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Can't black holes just be super dense objects? They could still be black (having the color of black never really required special physics, after all) and have a really strong gravitational field. If we suspect that it actually absorbs light due to its gravity then it is possible that there is gravity strong enough to capture light, not letting any radiate out when matter falls into this object.

I'm just having a hard time accepting that anything can exist with an infinitely large property as it would lead to infinitely large mass leading to infinitely large forces... that would just destroy the universe infinitely fast, wouldn't it? So I think a supermassive object can be a badass just as well without having to be a singularity.

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    $\begingroup$ A singularity is just where our laws of gravity and space-time break down, it has infinite potential because it cant be accounted for by our current rules, not because it has somehow amassed all the matter in the universe and become infinitely dense. $\endgroup$ – Dean Feb 19 '16 at 15:24
  • $\begingroup$ @Dean is correct. With our current math singularities are impossible. The equations we have show that if they existed their influence would extend out infinitely. The only problem is that nobody has yet determined the boundaries between classical physics and quantum physics. They appear to be completely separate and distinct right now. The math has not caught up to reconcile how a singularity could exist without exerting gravity on everything else in the universe. $\endgroup$ – SDsolar Jun 27 '17 at 7:56
  • $\begingroup$ @Dean Yes, my initial idea was that black hole phenomena is possible to be explained without singularities. Light cannot escape its gravity, so what? It looks like photons can be captured with a powerful enough gravitational field. Maybe we could find the threshold value, sure it's a huge number but why should it be infinite? $\endgroup$ – stevie Jun 29 '17 at 9:09
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This answer is to some degree opinion-based. I share your scepticism about the existence of strict mathematical singularities as General Relativity would predict. This is mainly, because the assumption of a strict singuarity ignores quantum theory. One approach to overcome the singularity is a Gravastar. Related is a Planck star. Both approaches try to overcome the paradoxes near the singuarity. A full answer might eventually be provided by a still to be defined quantum gravity.

Another difficulty for "real" black holes is rotation. The Schwarzschild solution is not to be expected to occur in real-world black holes.

Instead a Kerr solution (or might be a Kerr-Newman metric) will get closer to real astronomical objects, hence including at least rotation, and might be some residual electric charge.

A massless particle travelling with the speed of light results in an undefined energy according to Special Relativity. Nature opens a physics dedicated to massless particles by allowing them to take any energy to resolve this undefined range. In a similar way nature may open a new kind of physics inside black holes to resolve the paradox between General Relativity and quantum theory.

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  • $\begingroup$ Thanks, @Gerald, Gravastars to the rescue! Just what I needed. Finally, some theorists who are not comfortable with infinitely nasty objects populating our universe! You know, it's just pure feeling, but to me, black holes sound like some hypothesis from the middle ages with wormholes and all. $\endgroup$ – stevie Feb 19 '16 at 15:47
  • $\begingroup$ Well, wormholes (Einstein-Rosen bridges) would be consistent with General Relativity, and without need for singularities. But they may be instable for other quantum theoretical reasons, at least when assembled as a macroscopic "time-machine". Kip Thorne once discussed this in his book "Black Holes and Time Warps" in rather detail: en.wikipedia.org/wiki/Black_Holes_and_Time_Warps $\endgroup$ – Gerald Feb 19 '16 at 16:00
  • $\begingroup$ I disagree that this is opinion based. Maybe the OP didn't write it correctly but the issue is real. If there was a singularity in a black hole the event horizon would not be definable as being finite. It will take a real breakthrough to reconcile the quantum world with the classical world. Right now we don't even know the boundary conditions where the math must switch from one to another, let alone mesh smoothly. $\endgroup$ – SDsolar Jun 27 '17 at 7:59
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What is a singularity?

A singularity is a point in the universe where matter is infinitely dense. The singularity is at the centre of the black hole, and is often hiding behind an event horizon. So basically, a singularity is a point in space where a lot of matter is squeezed together in a very small space. When you have a singularity matter does not go to infinity. Density is what goes to infinity. Density and matter are different things.

A singularity is often created in a supernova explosion. A supernova is where a star is caused when there is so much matter in the centre of a star (at the end of its life) that the star cannot support its own gravitational force, and it collapses in on itself and explodes.

The second way a supernova can happen is in a binary star system - where if there is a white dwarf, it steals matter from the other star and eventually it accumulates so much matter that it explodes.

A singularity is caused by the big stars, not small ones. Small stars that explode in a supernova create something called neutron stars.

