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How thick is a black hole event horizon?

  1. Aug 5, 2012 #1
    hi, im not a physicist so sorry if this is a stupid question, its just curiosity.

    how thick is the light like event horizon of a schwarzschild black hole,
    for instance, what the closest distance scale that an infalling photon and an escaping photon be, and whatI is "inbetween"? ive heard that when observed from outside you would never see the photon reach the EH,it just slows and becomes flattened against it,but from the photons point of view nothing out of the ordinary happens,it just crosses the horizon into what seems like normal flat space time,though the maths shows something dramatic has occurred when you pass r = 2MG. is this "thickness" planck length or area or something else?
    Last edited: Aug 5, 2012
  2. jcsd
  3. Aug 5, 2012 #2
    As far as classical GR is concerned, it is a surface. That is, infinitely thin.
  4. Aug 5, 2012 #3

    thanks, i saw a couple of lectures about the entropy of such a BH and that the maximum entropy of any region of space is its,slightly smaller i think, bounded area in planck units and not volume and was just in wonder at how much goes on there.
    Last edited: Aug 5, 2012
  5. Aug 5, 2012 #4
    A somewhat puzzling aspect of black holes is that their information content is proportional to the surface area of the horizon rather than the volume of the interior....this is a general feature of such horizons, not just black hole horizons....a related feature is that everytime a larger horizon is envisioned, it too encapsulates all the interior information...in other words every time you look at a larger horizon all the information is THERE....so exactly where is the information content is located is somewhat puzzling....is observer dependent.

    While the entropy of a black hole is large, it is not maximu...it it were the black hole would be stable and in fact they slowly decay..via Hawking radiation...

    Seems we have much to learn about horizons, information and entropy.
  6. Aug 5, 2012 #5

    many thanks,i have a lot to learn before i can think intelligently about theoretical physics.
  7. Aug 5, 2012 #6


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    Just for added encouragement, cosmology has reached a stage where advanced knowledge of particle physics and quantum mechanics is extremely valuable - not to mention the Phd level skills you need in mathematics.
  8. Aug 5, 2012 #7
    thanks,you are right and hopefully during my lifetime i may gain some understanding,nevertheless i will always keep a towel handy.
  9. Aug 5, 2012 #8


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    I've repeatedly seen that kind of "thickness" or horizon fuzziness postulated and used in the analysis of BH. I'm sorry but I cannot recall if the tolerance used is always planck length or some multiple of it. This is just my unreliable impression but what I recall is that people analyzing the effect of dropping something into a BH consider that the particle (or whatever) is morally part of the BH as soon as it comes within this designated "thickness" of the event horizon. My impression is that this plays a significant role in some of the analysis.

    I may not be able to find a link for you which illustrates this. But someone else may find it easy to provide one.
    EDIT: Europa, I went and quickly looked at three recent LQG black hole papers where (among other things) they were analyzing what happens when you drop a test particle into a BH and it gets absorbed and the horizon area increases and settles at a new level etc. In no case could I find this "finite thickness" invoked. But I remember seeing it in several papers some time back, not necessarily LQG papers, possibly in some other framework. I strongly suspect that your intuition is correct, quantum BH analysis does require some planck scale indeterminacy of radius and/or horizon area. But I can't help you with a link to a paper where that is made explicit. Hopefully someone else will be able to.
    Last edited: Aug 5, 2012
  10. Aug 6, 2012 #9
    These mathematical constructs are not easy to unravel via some ascribed physicality and would seem to require you to specify the type of horizon one is discussing: absolute, relative or stretched.

    The stretched horizon does not seem to be frequently discussed in these forums:

    I saved this very concise explanation of the other two thus: {did not record the author}

    The apparent horizon is a local (instantaneous, discontinuous) phenomenon...It would not seem to have any
    dimension. The absolute horizon (continuous, forms at the center of mass and moves outward) is global...not likely it has a dimension either.

    And another related phenomena is the photon sphere. The position of the photon sphere is the apparent dividing line between black hole and sky..outside the event horizons.
    Wiki is down right now, but I know it is discussed there. I am not positive if this is considered a 'horizon'; I think not but am unsure exactly why.

    Physicsforums has a description...looks like it has no 'thickness'??
  11. Aug 6, 2012 #10
    And this gets weird when you have a cosmic event horizon. Once dark energy pushs a galaxy through the cosmic horizon then all of information is still there.
  12. Aug 7, 2012 #11
    Apparently even the Hubble sphere is an event horizon of sorts {despite repeated earlier claims in these forums that it is not} from which :

    Gravity appears to make horizon entropy finite. Space, time and quantum fluctuations in his perspective emerge from thermodynamic degrees of freedom considerations: Because the Hubble Horizon limits degrees of freedom, a symmetry which I suppose of a vacuum that is broken, 'stuff' becomes emergent!!!! Another way to say this, I think, is that analogous to GR where geometry creates gravity, a thermodynamic 'equipartition' view can show expanding geometry spawns particles.

    Feynman says "The truth is always simpler than you thought."
    I have not yet gotten THAT far!
  13. Aug 10, 2012 #12
    many thanks to @marcus,@Naty1 and @twofish-quant,
    im very happy that there is a lot of "strangeness" there,and your insights have pointed me in many areas to explore.
    thanks to your specifications i have much much food for thought!
    i have found a set of lectures covering this topic by Leonard Susskind,though the title is misleading(topics in string theory)it covers exactly the very features of interest that you guys described.
    There are 9 lectures,im only about 2 lectures in,and theres lots of areas of maths that i have to learn as i go along,so i had better settle down for a while.
    You guys are great! Horizons are great, and im going to start to learn physics which i should have done years ago
    thanks again
    Last edited: Aug 10, 2012
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