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B Neutron stars and black holes

  1. Apr 20, 2017 at 5:08 PM #1
    I was just wondering if there is anything to suggest that black holes are anything but giant neutron stars cloaked in an event horizon created by thier own gravity. I mean if a neutron star is just on the cusp of having enough mass to be a black hole, and then gains that mass, what's to say it doesn't just gain an event horizon at that point? Or is there a huge explosion and burst of energy as the star collapses into a black hole?
     
  2. jcsd
  3. Apr 20, 2017 at 5:18 PM #2

    Drakkith

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    Hmm... This paper may be of interest to you: https://arxiv.org/pdf/1307.3995.pdf
     
  4. Apr 20, 2017 at 10:06 PM #3
    I have trouble believing there is an actual singularity inside the horizon, and much less trouble believing there is an object of some finite maximum density in its place. Perhaps one of the exotic star candidates, or even the relatively new Plank Star theory.
     
  5. Apr 21, 2017 at 10:07 AM #4
    I don't think anyone believes that there is an actual singularity. Singularities tend to not be physical things, but simply holes in mathematics.
     
  6. Apr 21, 2017 at 10:33 AM #5
    Yes and that means if neutron stars have a maximum mass, and that mass doesn't create a horizon, then some further compressed state of matter must exist, but while we've evidence for thousands of neutron stars, we haven't seen any conclusive evidence of "quark stars" or other exotic stars. Which could potentially mean such stars have horizons.
     
  7. Apr 21, 2017 at 2:05 PM #6
    A compact star with a horizon appears to me to be the most logical answer for what's inside a black hole. Just a ball of disassociated fundamental particles behaving the way models describe the core of a neutron star.
     
  8. Apr 22, 2017 at 8:47 AM #7

    Ken G

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    It's funny how people kind of pick and choose the singularities they are comfortable with. There are certainly plenty of GR theorists of note, such as Kip Thorne, who do take the predictions of GR seriously enough to think there really is a singularity in there. The story goes that what we regard from our external perspective as radius is converted by the extreme curvature into what is locally regarded as time, so then anything that experiences a forward march of time must reach the central singularity (or the ring singularity). In that picture, to say that some kind of exotic matter could resist gravity would be like asserting that it could stop time. Of course we don't really know what happens in there, but people like Kip Thorne do think it's a singularity.

    What's more, the acceptance of singularities is not as rare as you might think-- people seem content with certain types of singularities. For example, many QED theorists are content to imagine that the electron really is a point particle, not just that it is small. Even more common is the idea that photons really have zero rest mass, which is also a type of singularity because then they have no rest frame. So we should at least be consistent-- we should reject all singularities of any stripe, or we should accept that any theory that works which includes singularities raises the possibility of real singularities. The hybrid approach where people pick and choose out of personal taste seems a bit disingenuous to me.
     
  9. Apr 22, 2017 at 9:49 AM #8
    Well Kip Thorne is certainly smarter than me, but I don't see how anyone could be "comfortable" with singularities. They may indeed be real physical objects, but they could just as easily mean our current theories are useless at such extreme scales. And for me, its a lot easier to believe that we don't know what's going on at scales we can't probe, and may never be able to probe.

    And anything that goes on inside an event horizon is unobservable to us, and any theory is potentially unfalsifiable. For all we know the "gravitational singularity" could just be Santa Clause shacking up with the tooth fairy. I suppose a singularity behind the horizon would look identical to us as any other object with a horizon, so that in some sense, its valid to call it a real singularity whether it is or isn't.
     
