  The above problem, commonly called ‘the measurement problem,’ arises in orthodox no-collapse quantum mechanics from two features that account very successfully for the behavior of microscopic systems: The linear dynamics of quantum states as described by the Schrödinger equation and the property assignment rule called ‘eigenstate-eigenvalue link.’ According to the eigenstate-eigenvalue link, a physical observable, i.e., a physical quantity, of a system has definite value (one of its eigenvalues) just in case the system is in the corresponding eigenstate of that observable (see the entry on quantum mechanics, section 4). Microscopic systems may be in a superposition state of spin components, energies, positions, momenta as well as other physical observables. Accordingly, microscopic systems may be in a state of indefinite z-spin, energy, position, momentum and various other quantities. The problem is that given the linear and unitary Schrödinger dynamics, these indefinite quantities are also endemic in the macroscopic realm. For example, in a z-spin measurement on a particle in a superposition state of z-spin ‘up’ and z-spin ‘down,’ the position of the apparatus’s pointer gets entangled with the indefinite z-spin of the particle, thus transforming the pointer into a state of indefinite position, i.e., a superposition of pointing ‘up’ and pointing ‘down’ (see Albert 1992, chapter 4, and the entry on collapse theories, section 3). In particular, in the EPR/B experiment the L-measurement causes the L-apparatus pointer to get entangled with the particle pair, transforming it into a state of indefinite position: