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I Do event horizons really exist?

  1. Mar 8, 2016 #1
    If not, it seems to me Hawking radiation can't exist also.
    ???
     
  2. jcsd
  3. Mar 8, 2016 #2

    ZapperZ

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    This is a puzzling question in light of the LIGO result.

    1. There's plenty of evidence that point to the existence of blackholes

    2. Consequently, since these event horizons accompany blackholes, then the evidence supports the existence of these event horizons.

    So what is the problem here?

    Zz.
     
  4. Mar 8, 2016 #3
    Dear ZapperZ,

    I reffer you to to the following publication, for instance (there are many others. as the matter is far from decided:

    Barceló, Carlos
    ; Liberati, Stefano; Sonego, Sebastiano; Visser, Matt

    Affiliation: AA(Instituto de Astrofísica de Andalucía, CSIC, Camino Bajo de Huétor 50, 18008 Granada, Spain), AB(International School for Advanced Studies, Via Beirut 2-4, 34014 Trieste, Italy and INFN, Sezione di Trieste, Italy), AC(Università di Udine, Via delle Scienze 208, 33100 Udine, Italy), AD(School of Mathematics, Statistics, and Computer Science, Victoria University of Wellington, New Zealand)

    Publication: Physical Review D, vol. 77, Issue 4, id. 044032

    "While the outcome of gravitational collapse in classical general relativity is unquestionably a black hole, up to now no full and complete semiclassical description of black hole formation has been thoroughly investigated. Here we revisit the standard scenario for this process. By analyzing how semiclassical collapse proceeds we show that the very formation of a trapping horizon can be seriously questioned for a large set of, possibly realistic, scenarios. We emphasize that in principle the theoretical framework of semiclassical gravity certainly allows the formation of trapping horizons. What we are questioning here is the more subtle point of whether or not the standard black hole picture is appropriate for describing the end point of realistic collapse. Indeed if semiclassical physics were in some cases to prevent formation of the trapping horizon, then this suggests the possibility of new collapsed objects which can be much less problematic, making it unnecessary to confront the information paradox or the runaway end point problem."
     
  5. Mar 8, 2016 #4

    ZapperZ

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    And so? Does it really claim that BH doesn't have event horizon?

    Besides, is there a reason why you accept this, and discard others? How did you pick and choose?

    BTW, next time, don't hide anything. You should have cited this paper in the very beginning, rather than ask a rather vague question.

    Zz.
     
  6. Mar 8, 2016 #5
    Dear ZapperZ,

    My doubts were about the existence (or not) of black holes, as the matter is far from settled. I was asking if Hawking radiation persist even in the case of much simpler black stars. Wikipedia gives the following explanation:

    "A black star is a gravitational object composed of matter. It is a theoretical alternative to the black hole concept from general relativity. The theoretical construct was created through the use of semiclassical gravity theory. A similar structure should also exist for the Einstein-Maxwell-Dirac equations system, which is the (super)classical limit of quantum electrodynamics, and for the Einstein-Yang-Mills-Dirac system, which is the (super)classical limit of the standard model.

    A black star need not have an event horizon, and may or may not be a transitional phase between a collapsing star and a singularity. A black star is created when matter compresses at a rate significantly less than the freefall velocity of a hypothetical particle falling to the center of its star, because quantum processes create vacuum polarization, which creates a form of degeneracy pressure, preventing spacetime (and the particles held within it) from occupying the same space at the same time. This vacuum energy is theoretically unlimited, and if built up quickly enough, will stop gravitational collapse from creating a singularity. This may entail an ever-decreasing rate of collapse, leading to an infinite collapse time, or asymptotically approaching a radius less than zero.

    A black star with a radius slightly greater than the predicted event horizon for an equivalent-mass black hole will appear very dark, because almost all light produced will be drawn back to the star, and any escaping light will be severely gravitationally redshifted. It will appear almost exactly like a black hole. It will feature Hawking radiation, as virtual particle pairs created in its vicinity may still be split, with one particle escaping and the other being trapped. Additionally, it will create thermal Planckian radiation that will closely resemble the expected Hawking radiation of an equivalent black hole.

    The predicted interior of a black star will be composed of this strange state of spacetime, with each length in depth heading inward appearing the same as a black star of equivalent mass and radius with the overlayment stripped off. Temperatures increase with depth towards the centre."
     
  7. Mar 8, 2016 #6

    ZapperZ

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    First of all, you shouldn't use "Wikipedia" articles with me. I don't buy them.

    Secondly, that was what I asked you earlier. You should have just come right out and stated that you are not convinced that blackholes exist.

    Thirdly, note that I said that there are evidence pointing to the existence of blackholes. NOWHERE in my post did I claim that this is settled (as if there's such a thing for this type of phenomenon). My question to you is, why do you believe or accept contrary evidence in some, but dismiss the evidence pointing to its existence in others? How did you pick and choose what to accept? That question that I asked to you earlier, you ignored.

    The degree of certainty in something like this is less than what you get in, say, superconductivity phenomenon. And there are more data that are still being sought after. So of course people still discuss, debate, and argue about it. But look at how much we know, how much evidence there is for and against, and you decide why you decide on one versus the other.

    Zz.
     
  8. Mar 8, 2016 #7

    PeterDonis

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    If you read the very quote from Wikipedia that you posted, you will find the answer to this question, at least to the extent that it has an answer, since "black stars" are a speculative concept. Thread closed.
     
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