Question

Energy can be created and not destroyed — it can only change from one form to another.

Answer 1

Actually the usual statement is the opposite: energy cannot be created or destroyed — it can only change from one form to another. That is the law of conservation of energy (the first law of thermodynamics).

What that means in practice
- In a closed (isolated) system the total energy remains constant. Energy can be transferred (work, heat) or converted (kinetic ↔ potential, chemical → thermal, nuclear → radiation), but the sum stays the same.
- Example: a swinging pendulum converts kinetic energy to potential energy and back; friction converts mechanical energy into thermal energy.
- In nuclear reactions mass and energy interconvert (E = mc^2), but the total mass–energy is still conserved.

Nuances
- Conservation applies locally and in closed systems. In general relativity global energy conservation can be subtle in an expanding universe, but local energy–momentum conservation still holds.
- Quantum processes may look like brief "creation" of particles (virtual particles), but they respect overall energy conservation in measurable interactions.

Summary: energy is not created or destroyed; it is conserved and only changes form (with the proper understanding of mass–energy and relativistic/quantum subtleties).