## Where Lies the Optical Properties of Matter in General Relativity?

In the December 12 1936 issue of *Nature*, Ludwik Silberstein publishes a letter entitled "Minimal lines and geodesics within matter: A fundamental difficulty of Einstein's theory."

The issue Silberstein address was that Einstein equates geometry $G$ with a material tensor $T$ so that one should know what the other is talking about. Otherwise, you would be equating apples and oranges.

Now according to Einstein's generalization of the Minkowski indefinite metric,

$$ds^2=g_{\nu\mu}dx^{\nu}dx^{\mu}$$