Event Horizons in General Relativity

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

  1. NotEinstein Valued Senior Member

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  3. RJBeery Natural Philosopher Valued Senior Member

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    What do you mean "without showing how I got there"? Do you agree that your high school, your computer, and your laboratory are in your past light cone?

    If no, WHY?

    If yes, do you agree that a theoretical micro black hole which was created in your laboratory is also in your past light cone?
     
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  5. Seattle Valued Senior Member

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    I'm sure I'm not as up on GR as you are but indulge me for a moment. What is the basic incongruity with GR that you are most concerned with?

    A light cone (past for future) only relates to light, right? Something can still exist even though you can't see it.

    The Event Horizon isn't a thing. It's a mathematical construct, a point, beyond which, light will not be visible.

    If you remove/dissipate a black hole, there no longer is that measurement known as the Event Horizon.

    What exactly is the issue that you are concerned with?
     
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  7. NotEinstein Valued Senior Member

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    I don't see the argument why the insides of the event horizon not being in ones causal past means the entire thing doesn't exist.

    I agree that those things can be in your causal past, yes.

    Obviously I agree with that, and I have already done that multiple times. Please don't re-ask such silly questions.

    No, it's not. Everything inside the event horizon cannot be in ones causal past; it's the definition of an event horizon.
     
  8. RJBeery Natural Philosopher Valued Senior Member

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    A light cone relates to more than light; it sets an objective ordering of causal events. If a black hole "has dissipated" and is no longer in existence then it has been placed firmly in our past light cones. Black holes cannot exist in our past light cones -- it contradicts the mathematics of GR. Black holes in GR must remain "outside of the causal past of future null infinity" (per Wald's definition), and any sort of dissipative process allows us to place them in our past.

    The contradiction is resolved if we claim that a dissipative process doesn't exist, or that black holes are never fully formed. The LHC Safety Assessment paper suggests that there is a consensus in the Astronomy community that a dissipative process does exist, so I'm left questioning that event horizons are ever formed at all.
     
  9. NotEinstein Valued Senior Member

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    No, the dissipative process allows us to place whatever remains behind in our causal past. By definition, the contents of the event horizon cannot ever be in our causal past.
     
  10. sweetpea Valued Senior Member

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    I don't know what this is suppose to ''prove'', but, I too took the time to play with the light cone picture, and found two cones ( A and B) with the event horizon in their past cones. So, what's suppose to happen now?
    Left picture from post #3 (with my alignments) and right picture is RJ's post #6.

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  11. RJBeery Natural Philosopher Valued Senior Member

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    But this micro black hole was in our laboratory yesterday, and today it is not. I think you're seeing this as a pocket of spacetime that continues to exist in our future light cone, embedded in our past...or something...but that breaks the continuity of spacetime. That is madness.
     
  12. sweetpea Valued Senior Member

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    What happened to the particles from the micro black hole's Hawking radiation? They are left.
     
  13. NotEinstein Valued Senior Member

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    Yes, that is what happens when a black hole evaporates.

    I have no idea what ever gave you that idea. I haven't been talking about the future light cone at all. Please stop making up random things and assuming it sprang from my mind.

    Yes, it indeed is. Which is why I didn't come up with it, nor do I agree with it.
     
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  14. Seattle Valued Senior Member

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    The contradiction is with GR, right? But GR doesn't talk about black holes dissipating does it? That was a Hawking concept.

    Given your scenario, I think I'd be questioning dissipation rather than event horizons. If there is some consensus (what does that really prove in this case) that a dissipative process does exist, I'm sure there is even more consensus that event horizons exist. Right?

    An event horizon is just a mathematically constructed area within which light doesn't escape due to gravity. If there is no event horizon then we would have no problem detecting "black holes" or to put it another way, there would be no black holes. What am I missing?
     
  15. RJBeery Natural Philosopher Valued Senior Member

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    OK, where does that black hole reside in the following picture showing our laboratory "yesterday" where a micro black hole was supposedly created and subsequently evaporated?

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  16. RJBeery Natural Philosopher Valued Senior Member

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    Yes! The problem here is that GR actually predicts micro black holes coming from cosmic rays, while you correctly pointed out that evaporation is not mentioned. Since we all agree that micro black holes are not showing up in our laboratories, as GR suggests they should, I am personally concluding that they are evaporating. The only other option that I see is to claim that GR is wrong in predicting micro black holes. In other words, I don't think the choice is "either black holes exist or evaporation exists" but rather "either GR is wrong or evaporation exists"...and if evaporation exists then black holes do not.
     
