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Thread: Can someone elaborate on what Leonard Susskind said about black holes?

  1. #1
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    Can someone elaborate on what Leonard Susskind said about black holes?

    Especially the part about black holes smearing things from 3D to 2D and the universe being a sort of hologram.

    I understand Susskind won this debate against Hawking and it's now accepted fact is this correct?

    I've even watched some of his lectures/talks on youtube and still can't wrap my mind around the concept/fact or which ever it is.

    http://en.wikipedia.org/wiki/Black_hole_complementarity

    http://physics.stackexchange.com/que...f-a-black-hole

    The black hole part of this I kind of understand, I still don't understand how the stretched horizon can exist. What I really don't understand out of this is the universe as a whole: http://io9.com/5860931/are-we-just-a...f-the-universe

  2. #2
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    First off this is based on the holographic principle which is only shown to be rigorously true for some formulations of String Theory. Susskind is a String theorist and takes String theory as correct as part of his starting point. That is not proven yet, not by a long way. So when he says it is accepted what he means is that in the String Theory community it is accepted. Outside that realm it is not so clear cut. The entropy of a black hole scales with surface area, not volume, and so in some sense you can describe a 2D boundary which 'contains' all the entropy. But showing that more rigorously leads you to the AdS/CFT correspondence which is relates conformal field theories (theories similar to QED/QCD) to Anti-de Sitter spaces (used in String Theories).

    So the whole "Universe is a hologram" thing is in itself a massive simplification of a very complex subject, the validity of which is mostly tied to String theory. You do see a lot of junk science articles about it, however. It is actually a rather complex piece of mathematics that is still not amazingly well rooted in physical theory.

  3. #3
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    Or put in more simplistic terms, as Bekenstein and Hawking argued in the mid-70s, the amount of information that a black hole can store is a function of its surface area, not its volume as one might naively expect. So the maximum amount of information that can be stored in any volume of space will always be bounded - the same amount of information that can be stored on the surface of a black hole that contains exactly that volume of space. So for any chunk of space, a full description of it could be encoded by the surface that encompasses it - much as a 2-dimensional hologram can encode all the information of a 3-dimensional scene. If this is true for a chunk of space, it should be true for the whole universe. (Of course, we're rather ignorant of any "surface" encompassing the universe.)
    Last edited by Cougar; 2014-Apr-28 at 02:12 PM. Reason: typo
    Everyone is entitled to his own opinion, but not his own facts.

  4. #4
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    Quote Originally Posted by Shaula View Post
    First off this is based on the holographic principle which is only shown to be rigorously true for some formulations of String Theory. Susskind is a String theorist and takes String theory as correct as part of his starting point. That is not proven yet, not by a long way. So when he says it is accepted what he means is that in the String Theory community it is accepted. Outside that realm it is not so clear cut. The entropy of a black hole scales with surface area, not volume, and so in some sense you can describe a 2D boundary which 'contains' all the entropy. But showing that more rigorously leads you to the AdS/CFT correspondence which is relates conformal field theories (theories similar to QED/QCD) to Anti-de Sitter spaces (used in String Theories).

    So the whole "Universe is a hologram" thing is in itself a massive simplification of a very complex subject, the validity of which is mostly tied to String theory. You do see a lot of junk science articles about it, however. It is actually a rather complex piece of mathematics that is still not amazingly well rooted in physical theory.
    The underlying mathematics is much more general than string theory or even physics in general, and derives from the Erlangen program in geometry. The Erlangen program, which is essentially the modern mathematical approach to geometry, considers a geometry to be a 2-tuple of a homogenous space and an automorphism group. So for instance affine geometry is the tuple of and the affine group (wich preserves points, lines and triangles), euclidean geometry is the tuple of and the euclidean group (which also preserves distances and angles), and so on. Now as it turns out there exist quite a lot of distinct geometries whose automorphism groups are isomorphic, so even though distinct the geometries do "correspond" in some way. This tends to lead to the existence of certain dualities between physics theories formulated in those geometries. One such example would be the isomorphism between the authomorphism groups of n-dimensional Anti-deSitter space and (n-1)-dimensional conformal space, leading to dualities between gravitational theories in Anti-deSitter space (such as some GR solutions) and conformal field theories in one-less dimensional space, which is the AdS/CFT correspondence. However this is not the only example of this. While this is far from trivial mathematics it is a very interesting thing to look into if you're interested in where these dualities come from.

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