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There are variations on this that are more problematic (which I've never seen solutions to). 1) It looks like it takes forever, to an outside observer, for any

There are variations on this that are more problematic (which I've never seen solutions to). 1) It looks like it takes forever, to an outside observer, for anything to reach the event horizon. So if no matter appears to reach event horizons, how can they (event horizons) form in the first place (i.e. exist in our universe)? 2) While it takes forever for matter to fall in (again to outside observers), it takes finite time for black holes to evaporate via Hawking radiation. So not only do black holes would take forever to form, they seem to have to extinguish before being able to exist (again in the sense of event horizons). I'm not a physicist, but my speculation is that Black Holes don't really exist, they're just a limit of a process that approaches but doesn't converge to a singularity, which I think is unphysical.

mehrdadn 8y ago
I'm also not a physicist but I think I can guess a plausible answer to these? 1. Because if you don't have a black hole yet then there is no event horizon to prevent you from falling in and creating one. I think the event horizon wouldn't form at exactly the same point whose crossing would finally increase density enough to create it, so this shouldn't be a problem. (Although, again, since the event horizon doesn't exist yet, I think it might still not be a problem even in that case?) I would also expect that quantum fluctuations can also inject matter into a black hole, just as they can remove matter from it. 2. Again, they wouldn't take forever to form -- see above. I'm pretty sure black holes have been observed indirectly, so the idea that they actually don't exist would require a pretty rock solid alternative explanation!
- pdonis 8y ago
  > I'm pretty sure black holes have been observed indirectly What we have observed indirectly are compact objects that emit no light, but contain a large enough mass in a small enough volume that they can't be anything else but a black hole, if we only take classical GR into account. As I responded in another post just now, it is possible that black holes, in the sense of objects with actual event horizons (boundaries of regions from which light will never escape, even in the infinite future), cannot exist when quantum gravity effects are taken into account. If that is the case (and it is not clear whether it is--as I said, this is an open area of research), then the objects we call black holes based on our current observations won't have actual event horizons--eventually, in the very far future, light will escape from those compact regions. But they will still have apparent horizons, i.e., surfaces from which outgoing light is not escaping now (or for a very long time in the future). And that, in itself, is sufficient to show that the issues raised by darkmighty are not valid.
pdonis 8y ago
> There are variations on this that are more problematic (which I've never seen solutions to). Then you haven't spent much time looking at actual textbooks on GR, since all of these issues are addressed there. (Not to mention in many peer-reviewed papers in the field.) > if no matter appears to reach event horizons, how can they (event horizons) form in the first place (i.e. exist in our universe)? This is the same fallacy as the fallacy that nothing can actually fall into a black hole because it looks like it takes forever from the outside. Oppenheimer and Snyder published a mathematical model way back in 1939 that shows how a black hole can form in a finite time from the gravitational collapse of a massive object, as seen by an observer falling inward on the surface of the object. The collapse appears to take forever as seen by a distant observer, but this is an optical illusion caused by the effect of spacetime curvature on the paths of light rays. This has been studied for decades and is thoroughly understood. > While it takes forever for matter to fall in (again to outside observers), it takes finite time for black holes to evaporate via Hawking radiation. Wrong. First, if matter is falling in, the hole is gaining mass, not losing it, so it will never evaporate even though, in principle, it is emitting Hawking radiation (in practice this radiation is many, many orders of magnitude too faint to detect for any black hole we can observe). Second, if you include quantum effects, and therefore Hawking radiation in your model, you've changed the model, and it is no longer true that a distant observer will never see anyone falling into the hole. Instead, the distant observer will see light signals emitted by objects falling through the hole's horizon at the same time the distant observer sees the hole evaporate. (At least, that is the case in the simplest model, the one Hawking used when he first published his prediction of Hawking radiation. More complicated models have been developed since, and we won't know which, if any, of them is really correct until we have an experimentally confirmed theory of quantum gravity.) > I'm not a physicist, but my speculation is that Black Holes don't really exist, they're just a limit of a process that approaches but doesn't converge to a singularity, which I think is unphysical. Your speculation is uninformed and wrong. It is possible that an actual black hole, with an actual event horizon, is impossible when all of the laws of physics, including the laws of quantum gravity, are taken into account. This is an open area of research. But if it turns out that actual black holes cannot exist, it won't be for any of the reasons you give.
- darkmighty 8y ago
grkvlt 8y ago
1. Because an 'event horizon' is not a physical boundary, it's more like a boundary that separates events that we can observe and events that we will never see in this universe, ever - that is, in this universe we can't know anything about events that happen beyond this horizon.

mehrdadn 8y ago

I'm also not a physicist but I think I can guess a plausible answer to these? 1. Because if you don't have a black hole yet then there is no event horizon to prevent you from falling in and creating one. I think the event horizon wouldn't form at exactly the same point whose crossing would finally increase density enough to create it, so this shouldn't be a problem. (Although, again, since the event horizon doesn't exist yet, I think it might still not be a problem even in that case?) I would also expect that quantum fluctuations can also inject matter into a black hole, just as they can remove matter from it. 2. Again, they wouldn't take forever to form -- see above. I'm pretty sure black holes have been observed indirectly, so the idea that they actually don't exist would require a pretty rock solid alternative explanation!

pdonis 8y ago
> I'm pretty sure black holes have been observed indirectly What we have observed indirectly are compact objects that emit no light, but contain a large enough mass in a small enough volume that they can't be anything else but a black hole, if we only take classical GR into account. As I responded in another post just now, it is possible that black holes, in the sense of objects with actual event horizons (boundaries of regions from which light will never escape, even in the infinite future), cannot exist when quantum gravity effects are taken into account. If that is the case (and it is not clear whether it is--as I said, this is an open area of research), then the objects we call black holes based on our current observations won't have actual event horizons--eventually, in the very far future, light will escape from those compact regions. But they will still have apparent horizons, i.e., surfaces from which outgoing light is not escaping now (or for a very long time in the future). And that, in itself, is sufficient to show that the issues raised by darkmighty are not valid.

pdonis 8y ago

> There are variations on this that are more problematic (which I've never seen solutions to). Then you haven't spent much time looking at actual textbooks on GR, since all of these issues are addressed there. (Not to mention in many peer-reviewed papers in the field.) > if no matter appears to reach event horizons, how can they (event horizons) form in the first place (i.e. exist in our universe)? This is the same fallacy as the fallacy that nothing can actually fall into a black hole because it looks like it takes forever from the outside. Oppenheimer and Snyder published a mathematical model way back in 1939 that shows how a black hole can form in a finite time from the gravitational collapse of a massive object, as seen by an observer falling inward on the surface of the object. The collapse appears to take forever as seen by a distant observer, but this is an optical illusion caused by the effect of spacetime curvature on the paths of light rays. This has been studied for decades and is thoroughly understood. > While it takes forever for matter to fall in (again to outside observers), it takes finite time for black holes to evaporate via Hawking radiation. Wrong. First, if matter is falling in, the hole is gaining mass, not losing it, so it will never evaporate even though, in principle, it is emitting Hawking radiation (in practice this radiation is many, many orders of magnitude too faint to detect for any black hole we can observe). Second, if you include quantum effects, and therefore Hawking radiation in your model, you've changed the model, and it is no longer true that a distant observer will never see anyone falling into the hole. Instead, the distant observer will see light signals emitted by objects falling through the hole's horizon at the same time the distant observer sees the hole evaporate. (At least, that is the case in the simplest model, the one Hawking used when he first published his prediction of Hawking radiation. More complicated models have been developed since, and we won't know which, if any, of them is really correct until we have an experimentally confirmed theory of quantum gravity.) > I'm not a physicist, but my speculation is that Black Holes don't really exist, they're just a limit of a process that approaches but doesn't converge to a singularity, which I think is unphysical. Your speculation is uninformed and wrong. It is possible that an actual black hole, with an actual event horizon, is impossible when all of the laws of physics, including the laws of quantum gravity, are taken into account. This is an open area of research. But if it turns out that actual black holes cannot exist, it won't be for any of the reasons you give.

darkmighty 8y ago

grkvlt 8y ago

1. Because an 'event horizon' is not a physical boundary, it's more like a boundary that separates events that we can observe and events that we will never see in this universe, ever - that is, in this universe we can't know anything about events that happen beyond this horizon.