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I Confusion about Frame Dragging

  1. Jan 2, 2018 #21
    I am interested in a case when a chain approaches infinitely the ergosphere, but is not touching it.

    Lets looks at a sequence of hovering points above and below the ergosphere. Below the ergosphere their path is spacelike and is physically impossible. Above the ergosophere it is timelike. Exactly on the ergosphere it is NULL.

    Then if we look at an infinite sequence of hovering observers, approaching but not touching the ergosphere, then the factor of time dilation for them will diverge to infinity (as it is infinite for a NULL path). A little bit weird but looks like it is true.
     
  2. Jan 2, 2018 #22
    I believe if would start to disintegrate - atom on a surface moves down - is pulled to the left very hard - escapes from surface. This is just one of many effects.
     
  3. Jan 2, 2018 #23

    PeterDonis

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    Yes, that is correct.
     
  4. Jan 2, 2018 #24

    nomadreid

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    I am not sure whether I should start a new thread with my question, because whereas it does not directly deal with the OP's question, my question deals with a related question. I leave the fate of the question to the moderators. (Feel free to move it or to omit it and have me re-post as a new thread.)
    So, the question: please point out whether the following reasoning is correct, wrong in certain steps, or simply off-the-wall.
    If one decides, instead of lowering a chain, to commit suicide by entering the event horizon of black hole that is spinning enough to have jets, one would indeed commit suicide in the attempt without however being able to enter, even when one makes the assumptions:
    [1] the black hole is a super-massive one, so that it is not the tidal forces outside the event horizon that would kill the suicidal astronaut) and
    [2] supposing there is no firewall at the event horizon,
    as the frame dragging would tear the astronaut apart before reaching the event horizon, unless she tried to enter when one does not experience any frame-dragging, but that would be at the poles, where the jets would mean a quick death.
     
  5. Jan 2, 2018 #25

    PeterDonis

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    Why do you think that? If tidal gravity is negligible at the horizon, then any difference in frame dragging between the astronaut's head and feet will also be negligible. So his whole body will be frame dragged by the same amount and he will feel no force due to it, so he won't be affected at all.
     
  6. Jan 2, 2018 #26

    nomadreid

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    Ah, thank you, PeterDonis. My fault for reading popular science sites.
     
  7. Jan 3, 2018 #27

    nomadreid

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    Ah, coming back to that answer.... if there were no significant frame dragging around a supermassive black hole, then where do the jets of those who have them (OK, only a small portion, but nonetheless) come from? I thought the consensus was that the frame dragging around them was one of the causes. ???
     
  8. Jan 3, 2018 #28

    Ibix

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    Peter isn't saying that frame dragging isn't significant. He's saying that the difference in the frame dragging effect between your head and your toes isn't significant (with the stipulation you made that this is a super massive black hole). You'll go round the black hole, and fast, but you won't be torn apart because your toes won't be trying to go much faster than your head.
     
  9. Jan 3, 2018 #29
    Yesterday I learned that there may be ergospeheres around neutron stars too. Wow.
     
  10. Jan 3, 2018 #30

    nomadreid

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    Thanks, Ibix. so
    I would end up spiraling into the black hole?
     
  11. Jan 3, 2018 #31

    Ibix

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    I wouldn't like to take a position on whether you would complete a full circle without doing some maths, but basically yes. If I drop you from a great distance from a non-rotating hole you will always be between me and the hole as you fall in. But with a rotating hole you would be dragged spinwards to some extent.

    In practice the accretion disc around the hole would have a straightforward frictional drag effect as well (not to mention cooking you extra-crispy). I suspect that would tend to make you orbit more than a vacuum GR analysis would suggest, but I haven't done the maths for that either.
     
  12. Jan 3, 2018 #32

    nomadreid

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    Many thanks for that, Ibix. That makes sense, even without the calculations. (The various things that can, as you put it, cook an astronaut extra-crispy, seem to do away with all science-fiction scenarios about an astronaut entering the event horizon alive.)
     
  13. Jan 3, 2018 #33

    PeterDonis

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    From where? Please give a reference.
     
  14. Jan 3, 2018 #34

    PeterDonis

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    The jets are believed to be coming out along the rotation axis of the hole; as I understand it, frame dragging basically collimates them to shoot out that way. But that is frame dragging considered globally, on the scale of the entire hole: as @Ibix said, that is very different from the difference in frame dragging between the feet and the head of an astronaut in a free-fall trajectory near the hole.
     
  15. Jan 3, 2018 #35
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