So what is a black hole?

A black hole is a area in space with a gravitational field so strong not even light can escape. Black holes don't have the colour black, they don't have a colour because we can't see any light coming back from them. So no, light is not absorbed. Black holes can also eventually dissipate, due to a mechanism called Hawking Radiation

There are multiple types of black holes including:

  • Supermassive black holes (often found at the centre of galaxies)

  • Stellar black holes

  • Primordial black holes

  • Rotating (Kerr) black hole

Finally, there are 3 main areas inside a black hole.

Diagram of a black hole

This diagram can show us many things, for example, how black holes don't have a gravitational pull which goes across the whole universe.

On a final note, we can work out the area where even light cannot escape by using the Schwarzschild radius. Seen as you said this answer shouldn't include relativity, I won't include any maths either. The Schwarzschild radius, in summary, is where the event horizon is in a black hole.

Summary

Sorry for providing a awkwardly long answer, I was having trouble understanding what you didn't understand, and I thought that if I explained a singularity - then I'd surely help you, sorry if I've just said things you already know. I've provided some Wikipedia links if you want to have a look at some of the terms I left in italics. You might also want to look at black hole thermodynamics

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    $\begingroup$ Okay so here is what I'm skeptic about: Apart from being able to imagine infinity as a mathematical phenomenon, it may not make sense in the physical universe when applied to objects. Just because something has an inhumanely large mass we have no reason to believe that it collapses into something infinite. Large values are nothing special in space. Is this idea for historical reasons, so when scientist first realized that there are huge masses out there, they supposed they might cause singular behavior? If so, why not also fantasize about gamma rays with infinitely high frequencies? $\endgroup$ – stevie Feb 19 '16 at 15:44
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    $\begingroup$ The singularity is a point with 0 volume. You are getting mixed up with some things, I think. Mass does not go to infinity. The mass becomes so dense, its volume becomes 0. You said "inhumanely large mass", the mass does not go to infinity. The density goes to infinity. A singularity is only created when the point has infinite density. @stevie $\endgroup$ – Daniel Cann Feb 19 '16 at 18:12
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I think you are starting off from the premise that there is some way of stabilising an extremely dense object inside the event horizon of a black hole. In the classical theory of General Relativity the singularity is inevitable. Once inside an event horizon an object is compelled to move towards, and reach, the singularity in a finite time, in the same way that you are compelled to move forward in time whether you like it or not.

Therefore it is impossible to have something in classical GR that looks like a black hole and has an event horizon, but which does not form some sort of singularity. However, we know that classical GR must break down on extremely small (quantum) scales so it is entirely possible that something happens to prevent singularities in a quantum theory of gravity.

So one is perfectly free to invent a version of General Relativity that allows for some sort of avoidance of a singularity. But there are (at least) two requirements. (1) It should explain all the other things that classical GR explains perfectly well. (2) It should have observable consequences outside the event horizon, because otherwise it's pointless navel gazing (IMO).

NB: Wormholes and such-like may be possible in (classical GR) rotating black holes, but my limited understanding is that these form despite the singularity (which is no longer a point). In other words, the singularity is still there, but matter does not inevitably end up in it (but goes somewhere else!)

The stuff you've written about infinite mass, infinite force etc. is just wrong. The mass of a black hole is well-defined as are its gravitational effects. A finite mass still arises from taking the integral of an infinite density over zero volume.

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It is common to describe a singularity as a point of infinite density, but really they are more general than that: they are some sort of pathological behaviour in the spacetime metric. Whether singularities are actual physical phenomena or they just demonstrate the point at which a theory (in this case general relativity) can no longer describe nature is a matter of debate.

Penrose's singularity theorem demonstrates that under a certain set of very reasonable assumptions that the gravitational collapse of an object like a star inevitably leads to a singularity. Note though the proof is of geodesic incompleteness (i.e. the time-lines of some free-falling object end abruptly). This is not (necessarily) the same as the apparent 'infinite density' curvature singularities that occur in idealized black hole solutions like the Schwarzschild metric.

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  • $\begingroup$ Does this mean that singularity can arise without inifinite values? It would mean that at very high values things might behave differently from our current knowledge? Without such values being infinite? $\endgroup$ – stevie Feb 20 '16 at 20:26
  • $\begingroup$ A curvature singularity is where the curvature "blows up" at some point in spacetime (i.e. there are infinite values that you can't get rid of). It isn't the only kind of singularity, for example it is easy to construct a spacetime that has no curvature, but still has a conical singularity. Singularity theorems tell you next to nothing about the exact nature of the singularity, it may be possible that curvature singularities are features of highly-symmetric metrics only, on the other hand it seems very plausible that they are not. $\endgroup$ – John Davis Feb 20 '16 at 20:39
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There's many good answers to this on a physical level, but I don't see this one addressed, so I'll give an answer briefly.

