We have on several occasions in this book noted the relationship between energy and size of particles. Large photons carry more energy than small ones. Electrons carry more energy than photons, etc.
This relationship between size and energy is no coincidence, and I will therefore propose that energy is in fact size of subatomic particles. The energy of matter in the closed state is carried entirely as size.
This is also true for matter in the open state.
What is strange about matter in the open state is that it can move at a variable speed. However, it is not the speed of particles that carry energy. It is their size. When we accelerate a particle in the open state, it grows in size by a tiny bit.
Energy is in other words entirely a function of state and size of particles. The state is either closed or open, while the size is variable.
It should be noted that this definition of energy is compatible with the principle of mass-energy equivalence, popularly known as E = mc².