Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do Black Holes *actually* exist?

  1. Mar 3, 2013 #1
    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:

    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.

    Last edited: Mar 3, 2013
  2. jcsd
  3. Mar 3, 2013 #2
    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.
  4. Mar 3, 2013 #3
    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.
  5. Mar 3, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  6. Mar 4, 2013 #5
    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.
  7. Mar 4, 2013 #6


    User Avatar
    Science Advisor
    Gold Member

    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.
  8. Mar 4, 2013 #7


    User Avatar

    Staff: Mentor

    "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?"
    Last edited: Mar 4, 2013
  9. Mar 4, 2013 #8


    Staff: Mentor

    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.
  10. Mar 4, 2013 #9


    User Avatar
    Gold Member
    2017 Award

    Last edited: Mar 4, 2013
  11. Mar 4, 2013 #10
    Do black holes exist? Is there a singularity at the center of a black hole?

    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 Schwarszchild 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.
  12. Mar 4, 2013 #11

    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...
  13. Mar 4, 2013 #12
    How would one go about observationally proving that black holes do not exist?
  14. Mar 4, 2013 #13
    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 existance.
    Its no secret that both GR and QM cannot describe inside a blackhole. 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 blackholes exist. What happens inside the event horizon is anybodies guess.
    As far as emptying into another universe for me thats 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.
    Last edited: Mar 4, 2013
  15. Mar 4, 2013 #14


    User Avatar
    Science Advisor
    Gold Member

    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.
  16. Mar 4, 2013 #15
    You will find this article on the information paradox interesting then.


    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.
    Last edited: Mar 4, 2013
  17. Mar 5, 2013 #16
    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.
  18. Mar 5, 2013 #17
    Ah that more sense now. It was near impossible for me to figure out from the math but that isn't too surprising lol.
  19. Mar 7, 2013 #18

    Jonathan Scott

    User Avatar
    Gold Member

    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.
  20. Mar 7, 2013 #19
    MOND may have some successes but even MOND requires dark matter in some of their models.





    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 blackholes 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.
    Last edited: Mar 7, 2013
  21. Mar 7, 2013 #20


    User Avatar

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook