Why can energy change forms without being created or destroyed?

The principle that energy cannot be created or destroyed, but only converted between forms, is known as the law of conservation of energy. This concept serves as the foundation for the first law of thermodynamics.12

Energy remains constant within a closed system because it is a fundamental physical quantity that can be transformed—such as from kinetic energy to potential energy or thermal energy—without being lost. This conservation principle is deeply rooted in the nature of physics; mathematically, it follows from the uniformity of time. If the laws of physics were different at different moments, energy would not be conserved.23

As scientific understanding has expanded, the definition of energy has been broadened to include new forms—such as electromagnetic fields and mass (via Albert Einstein's E = mc²)—to ensure that the total energy in a closed system remains constant. While energy can change forms freely, the second law of thermodynamics notes that these conversions are not always perfectly reversible, particularly when heat is involved.231