On possible embeddings of the standard model of particle physics and gravity in $E_8$

I investigate the structure of $E_8$ under the action of the subalgebra/subgroup $A_1+G_2+C_3$, as a potential route to unification of the fundamental forces of nature into a single algebraic structure. The particular real form $E_{8(-24)}$ supports a decomposition into compact $G_2$ plus split $A_1+C_3$, which allows a restriction from $G_2$ to $SU(3)$ for the strong force, together with split $SL_2(\mathbb R)$ to break the symmetry of the weak interaction and give mass to the intermediate vector bosons. The factor $C_3$ contains various copies of the Lorentz group $SL_2(\mathbb C)$ and extends the `spacetime' symmetries to the full group of symplectic symmetries of real $3+3$-dimensional phase space. Restricting $G_2$ to the Standard Model $SU(3)$ extends $C_3$ to $A_5$, in the real form $SU(3,3)$, acting on a complex phase space that includes both momentum and current. There is then a natural restriction from $SU(3,3)$ to $SO(3,3)$, describing the action of $SL_4(\mathbb R)$ on phase space. The resulting action of $SL_4(\mathbb R)$ on $E_8$ includes tensors that are equivalent to the stress-energy tensor, the Ricci tensor and the Riemann tensor, and therefore permits the formalism of general relativity to be developed inside $E_{8(-24)}$. The model then suggests unexpected and perhaps subtle ways in which general relativity and particle physics may be forced to modify each other, in order to produce a unified theory.