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I was hoping for an answer in general terms avoiding things like holonomy, Chern classes, Kahler manifolds, fibre bundles and terms of similar ilk. Simply, what are the compelling reasons for restricting the landscape to admittedly bizarre Calabi-Yau manifolds? I have Yau's semi-popular book but haven't read it yet, nor, obviously, String Theory Demystified :)

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See also on the nLab for more: ncatlab.org/nlab/show/supersymmetry+and+Calabi-Yau+manifolds –  Urs Schreiber Sep 15 '13 at 13:00
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3 Answers 3

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There is one simple reason: in such scenario the physics at the string scale has supersymmetry. Supersymmetry (more technically $N=1$ supersymmetry) has some nice phenomenological features that make it an attractive bridge between low energy physics and string theory. The existence of this symmetry translates directly to the requirement that the compactification manifold is Calabi-Yau.

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Well,yes I realise that superstring theory means supersymmetric string theory, but was unaware that restricted the manifolds of the extra dimensions in that way. –  Gordon Feb 10 '11 at 23:07
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There is extended supersymmetry at very short distances, but then part of it is broken by what manifold you choose for compactification. For Calabi-Yau manifold the remaining amount is what is most attractive for phenomenology, the minimal amount. –  user566 Feb 10 '11 at 23:10
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Since the word "supersymmetry" did not appear in your list of forbidden words let me give you this answer:

Because Calabi-Yau manifolds leave unbroken some part of the original supersymmetry, which is advantageous for model building.

But there are alternatives to Calabi-Yaus, like flux compactififcations or large extra dimensions.

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how can large extra dimensions be consistent with everyday experience? –  lurscher Feb 11 '11 at 16:20
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"Large" means "much larger than the Planck scale", but it still can be tiny. Interestingly, experiments do not rule out large extra dimensions of sub-millimeter size. See, for instance, arxiv.org/abs/hep-ph/0011014 –  Daniel Grumiller Feb 11 '11 at 19:01
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We can have compactifications over 7D manifolds with a $G_2$ holonomy, or an 8D manifold with an $SO(7)$ holonomy. We can have orbifolds, or flux compactifications. We can have warped compactifications like $AdS_5 \times S^5$.

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