Do Black Holes *actually* exist?

[Zelyucha]

So I found this article(albeit rather dated) questioning they physical existence of Black Holes and what really struck me were some of the comments by physicists Jose M Pecina-Cruz who may or may not be on physicsforums but nevertheless, this is what he had to say about it:

The answer is that they do not. Matter can not collapse in a point, since a point does not have dimensions. Before matter collapses formation of particles antiparticles take place preventing a black hole formation.

Assume you have two neutrons approaching or collapsing one into the other, this process has a limit. Those neutrons never collapses since Heinsenberg uncertainty principle prevent they reach a separation shorter than their Compton wave length. Oppenheimer and Snyder recognized that their conclusion are based on the validation of Fermi Statics and this fails for distances shorter than the Compton wave length of the assemble of particles.

So there is also a more recent (scholarly) article here arguing that the Oppenheimer-Snyder collapse of a uniform ball of dust does not result in a singularity. But Pecina-Cruz' arguments are based in quantum mechanics which begs the question: Does quantum mechanics bind GR? By that I mean that GR can never violate Quantum Mechanics but the converse isn't necessarily true.

Theory aside, one of the defining Black Hole characteristics(I'm not going to cover naked singularities and the cosmic censorship hypothesis for now) is an event horizon. Now the thing is, TMK there already is a known object whose event horizon has been physically detected(& partially observed already)! It also shows evidence of gravitational lensing, swallowing up stars, and fame dragging. That object is at or very near to Sagittarius A* in the Galactic Center. And now there is this scientific white paper making the case from observational data that a black hole exists in Sagittarius A* and drawing a plan to produce an image of this black holes event horizon. So it appears that despite theoretical objections to Black Hole formation, such an object does indeed exist without a doubt. I think I answered my question earlier about QM binding GR: clearly it doesn't.

I'll try not to speculate much further but perhaps when the scalar curvature of space time reaches a certain threshold limit during gravitational collapse(since the curvature is negative here then I am speaking in terms of absolute value), the uncertainty principle of quantum mechanics actually breaks down.
Thoughts?

 

[chill_factor]

Your problem is that GR is a classical theory and you are trying to compare apples to oranges when you start talking about QM. In the regime of weak gravity (weak relative to the Plank scale, which applies to the event horizon of a stellar or larger sized black hole) GR is amazingly successful. Due to both theory and observation, it is very likely that a black hole event horizon does exist.

However, the singularity itself probably doesn't exist.

 

[Zelyucha]

Your problem is that GR is a classical theory and you are trying to compare apples to oranges when you start talking about QM. In the regime of weak gravity (weak relative to the Plank scale, which applies to the event horizon of a stellar or larger sized black hole) GR is amazingly successful. Due to both theory and observation, it is very likely that a black hole event horizon does exist.

However, the singularity itself probably doesn't exist.

In the case of a Kerr Black Hole, where the singularity is 1-dimensional(according to the theory), then perhaps what it is in reality is some kind of spacetime tube of whose cross section is no larger than Planck length as is essentially an infinite gravitational potential wherein quanta can move around inside of it but can never escape it. And perhaps Hawking radiation(which has yet to be observed) is the result of quantum tunneling through the infinite potential barrier.

 

[Drakkith]

In the case of a Kerr Black Hole, where the singularity is 1-dimensional(according to the theory), then perhaps what it is in reality is some kind of spacetime tube of whose cross section is no larger than Planck length as is essentially an infinite gravitational potential wherein quanta can move around inside of it but can never escape it. And perhaps Hawking radiation(which has yet to be observed) is the result of quantum tunneling through the infinite potential barrier.

Let's not speculate please. Hawking radiation takes place at the event horizon, not the singularity. And it's practically guaranteed that black holes exist. How else would you describe a mass of several million Suns packed into a volume of space smaller than our solar system? That's the supermassive black hole at the center of our galaxy.

 

[Zelyucha]

How else would you describe a mass of several million Suns packed into a volume of space smaller than our solar system? That's the supermassive black hole at the center of our galaxy.

An object with a mass of several million suns whose volume is smaller than our solar system is certainly going to have a very strong gravitational pull...But that alone is not enough to make it a black hole. A black hole requires an event horizon which shrouds a singularity; where the scalar spacetime curvature is infinite.

Now Hawking radiation takes place at the even horizon. That much is true. But if a singularity(particularly the ring singularity found inside of a Kerr black hole)is indeed describable using quantum mechanics as an infinite potential well, then this lends to the possibility of quantum tunneling whenever matter falls into the singularity after passing through the event horizon.

