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The pinnacle of the theory is equation~\ref{einstein}, which is actually a set of 10 equations called {\bf Einstein's Field Equations}.   These equations describe gravitation as a result of space-time being curved by matter and energy.   Space and time are described through the so-called metric tensor (a tensor is a mathematical object analogous to but more general than a vector), which enters through  the quantity $G_{\mu\nu}$. The metric tensor allows to measure space and time. But the result of the measure does not only depends on space time itself, as expected in Newtonian mechanics: Mass and energy appear in Equation ~\ref{einstein} via the quantity $T_{\mu\nu}$, also called the stress-energy tensor. The stress-energy tensor basically measures the density and flux of energy and momentum.