Hi Alan,
Thanks for your comment.
In your essay you write;
"• The underlying mathematical language used to describe GR is the language of smooth
curved surfaces, using the "real" numbers, IR. There is nothing in Einstein's original
account of GR which lets a physicist "add" two states of space-time.
By contrast
• The language of SM is that of algebra over the "complex" numbers, C, using functions
on a flat space."
The reason for this is that complex numbers in quantum theory relate to the transverse wave components of the plane waves in 3D space. The scalar / real numbers relate to how matter is formed, where there are four solutions where these transverse waves cancel producing scalar spherical standing waves (the two spin states of the electron and its opposite phase positron). This is the spherical (ellipsoidal) geometry of matter found in GR.
I did not understand your comments about complex quaternions and Geometry. Apparently Clifford called his complex quaternions 'Geometric Algebra', and David Hestenes uses this term, there is no loss of geometry in using them.
Hope this helps (and sorry for very late reply!)
Cheers,
Geoff
PS - You will also find Declan Traill's essay interesting, it shows a deduction of Einstein's GR based on Euclidean space assuming the velocity of light varies with the energy density of space (waves in space are non-linear).
http://fqxi.org/community/forum/topic/1363