 

[Chronos]

You need only shrink a mass below its Schwarzschild radius to form an event horizon. GR classically predicts the mass will continue to collapse into a singularity, but, that remains under the microscope of modern physics. Its minimum size could be QM limited by its compton wavelength, Planck density, or some as of yet undiscovered law of condensed matter physics.

 

[russ_watters]

"Do black holes exist?" is a question of theory meeting observation. 'Can black holes exist?' would be a question on theory only. Why does the difference matter? As the OP mentioned, we've observed objects that fit a lot of the basic description.

The theory of what black holes are may have some holes (pun not intended), but it was good enough that physicists were able to theorize the existence of super-dense objects that weren't stars, then find them.

To me, the question isn't do/can they exist, it is "what exactly are they/how do they work?"

 

[anorlunda]

Do black holes exist? Is there a singularity at the center of a black hole?

Aren't those separate questions? The first could be true regardless of the second.

 

[DennisN]

Just want to point out that there also is a movie (movies?) showing stars orbiting a very large mass in the centre of the Milky Way. I have no link atm, maybe someone else here has one. Nevertheless, here's a page with info and links about the matter: ESO: A Black Hole at the Centre of our Galaxy.
EDIT: A clip about the movie I was talking about: ESO: Unprecedented 16-year long study tracks stars orbiting Milky Way black hole.

 

[Naty1]

Do black holes exist? Is there a singularity at the center of a black hole?

Aren't those separate questions? The first could be true regardless of the second.

yes... they are related but distinct questions...and also depend on your definitions of each.

I think there is general agreement that the horizons of black holes are fairly well described...there is little to suggest that at the Schwarzschild radius some theoretical or mathematical deficiency is evident. At the central singularity, however, I think it is fair to say there are uncertainties about exactly what is there...as nicely summarized by Chronos.

I know of no observational evidence to suggest black holes do not exist; so far, there seems to be observations strongly suggesting they do.

 

[Zelyucha]

To me, the question isn't do/can they exist, it is "what exactly are they/how do they work?"

I concur. The fact that there is no theory of quantum gravity shrouds the true nature of what lies beyond the event horizon a theoretical mystery.

However, from a mathematical standpoint the Kerr black hole(which best describes Sagittarius A* as there is observational evidence for both an event horizon and frame dragging) is a spacetime tunnel whose entrances are shrouded by event horizons that leads to whoknowshere...

 

[AnTiFreeze3]

... I know of no observational evidence to suggest black holes do not exist

How would one go about observationally proving that black holes do not exist?

 

[Mordred]

That last statement sounds like the Einstein Rosen bridge. Or generically the wormhole.
Even though the mathematics predict wormholes we have yet to confirm their existence.
Its no secret that both GR and QM cannot describe inside a black hole. Does the singularity reach an infinitely fine point?. Or is the Planch length the smallest size of the singuarity?
Thats one point GR and QM currently disagree on. The same applies to is spacetime granular or is it smooth at extremely small scales. No one really knows at this point.
I've even seen some articles that suggest that the wormholes and information end up in another universe. Not sure I buy that as its essentially unprovable. For now we know black holes exist. What happens inside the event horizon is anybodies guess.
As far as emptying into another universe for me that's about as likely as an electrician with a flashlights running around inside the EH looking for a circuit breaker.

I just had to use that last line from the big bang series lol.

 

[Chronos]

I would hedge my bet by asserting that event horizons exist - which has abundant observational support. Theoretically, what happens inside an event horizon is inaccessible from outside an event horizon. Oddly enough, the converse does not appear to be true - which I find interesting.

 

[Mordred]

You will find this article on the information paradox interesting then.

http://arxiv.org/abs/1209.2686

Its an article I am still puzzling through.
Particularly the statement in the conclusion that information is never in the EH.
Needless to say its been giving me hassles understanding it.

 

[Naty1]

Particularly the statement in the conclusion that information is never in the EH.

This is the Leonard Susskind argument that information can be thought of as smeared on the horizon...he calls it the stretched horizon about a Planck length outside th cosmological horizon...He describes such in his popular book THE BLACK HOLE WAR...it's a very interesting non mathematical treatment of his arguments with Hawking about information loss and his black hole complementarity theory.

 

[Mordred]

Ah that more sense now. It was near impossible for me to figure out from the math but that isn't too surprising lol.

