Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Transformation of Matter into Black Holes

  1. May 10, 2017 #1
    From Wikipedia:
    So, assuming we have a massive ball of water that keeps growing, but somehow manages to remain at a fixed density, the moment the Schwarzschild radius overtakes the physical radius, will the gravitational properties of the ball of water undergo a sudden, dramatic change?
     
    Last edited: May 10, 2017
  2. jcsd
  3. May 10, 2017 #2
    https://www.quora.com/Could-you-cre...re-and-more-mass-keeping-the-density-the-same

    Here explains in a bit detail.In here It says its possible.But I dont think it would be sudden (Not sure though), cause we are adding matter in some amount so probably as we add the matter things will change in a someway, like gravity affect on the center will increase an amount so it will keep increasing with addition of matter and finally it will collapse I guess
     
  4. May 10, 2017 #3
    but what if the schwarzschild radius exceeds the physical radius while the water ball is still at a constant density?
     
  5. May 10, 2017 #4

    russ_watters

    User Avatar

    Staff: Mentor

    There is nothing magic about black holes; they are just massive objects. They obey the same laws of gravity other every-day objects do. So from a distance, when you add that last bit of water, nothing noticeable will change about the gravitational field.
     
  6. May 10, 2017 #5
    what if you were taking a dip in that water when the Schwarzschild radius coincided with the physical radius?

    let's say you set your spaceship to hover above the water and then took a dive.

    and assuming you were just fine with the massive gravitational field strength.

    would you be able to climb back into your spaceship?
     
  7. May 10, 2017 #6

    russ_watters

    User Avatar

    Staff: Mentor

    No. But bear in mind you are stretching really far into non-physical assumptions that make the scenarios less and less realistic as you go.
     
  8. May 10, 2017 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You have your answer. Nothing magic happens.
     
  9. May 10, 2017 #8
    An observer who enters the region or near it will see no difference but outside observers should see a "real" black hole I think.
     
  10. May 10, 2017 #9
    true. but its a consequence of trying to simplify it.

    but that means if the Schwarzschild radius was 20 cm below the surface of the water, I would be able to climb out?
     
  11. May 10, 2017 #10

    russ_watters

    User Avatar

    Staff: Mentor

    I don't know what you mean by "the region", but notice I said "nothing noticeable will change about the gravitational field". What a distant observer sees with his eyes will change.
     
  12. May 10, 2017 #11

    russ_watters

    User Avatar

    Staff: Mentor

    Are you above or below the Schwarzchild radius? Are you a point particle or a person who is more than 20cm tall? Trying to descipher non-physical assumptions is hard...
     
  13. May 10, 2017 #12
    The region is event-horizon/near black hole.I see your point..
     
  14. May 10, 2017 #13
    While I don't know much about GR and black holes, I would say to an outside observer all Schwarzschild BHs of a certain size (mass) look identical no matter how they formed. In fact the mass is the only independent parameter needed to fully describe them.

    As for a ball of water I don't think it can possibly maintain any finite density as the event horizon surpasses its diameter, it will have to collapse into a singularity. Nothing inside an event horizon can be static, the inside can only be a vacuum with a central singularity.
     
  15. May 10, 2017 #14
    I'm an average height human, with my neck above the water. So the Schwarzschild level is at my stomach perhaps.

    So the Schwarzschild radius hasn't coincided with the physical radius, but is 20 cm below it.
     
  16. May 10, 2017 #15
    True, bu let's say that it maintains a constant density for the sake of simplicity.
     
  17. May 10, 2017 #16

    Dale

    Staff: Mentor

    If you have a perfect fluid in hydrostatic equilibrium at some radius 20 cm greater than the Schwarzschild radius then there is no event horizon. You can go down to the middle and come back up again.

    Note, the fluid in question cannot be water due to Buchdahl's theorem, it would have to be some unobtanium. The tidal forces would be enormous so you would have to be made of a different unobtanium.
     
  18. May 10, 2017 #17
    Ok. Let's say that someone keeps pouring the unobtainium-water onto the water-ball planet, causing it to grow.

    I still keep my head above the water, but the 20cm gap is shrinking.

    When the gap is 1 cm, I can still climb up to my spaceship and jet off, but the moment the gap decreases to zero, there is no way I can ever leave?
     
  19. May 10, 2017 #18

    Dale

    Staff: Mentor

    As the radius becomes less than 9/8 of the Schwarzschild radius the pressure st the center becomes infinite and even your unobtanium fluid collapses. As you get near that limit the system becomes unstable and even your treading water will cause the collapse.
     
  20. May 10, 2017 #19
    ahh, I see. Is there a name for this 9/8 factor? I'd like to read more about it.
     
  21. May 10, 2017 #20

    Dale

    Staff: Mentor

    Buchdahl's theorem or the Buchdahl limit
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Transformation of Matter into Black Holes
  1. Black Hole matter (Replies: 19)

Loading...