I'm surprised that I haven't written about heavy fermion s as a separate post before, so here we go. (It's a break from thinking about science and politics, anyway.) I've written before about " effective mass " for electronic excitations in solids ( wiki page here ).  From classical physics, we are used to the idea that inertial mass (m) is the ratio between an external force (\mathbf{F}) and the acceleration (\mathbf{a}) of some object, (\mathbf{F} = m\mathbf{a}), which is also the rate