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Just hundred years ago the world of physics (and not only) was shaken by a visionary work. One man alone brought to humankind a profoundly different perspective on the very nature of, arguably, the most relevant aspects of our perceived reality: space and time.  In his 1915 work called "General Theory of Relativity", Albert Einsten discussed the intimate connection between space, time, energy and mass. The picture that emerges is extremely elegant.   %geometric theory in which the force of gravity emerges naturally as the stretching and bending of space-time caused by the presence of %mass and/or energy.   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(described by $G_{\mu\nu}$)  are not static quantities, described through the so-called metric tensor (a tensor is a mathematical object analogous to  but are affected by more general than a vector), which enters the quantity $G_{\mu\nu}$. The metric tensor allows to measure space and time. But  the presence result  of mass the measure does not only depends on space time itself, as expected in Newtonian mechanics: Mass  and energy (enclosed appear  in $T_{\mu\nu}$). 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.