rest mass and
invariant mass are often interchanged, however i cannot reconcile these concepts:
Consider a photon inside a mirror-box, measuring the mass of the box in rest we must arrive at $m=E_p/c^2$ (disregarding the mass of the box itself), however we know that this measured mass is in fact
relativistic mass of the photon and is not invariant as it is dependent on our velocity and apparent frequency of the photon.
rest mass is not equal to
invariant mass in the case of the box.
Now someone may object that putting a box around a photon doesnt change anything, regarding mass of the box or a photon, however that is basically what elementary particles are. According to wikipedia the mass of proton consists of 99%
relativistic mass of otherwise massless gluons. One can imagine that due to the small size of proton we will never be able to tell the difference between its measured
rest mass (which is actualy
relativistic mass of the gluons) and its
invariant mass. However that doesnt change the facts that they are conceptually completely different and probably hold very different values.