View Poll Results: Does matter have to cross the EH to become part of the BH?

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Thread: Does matter have to cross the event horizon to become part of the black hole?

  1. #61
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    Consider a uniform hollow shell of matter falling within its Schwarzschild radius. There is nothing at the center, space is flat in the interior of the shell, and you will never have a hollow sphere with an empty event horizon inside. The event horizon will appear with a finite, non-zero radius as the shell collapses, so no, not all matter has to cross it to end up inside it. However, there is no scenario I can think of where it'd grow without matter crossing it. The growth of the event horizon and matter crossing it are not two separate things.

    The event horizon is not some physical barrier or object, it is a boundary that becomes meaningful once it contains a certain amount of matter. Its formation is no different from the surface gravity (or more accurately, the surface escape velocity) of an object increasing beyond some threshold as it compresses. The horizon will just appear when a certain threshold is reached, it doesn't have to start out at some point and expand from there. Real black hole-forming objects tend to be densest at the center and an event horizon will form at the center and expand from there, but this isn't fundamental to the nature of event horizons, just to the processes that form them in the real world.

  2. #62
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    Quote Originally Posted by cjameshuff View Post
    Consider a uniform hollow shell of matter falling within its Schwarzschild radius. There is nothing at the center, space is flat in the interior of the shell, and you will never have a hollow sphere with an empty event horizon inside. The event horizon will appear with a finite, non-zero radius as the shell collapses, so no, not all matter has to cross it to end up inside it. However, there is no scenario I can think of where it'd grow without matter crossing it. The growth of the event horizon and matter crossing it are not two separate things.
    I don't know...wouldn't that be all the matter in that shell falling towards the event horizon slower and slower, and never crossing it?
    Formerly Frog march..............

  3. #63
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    Quote Originally Posted by Frog march View Post
    I don't know...wouldn't that be all the matter in that shell falling towards the event horizon slower and slower, and never crossing it?
    Only in the frame of a distant observer, the matter and any infalling observers just go right through the horizon. But even for the distant observers...just don't make the shell completely empty, give it a low density filling, or just a density profile that increases in density toward the surface with a suitable curve. Spacetime's no longer completely flat inside, but there's not enough matter there to form an event horizon of its own before the collapsing shell forms one around it. That interior matter will end up within an event horizon without ever having to cross it.

  4. #64
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    Quote Originally Posted by Frog march View Post
    I don't know...wouldn't that be all the matter in that shell falling towards the event horizon slower and slower, and never crossing it?
    It would appear that way when viewed from afar. An observer falling in with the shell would find it passing the critical radius in a finite time. Such an observer would not detect any discontinuity or change of state, but would simply remain in free fall while becoming more spagettified over the course of the fall. For a supermassive black hole with a relatively gentle gravitational gradient, he would be well inside the Schwarzschild radius before being seriously stretched. All of my remarks are assuming I am not mistaken.

    This hypothetical scenario is more extreme than my uniformly dense contracting spherical body, and a sprinkling of matter in the center would already be inside the event horizon by the time the body reaches black hole conditions, without having crossed anything that was a functional event horizon. I feel reasonably certain that no such thing will happen on a large scale in the real universe, and that Luciano Rezzolla's findings will be valid for a dying star for all practical purposes.

  5. #65
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    Quote Originally Posted by cjameshuff View Post
    Only in the frame of a distant observer, the matter and any infalling observers just go right through the horizon. But even for the distant observers...just don't make the shell completely empty, give it a low density filling, or just a density profile that increases in density toward the surface with a suitable curve. Spacetime's no longer completely flat inside, but there's not enough matter there to form an event horizon of its own before the collapsing shell forms one around it. That interior matter will end up within an event horizon without ever having to cross it.
    I'm not sure. Say you stick a neutron star inside the shell, then the added curvature inside the shell will extend beyond the shell. Will that alter the radius at which time dilation is infinite?
    I would think it moves it outwards...although that doesn't seem to alter the situation.
    If time dilation rises as the shell collapses then the time dilation in the core of the neutron star would be the place where the time dilation was the greatest? So the surface of infinite time dilation should start inside the core of the neutron star, in side the shell, and expand from there..?
    Formerly Frog march..............

  6. #66
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    Quote Originally Posted by Hornblower View Post
    It would appear that way when viewed from afar. An observer falling in with the shell would find it passing the critical radius in a finite time. Such an observer would not detect any discontinuity or change of state, but would simply remain in free fall while becoming more spagettified over the course of the fall. For a supermassive black hole with a relatively gentle gravitational gradient, he would be well inside the Schwarzschild radius before being seriously stretched. All of my remarks are assuming I am not mistaken.

