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Thread: General relativity 101

  1. #1
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    General relativity 101

    How's this sound?

    An object is moving in 'empty' space at a constant velocity and then comes into the vicinity of a massive object.

    To explain the acceleration of an object entering a gravity field I would say this object (which would otherwise have proceeded in a straight line at a constant velocity) suddenly finds itself entering an environment where the clocks run slower and lengths are contracted such that it (from the perspective of an external observer) is suddenly accelerating as a result of the momentum it was initially carrying when it entered this new enivronment (i.e new frame of reference).

    I am trying to explain that the result of an object with a fixed momentum, entering a gravity field, is acceleration (at least from the perspective of an external observer hovering above the massive object).

    Thanks!

  2. #2
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    Quote Originally Posted by Cheap Astronomy View Post
    To explain the acceleration of an object entering a gravity field...
    Well, that's just it - it's always in a gravity field. There's no entry or exit. There's only a change of magnitude and direction based on their relative locations and velocities.
    Last edited by mugaliens; 2009-Feb-17 at 07:51 PM.

  3. #3
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    Mugs's objection would seem to apply to the use of the word "suddenly".
    I'd drop those suddenlys.

    I'm not sure about the GR aspects of your question, but an object does
    not accelerate "as a result of the momentum it was initially carrying".
    The object could as well start accelerating from a standstill, because
    as Mugs says, it does not move from a place of no gravity to a place
    of gravity. Gravity's reach is infinite.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  4. #4
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    Quote Originally Posted by Jeff Root
    Gravity's reach is infinite.
    What exactly is meant by that? The gravity emitted from the matter that became the Earth 13.7 billion years ago should only reach something like a comoving distance of 46 billion light years. Is it said to be infinite because every object in the universe has perturbed an object some 46 billion light-years away from itself? So this "hop-scotching" allows for the concept that gravity's reach is infinite? Or am I off base??

  5. #5
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    Quote Originally Posted by m74z00219 View Post
    Quote Originally Posted by Jeff Root
    Gravity's reach is infinite.
    What exactly is meant by that? The gravity emitted from the matter
    that became the Earth 13.7 billion years ago should only reach something
    like a comoving distance of 46 billion light years. Is it said to be infinite
    because every object in the universe has perturbed an object some
    46 billion light-years away from itself? So this "hop-scotching" allows
    for the concept that gravity's reach is infinite? Or am I off base??
    Infinity is a mathematical abstraction. In both Newton's law of
    gravity and general relativity, gravity is described mathematically
    in such a way that it always has some effect nomatter how great
    the distance. That is all that is meant. Neither Newtonian gravity
    nor general relativity take into account the finite age and probably
    finite size of the Universe. They don't need to. That isn't their
    function. If the Universe were infinitely old and infinitely large, the
    Newtonian and general relativistic descriptions of gravity would still
    work just fine, and gravity's reach would be infinite in those cases.
    The theories cover all the possibilities, even if those possibilities are
    not the actual case.

    On the other hand, if gravity is quantized, then it probably cannot
    have infinite reach. But there is no evidence that it is quantized.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  6. #6
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    Quote Originally Posted by m74z00219 View Post
    What exactly is meant by that? The gravity emitted from the matter that became the Earth 13.7 billion years ago should only reach something like a comoving distance of 46 billion light years.
    When did it first exist? That's an unanswered question. Even converting energy into matter does not change its gravitational effect, right?

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