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String Theory claims strings to be one dimensional objects vs. dimensionless point particles of the Standard Model. String Theory also needs 10 or possibly more spatial dimensions to operate in. There is also a popular view in cosmology of a Holographic universe. i.e. Leonard Susskind's "Holographic Universe". ( From what I am reading in journals reduces the spatial dimensions of our universe to two dimensions. ) Are we to assume the two views, one of the universe and one of fundamental particles are at odds. ?

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    $\begingroup$ I don't understand the question - why would you assume the two views are "at odds"? The holographic principle is about bulk information being encoded on the boundary, and string theory is, well, string theory, and even includes a version of holography in it. Where is the alleged incompatibility? $\endgroup$ – ACuriousMind Jul 26 '16 at 21:33
  • $\begingroup$ In the the end the AdS/CFT correspondence would like both to be exactly the same theory, with both the 3D and the 2D versions being only different interpretations $\endgroup$ – Wolphram jonny Jul 26 '16 at 22:12
  • $\begingroup$ The holographic principle says there exist 2 spatial dimensions, the third I assume is the projection? Are we to assume the projection is not real? or is the theory saying you don't need it as far as encoding but that the 3rd spatial dimension actually exists.? Then string theory says there are 10 spatial dimensions. You see the conflict? It rests on what "projection" means in the holographic view. If the holographic view says there cannot exist more than 2 spatial dimensions that then there is an obvious contradiction to the 10 dimensional string theory. $\endgroup$ – Sedumjoy Jul 26 '16 at 22:41
  • $\begingroup$ no, the extra dimension of string theory are compactified, so that string theory describes a "mostly" 3D universe (like any physical theory, they want to describe our experienced world). The holographic principle says that this 3D universe in turn can be described as a 2d universe. $\endgroup$ – Wolphram jonny Jul 26 '16 at 22:46
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    $\begingroup$ I also wondered about this... Came across this article that states: "The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity." $\endgroup$ – Dominic Comtois Sep 15 '17 at 3:05
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So actually they are compatible. It says that a string theory in 3D for AdS (anti deSitter) spacetime which describes a universe with quantum gravity can be mapped (i.e., there is some correspondence) to a conformal field theory (CFT) in 2D, in the spatial boundary of that universe. This gave rise to the holographic principle, unproven but also not disproven, that a solution with quantum gravity in 3D can be instead solved as a CFT in 2D, so solve whichever is easier to solve (usually the CFT problem) and then map to the corresponding solution for the other one (usually the string theory, which normally does not easily, or uneasily, yield exact solutions).

The mapping, or correspondence, though not fully determined (ie, how exactly to go from one solution to the other) so still work to do, it is proceeding slowly case by case, and so it is more a suggestive relationship than a real theory. I believe there's been a few, very small specific identified mappings.

The person who proved that one example of the correspondence (AdS to CFT) was Maldacena. A truly genial insight. Susskind and others have hypothesized that it is a general principle and can be used to find solutions that are otherwise hard to find. So far so good and there's been useful research done from it. It has given people also some ideas on how to try to figure out how to quantize gravity. It and a few other things have made CFT of great interest in theoretical physics.

Some people even hypothesized that the 3D world is an illusion and our real existence is in 2D. Most physicists ignore this (except for the few making money lecturing on it or writing speculative physics books on it), and do the real physics work.

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  • $\begingroup$ I love your last sentence Bob Bee. That forced me to give a thumbs up. Ain't it the truth though. $\endgroup$ – Sedumjoy Jul 28 '16 at 0:53
  • $\begingroup$ Thanks. I appreciate good physics in all its forms, and even science fiction, but not super-speculation (beyond the standard speculation). The AdS CFT correspondence is interesting without making us 2D beings $\endgroup$ – Bob Bee Jul 28 '16 at 1:37

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