It is generally agreed that it is impossible to enter a black hole and remain intact to experience it. What do you think would happen to a segment of space that was surrounded on all sides by a thin layer of black holes (but not touching them) i.e. if it was completely isolated from normal space but not within any of the black holes event horizons. Would physics just break down and be completely out of sync with the outside universe? I think it would be an expensive experiment!
By Newton's shell theorem there is no net gravitational acceleration anywhere within a shell of material (assuming constant density in the shell), though that is likely a massively inappropriate thing to use in the case of something so obviously relativistic. The the Kerr black hole (the spinning one) the singularity is actually a ring and as you approach the ring (no along the equator though) the space-time takes on the form of a cylinder, ie a wormhole-like throat. Whether or not you can trust what GR then says, about exiting in a asymptotically flat space-time which may or may not be our own, is up to you. There are no 'black hole shell' solutions in 4 dimensional general relativity, the restriction of spherical symmetry is extremely restrictive, not to mention such a configuration is unstable and would collapse into a point. I suppose you could construct a setup like an explosion where the outward force crushes a ball of material into a shell of 'black hole material', which then collapses and then you could ask what it looks like from the inside. Higher dimensional relativity and string theory might cover such things a little bit more. String theory and M theory involve 'black branes', where a brane is such that it forms an event horizon around it. They can come in all shapes and sizes and dimensions and while I can't think of something off the top of my head I'm absolutely certain someone has considered black branes wrapped on spheres somewhere in the AdS/CFT correspondence or in M theory. Unfortunately my M-theory textbook is in the office. /edit Damn it, it looks like I've lost my copy of Johnson - D-branes!!! Must have left it at university (along side another QM textbook!). Crappity crappity crap! It's a bit depressing it has taken me a year to realise that, shows I'm not reading my string theory books enough!
Yes I was thinking it would be constructed by firing a load of small black holes from different points in space so that they momentarily coincide as a sphere (or any closed shape) such that they never physically contact with the centre. Thanks for your input -it was just within my understanding!
Is the 4D topology of that configuration the same as a regular black hole, or is some part of the inner region permanently disconnected in some way from the region between the event horizons? I have only a layman's casual understanding of topology (ie mostly visual intuitive), so I hope that question makes sense.
My first thought is that the outside of the shell of black hole material would look the same but it is not a static configuration. However, if it is perfectly spherically symmetric than it is impossible for it to emit gravitational radiation and thus I don't think the fact it is static is too much of a problem to the outside observer, all that might happen is the event horizon might change size, depends whether the radius of the shell is large compared to the event horizon radius of a black hole of equal mass but concentrated in a single singularity. As for inside I think it is permenantly cut off but I've no idea what the topology of the space-time there would be.
I was thinking of it from the inside but now you have mentioned the outside I suppose the gravitational field might be seen by the outside universe as though it is one large 'solid' black hole. This is very weird indeed. I don't know about M theory - would the other dimensions mean that it is not actually a closed shape to the universe?
