# How do we know blackholes are dense? Could instead they be a vortex?

1. ### taonut

10
I was wondering if we decided that blackholes are dense due to the spin and gravity? Are there any other factors for determining density? And could those factors also be true if a black hole were simply a vortex- like a drain per se. A drain that shoots out in two opposite directions, that shreds material into gas and shoots it out back into the galaxy or area surrounding it. Like a torus 4D phenomenon? My mind has a hard time accepting singularities, save the French philosopher Jean Luc Nancy's idea of such. Can somebody line this out for me?

2. ### gabriel.dac

50
The denser the object is, the greater its escape velocity will be. If a object is dense enough the escape velocity can equal the speed of light, so that's why it is black. Theorically, anything can become a black hole if you increase the density somehow. If the Earth was a black hole its diameter would be about 2 cm. The mass would still be the same.

3. ### taonut

10
Ok, but could also a vortex pose similar effects? Creating darkness and appearing to have density? Could the factors that we use to determine that a black hole is dense be the same factors that could determine that a vortex, like a tornado, exists instead in its place?

4. ### phinds

8,533
We don't really know WHAT the inside of a BH is, and since the math leads to the conclusion that there is a "singularity" of finite mass but zero extent and thus infinite density at the center, it is believe that the math has failed at this point and what we really need is a verifiable theory of quantum gravity that better describes what is actually going on.

Although we don't know what is going on inside the event horizon, I don't think it would be legit to say that it could be just anything and I doubt it's a vortex (but that's just my opinion, not anything I or anyone else, can back up with empirical evidence).

5. ### Jonathan Scott

1,342
Black holes are theoretical solutions of the GR field equations and their external properties are determined entirely by the maths. There's no choice about "interpretation" involved.

Physics admittedly has nothing useful to say about the singularity so far, but as it's beyond the event horizon it doesn't affect the external properties. The idea for example that it might lead somewhere else has no basis in standard physics.

Even the existence of black holes depends on the unproven assumption that the field equations of GR hold in that extreme case, and it is still to be determined as to whether observed black hole candidates actually have the properties predicted for a theoretical black hole (such as no significant intrinsic magnetic field). If for example the effective value of G decreased close to an extremely dense mass, then it might be that actual gravitational collapse would not occur, but it might be difficult to distinguish the result from a black hole, as it would still be extremely compact.

So far, black hole candidates have also been generally very far from black, typically being surrounded by extreme luminosity, because of the extreme temperatures to which infalling material is heated by friction.

6. ### PAllen

5,466
This is not true. An object of any density can have any escape velocity, depending on its size. For the mass of the milky way galaxy, the escape velocity would be c (thus it would be a BH) at a density 100,000 times less than air.

The actual relation is that: escape velocity $\propto$ R√ρ, with ρ being density. [edit: I guess, for completeness, per Newton, ve = 2R√((2/3)πGρ). GR difference is very small, and, in fact, ve per this formula being c matches SC radius per GR.]

Last edited: Nov 21, 2013

### Staff: Mentor

The Schwarzchild black hole is the GR solution to any spherically symmetric (and uncharged, non-rotating) mass distribution concentrated inside the Schwarzchild radius, and for sufficiently large black holes there's nothing especially extreme about those conditions.

The field equations also predict that such a mass will collapse to a point singularity of infinite density and that's the part that depends on the assumption that they hold even under extreme conditions. It's not likely that they do - more likely that some other physical effects come into play - but that does not prevent the black hole from forming.

8. ### Chronos

9,951
It is often convenient to treat the density of a black hole is as a ratio of its mass to the volume enclosed by its event horizon. As noted by PAllen, it can be a surprisingly small number. This illustrates the possibility matter inside a black hole need not be squashed out of existence to form an event horizon. What we haven't yet figured out is how to avoid this outcome. The atomic forces resisting compression have finite strength and it is easy to show this strength is exceeded in a body with sufficient mass and density. If you subscribe to the idea that gravity becomes repulsive at some finite energy level, problem solved. This is how the big bang singularity is avoided in some versions of loop quantum gravity and you get a bounce. It would, however, seem we should see this same bouncing effect in ordinary black holes, which is not yet evident. There remains plenty of work to do.

9. ### gabriel.dac

50
Yes, that is true. But I'm still not wrong. If I make any object more dense, its escape velocity will increase. I didn't want to explain much in my other post so I wouldn't be too obnoxious

10. ### PAllen

5,466
Didn't mean to be obnoxious, sorry it seemed that way. We all catch each others goofs here.

