Before I even ask a question I'll say what I know just in case this confusion stems from not knowing the basics. My understanding is: -Nuclear reactions have changes in mass even though all particles are accounted for. -Fast moving particles approaching the speed of light have higher mass but still consist of the same basic components (1 proton becomes heavier, it does not become 2 protons). Now for the actual question. Thinking back to high school there was a thing called "mass defect" that accounted for the energy release in nuclear reactions. Does this concept of a mass and energy relationship apply to creating new particles, or only to changing the mass of already existing particles?