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A question about supposed faster than light movements.

  1. Mar 26, 2003 #1
    A question about supposed "faster than light" movements.

    I'm having a real problem understanding when people talk about the universe's expanding faster than the speed of light. Isn't that impossible? Also, I have heard that a black hole's gravitational pull works at a speed faster than that of light. Is this really true? If so, why doesn't someone arrive at the center of the black hole, before he/she arrives at the EH?

    Any help would be greatly appreciated.
  2. jcsd
  3. Mar 26, 2003 #2
    In regards to black holes, look at the following links (in order):


    More on this

    After reading those you should better understand what I've said in your time travel topic.
  4. Mar 26, 2003 #3
    Black holes dont work faster tham the speed of light, but merely have a gravitational pull stronger than the escape velocity of light, but if I put that in the wrong words please tell me.
  5. Mar 26, 2003 #4
    Oh, duh *slaps self on head*. I should have realized the obvious difference between sending a signal that travels faster than light, and having a curvature that creates an escape velocity beyond the value of c.

    So, how about the expansion of space problem?
  6. Mar 26, 2003 #5


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    The expansion of the universe is not a speed, it's an increment of space. Distant galaxies are not retreating from us faster than light, nor we from them. Rather the space between is increasing, and there is no natural limit on how fast that can happen. All the appearances, including doppler effect, work just as if they were moving (or we were) at high speeds.
  7. Mar 27, 2003 #6
    Could you be a little more clear on this, please? I don't understand how "the space between us is getting bigger..." is any different than "we are getting farther apart...".
  8. Mar 27, 2003 #7


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    The point is that neither we nor the remote galaxies are moving at those speeds. There are local motions but they are not what the expansion is about. What is happening is the space is being added in between (well, added everywhere away from gravity wells, including the space between us and old NCG whatzitzname). This addition is a new way to get farther apart, without using speed.
  9. Mar 30, 2003 #8
    That reminds me, even though galaxies are not actually traveling faster than light away from each other, the doppler shift of light makes it appear that way. This creates an aparent event horizon at a particular distance away - say about 13-billion-l-years. It is interesting to think that this apparent EH has absolutely nothing to do with how old the universe really is.

    If it did, the event horizon would have to recede from us over time. The very distance red-shifted galaxies would have less of a redshift over time. --What if it's not a coincidence, and the redshift has less to do with velocity than with gravitation.
  10. Mar 30, 2003 #9
    Special relativity places a velocity limit of c between observers in an inertial (zero acceleration) frame. General relativity allows for the gravitational curvature of spacetime and its equivalent non-inertial acceleration. When the universe expands due to inflation, Hubble effect, or cosmological constant, it is attenuating the arena (spacetime) of events. Thus even locally fixed observers in such a universe can, separated over an adequate amount of space, be travelling relatively faster than the speed of light, and beyond each other's horizons.
  11. Apr 2, 2003 #10
    The universe is supposed to exapand faster than c. Can it have an escape velocity? I know it sounds silly, but something like an escape velocity?
  12. Apr 2, 2003 #11


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    That implies that there is something you can escape TO. There isn't.
  13. Apr 3, 2003 #12


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    I repeat that the expansion is not a velocity, in the meaning of special relativity.

    Maybe try the old model of blowing up a balloon with dots on it representing the galaxies. Suppose there's a speed limit for dots on the balloon, no dot can move across the balloon at more than one inch per second as seen from any other dot (we have to suppose that Lorentz transformations apply to such motions). But that's moving across the baloon's surface; it doesn't factor in the possibility that the surface itself is increasing as you blow the baloon up.

    It's notorious that you can't push the balloon analogy very far, but this just shows the distinction I am trying to make.
  14. Apr 6, 2003 #13
    the gravitation never work faster that speed of light
    if we put a ball in vacuum....
    and put it with a force
    the ball will have a acceleration
    and become fast and fast....
    but when it reach the speed of light....
    no force can give it again because it's a limit
    like u run 10m/s and want to put the car which is move 10m/s too...
  15. Apr 7, 2003 #14
    Let's look at this problem with the other standpoint. This is my point where universe consists of time and light only, where all other effects this manifestation these two, where light this zero point.
    Immediately becomes obvious an absoluteness of speed of light. Hereinafter obviously that space and energy this effect of the change of time. The Mass --> Energy of the object this amount of Time which have the object. This is his Time Cycle. The Objects can interact by returning or takeing time. They lose its time through light radiation that reveals itself as expansion of space in our perception . This is the set of the past conditions of object. At termination of the Time Cycle the object stops its existence and moves to zero level. BH is object which does not spend its time. He is capable to take it only.
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