 

[Jonathan Scott]

I think the "black hole" stuff is possible but far from proven.

GR is a neat and self-consistent theory and has been confirmed to high precision in the solar system, but that's only a tiny correction to Newtonian theory. However, the PPN approximation normally used to analyze that case doesn't lead to black holes, and neither does the closely related exponential potential approximation.

The accurate experiments have essentially looked at an expression of the form (1 - Gm/rc^2 + k(Gm/rc^2)^2 + ...) and confirmed that k has the correct value to match GR in the appropriate coordinate system.

We can certainly say that galaxies nuclei contain huge compact masses which would have to be black holes (a) if GR is accurate to sufficient terms and (b) if the internal solution actually allows collapse (which so far hasn't been proved analytically as far as I know).

However, if gravitational collapse does not occur, for example because the potential close to the surface is actually more like exp(-Gm/rc^2) or 1/sqrt(1+2Gm/rc^2) rather than sqrt(1-2Gm/rc^2) as in GR, then it will be difficult to distinguish that from the predicted properties of a black hole except that it won't be black, and could emit radiation from the surface, and it could for example have a significant intrinsic magnetic field in violation of the "no hair" hypothesis (although some people have suggested that it might be possible for it to have a "fossilised" field frozen in).

Funnily enough, galactic cores and quasars emit extreme amounts of luminosity (the "continuum background") from the vicinity of the core, which is thought to be from infalling material, although the luminosity pushes all theory to the limits, and there have been suggestions of evidence of polarization of radiation passing close to the core in a way which suggests an extremely strong magnetic field.

Also, the weird success of the MOND empirical formula for galactic rotation curves suggests that rather than dark matter, the explanation may be that we don't yet fully understand gravity.

Black holes may be the leading candidate explanation for what we are seeing, but are far from proven.

 

[Mordred]

MOND may have some successes but even MOND requires dark matter in some of their models.

http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/

http://arxiv.org/pdf/1112.1320v1.pdf

http://arxiv.org/pdf/1205.4880v2.pdf

http://arxiv.org/pdf/1205.1308v2.pdf.

Above are various links discussing some of the problems with MOND. Granted the inflationary model also has holes in it. The current LambdaCDM model has far more overall successes.
However despite that I do not feel that we have completely ruled out MOND however MOND needs significant work to reach the predictability of LambdaCDM.

The question I have is if MOND and its TeVes companion cannot do away with dark matter entirely then it cannnot deal with its own premise of development. In that regard what purpose does it serve ?
I've never come across any MOND base models that state black holes do not exit? Is there such ?
If so I'd still would be curious enough to read it as I do keep an open mind on all theories whether I have faith in them or not.
I've found that they all have useful insights in them even the most outlandish.

 

[rbj]

didn't they see a star take a sharp left turn going around what they thought is the super-massive black hole in the center of the Milky Way?

sounds like physical evidence to me.

i thought they had also seen "Hawking radiation" or the like, which results from rapid compression (maybe it's not compression) of a neighboring star's gasses getting sucked in, before it hits the event horizon. maybe i got that wrong.

 

[Mordred]

Yes they have plotted stars orbitting the BH in the Milky way. However I don't believe they found Hawking radiation. You may be thinking of the accretion disk/ jets which have been measured by their temperature variations. Higher temp as you approach the event horizon. This also shoots out in high energy radiation and we have spotted those jets.

It took a bit of digging through my old flash drive but I found the article I was hunting for. Its perfect for this thread. Keep in mind its 91 pages, however its one of the best articles I' ve read on BH accretion disk and other measurable properties.

http://arxiv.org/abs/1104.5499

 

[Mordred]

http://m.tgdaily.com/space-features/59481-hubble-directly-observes-a-black-hole-accretion-disk

Here is a brief article describing one instance where they have measured the accretion disk. However the article mentioned in my previous post is a desription of possible processes.

However this article shows measurements taken

http://arxiv.org/abs/1101.0811

 

[Jonathan Scott]

There is no shortage of evidence that compact super-massive objects exist. However, it is much more difficult to determine whether they have event horizons.

Metric factors of the form exp(-Gm/rc^2) as used in the exponential approximation to GR or 1/(1+Gm/rc^2) as from Machian models can both get arbitrarily close to zero, but unlike the GR metric factor sqrt(1-2Gm/rc^2) they cannot actually cross zero and give imaginary factors, so they do not give rise to an event horizon. However, at the typical minimum orbital distance of an accretion disk there is probably not much difference between the resulting effects at least from the gravitational point of view.

