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F.t.l. question

  1. Sep 29, 2008 #1
    first, I would like to apologize for my lousy english
    second,I hardly know anything about relativity (well I understand why you can't travel faster than light, and i understand a few concepts,so I get the simplfied big idea , but I don't know the fine stuff of it)
    me and a friend were discussing f.t.l travelling yesterday.
    We have read about faster than light travelling by taking a shortcut through space time
    now so far evrything I have read is about light that travels in a straight line
    but what if light is bended (by the gravity of an extremly dense object, for example, a black hole)?
    i made a picture of this

    http://img72.imageshack.us/img72/8672/ftltravelispossibleun7.jpg [Broken]

    now thanks to the black hole, the light is not taking a direct way from point B to C
    and travelling more distance.
    Assume you would counteract the gravitational force of the black hole by igniting side rockets, so you *DO* go in a straight line
    faster than light travel mean : basically travelling faster than light, so in this example
    is it true that you are travelling faster than light?
    or are we horrible wrong here ?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 29, 2008 #2


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    Science Advisor

    In general relativity, it makes sense to say that we cannot travel faster than the speed of light, and also that we can - each meaning something different.

    In special relativity, which applies when there is no gravity and space is flat, we can draw space as a horizontal axis and time as a vertical axis. Then the spacetime path of a ray of light will be a straight line with a certain slope. A spacetime path with a slope less than than that of the path of light, represents a particle travelling faster than light, which is forbidden.

    When spacetime is curved, only very small parts of space are approximately flat, and within any small region of space, nothing can travel faster than light in the sense defined above.

    For large curved regions of space, the above definition no longer holds, and the meaning of the speed of light is ambiguous.

    Here is an article describing the different definitions of the speed of light in curved spacetime:
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