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String vs Black Hole

  1. Sep 5, 2012 #1
    There is a spaceship situated orbitting a blackhole at x million miles.

    We have an exactly x milllion mile long string (perhaps a bit longer)

    Lets not call it a string, but a flexible steel cable instead.

    Now, we tie a metal ball at its one end, and project the ball towards the blackhole such that it goes straight to the so-called centre of the BH.

    Now, the ball enters the Event Horizon. Will we be able to pull the ball out of the hole? If we are able to pull the ball out, will its composition remain the same(it wont be spherical, im talking about its material)?

    The spaceship has infinite fuel, i.e. if it gets closer to the hole, it can increase orbital velocity.
  2. jcsd
  3. Sep 5, 2012 #2
    The string's going to break, probably, or the spaceship can't exert sufficient force.
  4. Sep 5, 2012 #3


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    NOTHING comes out of the EH of a BH
  5. Sep 5, 2012 #4
    Expecting Support...

    Lets consider it can.

    The topic of discussion is actually what will happen if THIS:
    isnt wrong or right...
    Last edited: Sep 5, 2012
  6. Sep 5, 2012 #5
    The point is, we can consider a spaceship that can offer any finite amount of force. My guess would be that it would require an infinite amount of force to get it to escape, which is obviously impossible to produce.

    If you like, you can look over the proof relying on GR that nothing can escape the EH and see that it covers this case.
  7. Sep 5, 2012 #6


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    Algren, I'm not sure where you are going with this but as nearly as I can make out what you are asking is "if physical reality were not the way it is, what would happen?" Well, there would be unicorns.
  8. Sep 5, 2012 #7
    Right, well, erm, lets see

    I have not gone into some/any textual research into that matter. But, when i come to look at it at a very classical way, i see a force, which isnt unstoppable.

    U see, we place an object inside the Event horizon, it experiences a gravitational force F.

    We apply an opposite but larger force to pull it out, right? Will the object/particle come out, or not?

    EDIT: One sec, if i am applying a force equal and opposite to the force of gravitation of the BH, howcome am i exceeding speed of light?

    Or is it that so much energy being produced isnt possible?

    Well, thats a wrong example, unicorns are one-horned horses, thats biology's jurisdiction, not ours.
  9. Sep 5, 2012 #8


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    Which part of "NOTHING comes out of the EH of a BH " did you not understand? Do you think I'm making that up? There IS no force strong enought to get anything out of the EH of a BH. The escape velocity of ANYTHING inside the EH is greater than the speed of light.
  10. Sep 5, 2012 #9
    And how do you define escape velocity?

    The velocity required for an object to achieve at that point to reach infinity?

    Well, we arent doing THAT here, are we? We are simply pulling that object out very slowly, what part of it is contradicting/exceeding speed of light?

    I'm not saying that u are making it up, and here you said:

    Well really, so gravitational force just inside EH isnt Defined?
  11. Sep 5, 2012 #10


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    Fortunately, there are some really great folks on this forum who have both more knowledge and WAY more patience than I do. I'm sure one of them will take up this discussion.
  12. Sep 5, 2012 #11
    Algren, I think the problem is that you are approaching this problem with just the Newtonian picture of the universe. A black hole's event horizon is a thing that doesn't work when you look at it using Newtonian physics. You must apply General Relativity in order to understand what happens when you approach the event horizon of a black hole.


    Trying to think about something like a black hole and its event horizon just doesn't work. It has to do with the way gravity interacts with space-time, and how matter interacts with space time.

    In that wiki article, it shows a picture to the right which shows the possible world-lines (I think that's the name) of some particle as it approaches the black hole. Once it passes the event horizon, all of the possible world-lines for that particle lead it closer to the center of the black hole.

    Essentially, it's not that it's impossible to escape the event horizon due to some huge force, rather it's that it is impossible to escape the event horizon due to the shape of space-time at the event horizon and within.
  13. Sep 5, 2012 #12


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    Consider this, you will never see the ball cross the event horizon. It will become increasingly redshifted as it approaches the EH and increasingly time dilated until it appears to be frozen in time. Also, bear in mind that, by definition, the escape velocity at the EH is c, and no mass possessing object can ever travel at c. Of course it is true the rope will break before the ball reaches the EH.
  14. Sep 5, 2012 #13


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    Another thing to consider is that there are very strong tidal forces near a black hole. A tidal force is just a stretching of an object that arises when the gravitational force pulling on one side of it is much greater than the gravitational force pulling on the other side. In this case, the near side of the cable experiences a much stronger force than the far side. The cable will stretch, probably until it breaks.

    EDIT: but really thinking about it some more, what Chronos said seems like the most relevant point, and he already said the cable would break: I missed that.
  15. Sep 5, 2012 #14
    Relativity makes things go floppy.

    Near the event horizon, the gravity of the black hole will keep string from reacting to the force of the pull so the string will break.

    One other way of thinking about it. Suppose you have something traveling near the speed of light. If you push on it, it won't change very much because it will still be going near the speed of light.

    Near the black hole, the pressure waves within the string is moving close to the speed of light. If you add more force to it, because things are already relativistic, it won't change the behavior of the string.
  16. Sep 5, 2012 #15
    By applying a force to the string you are adding energy to the atoms in the string. As you add energy to the atoms, the atoms vibrate more quickly, and it works at that anywhere near a black hole, you have to add enough energy so that the atoms are vibrating at nearly the speed of light. Once that happens, the atoms start acting like photons and the string turns into gas.
  17. Sep 6, 2012 #16
    If you have a force carrier that transmits/propagates a force through some medium (in your case physical matter, a strong, low density alloyed cable connected in tension), the force will be successfully transmitted until the medium fails to be consistant from end to end (i.e gravitational/tidal forces + the same force oppositely exerted by you exceeds ultimate tensile strength in the material before subsequent rupture).

    Under such immense scales as the black holes we observe today, there isn't matter that we're aware of that can be constructed to withstand twice the force of a black hole at its event horizon plus a non-zero net force required to pull it out. But if you assume a medium, any medium that can reliably transmit a force within the environment of a black hole and isn't severed due to material limitations, then it's a question of the propagation limits of the force carrier in your medium. General relativity breaks down here where classical physics is quite solid and applicable, you just need to figure out what medium best conforms to your assumptions that matter has matterless physical properties.
  18. Sep 6, 2012 #17
    That's not true. Speed of light imposes limits on tensile strength.

    See problem 12 on this page http://www.lightandmatter.com/html_books/0sn/ch07/ch07.html [Broken]

    And if you assume such a medium then you contradict relativity, and if you contradict relativity then anything is possible. If you assume that nothing can travel faster than light then everything works out. You can't get stuff out of a black hole.

    If assume that there is a string that doesn't break when you dip it into a black hole then it turns out that that string can transmit information faster than light, at which point black holes don't exist.

    No need to bring in general relativity, you just need special relativity. If you assume that special relativity is true then you can't create a string that is strong enough. If you reject special relativity then at that point you are making up your own physics, and then you tell me what happens.
    Last edited by a moderator: May 6, 2017
  19. Sep 15, 2012 #18
    I can kinda understand what the OP was asking.
    I think.

    But you have to understand, when you start going into hypothetical situations that current theories say are impossible, you can't return back to those theories for the answers.

    You'd wind up with matter flying back in time, negative mass matter, etc. all things theorized to be impossible
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