Science requires a belief that there are constants of nature – invariable, unchanging conditions and measurements – used in scientific investigation, without which its models, theories, predictions fundamental to its laws of physic are meaningless.
One of the simplest but most powerful laws of physics is E = mc2. As Einstein himself put it: “mass and energy are both but different manifestations of the same thing.”
This is called, logically, the Theory of Relativity.
No conversion from energy to mass or mass to energy can happen anywhere in the universe until acted upon by a Force.

