Re: Resubmitted: What is the current explanation of mass and inertia?

Hi William,

first of all let me say a few words on mass in general. there are two "types" of mass, as is well known, inertial mass and gravitational mass. the gravitational mass of a particle is the property that couples to gravity, inertial mass is the resistance if you want to accelerate a particle. but also energy couples to and experiences gravity. Galileo Galilei and more recently Roland von Eötvös have shown experimentally that gravitational and inertial mass seem to be equal. therefore Einstein postulated the equality in his general theory of relativity. as a consequence of this axiom, one cannot distinguish between gravity and and accelerated frame, locally.

Does all matter have mass and is all mass matter?
somehow, you might define matter as a form of energy. all matter has energy, but not necessarily (rest)mass. photons don´t have mass for example. although, via Einstein energy interacts by gravitation and has an equivalent mass m=E/c^2.

it is a very difficult and interesting task to think about what inertia really is and where it comes from. you suggested to find its origin in the electromagnetic interaction. a famous suggestion has been made by Ernst Mach almost one hundred years ago. his opinion has influenced Einstein, when he created the general theory of relativity. this suggestion is called Mach's principle. Basically he suggests that inertia is due to the gravitational interaction of a body with the fixed stars. for detailed information on Mach's principle have a look at the question 981487202.Ph, What causes my spring to stretch? on this site.
in elementary particle physics, some massless particles are rendered massive via the socalled Higgs mechanism. this mechanism is due to spontaneously broken symmetries. these particles would be massless it the symmetry was conserved. but the spontaneous braking of the symmetry miraculously assigns some mass to that particle. examples are the massive W and Z bosons of the weak interaction.
you see that it is not settled what inertia "is". still a lot of interesting and controversial question can be discussed. I hope I could help you a bit with my answer,
Michael