 

[morghen]

Theoretically, what happens inside an event horizon is inaccessible from outside an event horizon. Oddly enough, the converse does not appear to be true - which I find interesting.

Maybe the Hawking radiation characteristics will be the way to learn/see beyound the event horizon...

 

[Naty1]

Maybe the Hawking radiation characteristics will be the way to learn/see beyound the event horizon...

Not according to what is understood so far. HAwking radiation does not come from within the event horizon...nothing does.

 

[Mordred]

Section 12.2 of the first manual I posted above covers a methodology to distinquish between a neutron star and a black hole.

12.2 Black hole vs. neutron star accretion disks

Little of what we have said so far has depended on whether the central compact object is a black hole or neutron star, provided only that the neutron star is compact enough to lie inside the inner radius of the disk rin. In this case, its presence will not be noticed by the disk except through its gravity, which will be practically the same as for a black hole (an exception would be if the neutron star is strongly magnetized [113]). However, this does not mean that accreting black hole and neutron star sources will be indistinguishable, as we have not yet fully addressed the question of what happens to energy advected past rin. For optically thick, geometrically thin Shakura{Sunyaev disks (Section 5.3), a signi cant fraction of the gravitational energy liberated by advection is radiated by the gas prior to it passing through rin. Thus, the total luminosity of thin disks will not depend sensitively on the nature of the central object. However, this is not the case for the ADAF solution (Section 7), for which much of the thermal energy gained by the gas from accretion is carried all the way into the central object. Narayan and his collaborators [225, 215, 195, 107]
have convincingly argued that this may allow observers to distinguish between black hole and neutron star sources. The key is that, for black hole sources, advection through the event horizon allows the excess thermal energy to be eectively absorbed without ever radiating. For neutron star sources, on the other hand, the presence of a hard surface ensures that the excess energy of accretion is released upon impact and must be radiated to infnity. This implies that for systems in the ADAF state, a
black hole source should be signifcantly less luminous that a neutron star one with the same mass accretion rate [225]. Perhaps a more important point is that the range of luminosities should be wider for a black hole source than for a neutron star one [215]. This is because, while the luminosity goes as L / m_ for all neutron star states and for black holes in a high accretion state, it goes as L / m_ 2 for black holes in the ADAF state for which m_ < 102 101. Furthermore, since the luminosity is also proportional to the mass of the central object L / M, at the highest accretion rates a black hole source should be more luminous than a neutron star one due to its higher mass (Figure 23). Another key point to this argument is that neutron stars can independently and reliably be confirmed if they display type I bursts, which are thermonuclear ashes occurring in material accumulated on the surface of the neutron star [141]. Thus one can compare known neutron star sources against suspected black hole sources. This has now been done in a number of otherwise similar sources and Narayan's expectations have indeed been confirmed [215, 195, 107]
(a recent example is shown in Figure 23). This provides compelling observational evidence for the existence of black hole event horizons, although this falls short of being a proof [12]. This topic is discussed further in the review article by Narayan and McClintock [219].

Now as of the date of this article I'd have to check and see where Narayan and McClintock's methodology lead to. I haven't seen any recent articles to further add to this methodology. However if they are correct then distinquishing between the two is possible. Although not by gravitatonal means.

 

[Jonathan Scott]

I think that the above discussion about distinguishing between a black hole and a neutron star primarily relates to identifying the types of stellar-mass objects whose mass is around the maximum mass before collapse, which appears to be based on the standard GR assumption that a black hole will indeed form if the object has sufficient mass.

However, one point here is quite interesting, which is to note that if there is an event horizon, it will absorb a lot of the radiation from infalling material, but if there is no event horizon (which means there can be a surface being heated by the infalling material) then the luminosity can be very much greater. Given the surprisingly high luminosities seen for supermassive objects, especially in cases where there is no visible accretion disk, this would actually seem to fit observations better if there are no event horizons.

 

[Mordred]

This article pertains to measurements to determine if BH candidates have an event horizon.

http://arxiv.org/pdf/1205.4640v1.pdf


http://arxiv.org/pdf/1202.0348v1.pdf

here are two articles related to measuring event horizons specifically. I'll grant they are not entirely conclusive however I feel that event horizons ar real. Although I could be wrong the research findings I've read on numerous articles are leaning toward that conclusion

 

[Mordred]

I think that the above discussion about distinguishing between a black hole and a neutron star primarily relates to identifying the types of stellar-mass objects whose mass is around the maximum mass before collapse, which appears to be based on the standard GR assumption that a black hole will indeed form if the object has sufficient mass.

