there has to be some master clock somewhere. There isn't any cure for being "stuck" that I've seen t except for abandoning the idea that there is no such master clock. Some people just cling to this incorrect idea, I don't know why.

    There are other ways of understanding SR, but the geometrical idea is one of the most powerful, and it's actually not any harder than the Euclidian geometry one probably learned at school.

    Some alternatives are the K-calculus approach, which only requires high school algebra. This involves some equations (though rather simple ones), so I thought the geometric appraoch might be a better attempt from the impression I formed from your initial post.


    The hardest part about SR seems to be"unlearning", not learning. The actual ideas are fairly simple, the hard part for most people seems to be abandoning old, incorrect ideas.
     
  9. Apr 29, 2007 #8
    I understand.
    So it the path itself (its geometrical structure) which affects the time.
    Another question is, why is it that when one travels at the speed of light, let's say, to Neptune and back, why is it that millions, possibly billions of years have passed on earth?
    I would have thought that since you traveled at the speed of light, barely seconds would have passed.

    P.S. I know humans can't (yet) travel at the speed of light, but let's suppose for this purpose.
     
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