Please Register or Log in to view the hidden image! In the context of the membrane paradigm which most graphic models of Black Holes are represented as, the more matter that falls into the center of the sheet, the deeper the well that is created, and the harder it is for matter to escape. According to Einstein's theory, if enough matter is packed into a small enough volume, the well will get so deep that the matter inside can never escape. Any matter that passes the point of no return can no longer escape to the outside world. It necessarily keeps collapsing, moving towards the center. The well gets deeper and deeper until finally a hole is literally torn in the fabric of spacetime! The density of matter at the center becomes essentially infinite, at least to the extent that Einstein's theory of gravity is still valid. But is a Black hole at its extreme really " a hole in the fabric of spacetime?" is it really: an area of space where the known laws of physics break down? Or do we have some new theories to explain what’s going on. And from artists representations I’ve seen of Black Holes I’m reminded of two dimensional Acme cartoon holes that Bugs Bunny might throw onto a wall to escape. But Spacetime is not a rubber sheet! It has an X, Y and Z-axis. So could you go behind a Black hole and look at it? What would you see as far as what can be seen? And could you look at it side on and see anything interesting? I know they are black against the black of space but then not all of space is black. Please Register or Log in to view the hidden image!
Firstly, this is not the membrane paradigm. That is when you consider what happens to a shell of matter very close to the horizon of the black hole. You can use it to calculate the black holes thermodynamic properties if memory serves me. No. Spacetime in GR is malleable but it cannot rip. The area within the horizon of a black hole is a perfectly legitimate area of space that has the same properties like local flatness as our region of space. Also, the horizon is not the one way barrier that you're envisaging when you consider quantum mechanical effects which does allow particles to escape the black hole. That's kind of the point. We know that GR breaks down at a singularity because it is a classical theory and so it doesn't include the regime where quantum gravity effects become important - an object that is both very massive and very small. A black hole singularity is an example of this. Artists representations should be taken with a healthy dose of scepticism. All the black holes that I know of are very bright objects because matter falling into them gets very hot and radiates light. The BH at the center of a galaxy is a good example of this. Suffice to say it's very far from as simple as the acme hole.
Haha, well spacetime can "sort of" rip: the mathematicians don't like to include singularities as part of the spacetime, it makes their theorems less nice. It isn't really anything physical though. Also there are areas inside the event horizons of some black holes that definitely aren't "perfectly legitimate area of space that has the same properties like local flatness as our region of space", such as the regions in a rotating black hole with closed timelike curves. Of course this is probably a bit of a mathematical game too, such things probably don't actually occur in real black holes. I don't remember what any spacetimes with a rotating black hole created via some realistic collapse scenario looks like though. I was thinking of the ideal Kerr geometry. Hmm. I don't think I've seen a Penrose diagram for such a thing. Maybe no-one has found an exact solution. I'm going to have to go and find out now...
Hmm ok it doesn't seem like everyone agrees on what would realistically happen. They all agree that all the fun stuff should disappear along with everything inside the inner horizon though. So probably the real ones are boring on the inside as Prom says.
you dont need a singularity to get a black hole. if matter could somehow resist the pressure a black hole would still form if enough mass were collected into a small enough area. its the event horizon that defines a black hole. not the singularity.
Does it matter what the object which collapsed was? You can construct odd things via unrealistic collapses (like an elongated mass collapsing into a black string) but any realistic collapse is going to end up having the No Hair theorem apply, as gravitational radiation will remove all perturbations away from being the ideal black hole solution. I don't have the Penrose diagrams for a Kerr solution but I do have the one for a R-N one, if you're interested.
The point is that there is no mechanism known in nature that could prevent the collapse when the star is above a certain mass. Obviously when you have an object that's small enough for vacuum fluctuations to have a significant effect you're well within the realm of quantum gravity.
One thing I thought I would point out that no one seems to have clarified, ScaryMonster, is that the rubber sheet analogy is a poor one because it uses direction to represent the greater attraction of gravity near the center of the BH - this is only because we are used to gravity attracting "downwards". It is further confusing to place the "rubber sheet" BH against a backdrop of space! In reality an ideal BH would be spherical with all attraction being directed toward its center - therefore it would look the same from all sides. I don't really have the energy to start a fight, Prom, but I have suggested in the past that pushing the collapse event out to the infinite future could be considered such a mechanism.
Unless vacuum fluctuations get stronger near a bh, I don't think vacuum fluctuations would be very significant - the energy density of the vacuum would be incomparably small next to the density of the black hole.:shrug:
A Spherical hole? I had envisioned that it had to be something like that! But it hurts my poor simian brain trying to imagine a ball like hole that’s also gravity well. So the depth of the BH would be consistent with the size of the object it was i.e. a large star before it collapsed? Or is it weirder then that?