11. ### gabriel.dac

50
Nah I was talking about me. It'd boring if I explained everything in all details. I'd rather make a short post.

12. ### taonut

10
ok. so i have this idea from hurricanes etc... being from hurricane alley in south louisiana.. the middle is empty. and strangely calm. it's crazy spinning at i guess what we would call a hurricane event horizon that does the most damage. We see black holes emit laser-like beams when it "eats" and we also notice that for our galaxy there somehow appears to be just enough gas to keep replacing stars as they die pretty much at an equal rate. I'm thinking the event horizon shreds stuff and shoots it back out to be recyclyed in a torus kind of geometry/motion. I think that the swirling of the milky way creates the math that could appear like there is infinite density in a place where in fact there is nothing at all. I was wondering how the math would play out in this scenario. I don't believe in wormholes or any fanciful stuff defying the laws of physics. I don't think we will defy them or see them defied- which a singularity would in fact be a defiance at some point eventually. if the mass is recycled back out that would provide dust for new star formation. and why can't the brown dwarves we keep discovering account for the matter that was previously unaccounted for? i hate that we talk about dark matter like it's real when it's never been directly detected. it's assumed to fill in gaps in calculations, as much of black hole theory is as well. it's deceiving to the general public. To me it's as bad a theory as any religious theory that fills in gaps and claims to be truth without any form of proof. I know it's off topic. But I would like some expert opinions on these things, as I have never found a satisfying rebuttal for the way my mind interprets these things. i stayed very broad academically and took from several disciplines so everything crosses over in my mind, and astrophysics is no different. I just don't know the math and haven't the time to go back for another degree at this moment :) please help!? And please explain it thouroughly like you are teaching a student. I love to learn!

13. ### Naty1

that's ok because none have been observed. That doesn't mean they are impossible, just so far experimentlly unverified. Sometimes math makes great predictions we fail to believe, other time math makes faulty predictions....that why experimental verification helps so much.

generally no and definitely yes, respectively. I assume the spin angular momentum [energy] of a rotating BH adds to the energy density, but a BH might form without spin....or very minor spin.

Anybody know an approximate range of values for spin energy...like " accounts for roughly 20% to 30% of a solar size BH energy"...I made up those numbers for illustration.....

While you might think of the BH as a 'vortex' analogy, I guess, it surely shoots nothing back out. If there is a white hole on the 'other side', that may emit what is swallowed from our side. Once something gets inside the BH even horizon, it is gone until the end of the universe when things get cold and the BH shrinks in size.

Here is some perspective I found useful...from experts of these forums:

From prior discussions: for a non rotating non charged BH.....the 'simplist' BH.....

It seems most in these forums think GR describes the outside and horizon and some of the inside of a BH 'realistically'....but as you get closer to the predicted singularity at the center, things become less clear:

PAllen:
PAllen:
Dalespam:
pervect:
PeterDonis
and I think from Bill_k:

As you may know, the 'Kerr solution' is the one for a rotating BH. Wikipedia describes it.

14. ### taonut

10
ok but a spin can occur around empty space. so if black holes are calculated as being dense just because there is spinning around it, then it seems kind of strange to me. the spinning could just be created by stars orbiting each other and falling into a synchronistic orbit around a certain point that could be empty... is there any reason to believe the space is not empty? is there any math that proves it cannot, in fact, be empty? like the eye of a hurricane?

15. ### taonut

10
what i am saying is that we know there are phenomenon that create spin around empty space already, so why do we have to assume there is gravity involved in every spinning system at the center? I think based on general knowledge of life and physics and phenomenon, it would be safe to assume that anything we figure to have infinite gravity could, in fact, be purely empty space created by other objects with gravity falling into a synchronous spin. I can't find anything that can rationally refute this notion. I understand why the math leads people to fill in the gap, but I am not satisfied with the answer that is popular and parrotted back and forth between folks who don't feel like thinking about other possibilities. Or maybe they are thinking too narrowly. Because things can spin around empty space as much as they can spin around mass. It's just the spinning is created by mass in both cases, just sometimes things of mass orbit each other with emptiness in between. Creating a drain effect. And certainly an event horizon spinning at a certain speed could rip anything to shreds and shoot it back out. I see no reason why this is implausible, if not more likely.

16. ### taonut

10
and i would like some answers for this previous post as well :)

17. ### taonut

10
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I'm not sure on this one because we observe that black holes "eat" infrequently" and we also see them shooting out material (gamma rays, right?) and we also see that our milky way somehow always has enough gas for perfect replacement of stars (as many are born as they die, I saw it in a physorg article). So to me there is no proof that a black hole "shoots nothing back out" to my mind as there is evidence that they probably do. We have not observed a black hole eat without observing lots of activity- they call it a "burp" but I call it stuff getting shredded and recycled- as there is no way to measure how much mass is emitted, why not assume all mass is emitted back out? Why assume something that goes against the laws of physics when there are other explanations that do not?

18. ### phinds

8,533
That depends entirely on how much matter is nearby and is not a characteristic of the black hole.

Wrong by a mile. Any emissions are from OUTSIDE the event horizon.

You are beginning to sound like a troll

Again, emissions are from outside the event horizon.

Last edited: Nov 22, 2013
19. ### taonut

10
"emmissions are outside the horizon" ok so that's what i'm saying- the even horizon itself could be shredding matierial like the center of a hurricane is most active and the center is totally calm and empty.

let's not get mean here. i'm just not finding answers that really refute my main ideas that satisfy me and am genuinely curious. I was hoping for answers to my questions. Not to get called names. I am a lady now. And I like to see some class from those who know more than me while they teach.

20. ### taonut

10
Could somebody kindly read posts 14 through 17 and respond to my actual questions?

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