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How Can we Find the Escape velocity of a black hole

  1. Feb 8, 2015 #1
    How Can we Find the Escape velocity of a black hole.
     
  2. jcsd
  3. Feb 8, 2015 #2

    Borg

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    Nothing can escape a black hole.
     
  4. Feb 8, 2015 #3

    phinds

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    We KNOW the "escape velocity" of a black hole. It is the speed of light. Since nothing can travel at the speed of light, nothing with mass can escape from a black hole (so it isn't really an "escape" velocity) and even light can only maintain a position exactly at the event horizon because locally it is traveling outward at c and globally, it is being held in place by the gravity of the black hole.
     
  5. Feb 9, 2015 #4
    In a very rudimentary way, it's the escape velocity equation that can tell us something about the nature of black holes. If we take the escape velocity equation-
    [tex]v_e=\sqrt{2Gm/r}[/tex]
    establish that the escape velocity is the speed of light (c) then rearrange relative to r, you get the Schwarzschild radius which is the coordinate radius for the event horizon (where the escape v is the speed of light)-
    [tex]R_s=\frac{2Gm}{c^2}[/tex]
    For a better understanding, you might want to check out the Schwarzschild metric.
     
  6. May 30, 2015 #5
    Yes if it's beyond Schwarzshild Radius
    Score 1 - 0 for me :smile:
     
  7. May 30, 2015 #6
    How is possible for x-rays to be emitted from a black hole, but not light?
     
  8. May 30, 2015 #7

    phinds

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    Not sure what you are talking about. Nothing comes out of a black hole, including any kind of photons, visible light, X-rays, microwaves, whatever.

    Perhaps you are thinking of the accretion disk, but it can emit any kind of electromagnetic radiation.
     
  9. May 30, 2015 #8

    Chronos

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    As Phinds noted, all radiation emitted by a black hole originates external to the event horizon
     
  10. May 30, 2015 #9
    Can I add a question here?

    Visible light travels at ≈ 300 000 km/sec
    X ray travels at 300 000 km/sec
    Microwave travels at 300 000 km/sec
    and...
    Gravity travels/propagates at 300 000 km/sec
    So nothing can escape black hole but Gravity? Because it looks like Gravity has the same characteristic as light.

    Please don't answer this, if this question should belong to another thread. I'll post it there someday
     
  11. May 30, 2015 #10
    Based on this type of picture, I was guessing the x-rays were coming from the center. Doesn't look like they're coming from the disk.
    490046a-f1.2.jpg
     
  12. May 30, 2015 #11
    The jets matter comes from the accretion disk, the circle with the blue color.

    And the accretion disk is beyond the event horizon.
     
  13. May 30, 2015 #12

    Chronos

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    Black hole jets emanate from the poles due to magnetic confinement. They still originate in the accretion disk.
     
  14. May 30, 2015 #13

    phinds

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    This is an easy point of confusion and you are certainly not alone in having it. The thing is, CHANGES in gravity propagate at c. The gravitational force itself propagates as it changes and develops but by the time a body, our sun for example, has formed, all the gravitational changes have propagated and the gravity field then exists unchanging. The same is true with a black hole. The gravitational field it exerts developed as it formed but now that it has formed, its gravitational field exists as is and does not need to propagate.

    Should our sun magically (and against all laws of physics) suddenly cease to exist, the change in gravity would propagate at c, reaching the Earth 8 minutes later.
     
  15. May 30, 2015 #14
    Can I add a question here?
    Please tell me if this question doesn't belong to this thread, I'll create a new thread.

    Gravity curvature.jpg
    No, it's not a light path.
    Does C orbit the barrcenter of A1, B, C or C orbits the barrcentre of A2, B, C?
    It's just that I was just thinking about this gravity thing. It propagates at the speed of light. Is it curved, too?
     
  16. May 31, 2015 #15

    phinds

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    n-body orbits are around the common center of gravity of all bodies involved. Gets very messy.

    It's better to start a new thread when you have a question that is a bit off-topic for the current thread.
     
  17. May 31, 2015 #16
    Thanks, Phinds.
     
  18. May 31, 2015 #17
    One decent analogy I recall for black hole jets is to image pouring a jug of water into a sink, in this case, there's no problem and the water will swirl down the plug hole. Now imagine taking a fire hose and firing this into the sink; the plug hole is too small to accommodate all the water and as a result, water will shoot up the sides of the sink. This (along with magnetic field lines) is pretty much what is happening with black hole jets. As it's been established, black holes are very compact and when a lot of matter is falling in, not all of it will pass the event horizon and as a result, be ejected at the poles.
     
  19. May 31, 2015 #18
    Wow, you're sure?
    Amazing :smile:
     
  20. May 31, 2015 #19

    phinds

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    There can be so much matter coming toward a black hole, and at an angle so that it's all rotating and the bits get in each others way and the accretion disk heats up and through a process I don't understand, but one of our more knowledgeable members will be able to explain I'm sure, it gets to a point where its interaction with the magnetic field of a rotating black hole shoots jets of matter/plasma out perpendicular to the accretion disk such that it appears to be coming out of the poles of the black hole (but of course it isn't)
     
  21. May 31, 2015 #20
    Rotating singularity?
    I can imagine a rotating 3D object.
    Cube, Prism, Pyramid. But it's hard to picture a rotatic sphere much less a rotating object with no size.
    The singularity. It rotates?
     
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