Where EXACTLY is the "Mass" of an Atom? Proton? Quark? Hadron? Gluon?? Just look at the simplest Hydrogen atom -- one Proton. The question is *where* exactly is the mass of this thing? Or *what* makes up the mass of this thing? Is it just Quarks? So where is the *mass* of those things? Where is the difference between mass and energy. Is there any? Is there really such a thing as mass, or is it all just energy appearing in different forms? Does anyone know? Is this the end of Physics and the beginning of Philosophy, again meaning no one really knows??? No one seems to have made this clear on Wikipedia either... http://en.wikipedia.org/wiki/Mass http://en.wikipedia.org/wiki/Quark#Mass In a hadron most of the mass comes from the gluons that bind the constituent quarks together, rather than from the individual quarks; the mass of the quarks is almost negligible compared to the mass derived from the gluons' energy. Really? Mass comes from gluon energy? If so is there really such a thing as mass, or is there only energy? Why do we think a rock has "mass"? Is a rock really a bunch of energy, but at our scale we perceive it as this so-called "mass" stuff?