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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    OK! Then do you also agree that it's a contradiction to say that there existed a micro black hole in our laboratory yesterday which has since evaporated due to Hawking Radiation (or some other form of evaporative/dissipative process)?
     
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  3. NotEinstein Valued Senior Member

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    In both cases of light-cones: no. Its existence in the SR light-cone case can be "observed" directly, as the black hole is inside the past light-cone. With malleable light-cones its existence can be deduced from the curvature existing in the past light-cone. In fact, the very presence of Hawking Radiation is a dead giveaway there was a black hole there.
     
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  5. RJBeery Natural Philosopher Valued Senior Member

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    OK so at least we understand where we disagree. You believe that the black hole formation exists in a part of spacetime that is completely disconnected from the traditional, continuous spacetime fabric. I'd have to point out that I've never read this in any literature, ever. The seminal textbook on GR -- Gravitation by Misner, Thorne and Wheeler -- makes no mention of this (I have it right here). I got the feeling that you came up with this explanation "on the spot" in order to refute the contradiction that I'm struggling with; is that the case or have you actually read this somewhere?

    Can we agree that if we demand that spacetime be continuous then the existence of black holes in an observer's past light cone is a contradiction?
     
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  7. NotEinstein Valued Senior Member

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    This is false. I do not hold that believe per se, and have not expressed any such believe here.

    Perhaps your knowledge of the literature isn't as extensive as you thought, because this is quite a commonly known viewpoint. See for example: https://en.wikipedia.org/wiki/Black_hole_cosmology

    Under the SR light-cone case this is trivially false, so no, we can't agree to that.
     
  8. NotEinstein Valued Senior Member

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    In fact, now that I have spend a couple of minutes on it, I do not hold this belief, because it's incompatible with the event horizon not being a physical barrier. It cannot be a break of spacetime, because clearly spacetime isn't broken at the event horizon. See, for example: https://en.wikipedia.org/wiki/Kruskal–Szekeres_coordinates
     
  9. RJBeery Natural Philosopher Valued Senior Member

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    Well, which is it? Either the black hole area is in a mysterious "elsewhere section," as you claimed or it is continuous on the traditional spacetime diagram. If it is the latter (and you'd better agree that it is!) then...do you believe it can exist in the past light cone of an observer in GR?

    (And please stop referring to SR light-cones; I've explicitly said that we are in the GR framework at least a half-dozen times, despite the original picture's shortcomings)
     
  10. NotEinstein Valued Senior Member

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    Please demonstrate that one has to exclude the other. I genuinely don't know.

    Why? You don't disagree with the first option yourself, so why should I? You are the one that gave the "photon's can't reach us, therefor, not part of the past light-cone" definition, which excludes black holes from the past light-cone. Since black holes obviously have formed outside of the future light-cone (see the NASA picture posted previously), and they can't be light-like, what other section is left? (I'm open to a different name for it, since indeed it might be confusing to call it "elsewhere", even though it fits the description.)

    I'm not sure if it is the latter, so I can't answer your question any different than I did in post #22.

    It's not about the original picture: it's about the red lines you drew in it. You are the one that introduced the notion (although admittedly, I went along with it initially because I didn't realize the other option). As long as you don't admit you made a mistake there and withdraw the picture with the red lines, I can't take the SR light-cones off the table, because you haven't done that either.
     
  11. RJBeery Natural Philosopher Valued Senior Member

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    What are we, children? hah
    By extending the light cones, I was showing how the author of that picture was determining the angle with which to tilt the cones. Specifically, they were being tilted such that the past light cone would never contain the event horizon. If you want to claim that in an SR world black holes could exist in past light cones...I would completely disagree with you but surrender the point because it has zero bearing on this thread. Forget the picture.

    OK so in post 22 you claim that we can deduce the existence of black holes in our past light cones because of Hawking radiation and "the curvature existing in the past light cone"...first of all, Hawking radiation suffers from the very same logical contradiction. It requires an event horizon to occur. This means that the event horizon must exist in the past light cone of the area experiencing Hawking radiation. Secondly, seeing a dark area "with curvature" is not proof of a black hole, but merely a black hole-like area that which may or may not contain an event horizon. Frozen stars were postulated to be the modern interpretation of black holes before the Kruskal coordinates were (in my opinion) used to improperly interpret the physical insignificance of the event horizon.
     
  12. NotEinstein Valued Senior Member

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    And I pointed out that this results in spacetime getting all bent out of shape over time, which to the best of my knowledge, doesn't happen. A point which you promptly ignored.

    Indeed, what are we, children?

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    I have never made such a ridiculous claim. Please stop putting words in my mouth; it's intellectually dishonest.

    And you are the one bringing up an SR world again and again, while at the same time complaining we should be sticking to a GR world.

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    And so would I.

    Yes, let's. Let's also ignore all those issues I raised with it that you failed to address. Let's indeed run away from that train wreck.

    Yes, that's the point. If there's Hawking radiation, there has to be an event horizon close by.

    Please demonstrate that this must be the case, because I'm not convinced this is true. In fact, I'm pretty sure it's false. Better yet, I'm pretty sure you yourself disagree with it, because of your definition of the past light-cone being all parts of spacetime from which photons can reach the observer.

    Should be easy enough to test. Measure the "strength" of the curvature, and see if there's only one item that fits the bill. Try to fly a spaceship roughly through the area, and see whether it makes it out at the other end (don't forget to check the measurements it took along the way!). Or shine a laser at it, and see whether it can pass through the area as predicted by various models.
    An event horizon is a pretty unique type of thing; shouldn't be too hard to identify using the right tools.

    So, after all this time, your beef is with established GR, even though you've been throwing the praises of "established features of GR" around like a mantra?

