Hi there, I'm trying to get a better intuitive handle on the concept of rest mass and rest energy - the energy term associated with rest mass. Introductory Physics textbooks often give statements along the lines of "mass is a form of energy" or "mass can be converted to energy" to explain phenomena. I've read a few papers (such as this one: http://home.comcast.net/~peter.m.brown/ref/baierlein.pdf [Broken]) that suggests that this is an overly naive view and that it's better to think about mass and energy as being attributes of systems that depend on their status rather than directly interchangeable quantities. My understanding is that one can think about the rest energy of a bound system of particles as in indication of what could be called its internal energy arising from microscopic kinetic energy of its constituent fundamental particles, binding energies of particles as well as the rest energy of those fundamental particles. The above article explains this in the case of a hydrogen atom, stating as follows: "The atom’s rest energy consists of the electrostatic potential energy of the electron-proton interaction, the kinetic energy of motions relative to the center of mass primarily the electron’s kinetic energy, and the rest energies of the electron and proton. According to the standard model, the electron’s rest energy cannot be dissected into distinct contributions. The proton’s rest energy could be described in terms of the rest energies of the constituent quarks, their motion internal to the proton, and their interaction energy." This explanation makes sense to me and it's helped me to understand why there should be changes in rest mass and rest energy if a collection of fundamental particles undergoes a re-arrangement so that binding energy changes as in a nuclear reaction, or if its temperature changes. _____________________________________________________________________________________ My question has to do with how to with how to interpret the rest energy of an isolated fundamental particle, like a free electron or a quark, both of which have rest energies that cannot be attributed to microscopic kinetic energies or binding energies. Is there an intuitive explanation for where this energy comes from? I'm so used to being there some kind of attribute of an object or a system that "explains" why it has energy - kinetic energy being associated with the motion of the system, for example, or gravitational potential energy arising from the gravitational interaction between components. Is there any kind of similar intuitive explanation for rest mass energy? Does it have something to do with the fact that particles that have non-zero rest energy also manifest as what we consider to be 'matter' - i.e. particles that have non-zero rest mass? I've read elsewhere that rest mass and rest energy for fundamental particles is just something that one has to accept about nature at a very fundamental level sort of like particle spin and to ask deeper questions about it is tantamount to asking questions like why gravity is an attractive force rather than a repulsive force. Is this the case, or am I missing something key? This has been quite a long-winded question, and I appreciate the effort anyone takes to read it and respond. Alex.