  The dynamical models for state-vector reduction were developed to account for state collapses as real physical processes (for a review of the collapse models and a detailed reference list, see the entry on collapse theories). The origin of the collapse models may be dated to Bohm and Bub's (1966) hidden variable theory and Pearle's (1976) spontaneous localization approach, but the program has received its crucial impetus with the more sophisticated models developed by Ghirardi, Rimini and Weber in 1986 (see also Bell 1987a and Albert 1992) and their consequent development by Pearle (1989) (see also Ghirardi, Pearle and Rimini 1990, and Butterfield et al. 1993). Similarly to orthodox collapse quantum mechanics, in the GRW models the quantum-mechanical state of systems (whether it is expressed by a vector or a wave function) provides a complete specification of their intrinsic properties and relations. The state of systems follows the Schrödinger equation, except that it has a probability for spontaneous collapse, independently of whether or not the systems are measured. The chance of collapse depends on the ‘size’ of the entangled systems—in the earlier models the ‘size’ of systems is predicated on the number of the elementary particles, whereas in later models it is measured in terms of mass densities. In any case, in microscopic systems, such as the particle pairs in the EPR/B experiment, the chance of collapse is very small and negligible—the chance of spontaneous state collapse in such systems is cooked up so that it will occur, on average, every hundred million years or so. This means that the chance that the entangled state of the particle pair in the EPR/B experiment will collapse to a product state between the emission from the source and the first measurement is virtually zero. In an earlier L-measurement, the state of the particle pair gets entangled with the state of the L-measurement apparatus.  Thus, the state of the pointer of the L-apparatus evolves from being ‘ready’ to measure a certain spin property to an indefinite outcome. For instance, in a z-spin measurement the L-apparatus gets entangled with the particle pair in a superposition state of pointing to ‘up’ and pointing to ‘down’ (corresponding to the states of the L-particle having z-spin ‘up’ and having z-spin ‘down’), and the R-apparatus remains un-entangled with these systems in the state of being ready to measure z-spin. Or formally: