
SpaceTime Invariance Group  Classify Lorentz × dilatation (i.e. Weyl) group orbits in the highdimensional vector space of PseudoRiemannian curvature structures on a pseudoorthogonal vector space, especially ( for the physical case of 4dimensional Minkowski space ) those of dimension 4. In
 H Tilgner The Group Structure of PseudoRiemannian Curvature Structures J. Math. Phys. 19 [1978] p 11181125
 H Tilgner Conformal Orbits of Electromagnetic PseudoRiemannian Curvature Tensors – Electromagnetic Implies Gravitational Radiation Lecture Notes in Mathema
tics 1156 [1984] p 317339 Weyl group actions were chased around decomposition and diagrams of Singer & Thorpe's (linear) space of all curvature structures.  Singer & Thorpe curvature structure theory  a great step for the mankind up from linear to multilinear algebra 
Gravitational and Electromagnetic Radiation  Gravitational waves are defined in terms of a lightlike vector [Lic], annihilated by the wave curvature structure ( lets drop the terminus structure in the following ). This is an orbit property with respect to the pseudoorthogonal group, even its Weyl group. Candidates of gravitational wave orbits are those special wave orbits which are not of electromagnetic origin – these are not strong enough to be measured and they can't be ,seen'. It should be possible to derive the frequency for every such pure gravitational wave orbit, in order to get an experimental verification of gravitational waves. Mathematically on a PseudoRiemannian manifold there is a LeviCivita connection which gives rise to a curvature. It remains to show, that a wave curvature lies the the subspace of Weyl curvatures only(?). The other curvatures, are parametrized by selfadjoint ( with respect to the Minkowski form ) transformations. Since their Jordan algebra is simple, there can be no such lightlike vectors at all!  ,curvature' means curvature structure
in the following 
Classification Still Unsolved  But which Weyl curvatures are determined by wave vectors? Since the (full) Weyl group acts transitively on the nullcone, and group actions permute with Weyl and Einstein projectors in Singer & Thorpe's curvature space, one perhaps can show, that there is only one wave orbit in the subspace of Weyl curvatures, which then would exhaust the space of Weyl curvatures, which is a ( awfully high ) n(n+1)(n+2)(n3)/12  dimensional subspace in the whole curvature space, for the dimension of which one has to add n(n+1)/2, the dimension of the ( Jordan algebra of ) selfadjoint endomorphisms. A structure theory usually means that there is a decomposition into ( the direct sum of ) subspaces in terms of projectors, like Singer & Thorpe's one of all curvatures, such that there is a structure theorem. A classification by orbits is a refinement of such a structure theory with the help of a transformation group, if such a group exists and if it commutes with the projectors onto the subspaces. If there is only one (gravitational) wave orbit there arises the question what are the other ones. Are one or two ( the number depends on the signature of the underlying bilinear form ) given by timelike or spacelike vectors of the underlying vector space? For this we need an explicite construction of a curvature in terms of these vectors, like we have one in the three cases by endomorphisms below. There is such a construction by using tensor products of such a vector, to get an endomorphism and insert it into the three constructions below. In the 'mixed' case  the electromagnetic one  we cannot arrive by this construction in the space of Weyl curvatures. In the other two cases we have to find 'odd' constructions in order to arrive only in the space of Weyl curvatures.  projectors are a great mathematical achievement,
wellknown as quantum mechanics' statistical operators
but never used
in gravity
and the symplectic phasespace formulation of classical mechanics
why? 
Categories Matter!  Electromagnetic orbits are given by two physical tensor fields, the field strengths E ( a 3vector E together with a skew 3×3 matrix H ) and the intensities B, in media without structure and special relativity's flat Minkowski space with a linear dependence in between. Physicists take them as skew 4×4 matrices – which is erronous! Because this means the introduction of a positive definite metric in spacetime and an Äther(category) which – doesn't exist! We have to formulate electrodynamics entirely in the Minkowski category, which means that E and B have to be skewadjoint with respect to the Minkowski form! Even exterior algebra formulations by exterior differential forms lead into a dead end – we have to take a Clifford algebra ( over the Minkowski space ) instead, wellknown in physics from Fermi statistics.  to get rid of Äther, reformulate Maxwell's equations 
Petrov's Classification  Petrov's classification by bivectors is one in the n(n1)/2dimensional Lie algebra of 2nd power elements in this Clifford algebra, which if formulated basisfree gives a very elegant formulation of the commutation relations of pseudoorthogonal Lie algebras. Therefore his eigenvaluebased curvature classification is one by orbits of the above Weyl group in this Lie algebra. Which of Petrov's orbits lie in the space of Weyl curvature and which not?  
Embedding Electromagnetic Fields into Gravity  Implementing electromagnetics into general relativity usually is done by inserting the energy momentum tensor into Einsteins gravitational field equations as a source ( i.e. on the right hand side ) and solving for metric, connection and curvature. Nomizu [Nom] has given a direct method to construct a curvature from two skewadjoint endomorphisms ( take electromagnetic E and B ) similar to the construction of curvature in terms of two selfadjoint endomorphisms in Singer & Thorpe's original structure theory of curvature. There remains to show that Nomizu's construction leads to a physical electromagnetic field, solving Einsteins field equations in terms of a (which?) electromagnetic energymomentum tensor. Post [Pos] remarks that if E and B are linearly connected ( like in most media ) the 4tensor relating them has the same symmetries as a curvature tensor, i.e. fulfills Singer & Thorpe's curvature axioms. Since he starts from only one field strength E, his method leads to a first question: Start in between Singer & Thorpe's and Nomizu's curvature constructions by one selfadjoint A ( Post has the identity ) and a skew electromagnetic field E, which, when inserted into the given curvature, delivers the skewadjoint ( with respect to the Minkowski form ) intensity B. In [♦ p 14] it was shown that for this ,mixed' case, there are only solutions in the space of nonWeyl curvatures, none in the space of Weyl ones, and if A especially is the identity, even these two vanish. In fact, the only possible curvatures constructed from such A's and E's are of the form E•A and the trace thereof, where the • is the natural (Lie on Jordan) algebraical adjoint adaction by inner derivations [♦]. So the only nontrivial embedding of electromagnetics into gravity remains Nomizu's! But this only is the case because all three constructions ( that by two selfadjoint or that by two skewadjoint endomorphisms or the mixed one ) start from the trivial scalar 1dimensional curvature.  
Post's Gravity Contribution to Electromagnetic Intensities  Post's construction inverted leads to more: Given an electromagnetic field strength E in a gravitational field, given by its curvature R, construct the electromagnetic field intensity B in the same way as the two mathematical approaches from the trivial curvature, but this time more general from the given R. Clearly this modification of R by E no longer is a curvature: Two of Singer & Thorpe's curvature axioms are not fulfilled  skew symmetry ( because E is skew adjoint ) and the Bianchiidentity ( that's why they are not interesting for mathematics ). But it is clear from the construction that the third still holds, i.e. the outcome B is in the pseudoorthogonal Lie algebra and hence may be interpreted as an electromagnetic field intensity. This has a physical consequence: The full gravitational field ( not only its scalar part, i.e. the trivial curvature ) contributes to electromagnetic field intensities. Since there is no point in spacetime without gravitation this contribution never vanishes. Half way between two adjacent galaxies, this contribution is neglectible, but near the center of a galaxy, near the event horizon of a black hole, or for a strong gravitational wave, these curvaturemodified intensities B can have measurable physical effects. Only in a universe with constant curvature, actually that one by Irving Segal, studied on this webpage, this contribution reduces to the usual one.  B(x,y) = R(x,Ey)
for all vectors x,y of Minkowski space

  
