in an English translation in 2021 as Helgoland: Making Sense of the Quantum Revolution. History and development Relational quantum mechanics arose from a comparison of the quandaries posed by the interpretations of quantum mechanics with those resulting from Lorentz transformations prior to the development of special relativity. Rovelli suggested that just as pre-relativistic interpretations of Lorentz's equations were complicated by incorrectly assuming an observer-independent time exists, a similarly incorrect assumption frustrates attempts to make sense of the quantum formalism. The assumption rejected by relational quantum mechanics is the existence of an observer-independent state of a system. The idea has been expanded upon by Lee Smolin and Louis Crane, who have both applied the concept to quantum cosmology, and the interpretation has been applied to the EPR paradox, revealing not only a peaceful co-existence between quantum mechanics and special relativity, but a formal indication of a completely local character to reality. The problem of the observer and the observed This problem was initially discussed in detail in Everett's thesis, The Theory of the Universal Wavefunction. Consider observer , measuring the state of the quantum system . We assume that has complete information on the system, and that can write down the wavefunction describing it. At the same time, there is another observer , who is interested in the state of the entire - system, and likewise has complete information. To analyse this system formally, we consider a system which may take one of two states, which we shall designate and , ket vectors in the Hilbert space . Now, the observer wishes to make a measurement on the system. At time , this observer may characterize the system as follows: where and are probabilities of finding the system in the respective states, and obviously add up to 1. For our purposes here, we can assume that in a single experiment, the outcome is the eigenstate (but this can be substituted throughout, mutatis mutandis, by ). So, we may represent the sequence of events in this experiment, with observer doing the observing, as follows: This is the description of the measurement event given by observer . Now, any measurement is also a physical interaction between two or more systems. Accordingly, we can consider the tensor product Hilbert space , where is the Hilbert space inhabited by state vectors describing . If the initial state of is , some degrees of freedom in