How generic can be derivation of energy?
In a system with gravity and masses – it is potential energy and kinetic energy.
What if a constraint would be specified that no mass and velocity should be allowed (or possible) to use? For example, suppose there is a system of massless particles. Is there a way to define and use energy in such system? If yes, then how?
In general, I am dealing with abstract systems, where no typical gravity and mass occur. So it is hard for me to use potential and kinetic energy. Yet, I think it is possible to define an energy in such system, a kind of intensity of a process. Is there such thing defined in physics? What are most abstract derivations of energy?