Nirvana

One of Newton's laws clearly states that "energy can neither be created nor it can be destroyed. It can only be changed from one form to another"
And we being energy are following the same.

Our material self (the body) arises and returns to the same source.
The life of a person eventually comes to an end along with the release of energy from its material self.
The cycle is continuously repeating; certainly, the forms of energy are changing, therefore, the flow is constant.

The energy released vibrates in the last highest frequency of that person and therefore it's always there in the universe, but whether the energy is freely moving in the universe or its mingled with the universal energy (attainment of nirvana) depends on the fact whether the frequency of the released energy matches with the universal energy or not.

Nirvana-
It is the liberation of oneself from all the material world. It is only possible when along with the body our energy also returns to its source.
Meditation helps to raise the frequency of the energy possessed by a person. The uniting of a person's energy with the highest energy (universal energy) is only possible when their frequencies match.
And this is what saints do. They meditate to raise their vibration to attain nirvana.

                                                         -Vartika Saxena