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. #1
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    Does matter have to cross the event horizon to become part of the black hole?

    Yes or no..?
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    Why is this a poll when the answer is one of physics?

    Although, it could depend on how you define "part of the black hole". Would you include the accretion disk, for example, as "part of" a black hole. If so, then the answer is no.

    But if by "part of the black hole", you mean "within the event horizon" then the answer is obviously yes.

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    But what about at the start of the formation of a black hole..? Before an event horizon has formed, then how does matter cross one?
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    There was a recent thread about this, I think, where Grant Hutchison(?) explained how the event horizon grows from the centre at light speed.

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    Quote Originally Posted by Strange View Post
    There was a recent thread about this, I think, where Grant Hutchison(?) explained how the event horizon grows from the centre at light speed.
    Yes, but how does it start?

    Take 100grams of matter, which would be tiny amount of matter at the centre of a collapsing star, at what point does it become inside an event horizon? How does it cross an event horizon that hasn't formed yet?
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    yes, it starts small, but as I asked, how does any matter fall through an event horizon before one forms? All right take the starting event horizon, suddenly 100 atoms are inside an event horizon; how did they get there? Did they cross an event horizon, with nothing inside it, or did they just suddenly appear inside one?
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  8. #8
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    matter cannot fall through an event horizon before it forms, it is physically and logically impossible.
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    Quote Originally Posted by Frog march View Post
    yes, it starts small, but as I asked, how does any matter fall through an event horizon before one forms?
    As the material in the linked thread says, the event horizon expands and "swallows" the matter.

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    Quote Originally Posted by Strange View Post
    As the material in the linked thread says, the event horizon expands and "swallows" the matter.
    Yes, but how does the event horizon start? It's a bit like evolution of life on the planet, we know quite a bit about that process, but not about the actual start.

    For an event horizon to exist, there has to be something inside it; some matter. So how did it get there, if there isn't already an event horizon?
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    Look at the diagram that Grant mentions (if you can't follow the math - much of it is over my head). The event horizon forms and starts to expand when the collapsing star meets the appropriate conditions. It then grows to enclose the material that caused it form.

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    I think the issue Frog March has is highlighted in this quote:
    An important property to deduce from (57) is that the event horizon starts from a zero radius...
    How much mass is located inside the zero radius which results in an event horizon? Can't divide by zero. So instead, how much mass is located in a radius equal to 1/10000000 the diameter of an electron which results in an event horizon? What exactly is that particle? If there's no such particle, then there can't be an event horizon that small. So what is the smallest possible event horizon, assuming particles packed as tightly as possible to fit into and define that horizon?
    Depending on whom you ask, everything is relative.

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    This thread has become such a hodgepodge of what I think are errors that I will not try to address each one individually. Let me give my take on the topic.

    Of course a sample of matter cannot cross an event horizon that does not yet exist. That does not mean it cannot be at or near a central location before further collapse of a surrounding spherically symmetrical body eventually creates the conditions that constitute a black hole. We can do thought exercise mathematical modeling of bodies that may or may not be physically possible in real life.

    Suppose a hypothetical collapsing spherical body is of uniform density at all times. In that case there will be no event horizon anywhere until the body has contracted to the Schwarzschild radius for its mass, at which time it will become a black hole with an event horizon at its surface. That is because the smaller amount of matter inside a fraction of the total radius would have to be more dense to create black hole conditions on its own. I think it is reasonable to say that the event horizon did not exist before this point in time, so all of the mass was inside it at the moment when it came into existence.

    Now suppose the body had most of its mass concentrated very close to the center, as in a typical star. If that core is dense enough, it could get inside its Schwarzschild radius while the envelope is still outside. Now we can have an interior event horizon that will expand as it eats the envelope. By letting the initial central density increase without limit, we could in principle make the initial event horizon have an arbitrarily small radius.

    Clear as mud?

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    Quote Originally Posted by Hornblower View Post
    This thread has become such a hodgepodge of what I think are errors that I will not try to address each one individually. Let me give my take on the topic.

    Of course a sample of matter cannot cross an event horizon that does not yet exist. That does not mean it cannot be at or near a central location before further collapse of a surrounding spherically symmetrical body eventually creates the conditions that constitute a black hole. We can do thought exercise mathematical modeling of bodies that may or may not be physically possible in real life.

    Suppose a hypothetical collapsing spherical body is of uniform density at all times. In that case there will be no event horizon anywhere until the body has contracted to the Schwarzschild radius for its mass, at which time it will become a black hole with an event horizon at its surface. That is because the smaller amount of matter inside a fraction of the total radius would have to be more dense to create black hole conditions on its own. I think it is reasonable to say that the event horizon did not exist before this point in time, so all of the mass was inside it at the moment when it came into existence.

    Now suppose the body had most of its mass concentrated very close to the center, as in a typical star. If that core is dense enough, it could get inside its Schwarzschild radius while the envelope is still outside. Now we can have an interior event horizon that will expand as it eats the envelope. By letting the initial central density increase without limit, we could in principle make the initial event horizon have an arbitrarily small radius.

    Clear as mud?
    so matter doesn't always have to cross an event horizon to be inside it?
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    Quote Originally Posted by Frog march View Post
    so matter doesn't always have to cross an event horizon to be inside it?
    Right, and you shouldn't think of the Event Horizon as being a magical boundary as shown in the TV series and movie "Star Gate". In the usual models related to General Relativity, an observer robust enough to stay in one piece near the EH wouldn't see local phenomena showing the boundary. That being said, your question about Event Horizon formation, to me, points out one of my big concerns about the "Fire Wall" models of black holes.
    Forming opinions as we speak

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    so does the event horizon start with zero radius, or no...?
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    Quote Originally Posted by Frog march View Post
    so does the event horizon start with zero radius, or no...?
    To expand upon my previous post, I would say no. The conditions for starting with zero radius would be mathematically undefined. As I pointed out, we can make the initial radius arbitrarily small by making a relatively small amount of mass exceedingly dense.

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    Quote Originally Posted by Hornblower View Post
    To expand upon my previous post, I would say no. The conditions for starting with zero radius would be mathematically undefined. As I pointed out, we can make the initial radius arbitrarily small by making a relatively small amount of mass exceedingly dense.
    but how would that matter get inside the event horizon? It is just in ordinary space, and then is inside an event horizon?
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    Quote Originally Posted by Frog march View Post
    but how would that matter get inside the event horizon?
    Gravity caused it to collapse to the state in question.
    It is just in ordinary space, and then is inside an event horizon?
    Yes.

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    Quote Originally Posted by Frog march View Post
    but how would that matter get inside the event horizon? It is just in ordinary space, and then is inside an event horizon?
    Because the event horizon is expanding. Passing through an event horizon, either because you fall through or it expands past you (or, more realistically, both) is a perfectly unremarkable occurrence. (I say that because I don't know what you mean by "ordinary space". As far as we know, the space inside an even horizon is also "ordinary".)

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    Quote Originally Posted by Frog march View Post
    so matter doesn't always have to cross an event horizon to be inside it?
    Yes, it does.

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    Quote Originally Posted by Strange View Post
    Because the event horizon is expanding. Passing through an event horizon, either because you fall through or it expands past you (or, more realistically, both) is a perfectly unremarkable occurrence. (I say that because I don't know what you mean by "ordinary space". As far as we know, the space inside an even horizon is also "ordinary".)
    Someone said that the time dimension becomes space-like, or something. Doesn't sound very ordinary to me...
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    Quote Originally Posted by Strange View Post
    Because the event horizon is expanding. Passing through an event horizon, either because you fall through or it expands past you (or, more realistically, both) is a perfectly unremarkable occurrence. (I say that because I don't know what you mean by "ordinary space". As far as we know, the space inside an even horizon is also "ordinary".)
    I think what this means is that since the event horizon (EH) is defined by the mass contained inside, an expanded EH requires additional mass - initially outside the original EH. The total mass defines a new EH mathematically, but it only exists as physical thing when both masses are contained inside the new EH. So, for example, the Schwarzschild radius for the sun is about 3KM. But the mass at the center of the sun currently within that 3KM radius is not in a black hole, because to be a black hole of that size, all of the Sun's mass would have to be inside it. If the Sun collapsed for some reason, it would not exist as a black hole until all its mass were inside that 3KM radius. When it is at 6 KM, there is no BH. When it is at 4 KM, no BH. Not until it is at 3KM does it define a black hole. Two such masses, have a mathematical EH of about 6KM. But until both masses are contained within that 6KM radius, you don't have a 6KM BH.
    Depending on whom you ask, everything is relative.

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    Quote Originally Posted by mkline55 View Post
    I think what this means is that since the event horizon (EH) is defined by the mass contained inside, an expanded EH requires additional mass - initially outside the original EH. The total mass defines a new EH mathematically, but it only exists as physical thing when both masses are contained inside the new EH. So, for example, the Schwarzschild radius for the sun is about 3KM. But the mass at the center of the sun currently within that 3KM radius is not in a black hole, because to be a black hole of that size, all of the Sun's mass would have to be inside it. If the Sun collapsed for some reason, it would not exist as a black hole until all its mass were inside that 3KM radius. When it is at 6 KM, there is no BH. When it is at 4 KM, no BH. Not until it is at 3KM does it define a black hole. Two such masses, have a mathematical EH of about 6KM. But until both masses are contained within that 6KM radius, you don't have a 6KM BH.
    I think that is not quite right. You can take any sphere from the centre and work out the Schwarzschild radius for it, and since it is going to be more dense in inner spheres of a collapsing star, they will be within that radius, before the whole star is.
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    Quote Originally Posted by Frog march View Post
    I think that is not quite right. You can take any sphere from the centre and work out the Schwarzschild radius for it, and since it is going to be more dense in inner spheres of a collapsing star, they will be within that radius, before the whole star is.
    But it is right.

    An event horizon marks the point where points inside versus outside are no longer causally connected. If the part of the sun within its Schwarzschild is not causally connected to the outside, how do you plan to explain that the energy of fusion is still released from the center??? Therefore a Schwarzschild radius and an event horizon are not the same thing.

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    Quote Originally Posted by ShinAce View Post
    But it is right.

    An event horizon marks the point where points inside versus outside are no longer causally connected. If the part of the sun within its Schwarzschild is not causally connected to the outside, how do you plan to explain that the energy of fusion is still released from the center??? Therefore a Schwarzschild radius and an event horizon are not the same thing.
    maybe my terminology wasn't correct.?

    But if a star was collapsing you can work out where the event horizon would be for spheres within the star..yes?
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    Quote Originally Posted by Frog march View Post
    maybe my terminology wasn't correct.?

    But if a star was collapsing you can work out where the event horizon would be for spheres within the star..yes?
    What?

    The Schwarzschild radius is when you imagine what the size of the event horizon would be if you could magically compress an object. The event horizon is a 'real' thing, since crossing it means you're not coming back.

    You can cross through the region of a Schwarzschild radius and come out again. You CANNOT cross through an event horizon and come out.

    You're talking about working out a Schwarzschild radius, and then you want to pretend it's an event horizon. It isn't.

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    Quote Originally Posted by mkline55 View Post
    I think the issue Frog March has is highlighted in this quote:

    How much mass is located inside the zero radius which results in an event horizon?
    Initially, none. My understanding of that paper is that when you have enough material in a volume of space to meet the requirements for creating a black hole, the event horizon does not suddenly spring into existence outside that mass. Rather, the event horizon starts from the centre and rushes out to encompass that mass.

  29. #29
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    Quote Originally Posted by ShinAce View Post
    What?

    The Schwarzschild radius is when you imagine what the size of the event horizon would be if you could magically compress an object. The event horizon is a 'real' thing, since crossing it means you're not coming back.

    You can cross through the region of a Schwarzschild radius and come out again. You CANNOT cross through an event horizon and come out.

    You're talking about working out a Schwarzschild radius, and then you want to pretend it's an event horizon. It isn't.
    yes, I under stand that. eg the Earth has a Schwarschild radius of 8mm or so, as someone said, and you could enter that centre of the Earth(bar molten metal) and come back, I understand that. I was just saying you could work out say a 10mile radius sphere of a collapsing star's Schwarzschild radius, maybe it would be 10m(Just a guess), and when that 10mile radius spherical section of the core had collapsed enough it ie down to 10meters radius, it would then be inside the event horizon, wouldn't it, and the rest of the star, outside that 10miles radius would follow soon behind. I'm saying the black hole would then be present before the whole star had collapsed...
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    Quote Originally Posted by Frog march View Post
    yes, I under stand that. eg the Earth has a Schwarschild radius of 8mm or so, as someone said, and you could enter that centre of the Earth(bar molten metal) and come back, I understand that. I was just saying you could work out say a 10mile radius sphere of a collapsing star's Schwarzschild radius, maybe it would be 10m(Just a guess), and when that 10mile radius spherical section of the core had collapsed enough it ie down to 10meters radius, it would then be inside the event horizon, wouldn't it, and the rest of the star, outside that 10miles radius would follow soon behind. I'm saying the black hole would then be present before the whole star had collapsed...
    The Schwarzschild radius of the Sun is about 3km. If the Sun collapsed down to 3km then (if I understood correctly) it would not immediately be within the event horizon. The event horizon would expand from the centre (at the speed of light) to encompass it.
    Last edited by Strange; 2017-Jul-28 at 10:51 PM.

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