  10. Apr 22, 2017 at 5:27 PM #9
    It is generally believed that neutron stars can only reach a certain size before they collapse into a "black hole'. Neutron repulsion studies over the years however might suggest that the size limits that are typically imposed of neutron stars may not be accurate;

    https://phys.org/news/2005-12-scientist-neutron-stars-black-holes.html
    http://arxiv.org/pdf/nucl-th/0511051

    Neutron repulsion observations are further supported by careful studies of the structure of neutrons which suggest that while they have a net zero charge, they have an 'Oreo cookie' type structure with outer and inner negatively charged layers with a positively charged layer sandwiched in between the negatively charged layers. That might explain why neutrons tend to repulse one another.

    http://www.eetimes.com/document.asp?doc_id=1167106

    Cosmological studies of neutron stars also show that they can be larger than we first believed, and they can produce powerful polar jets that were previous thought to be restricted to black holes:

    http://www.eetimes.com/document.asp?doc_id=1167106
    http://www.telegraph.co.uk/news/sci...er-detected-is-twice-the-mass-of-the-sun.html
     
  11. Apr 22, 2017 at 5:47 PM #10

    Ken G

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    I think it's fair to say that even Kip Thorne is a bit unclear on what he imagines a "singularity" actually is, but he certainly doesn't view it as just another kind of matter like a quark star. For example, this is the kind of thing he says at http://www.space.com/17086-bizarre-black-holes-kip-thorne-interview.html:

    "The matter of which a star is made, the atoms of which a star is made, are destroyed at the center of a black hole, when the black hole is created. The matter is gone, but the mass, in the sense of mass and energy being equivalent, has gone into the warped space-time of the black hole."

    But he also says:
    "And when you get right to the singularity itself, the laws of physics as we know them break down and the laws of quantum gravity take hold. Since we don't understand those laws very well yet, we can't say what the nature of the very core of the singularity is."

    So he seems to feel that "singularity" means that the laws we know break down, but he also thinks it is something that destroys mass as we know it and turns it into curvature. I conclude that he figures the situation is more than just a new law of physics there, but a very different behavior altogether. This probably means he is just as suspicious of the prejudice that singularities can't really exist, as he is of using GR in situations where you really need some kind of new theory.
     
  12. Apr 22, 2017 at 6:41 PM #11
    I can understand where he's coming from, but my perspective its a bit difficult to imagine a singularity making real physical sense when it arises from a theory almost universally considered to be incomplete. That both GR and QM break down in describing such extreme conditions make me suspicious of the singularity.

    But on the other hand, GR and QM make extraordinarily accurate predictions at all other scales to an extent it becomes scary. That they would be so accurate at everything else and so "incomplete" at those scales could certainly mean that perhaps these infinities DO exist, and we simply don't have the tools to describe them in a way we can make sense of, and it could very well be impossible for us to do so. Math describes the real world almost perfectly, and infinities naturally arise from mathematics. Infinite densities could very well be a reality that we simply cannot understand.

    At some point, I would suspect infinite something would have to have "existed", so why not infinite anything.
     
  13. Apr 22, 2017 at 6:42 PM #12
    I am suspicious that going North from the Earth's North pole makes no sense,
    yet at the equator you can go West or East forever, very suspicious that.
    However I do think it is credible idea that what GR describes as a black hole is a form of matter which is unknown to us.
     
  14. Apr 22, 2017 at 6:52 PM #13
    Indeed, perhaps we should ourselves of longitude.
     
  15. Apr 22, 2017 at 6:54 PM #14

    Ken G

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    Don't confuse coordinate singularities from essential spacetime singularities.
     
  16. Apr 23, 2017 at 3:01 AM #15

    Drakkith

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    Is that what a coordinate singularity is??
     
  17. Apr 23, 2017 at 7:00 AM #16

    Ken G

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    Yes, going north from the North pole is the commonly used example of a coordinate singularity. There is nothing physical about it, it is merely a glitch in some particular way addresses are generated. For example, see https://physics.stackexchange.com/q...rdinate-singularity-and-a-physical-singularit. The Schwarzschild metric has a coordinate singularity at the event horizon, and an essential singularity at the center. There is certainly something funky about the event horizon, given the topological difference between the geodesics that cross it and those that don't, so it's not a complete coincidence that the metric in Schwarzschild coordinates puts a coordinate singularity there, but that's a global property-- there's nothing locally singular there, and a free-faller experiences nothing strange there-- unlike at the center.
     
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