I'm just having a hard time accepting that anything can exist with an infinitely large property as it would lead to infinitely large mass leading to infinitely large forces...that would just destroy the universe infinitely fast, wouldn't it?

If black holes have infinities, which is only possible if they have a singularity, then those infinities are, by definition, infinity small and not infinite from any distance away, even a fraction of an atom's distance away. So "destroy the universe by infinite forces", sure, if the universe was smaller than an atom, or maybe, inside the event horizon where it couldn't do anything but fall towards the singularity, but at any safe and reasonable distance, black holes aren't dangerous and not a threat to the universe.

So I think a supermassive object can be a badass just as well without having to be a singularity

and

Gravastars to the rescue! Just what I needed. Finally, some theorists who are not comfortable with infinitely nasty objects populating our universe!

OK for starters, the laws of physics are what they are, and they don't care what we think. As to things being "nasty", that's in the eye of the beholder.

2000 years ago, "hellfire" was magma seen coming out of the occasional volcano and hell was inside earth. Today, the inside of the Earth generates a magnetic field that protects us and gives us plate tectonics, which is really useful for life bearing planets. The inside of the Earth hasn't changed, but our perception of it has changed enormously. Now we love the inside of the Earth, but 2,000 years ago people feared it.

100 years ago, everyone thought the Universe was the Milky way and 80 years ago, they thought the Universe was eternal, until that lousy Hubble suggested the Universe had a beginning and, well, so much for things being eternal. If Hubble had said that 300 years earlier he'd have been burned at the stake, no question about it, with a plaque that stated "blasphemer"

That's the problem with point of view. It's not a complete picture. Black holes make a great "boogie man", as something that eats everything and cannot be escaped, but that's only part of the big picture.

Einstein himself found black holes a distasteful idea (He wasn't too keen on quantum mechanics either), so Einstein imagined the Universe had some physical law that prevented black holes from ever forming and that might even be the case, but honestly, how different is it to be slowly squashed around the event horizon over an eternity vs quickly squashed in the singularity or whatever goes on in the center. From a certain point of view, it's pretty much the same thing.

Some theories have gone so far as to suggest black holes are magical places with entire baby universes inside of them. I find that more creative thinking than science, but the truth is, what happens inside the event horizon stays inside the event horizon and nobody knows.

On black holes in general:

They're very useful. The formation of black holes, and the gravitational collapse and rebound off the collapse, creates and distributes heavy elements across the galaxy and in the collapse that creates a black hole or Neutron star, something like 90% of the matter of the star gets blown off, recycled, if you will, back into the galaxy and only 10% or so, forms the collapsed core.

You can think of the black hole as a nasty little remnant of a star's death, but I find the fact that large stars recycle and distribute so much of their matter across the galaxy to be highly cool. Supermassive black holes also help in the formation of galaxies, so the simple truth is, black holes are very useful, even if you wouldn't want to cross paths with one.


Now on what happens inside a black hole, there's some good answers to that already and I don't want to make this too long, especially since I'm a layman, but I find the speculation of that exotic region inside the event horizon to be great fun to think about.

I personally don't believe in singularities. I (THINK), that the wave and field nature of quantum mechanics and the fact that empty space has properties which, for example, particle anti particle pairs can form essentially out of nothing, (which make hawking radiation possible), I think there's probably some kind of exotic space of never a full singularity.

I don't think there's what might be called a physical material inside a black hole. I think things would behave differently than that, more like the exotic nature of a proton or electron than the physical nature of a surface, but that's just my thoughts on the subject. Without a quantum theory of gravity, it's a bit like the blind looking at a map though. Nobody knows. (too long?)

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  • $\begingroup$ Okay so I guess my real problem is why scientists saw it necessary to estimate an infinite density here or singular behavior. Just because a mass is huge and concentrated in a little volume, it should be nothing special. Happens all the time in the universe :) What makes them believe there is more going on there than huge gravitational forces? $\endgroup$ – stevie Feb 21 '16 at 16:27
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I believe that black holes are singularities in the sense that matter in it, is not-defined to have any meaningful location or known velocities. I don't think it means singularity in the sense of having zero radius.

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