On the space-time split and 11D
There is an overwhelmingly good justification for: a space-time split, a space-extra dimensions split and 11 dimensions - the experimental evidence of physics.
1) There-exists over a century of experimental evidence for a split between space and time. The last physicists to think that they had discovered superluminal travel was possible were sacked because it turned out that they had a cable loose - no great surprise to me.
2) Extra dimensions can successfully unify particle forces with the force of gravity in extensions to General Relativity, where the success of GR is describing gravity gives it the status of a physical theory. The dimensions have to be compactified to get this effect, but also because there is no experimental evidence for macroscopic extra dimensions.
3) The particle charge count explaining the table of the experimentally discovered particles is - 3 colour 3 isospin 1 hypercharge = 7. By Noether's theorem, a conserved charge is associated with a continuous symmetry, and each extra dimension would give one extra symmetry. Hence minimum number of extra dimensions = 7.
The experimental evidence supports the conclusion that there-exists a minimum of 11 dimensions, which is why most extra-dimensional theories have arrived at 11: not orthodoxy, it's the conclusion from the physical evidence. For physical space being a manifold of S3, S7 in a physical GR theory, the chiral Higgs vacuum and the correct table of particles (S0) are derived - Witten was wrong about KK-theories because he didn't consider a closed universe. In GR, the dynamics of this closed universe is necessarily cyclical S1. The dimensional reduction of the extended GR derives 4D GR and the quantum field theory of the Standard Model. This is a physical theory as much as GR and the Standard Model are. Note that QFT is derived within my theory, and thus ALL QT predictions are also prediction within the scope of my theory - including ALL the spin correlation results, without exception. If QT predicts it, then so do I, because I predict QT. Thus ALL the experimental evidence of physics fits with the physical manifold of the universe being S0, S1, S3, S7.
Mathematically, this S0, S1, S3, S7 corresponds to the closed spaces in R, C, H, O and has dimension 11 - the justification for this is physics. In my book I ended the chapter with the conclusion that this "leaves the search for the unification of physics without arbitrary fields with nowhere left to go". In this year, nothing has changed - still true.
If a maths theory fails to produce the 3 features given above, then it just isn't physics - end of story. All of physics doesn't fit into 8 dimensions. No space-time split, then it's not physics. No distinction between spatial dimensions and extra dimensions, then it's not physics. I make no apologies for abiding by the old fashioned physics standard of physical evidence. I know this goes somewhat against the ideology of the last 40 years where Maths was assumed to trump Physics, but that route has proven to be a failure. Super-symmetry - dead, string theory - dead. In reality, Physics trumps Maths.
Michael