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B What are the biggest misconceptions about black holes?

  1. Apr 10, 2016 #1
    Hey guys, I'm currently doing a project on black holes and need some input on what you believe to be the biggest misconceptions about black holes, thanks! You can read more about the guidelines of the project at: http://www.aqa.org.uk/subjects/projects/aqa-certificate/EPQ-7993

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    Last edited by a moderator: Apr 13, 2016
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  3. Apr 10, 2016 #2

    ProfuselyQuarky

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    I personally think that the largest misconception is the fact that black holes do not just suck up everything that comes there way like a giant vacuum. Also, black holes are anything but empty. A lot of people think both these things. Rather, black holes are extremely dense with matter, which causes them to have such a massive gravitational force. I know people who have sometimes said that if the sun turned to a black hole, Earth would be sucked up and we would all die. No, that's not exactly true. If the black hole had the same mass as our sun right now, Earth would still orbit in its same path just as always (of course we would all still die a freezing death, but that's different matter).
     
  4. Apr 10, 2016 #3
    Thanks for the input, do you mind if I use your contribution for my project?
     
  5. Apr 10, 2016 #4

    ProfuselyQuarky

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    Not so sure what you mean by "use your contribution", but sure. None of this is my own information or anything—it’s just what I think to be the biggest misconception. If you do some research, you’ll find much more and more PF members will probably add there own input, as well.

    There's a lot you can find on the internet. You just have to look for it.
     
  6. Apr 10, 2016 #5

    Chronos

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    One of my favorites: a black hole is impossible because an infinite amount of time is required to form an event horizon.
     
  7. Apr 10, 2016 #6

    phinds

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    Another is that there is a contradiction that since nothing can escape a black hole, it can't have any gravitational effect since gravity could not escape it.
     
  8. Apr 10, 2016 #7
    Isn't that just a theory, not a misconception though?
     
  9. Apr 10, 2016 #8
    Can you elaborate? Why does an event horizon take an infinite amount of time to form?
     
  10. Apr 10, 2016 #9

    phinds

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    I don't even know what you mean about that being a theory.
     
  11. Apr 10, 2016 #10
    We
    Well I'm not sure how to say it but is it really a misconception?
     
  12. Apr 10, 2016 #11

    ProfuselyQuarky

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    Your probably thinking of gravitational pull as something between two objects. Well, if we had one body of mass in open space without any object "nearby", would it still have gravitational force? Read this: https://briankoberlein.com/2015/08/21/how-does-gravity-escape-a-black-hole/
     
  13. Apr 10, 2016 #12

    phinds

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    Well, do you think it's CORRECT? It is something that people have come here and asked about so, yes, it definitely IS a misconception.
     
  14. Apr 10, 2016 #13
    I think one of the biggest misconceptions is that there is some kind of physical object called a 'singularity' at the centre of a black hole.
    What the term 'singularity' actually means is that our current best theories are inapplicable for the centre of a black hole.
    We don't really have much of a clue what happens to matter there, and attempting to extrapolate from theories which are otherwise sound produces nonsense results.
     
  15. Apr 10, 2016 #14

    Chronos

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    Remember Bob, Sally and the black hole? Bob volunteers[?] to cross the event horizon while Sally stays on the ship and watches. Bob never makes it, Sally watches as Bob slows to a halt and freezes upon reaching the EH due to time dilation. So, how can a black hole form when infalling matter takes an eternity to cross the EH?
     
  16. Apr 10, 2016 #15

    ShayanJ

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    I don't think that's true. For example for a Schwarzschild black hole, the light cones inside the Schwarzschild radius are pointing towards the center. Let's imagine there is some matter at a radius ## R<R_S ##, then it has to move faster than light in order to stay there, otherwise it will just "hit the singularity" like anything else.
    Now if, instead we imagine that there is some sphere of matter concentric with the event horizon, with a radius ## R<R_S##, the metric inside it will be different from Schwarzschild's but still continuity of metric requires that the metric on its surface is Schwarzschild's and so still its surface should move faster than light in order to stay there. But we know that can't happen and so we need to accept that the inside of a black hole has to be empty, at least according to GR.

    The misconception I want to mention is exactly what triggered the above response. Its actually not that a black hole's gravity is so much stronger than other things and that's the reason for its strange properties. The reason for such properties is the strange causal structure of the spacetime region past the event horizon of black holes.
     