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  13. RJBeery Natural Philosopher Valued Senior Member

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    No, a frozen star would appear virtually identical to a proper black hole, but the event horizon would "not quite exist" at the center of its mass.
    No, my belief is that GR has been misinterpreted, not that GR is wrong. Einstein did not ever sign off on black holes, for example. They are a relatively modern (mis)interpretation of GR.
     
  14. NotEinstein Valued Senior Member

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    So you can fly to it, land on it, and fly back to the remote observer. Sounds quite different from a black hole to me!

    Then please demonstrate how the interpretation is wrong (as in: not what GR predicts).

    That's an argument from authority; it carries no weight in the scientific arena.

    Ah, you know it better than all those thousands of experts in the field, got it.

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    And there's nothing modern about them: check your fabled literature for the date on the Schwarzschild solution.

    Also, any chance of a response to the rest of my post?
     
  15. RJBeery Natural Philosopher Valued Senior Member

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    You quote and respond too much. It confuses the discussion for me. And you can't claim "argument by authority" in the same breath as "thousands of experts in the field" with snark and sarcasm. My objection has to do with a logical contradiction and I don't care if The Almighty God himself came down and argued otherwise. Black holes cannot exist in our past light cones in GR, ACCORDING TO GR.
     
  16. NotEinstein Valued Senior Member

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    I'm quoting what you write, and responding to that only once. If I quote and respond too much, you are writing too much.

    Then perhaps you should stick to the topic better?

    Actually, I can. See, you made the argument that you were right, because Einstein said so. That's a classical argument from authority.

    I didn't say you were wrong because of thousands of experts in the field disagreeing with you. I wasn't using it as an argument, and therefor it cannot be an argument from authority. In fact, you did realize this, because you pointed out the (as you call it) "snark and sarcasm", insinuating you know that it wasn't an argument.

    A logical contradiction you claim is there, but you keep failing to prove that it is.

    So... you are saying you are completely close-minded on this topic?

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    And I have never argued otherwise.

    So that's a "no" to responding to all those issues I brought up (and thus a "no" to displaying intellectual honesty)?
     
  17. arfa brane call me arf Valued Senior Member

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    I think introducing this SR-lightcone, malleable-lightcone idea is misleading.

    It's misleading because the spacetime diagram RJBeery posted is of lightcones which look like they've been bent out of shape; this is only because there is a rest frame somewhere in the picture where we can define an "SR" lightcone. Everything is relative to this.

    Moreover, I would say a black hole "existing" in a past (lightcone) is also a bit fraught, because what's inside the event horizon can't be given an "existence", in terms of events. Perhaps this thread is about the semantics of the word "exist".
     
    Last edited: Apr 19, 2019
  18. arfa brane call me arf Valued Senior Member

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    So here I'm going to address the thread with this:

    Saying an event horizon cannot exist in a past lightcone is a bit inaccurate, it should say what's behind an event horizon (after black hole formation) can't be in the past lightcone of an external observer. The difference between behind the horizon and in the external universe presents several problems for physics, which I'm sure you've heard about.
     
  19. RJBeery Natural Philosopher Valued Senior Member

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    Assuming "never argued otherwise" means that you agree that black holes in GR cannot exist in our past light cones, and you agree that we're restricting spacetime to be continuous with no "elsewhere sections" (i.e. any two events are either like-light, space-like or time-like separated) then I believe we are in agreement that there is a logical contradiction as outlined in the OP of this thread.

    I acknowledge that there is a semantic issue here, but I'm not addressing it. I'm pointing out a literal, logical contradiction. The only way to avoid the contradiction is to claim that micro black holes are not created by cosmic rays, as origin did...but then we're left with claiming that the consensus in the field of Astronomy is wrong (and that GR is wrong in its prediction of micro black holes).
     
  20. NotEinstein Valued Senior Member

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    Your words, not mine.

    Again, your words, not mine.

    (Note: without any "elsewhere sections", you don't have GR, or even SR. You've misformulated that.)

    That conclusion doesn't seem to follow, even with your assumptions about my position. Why is it logically impossible?
     
  21. RJBeery Natural Philosopher Valued Senior Member

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    Because a micro black hole in our laboratory yesterday, which we can all agree does not exist today, necessarily exists in our past light cones. This is a requirement of spacetime continuity. There isn't some sort of magical tear in spacetime that places that micro black hole outside of our spacetime fabric, like Purgatory or the 8th dimension of Buckaroo Banzai -- our past is completely enclosed within our light cones. We can walk all around the lab; there isn't a pocket for the black hole to hide within. Either GR is wrong in predicting micro black holes (doubtful), or these micro black holes somehow dissipate prior to their formation (which is what I believe is going on). The irony here is that the folks arguing for the status quo are actually arguing against GR in an attempt to save the existence of black holes.
     
  22. NotEinstein Valued Senior Member

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    This is part of your conclusion, which means your assumptions have to hold for this to be true.

    How so? Are you saying that spacetime isn't continuous on the border of the past light cone?

    Indeed; that's ridiculous, which is why in the context of GR nobody states that.

    I think this is meaningless, because it's a tautology. This is how "our past" is defined.

    Again, this appears to be only true due to your assumptions.

    Why would that be doubtful? You are currently constructing an argument against it on pure GR grounds!

    Do you have any evidence for this?

    (Irrelevant, since I'm not arguing for the status quo.)

    I take it that's a "no response" on the part of my post you quoted?
     
  23. RJBeery Natural Philosopher Valued Senior Member

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    If you don't understand my explanation of the logical contradiction then we'll agree to disagree (although to disagree on something you first have to commit to a position...which you have conspicuously not done). This back-and-forth high school debate format isn't interesting to me.
     

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