Wigner's friend and continuous time evolution of the state of a closed system and the probabilistic, discontinuous collapse of the state of a system upon measurement. Wigner's friend is therefore directly linked to the measurement problem in quantum mechanics with its famous Schrödinger's cat paradox. The thought experiment posits a friend of Wigner in a laboratory and lets him perform a quantum measurement on a physical system (this could be a spin system or also Schrödinger's cat). This system is assumed to be in a superposition of two distinct states, say, state 0 and state 1 (or "dead" and "alive", in the
Introduction to quantum mechanics an atom that was spun up to spin down: measuring its spin about the horizontal axis collapses its wave function into one of the eigenstates of this measurement, which means it is no longer in an eigenstate of spin about the vertical axis, so can take either value. [[File:Wolfgang Pauli young.jpg|right|thumb|upright|[[Wolfgang Pauli]]]] In 1924, [[Wolfgang Pauli]] proposed a new quantum degree of freedom (or [[quantum number]]), with two possible values, to resolve inconsistencies between observed molecular spectra and the predictions of quantum mechanics. In particular, the [[hydrogen spectrum|spectrum of atomic hydrogen]] had a [[Doublet (physics)|doublet]], or pair of lines differing