Event Horizons in General Relativity

Discussion in 'Astronomy, Exobiology, & Cosmology' started by RJBeery, Apr 17, 2019.

  1. RJBeery Natural Philosopher Valued Senior Member

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    Black holes certainly do take an infinite amount of 'external time' to form. This isn't controversial. And an object falling into a black hole-like area would evaporate before ever reaching the event horizon...if such an evaporative process existed.

    Anyway, Seattle, I guess I'm going to bounce the question back to you. I just gave you a resource that contained something you disagree with. You're an objective person; are you simply more informed than author of the link I provided?

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  3. Seattle Valued Senior Member

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    As to your second question, I'll go back and read the link before I answer that question.

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    What is "external time". I get the different reference frames as they apply to the falling in example but that's not the formation of the black hole.

    You don't see the falling object go past the event horizon because no light can escape. To the observer they just appear to go slower and slower and then due to their being fewer and few photos, they just gradually fade to "black".

    The reality from their perspective is that they just keep falling until they die. They may die due to the gravity gradients before the event horizon, it depends on whether it's a stellar black hole or a black hole in the center of a galaxy.

    I'll read the link...
     
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  5. Seattle Valued Senior Member

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  7. arfa brane call me arf Valued Senior Member

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    Here's something to chew on:

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    Note how the event horizon is inside some other horizons; you see you have to ask, what does time mean at the singularity?
     
  8. RJBeery Natural Philosopher Valued Senior Member

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    Hi Seattle. I assume we can agree that all black hole definitions should be equivalent, presuming they come from a reputable source. Wald definitely qualifies in that regard, but let's table this for a second.

    IF we accept Wald's definition of a black hole (i.e. "that area of spacetime which is not in the causal past of future null infinity") then do you agree that the idea of evaporation of black holes in finite time precludes event horizons from existing at all?
     
  9. Seattle Valued Senior Member

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    Wald doesn't agree with that. What about the two links that I posted don't you agree with?

    What does "future null infinity" even mean?

    A black hole is something so dense that light can't escape. The point where gravity becomes that strong is the event horizon. What about that description do you disagree with?
     
  10. arfa brane call me arf Valued Senior Member

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    Lightlike geodesics have a past and a future boundary, at infinity. In the diagram they intersect, on the right. But still at infinity.

    I recall Seattle objecting to the notion of infinity as a number, like any number. So you can divide with it, say.

    But the notion of the limit of a function implies this numerical property: a function of x, defined as the limit of x "going to" infinity, can be well-defined. How come when infinity can't be a number, and once you're "there" you can't come back? My my, hey hey.

    But you can project to infinity, and stay where you are, with nice straight lines, circles or spheres with finite radius, etc. Then infinity is "only" a boundary, and you are where you are because you can define some coordinates.
     
    Last edited: Apr 25, 2019
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  11. arfa brane call me arf Valued Senior Member

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    "The so-called teleology of the black hole event horizon is an artifact of the way in which physicists define an event horizon: the event horizon is defined with respect to infinite future elapsed time, so by definition it 'knows' about the entire fate of the universe," Engelhardt told Phys.org. "In general relativity, the black hole event horizon cannot be observed by any physical observer in finite time, and there isn't a sense in which the black hole as an entity knows about future infinity. It is simply a convenient way of describing black holes."

    --https://phys.org/news/2015-09-law-implies-thermodynamic-black-holes.html
     
  12. RJBeery Natural Philosopher Valued Senior Member

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    Wait, what? Why are you claiming to know what Wald believes? I'm confused. The links you posted are completely fine, and I'm OK with the definition of a black hole to be tied to an area where light cannot escape; my earlier point was that, the "future null infinity" definition is still valid. We can use one, the other, or both when discussing the topic. I think your definition is less useful when discussing evaporation, for example.
    It means "in the infinite future." In a closed universe, where time does not extend forever, black holes would be forbidden in GR without reference to evaporation, as the earlier quote from mathpages suggested.
     
  13. Seattle Valued Senior Member

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    Regarding Wald, because he didn't write about it.

    A closed Universe doesn't tell us anything about whether future time is infinite or not
    You are the only one concluding that black holes are forbidden in GR without reference to evaporation.
     
  14. RJBeery Natural Philosopher Valued Senior Member

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    I think you missed this, but I posted it earlier. It's from Kevin Brown's mathpages resource (although some of the material is written by other qualified authors):
    In this case, "closed universe" means that time itself is finite, not that gravity pulls all matter into a Big Crunch. In other words, when time is finite, black holes cannot form; and because time is finite for a black hole-like region which is evaporating, the black hole itself can never form. You still disagree?
     
  15. Seattle Valued Senior Member

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    Of course I disagree as does most of "mainstream science". You can change the definitions of anything that you want to until you get the inconsistencies that you are looking for to reach your looked for conclusions.
     
  16. RJBeery Natural Philosopher Valued Senior Member

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    I'm not changing definitions, I'm using an alternate, perfectly valid, definition by one of the mainstream scientists you appear to be speaking for. Secondly, you said that I was the only person making the claim that black holes could not exist in a closed (finite-time) universe...so I provided a quote and a link saying explicitly that very thing...and you have responded like you didn't even read it.

    I have to say that I find your responses odd, Seattle. I'm posting quotes from Brown, Wald, and Hawking while you appear to be claiming that "mainstream science" contradicts those quotes.
     

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