Impressionism, Realism, and the aging of Ashcroft and Mermin

Mermin replies: I am a realist. But my model of reality necessarily rests on what I have experienced, either directly, or indirectly through the reports of others. For all practical purposes (FAPP, John Bell’s famous adverbial acronym), it doesn’t matter if, like most physicists, I confer reality on such theoretical abstractions as quantum states or energy levels that enable me to calculate the likelihood of my subsequent experience. But for resolving obstinate conceptual conundrums (FROCC), such as “the quantum measurement problem” or “quantum nonlocality,” it is crucial not to reify our intellectual tools. “Filled bands are inert” means FROCC that “if the electronic state I assign to a crystal is an antisymmetrized product of Bloch levels, then, in calculating the odds on what I am likely to experience when I subject the crystal to a sufficiently weak intervention, I can ignore levels from bands entirely below the Fermi energy.” I leave the FROCC view of electrons, crystals, Bloch levels, bands, Fermi energies, excitons, and effective masses as exercises for the reader.