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#1




Does a black hole have to have a pointlike singularity at its core?
It seems to me that everything I read about black holes mentions that at the centre there is an infinitely dense onedimensional point called a singularity.
As far as my basic understanding goes  when a suitable massive star reaches its end of life, gravity forces all its electrons, protons and neutrons to fuse together causing a neutron star. If the star was too massive to begin with these neutrons will then be forced together by gravity into a infinite density causing a singularity and a black hole. What would happen if there was something type of matter that was denser than neutrons so that a star could form that had an escape velocity greater than the speed of light. Could we not have a black hole that had a solid surface somewhere inside the event horizon? Is there any reason that such a dense form of matter could not theoretically exist? Is it because of the speed of light/spacetime curvature/something else that anything inside the event horizon must end up in a singularity. A possible alternative way of looking would be  what if the speed of light was only 10km/s wouldn't the earth be a black hole but with a solid surface inside the event horizon? Thanks 
#2




There are postulated forms of matter even denser than neutron stars, resulting in even smaller objects called quark stars.
But for there to be anything nonpoint inside of a black hole would defy all that we know of physics. Simply put, matter fundamentally cannot be that strong. It's beyond even being a matter of strength: We're into comicbookSuperboychangingrealitybypunchingthefabricofspace territory, here. Which is not to say that our understanding of physics can't be wrong. Most attempts at a theory of quantum gravity predict that there's something of very small (but nonzero) size in there. But we can't yet get any theory of quantum gravity to work, so that's all just speculation. 
#3




I'm not disagreeing with you when you say that matter cannot be that strong but my (simplistic) train of thought is that if there was something theoretically twice as strong/hard as neutrons then this star would be twice(?) as small as a neutron star. If we then had something twice as strong/hard as this new material then that star would be smaller and so on. It would then seem that at some stage something would be hard enough to form a star where the escape velocity is greater than c and then we have my black hole with a surface.
Where is my thinking incorrect? Is it the fact that the escape velocity is greater than c that means this matter cannot exist and if so, does that it would be theoretically possible we could have a sphere where the escape velocity is 0.99999999c just not one where it is c? Last edited by Zerc; 10132016 at 08:17 AM. Reason: Terrible spelling 
#4




Just thinking a bit about this  is this because the entire volume of the star inside the black hole is going to end up in the singularity because of the switching of space and time inside a black hole?
The same way the singularity is in the future inside a black hole just as Tuesday is in my future and also just as inevitable? 


#5




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To your other question, there's no fundamental reason why you couldn't have a substance or object that's just barely shy of becoming a black hole In fact there has been some serious scientific speculation about just such objects, usually called "gravatars". Most mainstream physicists consider the idea absurd, because it just requires far too much finetuning and handwaving, but it's not completely in the realm of crackpottery. 
#6




Also, there's a significant number of physicists who think that a black hole doesn't actually collapse to a singularity, that a quantum theory of gravity will show that some other effect intervenes. The problem is that we don't have a working quantum theory of gravity yet, so this is pretty much just speculation. For example Loop Quantum Gravity, one proposed theory, predicts that a star would start to collapse to a black hole, then quantum gravity effects would cause it to stop and bounce back, but because of time dilation the bounce would take too long for us to observe directly. http://www.nature.com/news/quantumb...xplode1.15573

#7




I must say that a lot of things to do with black holes and relativity seem very weird/counterintuitive to me but that's obviously my problem. The one thing that really seems impossible to me is how can the singularity be infinitely dense? It just seems "obvious" () to me that would have to be a certain finite density. Loop quantum gravity mentioned in the article by Pantastic seems like at least a nice alternative
Still very interesting to think about and try to understand even if I realize my understanding of these topics is only skin deep/ Thanks for the answers. 
#8




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What's less intuitive to me personally is how two of them can have different masses when their density is infinite and their size is the same  but I guess that's probably because 'infinite' itself isn't an intuitive concept, and 'the same' probably doesn't apply here. 
#9




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So a singularity in a black hole is a point where our functions to describe reality blow up: Finite mass in zero volume is a dividebyzero moment. In a deep sense, the fact we have a singularity is more of a reflection of our ignorance than any physical reality, because the physical world makes sense. The physical world doesn't have any nonsensical parts, just parts we haven't learned how to make sense of yet. Quote:
__________________
"Ridicule is the only weapon that can be used against unintelligible propositions. Ideas must be distinct before reason can act upon them." If you don't stop to analyze the snot spray, you are missing that which is best in life.  Miller I'm not sure why this is, but I actually find this idea grosser than cannibalism.  Excalibre, after reading one of my surefire millionseller business plans. 


