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There are M2 branes and M5 branes in M theory. M2 branes can turn into strings. I heard that Protons and nuetrons are wrapped M5 branes.

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  • $\begingroup$ -1. No research effort. ... Where did you "hear" this? $\endgroup$ Jan 15, 2017 at 2:38

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In every example I can think of, the elementary particles are M2-branes stuck to something. For example, in the "G2-MSSM", the gauge bosons are M2-branes stuck to singular hypersurfaces (a crease in the compact dimensions), and the standard model fermions (therefore electrons, and quarks too) are M2-branes stuck to cusp-like points on those surfaces. Similar comments apply if you uplift Type IIA string theory or heterotic string theory to M-theory.

Protons and neutrons are made of quarks and gluons in some way that is not really understood. There are many models, based on different approximations to QCD. In one such model, they are a type of soliton called a skyrmion.

In Sakai and Sugimoto's holographic QCD, which is a string-theory approximation to QCD, the skyrmion actually is a wrapped brane as you say, a D4-brane; which should correspond to an M5-brane. However, Sakai and Sugimoto use D8-branes as the base for the open strings that are the quarks in their model, and the last I checked, no-one knows how to represent D8-branes in M-theory.

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    $\begingroup$ Don't D8-branes come from 'end-of-the-world' M9-branes? $\endgroup$ Jan 16, 2017 at 15:22
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    $\begingroup$ @Meer Ashwinkumar They probably come from some sort of M9-brane (doesn't have to be end-of-the-world), but M9-branes themselves are not understood... Thanks for reminding me about this. $\endgroup$ Feb 3, 2017 at 12:39
  • $\begingroup$ @MitchellPorter Are these M2 branes wrapped to become fundamental strings? Or are they unwrapped M2 branes? $\endgroup$
    – user86072
    Feb 18, 2017 at 17:19
  • $\begingroup$ @2117 It depends. In a IIA uplift, the 11th dimension is a circle and M2s are wrapped around it. In heterotic M-theory, the 11th dimension is a gap between 10-dimensional boundaries sukidog.com/jpierre/strings/mtheory.htm and the M2s stretch across it. In M-theory on a G2 manifold, the M2s are unwrapped. $\endgroup$ Feb 19, 2017 at 2:49
  • $\begingroup$ @MitchellPorter When we finally figure M theory out, and it turns out that protons and neutrons are infact M5 branes, wouldnt that conflict with observation? Since protons and neutrons have a measurable size that we can detect, and M branes are supposed along the planck scale. $\endgroup$
    – user86072
    Feb 25, 2017 at 17:11
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That's going too much in one step. So first see the reduction to string (say IIB) as you already stated. Then how you get more elementary particles from there. Finally you get from those to protons and neutrons. Those are steps in order of decreasing difficulty.

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