Given the prior discussion of Joy Christian's work on FQXi I thought it might help to clarify how my S10 unified field theory arrives at the same conclusion: it is all about the Hopf spheres S0, S1, S3 and S7. This might initially look like yet another case of putting mathematics before physics - the cart before the horse - that many of the essay entrants have pointed at as being a problem with physics. However, I arrive at this conclusion from the physics side, where the crux is identifying the form of Quantum Theory as being that due to a change in mathematical representation from natural-number terms denoting particle numbers, to real-number terms denoting particle numbers, in order to escape Gödel's incompleteness theorem where the wave property of a particle is the non-derivable feature in classical physics. This would mean that the matter fields of Quantum Theory are not fundamental and so cannot be just introduced into a unified theory of physics - matter must originate by some other means. This suggests re-considering extensions to GR, but not to forget the physics!
The metric of GR is conceptually a grid laid out over a surface in order to define distance measurements. An example which grounds the physics is to imagine an inflated party balloon and drawing lines of latitude and longitude on the balloon with a felt tip pen. This grid defines a metric field for the surface which expands and contracts as the balloon inflates and deflates. The Einstein tensor gives how this metric field changes with the volume of the balloon, where the form of the Einstein tensor is based upon the physical assumptions that the space is both homogeneous and isotropic. Add the physical conditions that the space is finite but without a boundary, and spheres are the simplest surfaces meeting these conditions. Now to say that there is no space-time in GR is analogous to imagining that the balloon blinks out of existence leaving the ink of the pen lines hanging in thin air - a mathematical map without its physical territory!
So we leave the territory where it is, keep the physical conditions specifying spheres, and then remember that Kaluza and Klein successfully unified gravity and electromagnetism by extending the number of dimensions with a closed S1 dimension associated with the U(1) symmetry group of electromagnetism. The condition of space being a closed sphere S^N and the S1 group space of electromagnetism gives the key to particles, as any theory where a symmetric space S^N is broken in some way to give a space containing S1 will give rise to topological monopoles. Such particle-like objects would be of the form of a hole in the space, like an air bubble in water.
In the same way the U(1) electromagnetic group space S1 corresponds to a compactified dimension in the orginal Kaluza-Klein theory, the S3 group space of the SU(2) isospin group would also correspond to compactified dimensions. There is the apparent problem that the colour group SU(3) doesn't have a simple correspondence to some space, so we will just denote it X for now. If the operative 'hole' of a wormhole is inserted into some sphere S^N it will change the topology to that of a higher dimensional torus S^3*S^M where the form of the closed spatial universe is the sphere S3. The problem is then to solve for S^M to get the particles as topological defects and for the colour space X to be something sensible - the solution is S^7 for which the colour space X = S^3 corresponds to colour group SO(3). As the space of monopoles and anti-monopoles is S^0 = {-1, 1} the Hopf spheres S0, S1, S3 and S7 are all physically realised. So physics arrives at the mathematical condition, it is just then a lot simpler to say it as the meta-principle: it is all about the Hopf spheres!
The S3 is the physical space of a closed universe and S7 consists of the compactified dimensions of a Kaluza-Klein theory, which I refer to as the particle space as it gives the properties of the topological monopoles as particles. The S^3 closed universe is locally flat R^3 and has S^7 particle dimensions at every point x. However, to get the conditions for particles as topological monopoles, there must exist a non-trivial global map from S7 to the spatial S3 universe. In local terms in R^3, this means that the orientation of S7 changes between two spatial points x1 and x2; in GR this change would be denoted by the metric, whereas in the dimensionally reduced theory it would be called the Higgs field. This physical S^7 space at every point x in the locally flat R^3 space apears to give the point at which to start considering comparisons with Joy's work.
Michael James Goodband