#10




How in the world would infinite curvature and a singularity actually look in 4D space? The singularity would be at what time? Time = infinite?
How does that reconcile with the fact that black holes seem to radiate? If a black hole radiates x amount of massenergy at t = y wouldn't that imply that the mass distribution would reflect that? At the instant right before y the black hole is mass M+x and at the instant y the black hole is now mass M how can the components of the black hole be chilling at the singularity? Plus how can you argue with Stephen Hawking? http://www.nature.com/news/stephenh...holes1.14583 
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#12




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The usual way of representing black holes in 3+1 dimensions is via Penrose diagrams. 
#13




Maybe someone could explain this to me. From the POV of an observer falling into a black hole, because of time dilation, the rest of the universe around you would speed up so much that the entirety of the universe's life would play out while you're still falling. Doesn't that mean that from our POV outside the black hole, that the things falling in never finish falling in?
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#15




Penrose proved that, in general relativity with the assumption of nonnegative mass density, there is always a singularity where there is a black hole. However there is nothing to say that a singularity must be "pointlike" or "ringlike" (as in the case of a Kerr black hole) or any designation that suggests it is somewhat similar to a spatial location/shape.

#16




This is not true. If you take Schwarzschild example: for a given observer crossing the event horizon at a certain time there's a very definite and finite limit as to how far in the future of the black hole exterior they will be able to see before they hit the singualrity.
Last edited by Asympotically fat; 10142016 at 03:00 PM. 
#17




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#18




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My attempts to gain an intuitive nonmathematical insight into this apparent paradox may or may not be sound, so I invite appropriate criticism. They have centered around the idea that this apparent freezing due to infinite time dilation is merely an artifact that arises from the use of inappropriate coordinates and implicit but incorrect assumptions about simultaneity. The Schwarzschild solution to the Einstein field equations for a nonrotating black hole (and the equivalent Kerr solution for a rotating one) are valid as one approaches the event horizon and can be used to show the aforementioned switching of the time dimension with the radial spatial dimension beyond the EH, but it's illbehaved at the EH itself, where it blows up and appears to show the time dilation factor tending to infinity as the distance to the EH tends to zero. However under the appropriate geometries and coordinate systems the spacetime curvature at the EH can be shown to be wellbehaved and finite. There seem to be multiple different ways of trying to grasp this intuitively. One way is that from the perspective of an external observer, time dilation really does freeze the infalling object  in space. But the Schwarzchild coordinate system that leads to this conclusion also leads to the conclusion that space itself is rapidly flowing into the black hole, exceeding the speed of light beyond the EH (which space is allowed to do, as distinct from objects in it), where time and the spatial dimension pointing to the singularity have correspondingly switched places and the singularity becomes the object's future. At no point would you actually see the intrepid astronaut frozen forever at the EH (he'd be redshifted out of detectability anyway), but more importantly, I'm suggesting that the idea that the astronaut would forever be at rest at the EH as seen by an external observer seems to stem from a misapplication of the Schwarzchild solution. 
#19




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But as Chronos says, this is theoretical. As a practical matter, the light from the infalling matter dims to invisibility in a finite amount of time. Mark 


#20




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I would argue with your premise, however. As I tried to suggest above, despite the Russian name and the illusion of slowdown to a complete halt of an infalling object, the "freezing" really is an illusion. What we see as the wavelength gets increasingly redshifted and the photon arrival increasingly longer does not reflect what is happening locally. 
#21




Yes, of course in the frame of the infalling matter, it reaches the event horizon and indeed reaches the singularity in a finite time. I'm not fully understanding your objection  are you just unable to accept that different observers experience different proper times between two events? If a person holding a clock falls into a black hole, he will of course see his clock ticking normally. According to General Relativity, an external observer would see the clock slow down more and more as it approaches the horizon. Do you accept that?
Mark 
#22




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This is as good an article as I've found trying to explain this apparent paradox. The key quote: If we program a space probe to fall freely until reaching some randomly selected point outside the horizon and then accelerate back out along a symmetrical outward path, there is no finite limit on how far into the future the probe might return. This sometimes strikes people as paradoxical, because it implies that the infalling probe must, in some sense, pass through all of external time before crossing the horizon, and in fact it does, if by "time" we mean the extrapolated surfaces of simultaneity for an external observer. However, those surfaces are not wellbehaved in the vicinity of a black hole. It's helpful to look at a drawing like this.Another quite interesting intuitive explanation from the same site is found here. Interpreted strictly in terms of Schwarzchild coordinates, one can regard an infalling object as taking infinitely long to pass the event horizon, and then (recalling what happens to the time dimension beyond the EH in the Schwarzchild solution) traveling back in time to the present in Schwarzchild coordinate time as it progresses from the EH to the singularity, yielding a net transit time to the inevitable singularity as seen by an external observer to be not much different from its own proper time. I have no idea if this interpretation makes physical sense, but not much beyond the event horizon really does. If we launch a massive object into a black hole and very soon observe that the radius of its event horizon has grown, then something like this must be true. 
#23




Only somewhat related...but apparently some folks actually saw a star turn into a black hole...sorta...
http://www.mnn.com/earthmatters/spa...olefirsttime 
#24




That's not just tangential, it's actually extremely relevant to the above conversation. Perhaps more knowledgeable posters than I will want to comment, but ISTM that if the interpretation is correct and this is what they think it is  namely that a dying star well beyond the Chandrasekhar limit has suddenly disappeared  then it's proof positive that matter can be observed to fall into a black hole in a short time from a frame of reference at an effectively infinite distance outside the gravity well. The only way a massive collapsing star can suddenly disappear is if it is engulfed by its own event horizon.



#25




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https://pbs.twimg.com/media/BcxA5rGCQAA98_.png We can ignore the bottom (white hole interior) and left (parallel universe) regions of the diagram(s) as they are features of the extended solution. In a Penrose diagram light always has worldlines at 45 degree angles to the vertical axis, and the worldlines of objects moving at less than the speed of light are always at an angle that is less than 45 degrees to the vertical axis. It should be easy to see that nowhere on the BH singularity (upper zigzag) is causally connected to the whole of the BH exterior region (right region), which means an observer hitting the singularity cannot see the entire future of the Universe played out before they hit the singularity. The misapprehension that you are able see the entire future of the Universe before you hit the singularity comes from taking Schwarzschild coordinates (the dotted lines in the BH exterior region. NB as illustrated the lines do not mark out equal amounts of coordinate time or distance) too literally. Schwarzschild coordinates map all the events that lie on the event horizon to the same time coordinate* as timelike future infinity (i^{+}), to which all events in BH exterior region are causally connected and in the past (therefore if you make it to i^{+} you are able to see every event that happened in the BH exterior region). *Strictly speaking they don't because the Schwarzschild time coordinate goes to infinity at both the event horizon and at i^{+}, so the problem is more from making faulty assumptions by incorrectly taking limits when the coordinates are singular. 
#26




Is there a way to easily explain the difference between the singularity at the center of a very massive black hole versus the singularity at the center of a small black hole? If both are considered infinitely dense, how is there a difference between them?

#27




They have different masses. No, really, that's it.
Well, they could also have different electric or magnetic charges and/or different angular momentum, but that's independent of size. 
#28




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BTW...instead of an infinitesimal geometric point possesing infinite density, a singularity of one planck mass per cubic planck distance would create a non"divide by zero" situation. It also occurred to me, instead of a 3D tiny object, it could be a 4D object as per General Relativity, 11D as per MTheory, 26D as per Bosonic String theory, or even a 248D as per Exceptionally Simple theory of Everything. 
#29




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Mark Last edited by markn+; 10172016 at 03:41 PM. 


#30




That's a peculiar definition of "still emitting photons". At a very short time after the object crosses the event horizon, the last photon from it will be received. No other photons, no matter how redshifted, will ever come out.

#31




But I thought that nothing ever does cross the event horizon, from an external viewpoint. Infalling objects just asymptotically approach it. No?
Mark 
#32




Well, no photons or information specifically tied to that particular object that fell in. There's still predicted to be Hawking radiation, though as I understand the mechanism that starts off as particles, even though they annihilate and become photons nearly immediately. Photons are predicted to be emitted from the area of the black hole, at any rate.

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#34




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Last edited by Some Call Me... Tim; 10182016 at 12:46 AM. 


#35




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#36




Yes, that's more or less true, and thank you for the correction as it was wrong for me to imply that tidal effects are the sole cause of what are actually very complex phenomena.
However I would ask whether you don't agree it's true that tidal effects (i.e. the amount of gravitational gradient near the EH), black hole rotation, and mass accretion rate are among the factors governing the behavior of the visible phenomena produced by infalling objects, like accretion disks and jets of ejected matter and energetic radiation. For example, even for supermassive black holes, tidal effects are significant if the object in question is large enough, like a nearby star, leading to tidal disruption events that shred the star into an accretion disk. It's been suggested that a star falling into a sufficiently large ultramassive black hole might disappear with much less visible effect. By the same token, a much smaller object that was small relative to the gravitational gradient of the BH might fall through the EH more or less intact. I think perhaps the key here is that in such cases, most of the energy of the object's gravitational potential would vanish beyond the event horizon and would never be seen. Perhaps somewhat nitpicky points, but I'm just suggesting that the gravitational gradient isn't by any means irrelevant, though you're absolutely right that it's not usually the primary cause of Xray or gamma ray emission. 
#37




Yeah, tidal effects can shred stars into an accretion disk, but there are a lot of other effects that can give you an accretion disk, too. I suspect that, for a supermassive black hole, dynamical friction is the dominant one, though I'm not certain on that.

#38




TBH I don't think it is entirely known what causes the emissions from quasars, though it is undoubtedly related to accretion. It has been suggested a significant portion of the emission may be related to the Penrose effect, which is the classical limit of Hawking radiation.

#39




You mean, that the accretion is spinning down the black hole? I've never heard that one. Wouldn't that require that the hole and its accretion disk somehow form with opposing angular momentum?



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#41




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See https://ru.wikipedia.org/wiki/%D0%A7...8B%D1%80%D0%B0 
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