Question

What is the relation between rest mass and effective mass?

Answer 1

Newton's second law takes a different form when relativity is taken into account, so it's not helpful to compare rest mass and inertial mass, except of course in the rest frame of the particle. ... Its rest mass is the mass when it's at rest with respect to an observer, and is then equivalent to its inertial mass

Answer 2

Answer:

The effective mass tensor diverges (becomes infinite) for linear dispersion relations, such as with photons or electrons in graphene. (These particles are sometimes said to be massless, however this refers to their having zero rest mass; rest mass is a distinct concept from effective mass.)