Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What happens to matter that was inside an event horizon before the collapse?

  1. May 2, 2012 #1
    Hey guys this is my first post so I hope this doesn't come out stupid since I only know slightly more than a layman. Recently I have been trying to wrap my mind around what happens inside an event horizon. Specifally I have been confused with the matter which resided inside the schwarzchild radius before the collapse. I know that matter falling into a black hole accelerates until it hits the event horizon, but matter which was already inside the event horizon may be able to move freely. I know there is no actual evidence about what happens in there, but I want to know if there are credible models or theories which predict what may happen inside the EH concerning the matter which resided and still resides inside the shrwazchild radius?
  2. jcsd
  3. May 2, 2012 #2


    Staff: Mentor

    Before the collapse (assuming that by "collapse" you mean "the collapse of the object that forms the black hole in the first place") there is no event horizon; the event horizon is *formed* as part of the collapse process.

    Also, matter inside an event horizon (once it has formed) is *not* able to "move freely". Anything inside the event horizon must fall into the singularity at r = 0.
  4. May 2, 2012 #3
    Each particle has a world line in spacetime, in the case of a particle falling behind the event horizon this world line simply stops at the singularity and thus the particle has no more future. So popularly speaking we could say that matter behind the event horizon is doomed to disappear.
  5. May 2, 2012 #4
    I understand there is no event horizon before the singularity, but my understanding of a black hole (which may be naive) is that is is a super dense object. I interpret that as meaning that the matter which makes the object does not change, it just becomes denser. That's the idea of the schwarzchild radius right? (I'm asking not being condescending) This is the radius an object needs to become to create a singularity. To me this says that object still exists on a macro scale, but is unreachable and inescapable. It is the matter which already exists inside the event horizon that I have a question about, not the matter approaching it.
    My question came from trying to relate the inside of a black hole to the real world. My thought experiment was this: view it in terms of the Earth. On the surface we all experience 1g; this would be the surface of the singularity where Vesc>C. As we near the center of Earth, wouldn't we experience less than 1g because there is less matter pulling us directly down; and thus inside the singularity matter experiences Vesc<C.
  6. May 2, 2012 #5
    Oh quick vocab check: is a singularity the object which causes the event or the event horizon itself? because I've been using it meaning the object which causes it.
  7. May 2, 2012 #6


    Staff: Mentor

    The singularity is neither of those things. It is formed inside the black hole (i.e., inside the event horizon) during the collapse process, and represents an "edge" to the spacetime--any object that hits the singularity simply "stops" there and ceases to exist. (Technical point: this is what the "classical" theory of GR says happens. Actually, most physicists expect that the classical theory is no longer valid very close to the singularity, and what really happens there is governed by quantum gravity; but we don't have a good theory of quantum gravity yet.)

    It isn't. See below.

    This is probably not a good strategy. The inside of a black hole is very, very different from any "normal" place that I think is what you mean by "the real world".

    The black hole is not at all like the inside of a planet or star. The inside of a black hole is vacuum, all the way down to the singularity at the center (r = 0). (Strictly speaking, there is a short period, after the collapsing object that forms the hole falls below the event horizon but hasn't yet formed the singularity at the center, where there is indeed matter inside the hole; but this phase is very short, and it ends with all of the matter in the original object reaching the center at r = 0, forming the singularity there, and ceasing to exist. After that the entire spacetime is vacuum; there is no matter anywhere.)
  8. May 3, 2012 #7
    Perhaps my confusion is with with term r=0. Is this meant to mean that radius=0, because if that's the case doesn't that violate newton's laws. Whatever creates the event horizon has volume and density, therefore it should be measurable. I understand we cannot actually go in and measure it, but the matter does not vanish, it is there, just unreachable.

    *Please do not take my questioning as some misguided attempt at undermining current theory. I am only trying to understand.

    What I struggle to get my mind around is how a measurable amount of matter can cease to be measured. To create a black hole, a definable amount of matter collapses to a density where light cannot escape. By my understanding the event horizon is a condition of acceleration over distance; matter accelerates until in reaches light speed. Once it reaches this speed, acceleration stops.

    I refer to my Earth thought experiment: the particles on the surface of the Earth are moving at their maximum acceleration, 1g. Inside that distance they have to be moving at less acceleration. If a measurable amount of matter creates an event horizon, then inside that horizon the acceleration should be less. This depends on the question: Can matter occupy zero space? If this is the case, than logic would suggest that matter can collapse in on itself infinitely. I don't understand this because in our universe, matter takes up space. This means, that there is a finite density matter can obtain.
  9. May 3, 2012 #8


    User Avatar
    Science Advisor

    Maybe this helps:

    Imagine there is a really massive hollow sphere around the earth, collapsing under its own gravity. It is so massive, that if you send a rocket from earth through the shell, it will finally not overcome its gravity but fall back to earth. If this happens to all possible rockets and light signals, the Earth is already inside the horizon. There's nothing different on earth by that time, everyone can go freely, no anomalous acceleration or whatever.
    The definition of an horizon is not that there is something weird happening inside. The definition is that nothing can escape. So there's nothing happening until the incoming shell crosses the earth surface and starts pulling everything towards the center.
  10. May 3, 2012 #9


    Staff: Mentor

    It does mean that radius = 0. Note that Newton's laws may indeed be violated because they are not correct; they are only approximately correct, and the approximation breaks down when gravity gets strong enough, as it does near or inside the event horizon.

    Not according to GR. According to GR, when the matter that creates the black hole reaches r = 0, it does vanish from the spacetime. It had volume and density before, but once it vanishes into the singularity, it no longer does, because it's no longer in the spacetime at all.

    That is indeed one reason why most physicists believe that GR is no longer valid when you get too close to the singularity, as I said in an earlier post. But as I said then, we don't have a good theory of quantum gravity to take over from GR in that regime. So there's not much else we can say about it. The best we can say is that hopefully our theories of physics will at some point improve to give a proper explanation of what happens in this case.

    In view of this, if you like, you can replace "singularity" in everything I've said with "region sufficiently close to r = 0 where we no longer have a good theory of physics", and "falls into the singularity and disappears" with "reaches the point where our current theory of physics breaks down and we can no longer correctly predict what happens", and so on. But that does *not* mean that we can reason correctly on the assumption that the matter that "falls into the singularity" is still "there" somehow. It is still correct that the matter vanishes *as far as the rest of the spacetime is concerned*.

    This is not quite a correct description of the event horizon. The event horizon is a surface in spacetime where outgoing light rays stay at the same radius forever. So to an object that is falling inward through the event horizon, the horizon itself appears to be moving outward at the speed of light. But the infalling object does not stop at the horizon; it keeps falling inward until it reaches the singularity at r = 0.

    Also, if observers who are "hovering" at a constant radius just outside the horizon watch the infalling object pass them, they will see it moving closer and closer to the speed of light as the "hovering" observers get closer and closer to the horizon. But there is *no* observer who can "hover" actually *at* the horizon; such an observer would have to move at the speed of light. So there is no observer who actually observes the infalling object pass him *at* the speed of light. So it's incorrect to say that "matter reaches light speed" at the horizon.

    As I said before, this is *not* a good analogy for how the spacetime around and inside a black hole works. Inside a black hole it is vacuum all the way down to the singularity. That's true even if we don't know what actually happens *at* the singularity (as above).

    Once again, whatever happens at the singularity, it is *not* a good strategy to try to compare it with how normal matter works. According to standard GR, all the matter in the object that originally formed the black hole does, in a sense, "collapse on itself infinitely"; once it reaches r = 0, it no longer takes up space. But that does not mean it is permanently sitting at zero size and infinite density at r = 0; once it reaches r = 0, according to standard GR, it *disappears*.

    As I said before, most physicists do believe that standard GR is no longer valid at the singularity; but that doesn't mean the matter that formed the hole really does stay in the spacetime at nonzero size and finite density after all. It means we don't *have* a good way of describing what really happens at r = 0.

    There's another point about this as well. Strictly speaking, r = 0 is not really a "place"; it is a "moment in time". That is, the set of all points in the spacetime with r = 0 is not a timelike line, as it would be if r = 0 were a place. Instead, it is a spacelike surface, like the set of all events that occur at a particular time. So the real reason that nothing that falls inside the event horizon can avoid hitting the singularity is that once you are inside the horizon, the singularity is in your *future*--you can no more avoid reaching it than you can avoid reaching tomorrow.

    So when I say that GR breaks down at the singularity and we don't have a good way of describing what really happens there, it's not really correct to view that as saying that there is a small region in space around r = 0 that GR can't properly describe; it's saying that there is a region to the *future* of all objects inside the horizon that GR can't properly describe--at some point in the travel towards tomorrow of any such object, it will reach a point where GR breaks down. That is what really happens to all the matter that originally formed the hole; it has disappeared into a *separate future* than the future that everything outside the horizon goes to. And that separate future is going to be there, and be separate from the future of objects outside the horizon, regardless of what theory of physics we finally arrive at to describe the separate future in detail.
  11. May 3, 2012 #10


    User Avatar
    Gold Member

    "Is there life inside black holes?", Vyacheslav I. Dokuchaeva


    I haven't read it in detail, only looked at the pictures.
  12. May 3, 2012 #11


    User Avatar
    Gold Member

    When black hole supposedly forms event horizon expands from a single point. So all the matter that is inside black hole gets there by falling in it.
  13. May 3, 2012 #12
    Can't say I disagree with that however if I recall correctly it was you who recently adamantly supported the view, a view I disagree with, that r=0 was the middle of the black hole. This view however seems to me inconsistent with what you write above here.
  14. May 3, 2012 #13


    Staff: Mentor

    Not inconsistent, but it does require carefully specifying the meanings of terms. In the previous discussion you refer to, I argued that r = 0 was the "center" of a particular *spacelike slice*, a slice of constant Painleve time. This is true--the set of all points with r = 0 within that particular spacelike slice consists of a single point at the center of the slice ("center" because it's the center of a family of nested 2-spheres that cover the entire spacelike slice).

    In this thread, I'm talking about the set of all points in the *spacetime* with r = 0. This set is a spacelike line (I said "surface" before, but "line" is better because even with the angular coordinates included the set of points with r = 0 is still only one-dimensional, not 3-dimensional like the sets of points with r = any specific positive value). Viewed as a line in the spacetime as a whole, I agree that "center" is not a good word to describe this set--but then again, I wouldn't say that "center" is a good word to describe any set of points in the *spacetime* (as opposed to in a particular spacelike slice).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook