blocks fulfill the Onsager constraints, they are natural building blocks to derive effective media constitutive tensors by homogenization that also satisfy the Onsager relations.

Optical theorem
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It is well known in point scattering theory for electric dipoles that polarizability tensors are not solely limited by reciprocity and spatial symmetry, but also fundamentally by energy conservation. Indeed, energy conservation imposes an ’optical theorem’ that constrains the polarizability of an electric dipole scatterer to ensure that (in absence of material absorption) extinction equals scattering [@Pedro98]. We proceed to examine these constraints imposed on $\boldsymbol{\alpha}$. Let us first recapitulate the well known case of a scalar electric scatterer [@Lagendijk96; @Pedro98]. An electric scatterer will absorb and scatter part of the incoming light, that together make up the extinction of a dipole. Extinction for an electric scatterer corresponds to the work done by the incident field $\bm{E}$ in order to drive the dipole $\bm{p}$. The work per optical cycle needed to drive $\bm{p}$ equals $W= \ll\mathrm{Re}{\bm{E}}\cdot \mathrm{Re}{\frac{d\bm{p}}{dt}}\gg$, where $\ll\gg$ indicates cycle averaging. Evaluating the work per cycle, and dividing it by the incident intensity $I_{\mathrm{in}}=|\bm{E}|^2/2Z$ ($Z$ the impedance of the host medium) leads to $\sigma_{\mathrm{ext}}=W/I_\mathrm{in}=4\pi k
\mathrm{Im}\alpha_{EE}$. Scattering corresponds to far field radiation 