  17. Apr 10, 2016 #16

    Vanadium 50

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    I think this is not going to work well - a list of misconceptions is not the way to learn something. If someone tried to explain baseball to you by explaining the ground rule double, the difference between interference and obstruction, and the infield fly rule, would you understand how the game is played?
     
  18. Apr 11, 2016 #17

    martinbn

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    I agree with this, but the next part

    is a bit of a misconception itself.
     
  19. Apr 11, 2016 #18

    Ken G

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    I think the biggest misconception about black holes is that physicists agree on what they are. I know of no example of mainstream physics literature where the experts in the field disagree more completely than on the topic of what goes on inside an event horizon.
     
  20. Apr 12, 2016 #19
    I think the biggest is that all black holes give off no light. Although they themselves don't, the infalling material can create x-ray radiation and other bursts. Especially with black holes at the center of the galaxy.
     
  21. Apr 12, 2016 #20
    Probably the greatest misconceptions about black holes comes from what people see in movies.
    That it's possible to pass through a black hole and come out the other side into another time or place. That black holes have infinite gravity and are like bottomless pits and violate dimensional space.
    Black holes do not have infinite gravity, they have enough gravity to not allow light to escape but that's not infinite gravity. An object with infinite gravity would suck in the whole universe.
     
  22. Apr 13, 2016 #21

    Demystifier

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    Even though it is a very common misconception, I have never seen a simple explanation of why exactly it is wrong. Two simple possible explanations come to my mind. One is that pure gravity is non-linear, so that the source of outside gravity is outside gravity itself. Another is that the outside gravity is a static remnant of the field created before the creation of horizon. Would you agree with one of those explanations? Or is there a better one?
     
  23. Apr 13, 2016 #22
    Under Newton gravity was a force.
    Einstien proved that gravity is an effect. The effect of the bending of space by mass.
    We feel gravity because the material of the surface of the Earth prevents us our falling towards the center of the Earths mass.
    Everything in the universe follows the curvature of space in 4 dimensions, space and time ( enen light ). The greater the mass of an object, the more pronounced the curve ( or warping ) of space around it.
     
  24. Apr 13, 2016 #23

    phinds

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    I agree that the gravitational field of a BH is static and is formed as the BH is being formed so the EH is irrelevant. As more matter moves in then, at every point of its existence, everything farther away from the BH that the matter is sees that particular matter as contributing to the gravity field of the BH and of course this persists even after it enters the EH, so this whole "escapes the EH" is nonsense.
     
  25. Apr 13, 2016 #24

    Ken G

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    It depends on whether one uses general relativity for gravity or not. Certainly that is our only current theory of gravity that has been verified at a high level of precision, but for some reason most elementary-particle theorists seem to doubt it is a correct description of the situation. They imagine a theory where gravity is carried by a force-carrying virtual graviton. They also imagine that, if one takes the virtual particle picture for the force carrier, virtual particles are not bound by the usual rules (indeed, virtual particles do not need to even propagate outward from the source of the force, they can propagate inward for the case of attractive forces, as gravity is). Of course, we are free to reject the graviton idea if we like, or we are free to reject the virtual particle picture. Every "why" answer we can give must always be in the context of some chosen theory.
     
  26. Apr 13, 2016 #25

    BiGyElLoWhAt

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    I like the latter, personally. An outside observer can never observe matter cross the event horizon, so I think we would necessarily have to measure the source of the gravitational field source as approaching the EH. So perhaps not necessarily exactly what you said, in the sense that the field can still be generated, but that the infalling matter "lodges" itself near the event horizon, and this can still happen even after the EH has been created. I'm no expert on GR by any means, but seeing as how both relativities are observer based, I think all of our observations need to be consistent in and of each other. If we "see" matter near the event horizon, we should also measure a gravitational field source near the event horizon.
    Perhaps the bulk of the field, i.e. the quantity of matter that caused the creation of the EH, would follow what you were saying. Perhaps that's what you meant. I don't see any reason why it can't increase, however. (by the addition of matter along the EH)
    Edited-*poorly worded post is poorly worded*

    Are the fields necessarily static? I think that's an assumption in the solution of a lot of metrics, as it completely eliminates the dependence of the metric on 1/4 of the coordinates, but I don't see any reason why this should be necessarily for say, a binary black hole system.
     
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