Are mass and charge manifistations of the same thing? Even though mass and charge are two different physical properties of matter, have there been any attempts at unifying them? say, by defining a physical property of matter such that mass and charge are naturally derived from it, and are regarded as different manifestations of the same physical phenomenon?
Kind of like how the electric and magnetic fields are related by Maxwell's equations (also the fact that a magnetic field is basically an electric field in a different reference frame), or how mass and energy are related.
 A: Yes, there have been attempts to unify these concepts. The earliest was done by Kaluza and Klein, who imagined that we lived in a $5$ dimensional universe, and the electromagnetic field was really a manifestation of gravity in the extra, fifth dimension.
Gravity, which is a manifestation of the curvature of spacetime, is described by the metric of spacetime $g_{\mu\nu}$, a $4\times 4$ matrix. In this five dimensional universe, we have a five dimensional metric $\tilde{g}_{ab}$, a $5\times 5$ matrix, parametrized by
$$\tilde{g}_{ab}=\begin{pmatrix}g_{\mu\nu}+\phi^2A_{\mu}A_{\nu} && \phi^2 A_{\mu}\\ \phi^2 A_{\nu} && \phi^2 \end{pmatrix}$$
Where $g_{\mu\nu}$ is a four dimensional metric, $A_{\mu}$ is a four dimensional vector potential, which describes the electromagnetic field, and a scalar field $\phi$. If $\phi$ is constant, the five dimensional Einstein field equations
$$\tilde{R}_{ab}-\frac{1}{2}\tilde{g}_{ab}\tilde{R}=0$$
miraculously reproduce both the four dimensional Einstein field equations, and Maxwell's equations.
And so in this five dimensional universe, charge is just a different manifestation of the five dimensional energy-momentum tensor $\tilde{T}_{ab}$ that would appear on the RHS of the above equation.
