  Einstein, Podolsky and Rosen (1935) thought that quantum mechanics is incomplete and that the curious correlations between distant systems do not amount to action at a distance between them. The apparent instantaneous change in the R-particle's properties during the L-measurement is not really a change of properties, but rather a change of knowledge. (For more about the EPR argument, see the entry on the EPR argument, Redhead 1987, chapter 3, and Albert 1992, chapter 3. For discussions of the EPR argument in the relativistic context, see Ghirardi and Grassi 1994 and Redhead and La Riviere 1997.) On this view, quantum states of systems do not always reflect their complete state. Quantum states of systems generally provide information about some of the properties that systems possess and information about the probabilities of outcomes of measurements on them, and this information does not generally reflect the complete state of the systems. In particular, the information encoded in the spin singlet state is about the probabilities of measurement outcomes of spin properties in various directions, about the conditional probabilities that the L- (R-) particle has a certain spin property given that the R- (L-) particle has another spin property, and about the anti-correlation between the spins that the particles may have in any given direction (for more details, see section 5.1). Thus, the outcome of a z-spin measurement on the L-particle and the spin singlet state (interpreted as a state of knowledge) jointly provide information about the z-spin property of the R-particle. For example, if the outcome of the L-measurement is z-spin ‘up,’ we know that the R-particle has z-spin ‘down’; and if we assume, as EPR did, that there is no curious action at a distance between the distant wings (and that the change of the quantum-mechanical state of the particle pair in the L-measurement is only a change in state of knowledge), we could also conclude that the R-particle had z-spin ‘down’ even before the L-measurement occurs.