AH: Haha, let's just stick with remotely realistic cases for now, and cool, sure send me the RN collapse diagram, I wouldn't mind seeing it. Scary-monster: Don't so much think of it as a hole, just think of it as a black spherical surface through which you can't escape. Of course you'll never be able to actually pass through the surface, or at least it won't appear that way to you if you try. You still won't be able to get out but you won't realise when you pass this point. Btw people called black holes "frozen stars" before someone, like Wheeler maybe, came up with the cooler sounding term, which may be a better mental picture than a spherical hole. Mental pictures tend to not work so well though once space differs enough from flat space.
And I still believe what you say about this is not physical. The infinite time taken for an observer to fall from infinity to the horizon is simply a symptom of the fact that the Schwarzschild metric is singular at the horizon. The space time is not singular at that point.
That's an interesting thought. Well, you may be right that what I describe is non-physical and if I'm ever convinced of that fact I will admit it. For clarification, however, it is not the "fall from infinity" that takes an infinite time - it is the calculated time for any object to cross the event horizon from an observer at infinity. That same observer would make the same calculation at any point arbitrarily close to the event horizon himself if he is accelerated such that he is static relative to the BH. The standard counter to this position is that, if we ignore tidal forces, the free falling frame would experience crossing the event horizon without trouble and seemingly quickly. This fact should not invalidate the calculations of the infinite observer! Why many Physicists freely accept the accuracy of Relativistic time calculations when discussing interstellar travel (e.g. the twin's paradox), yet reject them as "an illusion" or a "failure of the metric" when it comes to the event horizon of BH's is confusing to me. If my arm were twisted to provide an explanation, I suspect* the final solution to this will be that the objects in the free falling frame are destroyed as they approach the horizon due to interacting with particles of energy levels approaching infinity. Therefore, no observer nor information-carrying object whatsoever could withstand the journey, so trying to describe what that frame would "experience" is itself a nonsensical supposition. *Based on nothing but armchair supposition and perhaps a little Scotch. I am not a Physicist! (I am, however, back in school so give me a few years)
http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html Below the rocket, something strange is happening... an object dropped by the accelerating rocket would not notice anything unusual when it passed the 'black hole' observed by the rocket but once past that point it can never again interact with the rocket. even a photon emitted at that point could never catch up with the rocket.
I don't mean to change the subject, but is this paragraph correct? I haven't read the entire page, certainly haven't checked the math, and I know intuition is a poor guide when dealing with Relativity, but the part about turning the ship around half-way through the journey only doubling your travel time T doesn't "feel" right to me.
this is a math thread..and your discussing black holes.. so perfect time for my question.. what is the math for calculating gravity of a black hole relative to its mass?? at what point is the gravity emmited from a black hole substantial enough to compact matter around it? (provided there is enough matter to compact..)(outside the event horizon)
I found this qoute when I was Googling about this subject, reading up on "Gravitational time dilation", the "Twin paradox", Penrose diagram's, No hair theorem and the Schwarzschild metric! And very interesting reading they were.I know Intuitive thinking (so I’ve been told) doesn’t always work when talking about GR. But I did feel that this quote related to the subject at hand. "Q:Won't the black hole have evaporated out from under me before I reach it? --------------------------------------------------------------------- A: We've observed that, from the point of view of your friend Penelope who remains safely outside of the black hole, it takes you an infinite amount of time to cross the horizon. We've also observed that black holes evaporate via Hawking radiation in a finite amount of time. So by the time you reach the horizon, the black hole will be gone, right? Wrong. When we said that Penelope would see it take forever for you to cross the horizon, we were imagining a non-evaporating black hole. If the black hole is evaporating, that changes things. Your friend will see you cross the horizon at the exact same moment she sees the black hole evaporate. Let me try to describe why this is true. Remember what we said before: Penelope is the victim of an optical illusion. The light that you emit when you're very near the horizon (but still on the outside) takes a very long time to climb out and reach her. If the black hole lasts forever, then the light may take arbitrarily long to get out, and that's why she doesn't see you cross the horizon for a very long (even an infinite) time. But once the black hole has evaporated, there's nothing to stop the light that carries the news that you're about to cross the horizon from reaching her. In fact, it reaches her at the same moment as that last burst of Hawking radiation. Of course, none of that will matter to you: you've long since crossed the horizon and been crushed at the singularity. Sorry about that, but you should have thought about it before you jumped in." by Ted Bunn (Black Holes). So pulling out what was said about the “Schwarzschild metric” and Gravitational time dilation is the person to fall from infinity to the horizon really frozen in time? Or is it as the above quote says and someone else suggested an illusion? And it will all happen in quick succession once the BH evaporates and the gravity is gone?