    This hypothetical scenario is more extreme than my uniformly dense contracting spherical body, and a sprinkling of matter in the center would already be inside the event horizon by the time the body reaches black hole conditions, without having crossed anything that was a functional event horizon. I feel reasonably certain that no such thing will happen on a large scale in the real universe, and that Luciano Rezzolla's findings will be valid for a dying star for all practical purposes.
    Yeah, you would need a spherical shell of matter...probably dust or something to allow it to freely fall inward...with a total mass on the order of a galaxy or so, enough that the density of the resulting black hole would be lower than a giant hollow sphere of mostly vacuum. It would collapse directly into a black hole larger than any known supermassive black hole, without so much as heating up notably first. This is not a plausible configuration to encounter in the real world, it's a contrived setup only meant to illustrate that the laws of physics do not prevent matter from starting out inside the event horizon.

  7. #67
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    regardless of how big a cloud of dust was to start with, for the very distant observer, the surface of infinite red-shift would start somewhere in the centre, would it not?

    And for a sphere of uniform density, the same would apply?
    Formerly Frog march..............

  8. #68
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    Quote Originally Posted by Frog march View Post
    I'm not sure. Say you stick a neutron star inside the shell, then the added curvature inside the shell will extend beyond the shell. Will that alter the radius at which time dilation is infinite?
    I would think it moves it outwards...although that doesn't seem to alter the situation.
    If time dilation rises as the shell collapses then the time dilation in the core of the neutron star would be the place where the time dilation was the greatest? So the surface of infinite time dilation should start inside the core of the neutron star, in side the shell, and expand from there..?
    In that setup, the neutron star is unchanging. Its density never reaches a point where it fits within its Schwarzschild radius. It remains a normal neutron star until the shell collapses to the Schwarzschild radius of their combined masses.

  9. #69
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    Quote Originally Posted by cjameshuff View Post
    In that setup, the neutron star is unchanging. Its density never reaches a point where it fits within its Schwarzschild radius. It remains a normal neutron star until the shell collapses to the Schwarzschild radius of their combined masses.
    yes, but what about the surface of infinite red-shift? The co-ordinate must start inside the core of the neutron star, for the distant observe, mustn't it.?
    Formerly Frog march..............

  10. #70
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    Quote Originally Posted by Frog march View Post
    regardless of how big a cloud of dust was to start with, for the very distant observer, the surface of infinite red-shift would start somewhere in the centre, would it not?
    It would not.


    Quote Originally Posted by Frog march View Post
    And for a sphere of uniform density, the same would apply?
    I think uniform density is a simplification that has the misfortune of being a limiting case for two different results. There is a continuum of possible cases from the center being less dense and the event horizon first appearing as a sphere to the center being more dense and the event horizon growing from a point. Getting arbitrarily close to uniform density from one side will have one result, from the other side you get the other.


    Quote Originally Posted by Frog march View Post
    yes, but what about the surface of infinite red-shift? The co-ordinate must start inside the core of the neutron star, for the distant observe, mustn't it.?
    Again, the whole point of the example is to show that it would not.

  11. #71
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    Quote Originally Posted by cjameshuff View Post

    Again, the whole point of the example is to show that it would not.
    why not? You have a time dilation gradient from outside the shell to a very high time dilation at the shell, and then you have a shallow gradient to the neutron star, and then a steep gradient into the core of the neutron star, so all I can see is that it is greatest inside the core of the neutron star.
    Formerly Frog march..............

  12. #72
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    What about Hawking radiation?

    As has been stated upthread, any realistic collapsing body is densest at its centre. The event horizon will start there, with most material outside.

    The BH as it forms is initially microscopic (shot-lived). It then basically explodes in a burst of intense radiation.

    That will blow infalling matter further away.

    When the Hawking radiation burst is finished, matter falls back and the micro BH forms again. Another burst of radiation and the process repeats.

    This could go on for a long time, with repeated and very rapid cycles of micro-BH formation and rapid BH evaporation.

    The surrounding layers of matter absorb the radiation and become hotter and hotter as the cycles continue.

    As to what happens next, could be a variety of options depending on starting conditions. It may even be that a substantial proportion of the matter is eventually converted into radiated energy. Whatever the route, the end result is the object no longer has enough mass to collapse to a BH, and the cycles stop.

    No long-lasting BH is formed.

    How's that for a theory?

  13. #73
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    Quote Originally Posted by kzb View Post
    What about Hawking radiation?
    yes,
    here is a Hawking Radiation calculator.

    http://xaonon.dyndns.org/hawking/


    For a mini black hole of mass 1 ton, the radius would be 1.484852e-15 nano meters,(an atom is 0.5 nanometers across) and the luminosity would be 3.563442e+20 megawatts.

    but the surface gravity would be 3.026414e+40 m/s/s so that would be quite a weight pressing down.
    Last edited by WaxRubiks; 2017-Aug-01 at 12:29 PM.
    Formerly Frog march..............

  14. #74
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    Quote Originally Posted by Frog march View Post
    yes,
    here is a Hawking Radiation calculator.

    http://xaonon.dyndns.org/hawking/


    For a mini black hole of mass 1 ton, the radius would be 1.484852e-15 nano meters,(an atom is 0.5 nanometers across) and the luminosity would be 3.563442e+20 megawatts.

    but the surface gravity would be 3.026414e+40 m/s/s so that would be quite a weight pressing down.
    But what I mean is, there must be a micro-BH mass at which the luminosity would repel matter in any given situation. This is because the EH starts off almost infinitely small. I am questioning if any BH with an appreciable lifespan could ever form in the real universe.

    Perhaps the answer lies with time, I guess the EH can expand faster than light whereas Hawking radiation cannot. Maybe that is the answer and it allows BH's to grow until they have sufficient lifespan.

  15. #75
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    Quote Originally Posted by kzb View Post
    But what I mean is, there must be a micro-BH mass at which the luminosity would repel matter in any given situation. This is because the EH starts off almost infinitely small. I am questioning if any BH with an appreciable lifespan could ever form in the real universe.

    Perhaps the answer lies with time, I guess the EH can expand faster than light whereas Hawking radiation cannot. Maybe that is the answer and it allows BH's to grow until they have sufficient lifespan.
    A black hole will only expand at the rate matter falls into it(If matter has to cross the EH for the BH to grow), so take a star of 350,000 km radius, which would end up as a BH of radius around 3km, then the rate at which the BH would grow would be 3/350,000th the speed at which the star was collapsing....I think.



    --------------
    But...
    The massive Hawking radiation thing was one of the arguments against a CERN BH being dangerous. It would have supposed to evaporate before it would have a chance to grow.
    Last edited by WaxRubiks; 2017-Aug-01 at 01:00 PM.
    Formerly Frog march..............

  16. #76
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    I'm late to this thread an it hurts my head to try to read through it following the arguments, but I think it's worth pointing out that there's a pressure term in the GR equation for gravity, which may help address concerns about the zero-radius black hole appearing at the centre of a collapsing star. Pressure causes gravity. And the zero-radius point at the centre of the collapsing star will be associated with a specific (and very high) pressure, even if it has no mass.

    An increase in pressure will increase gravity, which will expand the event horizon. That's the essence of the point about the event horizon expanding outwards, recruiting new mass, rather than just sitting there waiting for mass to fall in.

    Grant Hutchison
    Last edited by grant hutchison; 2017-Aug-01 at 05:00 PM. Reason: Last para
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  17. #77
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    Frog march,

    The paper referred to says "the event horizon starts from a zero
    radius and then progressively grows to reach R=2M", but I don't
    know all of the assumptions behind that conclusion. I suspect
    that the gravitating object is treated as a mathematically perfect
    entity, with a precisely located center and absolute symmetry
    around that center. The density of such an object would increase
    smoothly to the center, which would be infinitesimally more dense
    than any other part of the star. In any real object, the density
    would vary chaotically, in time as well as in space. Relatively
    large volumes would have about the same density throughout
    the volume. Such a volume would reach the critical density at
    which an event horizon begins to exist. Density is a property of
    matter in a volume, not a property of a point. In any real object,
    one or more volumes near the center would instantaneously be
    within an event horizon when the density of matter in that volume
    reaches the critical value. That would only happen when the object
    is collapsing, so the density everywhere is increasing and separate
    regions enclosed by event horizons would rapidly grow and merge.

    I think you are looking at the formation of an event horizon as too
    much like a precise geometric construction and not enough like a
    messy physical event.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  18. #78
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    Remember that event horizon is not a 'thing'; it's not defined by any physical properties. It is simply a mathematical volume that shows us where gravity is so strong that light cannot escape it.

    So, there's nothing wrong with envisioning a collection of atoms being squished by not only their own gravity but the weight of gigatons of mass on top of it, then, at some point, the matter gets so dense that light inside that area has no point that leads to outside.

    It is not a physical property that is expanding at the speed of light; it is simply a geometry that defines - for us to imagine - where light can't escape. The lgiht stll travels, it's just that its path curves back on itself.

    So: There should not be any confusion about some sphere wherein some atoms are inside and some are outside and they experience different things.

    Nor should there be any confusion about whether light behaves differently on either side. It's a continuum. This photon is on a curved path but does not intersect the sphere again, whereas this one, a bit further in, is on a more curved path, and does intersect the sphere (i.e. never leaves).

    If you're thinking of different properties, then you're taking the event horizon too literally.

    Attachment 23310
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    Last edited by DaveC426913; 2018-May-10 at 04:57 PM.

