I feel i need to ask this fundamental question, because everywhere i look i just get a very bland definition with no discussion. So what actually IS energy? Is it just a mathematical thing that we have worked out that just works in a kindo of"number crunching" fashion, or is it more fundimental than that, more like an "entity" of some sort? I dont see how it can just be a number cruching principle, since in relativity, mass is converted to energy and vice versa - is mass if lost then i must become SOMETHING - that something is obviously energy but, but what is energy? :rofl: Please feel free to discuss this and elaborate on this as much as possible - I feel that it is a key part of physics to really unddertand properly. Thanks in advance.