  17. Seattle Valued Senior Member

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    I think there is much more evidence for GR than for evaporation. Regarding predicting micro black holes coming from cosmic rays. That could either be a case of something that we just haven't been able to detect so far or it's just a prediction that didn't turn out to a good one.

    I don't see the logic is jumping to the conclusion that there are no black holes, no event horizons, etc.

    This is just you're own spin on things isn't it? I mean, the scientific community's consensus isn't in alignment with your views on this is it?
     
  18. NotEinstein Valued Senior Member

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    That's a picture of a(n SR) light cone. The black hole would indeed be in the past light cone, but that light cone doesn't take into account the curvature of spacetime, and therefore gives the wrong impression. Something which I've already pointed out to you multiple times now. Notice how the black hole would be in the past light cone, but still isn't part of the causal past? That's because "causal past" is the GR generalization of the past light cone, and that's the one that counts in GR.

    This is explained in detail in chapter 31 of Gravitation which I pointed you to repeatedly.
     
  19. RJBeery Natural Philosopher Valued Senior Member

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    Correct, but I've talked to many people and nobody who has intimate knowledge with GR can give me a satisfactory answer. One prominent physicist snidely told me to jump into a black hole if I was so sure they didn't exist. hah

    What you're describing is a physical impossibility. The black hole is completely enclosed within the past light cone; there is no amount of curvature through which it can escape to future null infinity because there is no path for it to do so. The evaporative demise occurs at a specific date and time, beyond which it no longer exists.
     
  20. NotEinstein Valued Senior Member

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    I never said the black hole would escape? Please stop trying to put words in my mouth.

    And I've already explained multiple times why using a(n SR) light cone isn't correct in this situation. Yet you continue to do so. Why?

    Yes, obviously. And I've never disagreed with that, so I have no idea why you are bringing this up.
     
  21. Seattle Valued Senior Member

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    I'm sure you are objective so let me ask you why do you think the consensus doesn't have your viewpoint? Are they not able to understand the inconsistencies? That's not the case, right?

    Are they not as informed as you? That's not the case, so what are they weighing more heavily than you?
     
  22. RJBeery Natural Philosopher Valued Senior Member

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    I don't have these answers, I'm just extremely stubborn when it comes to understanding an answer to something. It's not like I'm getting answers that I don't like, I literally am not getting answers at all. If you look at the literature you'll find that almost all of it is on static black holes -- in other words, it makes no mention of where they came from, or how their coming-into-existence makes any sense. I think that physicists are caught up in studying the mathematics of black holes and perhaps just don't step back and answer the question as to whether they are more than a mere mathematical construct. I might be wrong, but that's my impression.

    Check this out:
    That comes from here: https://www.mathpages.com/home/kmath339/kmath339.htm
    Now, whether a black hole "runs out of time to form" because the universe is closed or because the area evaporates before the process is complete, it is logically inconsistent to say that they exist.
     
  23. Seattle Valued Senior Member

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    It don't think that is correct. It doesn't take an infinite amount of time for a black hole to form and certainly not for something passing the event horizon to become part of the black hole. The later takes seconds.

    Of course our understanding of them is largely just a mathematical construct and I'm sure the explanation (math) does break down when it results in a singularity.

    That's the compatibility problem between quantum physics and GR. No one is disputing that they are both widely successful in what they predict over most of the range for which they were constructed.

    Just because the explanation breaks down as to what happens at the extremes doesn't mean the theory isn't successful.

    I could have an algorithm that predicts how much longer (on average) someone will live once they pass a certain age. I could determine that they will live 20% longer but if you find someone that is 110 years old, that prediction may break down.

    Does that mean that the algorithm isn't useful or largely accurate?

    It seems to me that there is little doubt that black holes exist and if they do the concept of the event horizon is a must. Just because we don't know whether there is an actual singularity or a very small core doesn't change much in reality.

    GR works well but maybe it isn't entirely accurate when something is moving close to light speed with mass. We don't know what happened just before the Big Bang but that doesn't change the validity of all the predictions since the BB.

    Maybe quantum physics is more deterministic than we think (doesn't appear to be the case though). Maybe the probabilities are just because that's all we have rather than reality. The results are still accurate.

    Our view of gravity has changed, in terms of the explanation, but no one questions gravity itself and that it exists and what the results are. I think the same could be said for what is being talked about in this thread.
     
    Last edited: Apr 24, 2019

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