: I have been hearing alot of being able to travel the light of speed if we were pure energy. But the only problem is that, we really don't know what energy is. I have done some hardcore searching for a good definition of energy. We know that energy cannot be created nor destroyed, only changed in form, but there was a man who created a generator in which the total output energy was far greater than the total input energy. But my question is what is energy? I don't want to know about the properties of energy or laws of energy, I just want to know what exactly makes up energy.