formalism. But, Rovelli points out, this form of correlation is precisely the same as the definition of information in Shannon's theory. Specifically, an observer O observing a system S will, after measurement, have some degrees of freedom correlated with those of S. The amount of this correlation is given by log2k bits, where k is the number of possible values which this correlation may take the number of "options" there are. All systems are quantum systems All physical interactions are, at bottom, quantum interactions, and must ultimately be governed by the same rules. Thus, an interaction between two particles does not, in RQM, differ fundamentally from an interaction between a particle and some "apparatus". There is no true wave collapse, in the sense in which it occurs in some interpretations. Because "state" is expressed in RQM as the correlation between two systems, there can be no meaning to "self-measurement". If observer measures system , 's "state" is represented as a correlation between and . itself cannot say anything with respect to its own "state", because its own "state" is defined only relative to another observer, . If the compound system does not interact with any other systems, then it will possess a clearly defined state relative to . However, because 's measurement of breaks its unitary evolution with respect to , will not be able to give a full description of the system (since it can only speak of the correlation between and itself, not its own behaviour). A complete description of the system can only be given by a further, external observer, and so forth. Taking the model system discussed above, if has full information on the system, it will know the Hamiltonians of both and , including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to , if measures . The only reason that will perceive a "collapse" is because has incomplete information on the system (specifically, does not know its own Hamiltonian, and the interaction Hamiltonian for the measurement). Consequences and implications Coherence In our system above, may be interested in ascertaining whether or not the state of accurately reflects the state of . We can draw up for an operator, , which is specified as: with an eigenvalue of 1 meaning that indeed accurately reflects the state of . So there is a 0 probability of reflecting the