The Law of Conservation of Energy

https://en.wikipedia.org/wiki/Conservation_law

• https://en.wikipedia.org/wiki/Conservation_of_energy
• https://en.wikipedia.org/wiki/Mass–energy_equivalence

energy cannot be created or destroyed; only converted.

energy is a measure of the ability to perform work while entropy is a measure of energy not available to do work.

E = MC²
— https://en.wikipedia.org/wiki/Mass–energy_equivalence

The mass-energy equivalence relationship asserts a system's energy (E) is equivalent (=) to mass (M) * the speed of light squared (C²).