The law of conservation of energy states that energy cannot be
created or destroyed. While it can be transformed from one form
to another, the total amount of energy in the universe remains
constant. Scientists in all fields, including astronomy, biology,
chemistry, physics, and Earth science use this idea every day:
The total amount of energy remains constant no matter what changes
happen in any physical system.
The law of conservation of energy is an abstract expression of an observation that has been made over and over again in nature: a measurable quantity, which we call energy, is neither created nor destroyed in physical interactions. This law is a useful and powerful tool since in most commonplace energy transformations, changes in mass are too small to measure, and so scientists know that energy that seems to be lost must actually still exist, perhaps in a different form, even if its existence is no longer obvious.
Some processes that occur at very small scales, such as the nuclear fusion that powers the Sun and the nuclear fission that provides some of our electricity, convert mass directly to energy. Albert Einstein formulated a profound relationship regarding energy and matter: E = mc² (Energy equals mass times the speed of light squared), which tells us how energy and matter are related. So, to be complete, the law of conservation of energy should be restated as "the sum of the mass AND energy in a system remain constant."