This entry contains one chapter of geometry of physics. See there for background and context
previous chapter geometry of physics – manifolds and orbifolds
given a -principal bundle
a reduction of the structure group along is
reduction of the structure group along
is vielbein: definition of an orthonormal frame? at each point
example: the other 2 Maxwell equations: .
Given a homomorphism of groups , a G-structure on a -manifold is a lift of the frame bundle of prop. through this map
As in remark , it is useful to express def. in terms of the slice topos . Write for the given map regarded as an object in the slice. Then a -structure according to def. is equivalently a choice of morphism in of the form
In other words, is the moduli stack for -structures.
A choice of framing , def. , on a -manifold induces a G-structure for any , given by the pasting diagram in
or equivalently, via remark and remark , given as the composition
We call this the left invariant -structure.
For a -manifold, then a G-structure on , def. , is integrable if for any -atlas the pullback of the -structure on to is equivalent there to the left-inavariant -structure on of example , i.e. if we have an correspondence in the double slice topos of the form
The -structure is infintesimally integrable if this holds true at at after restriction along the relative shape modality , def. , to all the infinitesimal disks in :
Consider an infinity-action of on which linearizes to the canonical -action on by def. . Form the semidirect product . Consider any group homomorphism .
A -Cartan geometry is a -manifold equipped with a -structure, def. . The Cartan geometry is called (infinitesimally) integrable if the -structure is so, according to def. .
For an abelian group, then in traditional contexts the infinitesimal integrability of def. comes down to the torsion of a G-structure vanishing. But for a nonabelian group, this definition instead enforces that the torsion is on each infinitesimal disk the intrinsic left-invariant torsion of itself.
Traditionally this is rarely considered, matching the fact that ordinary vector spaces, regarded as translation groups , are abelian groups. But super vector spaces regarded (in suitable dimension) as super translation groups are nonabelian groups (we discuss this in detail below in The super-Klein geometry: super-Minkowski spacetime). Therefore super-vector spaces may carry intrinsic torsion, and therefore first-order integrable -structures on -manifolds are torsion-ful.
Indeed, this is a phenomenon known as the torsion constraints in supergravity. Curiously, as discussed there, for the case of 11-dimensional supergravity the equations of motion of the gravity theory are equivalent to the super-Cartan geometry satisfying this torsion constraint. This way super-Cartan geometry gives a direct general abstract route right into the heart of M-theory.
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Created on March 19, 2015 at 22:06:34. See the history of this page for a list of all contributions to it.