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Gravitational collapse

  1. Dec 11, 2009 #1
    hi, I have a question.

    Is it a reasonable assumption that there is no force that will prevent collapse to a singularity?

    Without knowledge of the physics involved, and with the impossiblity of singularities in nature (which again might be suspect reasoning), why are black holes considered to be a real phenomenon?

    Does anybody think black holes are probably impossible? and is there any chance that particle accelerators could answer this question?
     
  2. jcsd
  3. Dec 11, 2009 #2
    Singularities may or may not exist (we need quantum theory of gravity to figure out what exactly is going on), but there's no physical reason why you can't have enough mass packed into a small area of space to create an event horizon. Once you have an event horizon, it looks like a black hole for all intents and purposes, for all external observers.
     
    Last edited: Dec 11, 2009
  4. Dec 11, 2009 #3

    Chronos

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    I think it is unreasonable to assume no physical forces can prevent a singularity. Nature abhors infinities. Even by modern physics, a singularity would take forever to form. An event horizon, however, can form in temporal space. It takes much less time. Appears I am agreeing with hamster.
     
  5. Dec 11, 2009 #4
    Thanks for the interesting replies. What you say about singulraties taking forever to form makes a lot of sense. Forever being until the end of time?

    So when time is done, the singularities can exist.
     
  6. Dec 11, 2009 #5
    Time is relative, even in SR, and especially in GR.

    What 'Chimps' means is probably that an external observer (someone entirely on the outside of the black hole) will never see the singularity form. From his point of view, the "black hole" always appears to be frozen at the last stage of formation.

    An observer on the collision course with the black hole will see things differently. He will hit the singularity (or whatever there really is at the center of the black hole) in finite time.

    The lesson of GR is that spacetime is no longer the trivial R^3 x R^1, it's something entirely different. Even if GR's predictions with regard to singularities are wrong. Things are sufficiently complicated, even if singularities can't really form because of some yet unknown quantum-level behavior.
     
  7. Dec 11, 2009 #6
    So if I fell into a black hole I would experience the end of time along with everything else that ever fell in to every black hole.
     
  8. Dec 11, 2009 #7
    I believe this had been discussed a lot in multiple threads - singularities DO NOT take forever to form. This is true only in the coordinate system of the distant observer. Free falling observer reaches singularity in finite (and quite short) time.
     
  9. Dec 11, 2009 #8
    This is not true either, while for the distant observer it takes forever for your light to reach him, so while for the distant observer you look 'frozen', still you dont see the all the future history of the universe.

    You will be see some of the events outside of the Black hole AFTER you crossed the horizon (because light from these external events will also fell inside and reaches you when you are already inside the horizon) - but not for a long time.
     
  10. Dec 11, 2009 #9
    But surely if I reach the singularity that would have to mean infinity was reached and therefore time doesn't exist anymore. Therefore everything else has also come in as well. Why am I wrong here? I am confused.
     
  11. Dec 11, 2009 #10
    You are wrong at "Therefore everything else has also come in as well"

    Time is not global
    In 'classical' GR (without Quantum Gravity, so with infinities and singularities) worldlines end at singularity. So what? How does it affect the world outside?
     
  12. Dec 11, 2009 #11
    I'm not talking about the world outside though. I'm talking from my perspective and i'm at a singularity where time doesn't exist. So how could it come in after me?
     
  13. Dec 11, 2009 #12
    I dont understand your question

    For the falling observer singularity is in the future. You approach singularity and tidal forces increase, ripping you apart sooner or later.

    In GR, time exist everywhere except the singularity, where physical laws fail to predict anything. So time exists everywhere physical laws are applicable.
     
  14. Dec 11, 2009 #13
    Think about it. You are saying that I wouldn't see much of the future as I fell in and reached the singularity. Time would not exist at the singularity so logically nothing can reach it before or after anything else that fell into it.
     
  15. Dec 11, 2009 #14

    Wallace

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    The biggest misconception is thinking of 'a singularity' as a thing, a point, an object. This is not the case. A singularity is a mathematical phenomenon, like a divide by zero error. It is a situation where the maths of your theory gives you a non-sensical result. Dmitry has done a fine job of trying to explain this (as well as the other common misconception about black holes taking an infinite time to form). I know that it's pop-sci bread and butter to muse about 'the singularity' as if it was something actually predicted by a working theory, but none the less it's nonsense.

    It's meaningless to talk about the 'singularity', but what does make sense to speak of is the event horizon, that is something that is real, and something that has been observed (at least the consequences of it). You can even sensibly work out what an observer sees and feels once they have passed the event horizon (after which they cannot leave again). However once your radial co-ordinate goes to zero the game is over, GR stops telling you anything meaningful.
     
  16. Dec 13, 2009 #15

    Chronos

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    I agree to the extent that time becomes meaningless to an infalling observer upon reaching the singularity.
     
  17. Dec 17, 2009 #16
    With all due respect, I can't see how Dmitry, or yourself, has done a fine job at all.

    As you approach the singularity then all time must compress so that everything reaches the singularity together. How can it be any other way? There is no point at which it can go from being not a singularity to a singularity. So I agree with Chronos on this one.
     
  18. Dec 17, 2009 #17

    Wallace

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    As I've tried to explain, there is no such thing in nature as a 'singularity'. It is something that can occur in mathematical equations, and if those equations describe physics it tells you that you're theory is not behaving sensibly at some point.

    Musing about what goes on 'at the singularity' is like asking what it would be like to be minus eleventy six years old, or how long your toes would be if you didn't have feet. These things are not well posed concepts, so you can't rationally discuss them. The only thing we do know exists (or are at least pretty sure of, based on observations) is the event horizon of black holes. We can sensibly talk about what will and won't be seen by observers crossing those. But not 'singularities'.
     
  19. Dec 17, 2009 #18
    I'm not talking about the singularity.

    Of course we can't know the physics of a singularity. I understand that, but we can still speculate about what is happening between the event horizon and the singularity. That is what I was referring to when I said that time must compress for the observer heading towards a singularity.
     
  20. Dec 17, 2009 #19

    Wallace

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    You have to be careful when you are talking about 'time'. You can only talk about how time is altered by speaking in relative terms, your own time is always just your time and never changes. In this case, if you fall into a black hole you do not see, the time tick rate of someone else, say a distant observer, compress to an infinitely slow (or fast?) rate.

    Have a look at http://casa.colorado.edu/~ajsh/quiz.html" [Broken] site, particularly the answer to quiz question number 5. Note that site uses some unfortunate terminology, like 'what happens when you get to the singularity' but all the observational stuff is spot on. Some good movies and pics to have a look at as well.
     
    Last edited by a moderator: May 4, 2017
  21. Dec 17, 2009 #20
    I had a look and it agrees with you and Dmitry but I think it is incorrect.

    I think you are making a logical error with regards to your understanding of time.

    We all agree that we can't know what happens at a singularity, but we seem to disagree about what you would observe as you approach a singularity. There is no point at which a singularity can be reached in time, but an observer can head towards it. Time would compress and so the the idea of a singularity forming or being reached in finite time is false.
     
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