The standard interpretation of quantum mechanics associated with the ideas of Niels Bohr, who set up the Institute for Theoretical Physics in Copenhagen and discussed these matters at great length with his colleagues. In this interpretation a system (e.g. a particle) can be described by a wave function. This is a complex function—i.e. there is no real value, but the physical significance is that the square of the wave function is proportional to the probability of a particular definite state. In the Copenhagen interpretation a particle does not have a definite position or spin, for example, until it is observed—i.e. until a measurement is made. The idea is that the measurement ‘collapses the wave function’, leading to a definite measurement of the state. However, any prediction of the state of a system can only be probabilistic. The Copenhagen interpretation is the one most generally accepted by physicists but it does imply certain apparent paradoxes. See Schrödinger’s cat; EPR experiment. See also many-worlds interpretation.