  19. #79
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    Quote Originally Posted by etlini View Post
    Considering how black holes form from the collapse of a spinning star, or with gas spiralling into the black hole, we expect that most black holes would spin (have angular momentum).

    This distorts the shape of the event horizon into an ellipsoid, just like the spinning Earth has a tidal bulge at the equator.
    Well, it depends on the coordinates you choose, and what you decide to plot as defining the "shape" of the event horizon - the curvature values are incompatible with radius values, for instance, because spacetime is so strongly distorted in the vicinity of a black hole.

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    Quote Originally Posted by DaveC426913 View Post
    Nor should there be any confusion about whether light behaves differently on either side. It's a continuum. This photon is on a curved path but does not intersect the sphere again, whereas this one, a bit further in, is on a more curved path, and does intersect the sphere (i.e. never leaves).
    Though that isn't what defines the event horizon. There is a very specific point at which photons can make no progress outwards at all, and below which even outward directed photons fall inwards. That's the horizon.

    Grant Hutchison
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  21. #81
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    Quote Originally Posted by grant hutchison View Post
    Though that isn't what defines the event horizon. There is a very specific point at which photons can make no progress outwards at all, and below which even outward directed photons fall inwards. That's the horizon.

    Grant Hutchison
    Isn't that what I said? It's what I meant.

    I added a diagram. It's simplified, granted*, but I hope it gets the point across.

    *see what I did there?

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    Quote Originally Posted by DaveC426913 View Post
    Isn't that what I said? It's what I meant.

    I added a diagram. It's simplified, granted*, but I hope it gets the point across.
    It's the purple arrows going out and then falling back again, in the right diagram, that are the problem. In the Schwarzschild metric, no photon within the event horizon can make any outward progress at all. There really is an absolute difference between the region outside the horizon (in which the radial direction is spacelike and it's possible for photons to escape) and the region inside the horizon, in which the radial direction is timelike, and everything always moves towards the singularity.
    Here's what the photon trajectories should look like, either side of the event horizon:
    photonseventhorizon.jpg
    (If the black hole is accreting, there is a region between the apparent and absolute horizon in which photons can make temporary outward progress, but that region does not exist in a non-accreting black hole.)

    Grant Hutchison
    Last edited by grant hutchison; 2018-May-10 at 07:20 PM. Reason: diagram
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  23. #83
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    Quote Originally Posted by WaxRubiks View Post
    Yes or no..?

    No it doesn't. In fact, some theories of black holes suggests there is no ''inside'' as such and in fact it would be like hitting a brick wall in space. These are holographic theories of black holes which contain all their information on the horizon.

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    Quote Originally Posted by Dubbelosix View Post
    No it doesn't. In fact, some theories of black holes suggests there is no ''inside'' as such and in fact it would be like hitting a brick wall in space. These are holographic theories of black holes which contain all their information on the horizon.
    That is really not what those theories mean. Those theories say that the contents of a volume of space can be fully described by the surface of that volume, that the complete information content of the matter that has fallen into the black hole is available at the horizon and no information is destroyed by matter falling through the horizon. They aren't at all saying that the interior doesn't exist or that the surface is any sort of obstacle.

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    Quote Originally Posted by cjameshuff View Post
    That is really not what those theories mean.

    You will find one exact theory out there that does treat it this way. Also systems that not only encode information on the horizon have also been modeled with interiors, but that is not what I am talking about. A theory does exist where the idea of the interior is removed altogether. You can only do this because black holes are one of a few mathematical objects capable of doing this.

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    '' Recently, however, the hypothesis that the BH interior does not exist has been gaining traction... ''

    https://arxiv.org/abs/1701.07444

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    Quote Originally Posted by Dubbelosix View Post
    You will find one exact theory out there that does treat it this way. Also systems that not only encode information on the horizon have also been modeled with interiors, but that is not what I am talking about. A theory does exist where the idea of the interior is removed altogether. You can only do this because black holes are one of a few mathematical objects capable of doing this.
    Can you provide a reference to this one theory? Thanks.

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    Quote Originally Posted by Strange View Post
    Can you provide a reference to this one theory? Thanks.
    There has been one provided, only a few posts back, was in fact the last post before the one you posted.

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    That's just one reference I hastily found the other day, I do read a lot, especially physics papers. This idea has been around for not too long... personally, I believe black holes have interiors. But as I said, it is one of those strange mathematical objects that can encode everything on the boundary potentially.

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    Quote Originally Posted by Dubbelosix View Post
    There has been one provided, only a few posts back, was in fact the last post before the one you posted.
    I looked at that. It is about what states of matter could exist in a black hole and what information gravitational waves could give us about that. It makes passing reference to various other models but nothing very specific. And certainly nothing like "hitting a brick wall".

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