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Do black holes really exist?

  1. Jul 4, 2010 #1
    Do we have empirical evidence proving that black holes, rather than stars with density close to black holes, exist in our universe? To put it more clearly, are astronomers able to distinguish a black hole from a compact object with a surface gravitational redshift of, say, z=100? Of course, if you consider the (not well understood) neutron equation of state, then you would consider such an ultra-dense non-BH star to be impossible. However, I'm asking for observational evidence alone, without reference to any theoretical "plausibility argument".
     
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  3. Jul 4, 2010 #2
    How long do you think it would take for causal evidence of an event hozizon to reach an observer?
     
  4. Jul 4, 2010 #3
    Infinite for sure. However, if we can detect regions near the event horizon with enormous redshift, it would also add credibility to the existence of black holes. However, I'm not aware of any observation of grav. redshifts exceeding z=100 (I have no clue about actually numbers, so feel free to correct me). For example, the accretion disc could emit ultraviolet radiation, redshifted 100-fold to infrared, and get received by our infrared telescopes. If at such high redshift, we still only observe the accretion disc rather than a star surface, then the existence of BHs would be much more compelling.

    I agree with you that the direct observation of the event horizon may be impossible due to the infinite time needed (maybe with the exception of Hawking radiation which is too weak for current technology), but I'm unsatisfied with the fact that the experimental proof of BHs remains shaky, with nothing like what I described in the previous paragraph. Of course, I'm aware that experiments are limited by technology, but scientists should be honest about what has been proved, and what has not been proved beyond reasonable doubt.
     
  5. Jul 4, 2010 #4

    George Jones

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  6. Jul 4, 2010 #5
    The faithful regurgitation of doctrine is not one of my common sins, though the infatuating mystery of black holes is strongly expressed on this forum. There is no evidence, that I am aware of, that can distinguish a black hole from an incipient black hole and none of the faithful have been forthcoming to correct my weak mindedness.
     
  7. Jul 4, 2010 #6

    bcrowell

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    If this is the standard of proof you're looking for, then I think the answer is no, there is absolutely no empirical evidence for the existence of black holes that provides that level of model-independent proof. There have been a ton of exotic objects proposed that would be more compact than neutron stars but that would not be black holes. These include black stars, gravastars, fuzzballs, quark stars, boson stars, and q-balls. If you take semiclassical gravity seriously, then there's considerable doubt about whether black holes actually form; there was an article about this in Scientific American, Oct. 2009, by Carlos Barcelo, Stefano Liberati, Sebastiano Sonego, and Matt Visser.
     
  8. Jul 4, 2010 #7
    Thanks George. That's exactly the sort of thing I was looking for. Seeing a small dark (instead of bright!) spot surrounded by hot luminous accretion gas would certainly be very convincing evidence. I've always wondered whether this can be done for Sgr A* which is the only BH with a relatively large angular diameter. It does seem to be within our reach in the not-too-far future. I'm looking forward to learning about the actual results they get.
     
  9. Jul 4, 2010 #8
    Right, so what is the point of considering something for which direct causal evidence is never going to be obtained and indirect evidence is very hard if not impossible to distinguish it from near collapsing stars but not quite, like neutron stars?
    I think the media and popular science has done a lot of harm, but the appeal to mistery is very strong, I guess.
     
  10. Jul 4, 2010 #9
    There is more at issue here than a standard of proof.

    Would you argue that all black holes are older than the universe? An event horizon cannot, in principle, have a causal influence in a universe that has existed for finite time.
     
  11. Jul 5, 2010 #10
    Of course popular science can distort things in the effort to simplify, but you seem to be blaming the media for something else entirely here which doesn't make sense to me.

    Science tries to come up with predictive models/theories that consistently fit with all known observational data. Scientists want to understand how the universe "works". If you feel fault is on order here because people think science backs up the existence of blackholes, I don't understand your view of science.

    I could ask if there is direct proof quarks exist. Have you ever held one? Have you ever seen one? Can you prove without indirect evidence that they exist? If we want to get ultra strict, ALL measurement has to assume some kind of model for how the measuring device worked. How do we know the reading is even correlated with what you actually think we're measuring? Even concepts such as a measurement being more "direct", just comes down to being more "model independent", which in discussing scientifically just means we're using an even more general model which subsumes many theories.