However, one point here is quite interesting, which is to note that if there is an event horizon, it will absorb a lot of the radiation from infalling material, but if there is no event horizon (which means there can be a surface being heated by the infalling material) then the luminosity can be very much greater. Given the surprisingly high luminosities seen for supermassive objects, especially in cases where there is no visible accretion disk, this would actually seem to fit observations better if there are no event horizons.

.

Sorry I didn't see this post, you raise an interesting point here I'll have to think about this.
I've been trying to find something more conclusive in regards to this methodology. However that's become quite a challenge.

Edit: after a few hours searching I'll have to concede that their is nothing one can infer as conclusive, although their are good indicators they all state that their is room for error or other interpretations.

 

[morghen]

Not according to what is understood so far. HAwking radiation does not come from within the event horizon...nothing does.

I don't get the negativism and the will to say it isn't so on this forum. What exactly has been observed or measured about the Hawking radiation? Nothing..or not much. So we don't know much about it yet you're sure it won't give us information about what's beyound the EH. How can you narrow your mind so much to exclude the fact that even if the radiation is coming from a surface outwards its characteristics can hint us on what's behind it? Perhaps there are differences in the characteristics of the radiation between BHs or between the studied portion on the EH. Perhaps there is something to it...perhaps not. But being one of the few things we think we know abou the BHs its narrow minded to say, NO it does not matter, we will not look into it.

Good day.

 

[Chronos]

You may find sonic event horizons interesting. It's basically a phenomenological test of GR. The most interesting part of this is space, time and gravity appears to be emergent properties of the universe. I find that highly interesting.

 

[Mordred]

That does sound interesting Chronos. Could you post some articles on it?

 

[Drakkith]

I don't get the negativism and the will to say it isn't so on this forum. What exactly has been observed or measured about the Hawking radiation? Nothing..or not much. So we don't know much about it yet you're sure it won't give us information about what's beyound the EH. How can you narrow your mind so much to exclude the fact that even if the radiation is coming from a surface outwards its characteristics can hint us on what's behind it? Perhaps there are differences in the characteristics of the radiation between BHs or between the studied portion on the EH. Perhaps there is something to it...perhaps not. But being one of the few things we think we know abou the BHs its narrow minded to say, NO it does not matter, we will not look into it.

Good day.

Because what MIGHT happen doesn't matter. There are an infinite amount of possibilities when talking about something that hasn't been observed, so unless we narrow it down to what we think SHOULD happen, what we believe will PROBABLY happen, we can't have a meaningful discussion. According to the model and the math, hawking radiation shouldn't tell us anything about what's behind the event horizon. Will it? Can it? We have no idea. But IF our current knowledge of black holes is sufficiently accurate, it will not. At least as far as I know.

 

[Mordred]

I don't get the negativism and the will to say it isn't so on this forum. What exactly has been observed or measured about the Hawking radiation? Nothing..or not much. So we don't know much about it yet you're sure it won't give us information about what's beyound the EH. How can you narrow your mind so much to exclude the fact that even if the radiation is coming from a surface outwards its characteristics can hint us on what's behind it? Perhaps there are differences in the characteristics of the radiation between BHs or between the studied portion on the EH. Perhaps there is something to it...perhaps not. But being one of the few things we think we know abou the BHs its narrow minded to say, NO it does not matter, we will not look into it.

Good day.

Let me explain some aspects of Hawking radiation. For one its a process of virtual particle pair creation where one of the pairs falls into the BH. This particle is lost to us. The other particle becomes part of the blackbody temperature. All this occurs outside the event horizon and does not originate inside the BH.

Now the part where Hawking states that BH's will eventually radiate and decrease in size has to do with tachyons. First I should point out that this is an extremely slow process. A process slower than the entire history of the universe. The exact magnitude Id have to look up.
I should also note is the tachyon is only theorized and has never been measured in any process.
Hence its not part of our standard model. However even if
we did discover tachyons it still would not provide any information on what's inside a BH.
Hope this helps

 

[enceladus_]

If BH's did not exist, how could one explain the orbits of stars at the center of the Milky Way? I don't know enough about them, but I haven't read that Neutron Stars, Magnetars, or Quark Stars(?) would be capable of making stars orbit at several million miles per hour.

Also, wouldn't the entire orbit of the Milky Way (or any galaxy centered around a supermassive Black Hole) be a big question as well?