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Thread: Question about Black Holes and singularities

  1. #1 Question about Black Holes and singularities 
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    Imagine a star went supernova and was 1kg short of being a black hole, so instead it became a neutron star.
    Then an asteroid hit the neutron star putting it just over the mass limit.
    Would we expect that an event horizon would form just outside the surface of the neutron star, so it would suddenly become dark, whilst remaining a neutron star?
    Why is it necessary to also undergo additional collapse into a singlularity at the same time that an event horizon forms just outside its surface?

    Thanks



    I kind of asked the above on the introductions page which was probably the wrong place:

    I am a new member and I joined to ask a quick question about black holes:
    I read a few years back that black holes do not have to be singularities, and that even a body of water with a diameter as large as the orbit of Pluto would be a black hole.
    Is this true, do black holes have to be singularities?
    For example if a neutron star became large enough that an event horizon formed just outside the diameter of the neutron star then it would be a black hole without having to be a singularity.
    It would seem like a coincidence if the point of collapse to a singularity also happened at the same point as an event horizon forming.

    Wiki only describes black holes with singularities:
    https://en.wikipedia.org/wiki/Black_hole

    However here they say that the observable universe's mass has a Schwarzschild radius of approximately 13.7 billion light years, and the universe is not a singularity.
    https://en.wikipedia.org/wiki/Schwarzschild_radius
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  2. #2  
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    The estimated maximum mass for a neutron star is ~3.2 solar masses and would have a radius of ~18 km.
    The Schwarzchild radius for an object of that mass is 9.44 km.

    So what happens if you add just that little bit of matter to a neutron star just sitting on the edge of maximum mass, is that you push it over the tipping point of where it interior can support the weight of the star and it starts to collapse in on itself until it shrinks below its Schwarzchild radius and forms the black hole. The process of collapse starts before the black hole is formed.
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  3. #3  
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    Janus, thanks for reply.

    So saying that black holes can be made up of large amounts of ordinary matter, like for example a body of water, is not useful because they would immediately collapse to something close to a singularity?
    Perhaps they meant that in such examples a black could form as a result of these bodies.

    Is it also correct that no one currently knows how small the resulting object is after collapse to a BH? I presume that a true singularity is not a possibility?
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    Quote Originally Posted by jademann View Post
    Janus, thanks for reply.

    So saying that black holes can be made up of large amounts of ordinary matter, like for example a body of water, is not useful because they would immediately collapse to something close to a singularity?
    Perhaps they meant that in such examples a black could form as a result of these bodies.

    Is it also correct that no one currently knows how small the resulting object is after collapse to a BH? I presume that a true singularity is not a possibility?
    At present, we know of nothing that would prevent the collapse all the way down to a singularity. But a singularity just represents where the laws of Physics as we understand them break down. It is expected that a theory that combines QM and GR will avoid that singularity.
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  5. #5  
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    Do gravitational waves interact in any special way with Black Holes?

    Is it all just grist to the mill?
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