    Science is not just built on observations. If it was, then science would be nothing more than a list of measurements. Science necessarily includes models. No one is claiming these models can't be wrong. But to reject something because it is based on indirect evidence from a model that has yet to mismatch with experiment, and furthermore rejecting this model without a suitable replacement ... how can you possible feel this would be better for the media to do when presenting science to the public?

    It sure had a causal influence on the star it began from. So it clearly had a causal influence in the universe.

    If you have a better model that can fit all the observational data and doesn't lead to blackholes, go ahead and publish it. Technically, science can never prove a model, but only disprove models. I don't feel the ultra-strict sentiment you are pushing here has any advantage.

    Maybe I'm reading into it to much. Do you just mean to encourage the sentiment: remain curious, always question, always explore, etc.? I think everyone here agrees science isn't a religion with the equivalent of infallible statements. So you don't need to convince us.
     
  12. Jul 6, 2010 #11
    I've seen some exotic proposals for "denser than neutron" matter. But do any of these "exotic matter" equations of state allow dense enough material to actually push the critical mass beyond that of the super massive blackholes at the center of galaxies?

    I think I remember reading that. If it is the paper I remember, I thought the authors made it very clear this would not prevent all blackholes. It was in essence a "dark star" solution ... a proposal for a quantum back reaction that could infinitely delay collapse of some black holes (essentially give a different equation of state density limit once including this back reaction).

    Even if I'm unfortunately remembering another paper, the conventional take was that you should always be able to scale up a blackhole to make it "classical" regardless of the quantum effects. An event horizon is only a global entity. For a quantum effect to break all blackholes, it would somehow need to violate Lorentz symmetry on these huge scales ... for a free falling observer (in falling with dust which in the far future would become a singularity) could know about the pending event horizon from local measurements.

    I remember reading a paper once about blackholes in DeSitter spacetime which claimed there could be some strange issues. I don't remember the details. I know that is vague, but by any chance do you remember that discussion?

    -------------
    EDIT:
    The "Dark star" Scientific American paper doesn't appear to be freely available now. The "teaser" they give sounds like what I remember, but I found the paper the SA article is most likely based on, and they makes claims differently than I remember (I probably just remembered wrong, or maybe they toned down some claims for the SA article).

    Fate of gravitational collapse in semiclassical gravity
    Author(s): Barcelo C (Barcelo, Carlos)1, Liberati S (Liberati, Stefano)2,3, Sonego S (Sonego, Sebastiano)4, Visser M (Visser, Matt)5
    Source: PHYSICAL REVIEW D Volume: 77 Issue: 4 Article Number: 044032 Published: FEB 2008

    There is a free arxiv copy on the same subject by those authors here: http://arxiv.org/abs/0902.0346
    They mention the equivalence principle, and say it is implicitly held. I don't see their point of view here. It seems their proposal strongly violates the equivalence principle, since they claim the effects don't go away even if the size of the "would be" black hole is scaled up. This seems very suspect for reasons already mentioned.

    Their paper was cited by:
    "Radiation from collapsing shells, semiclassical backreaction, and black hole formation"
    Author(s): Paranjape A (Paranjape, Aseem)1, Padmanabhan T (Padmanabhan, T.)2
    Source: PHYSICAL REVIEW D Volume: 80 Issue: 4 Article Number: 044011 Published: AUG 2009

    Abstract: We provide a detailed analysis of quantum field theory around a collapsing shell and discuss several conceptual issues related to the emission of radiation flux and formation of black holes. Explicit calculations are performed using a model for a collapsing shell, which turns out to be analytically solvable. We use the insights gained in this model to draw reliable conclusions regarding more realistic models. We first show that any shell of mass M, which collapses to a radius close to r=2M, will emit approximately thermal radiation for a period of time. In particular, a shell that collapses from some initial radius to a final radius 2M(1-epsilon(2))(-1) (where epsilon < 1) without forming a black hole, will emit thermal radiation during the period M less than or similar to t less than or similar to Mln(1/epsilon(2)). Later on (tMln(1/epsilon(2))), the flux from such a shell will decay to zero exponentially. We next study the effect of backreaction computed using the vacuum expectation value of the stress tensor on the collapse. We find that, in any realistic collapse scenario, the backreaction effects do not prevent the formation of the event horizon. The time at which the event horizon is formed is, of course, delayed due to the radiated flux-which decreases the mass of the shell-but this effect is not sufficient to prevent horizon formation. We also clarify several conceptual issues and provide pedagogical details of the calculations in the Appendices to the paper.
     
    Last edited: Jul 6, 2010
  13. Jul 6, 2010 #12
    Clearly this is a distraction. Need I point out that the question is one of causal influence on matter on this side of the event horizon?

    Now we're getting somewhere. What are the observational differences of an incipient event horizon and an event horizon. And define what you mean by 'lead to'.

    My sentiments oppose an abuse of written and spoken language that leads directly to conceptual error--that is, bad science.
     
  14. Jul 6, 2010 #13
    Ugh. You seem to have missed the point. We have models/theories that fit observable data well.

    "Keeping an open mind" does not equal throwing out our current best understanding of matter in neutron stars and GR.

    No, steadfastly refusing the predictions of our current best understanding of the universe is bad science.

    If we learn something about quantum mechanics or gravity or evidence of new forms of matter, and someone comes up with new models that cover these observations better, then science as a whole will shift to these new models. Seeking new and alternate explanations keeps things fresh, and is good for the scientific pursuit. But not being open minded enough to even seriously consider the current models is, "bad science".

    Do you agree with the statement:
    To the best knowledge of our current scientific understanding, black holes exist.

    That is all we are saying. No is claiming there can't exist some exotic forms of matter stable at higher densities than neutron stars. We are not being close minded here. But on the other side you seem to be close minded and unwilling to accept current scientific understanding.

    What exactly is your prejudice against event horizons anyway? Do you understand that in GR they are just a global feature, and there's nothing special happening there locally? Even in Semi-Classical treatments in Swarzschild vacuum, it has been proven nothing special happens at the event horizon: as it should just based on expectations from the equivalence principle in classical GR.
     
  15. Jul 6, 2010 #14
    Then you should keep in mind that the map is not the territory.

    Excuse me?? You have been very mistaken at your attempt to understand my points.

    In the domain physics, and more so in regards to causal horizons, such questions such as "Are there black holes?" are ill posed questions. You can present a nice detailed drawing of a black hole with little light cones included, but as I've alluded, this is the map. If we were both to talk about the map, we would be in fine agreement. The verbiage must change to talk about the physical world.
     
  16. Jul 6, 2010 #15
    Given our current understanding, start with a star and let it evolve: will a blackhole form? The language of science is mathematics and the question is not ill posed.

    Are you saying that you consider the question "What is Sagitarius A*?" to be outside the realm of science?

    Your whole "map" comments just sound like vague philosophical complaints. Make your complaints more precise. Seriously, do you understand that in GR nothing special happens at the event horizon? That spot is like any other on the spacetime manifold. Why do you want to ad hoc say that our understanding of the universe fails at that location?
     
  17. Jul 6, 2010 #16
    See previous post. This statement would be equally and validly answered as "black holes will never form." You must specify a coordinate system first.

    However, there are wrong statements that can be made. When images of the centers of galaxies are presented and statements made to the effect that an accretion disk has formed around a black hole, such statements require the presumption of a black hole that has not only existed for infinite time on the cosmological clock, but has the same observed radius as had at time of the big bang.
     
  18. Jul 6, 2010 #17
    Is there a future?
     
  19. Jul 6, 2010 #18
    Oh come on:
    Will the sun rise tomorrow?
    Can you prove you exist and aren't just another being's dream?

    What, now you want to debate what it means to "exist" and don't even accept induction since we must assume it since it can't be directly measured or proven?

    You are throwing around vague crackpottish philosophy nonsense as if you were arguing scientifically.

    If you were claiming there are better models than our current scientific understanding that would be one thing. But you claim if we talked about the model of scientific theories regarding our universe, that this isn't talking "about the physical world". The whole goal of science is to understand the physical world, and we do so through experiments and predictive models. I'm not saying our understanding is proven in stone. Science can't talk about philosophical Truth. But if you think science isn't talking "about the physical world", then you are rejecting the whole scientific method.

    Imagine you were going to dive into a black hole. You are claiming it is outside the purview of science to predict what you will encounter crossing the event horizon. Why? Please answer with science instead of vague philosophical nonsense of your struggling with the word "exist".
     
    Last edited: Jul 6, 2010
  20. Jul 6, 2010 #19
    Answer the question.
     
  21. Jul 6, 2010 #20
    Induction is a necessary assumption of science. We could not do science without it.

    So yes, without the ad hoc assumption that the universe will suddenly stop existing, I will use induction to say that there is a future.

    EDIT: Yep, the ultra basic parts of our scientific model of the universe were correct. There was a future. Yeah science!
     
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