**1. Introduction**

The energy is a form of a manifestation of matter in motion, the widely used definition of which is that the energy of a system is its capacity to perform mechanical work, when it passes from an existing state to a reference state [1, 2].

The energy, as defined in the literature, as well as on numerous websites, in various languages of international circulation, is the ability of a physical system to produce mechanical work [3].

Occasionally it is mentioned in the definition of energy, the capacity of a physical system to produce heat, as well.

However, these definitions refer only to the production or conversion of mechanical work or heat, but these represent only two of the many forms of existing energy [4].

The notion of energy is much more complex being obviously associated with other systems besides the physical ones, namely, biological and chemical systems, etc. [5].

Sometimes the literature considers that the energy is involved in all processes that require any kind of change or conversion, being responsible for the production of those changes [6].

It can be even considered that the matter itself is a "condensed" form of energy and that energy is stored in the atoms and molecules of which matter is composed [7–9].

The goal of the study is to investigate the connections between the fission, fusion, and solar energy through Einstein's equation.
