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
Two-body Dirac equations The four-vectors formula_120 and formula_121 have only three independent components since The similarity to the three-dimensional structure of the nonrelativistic Schrödinger equation can be made more explicit by writing the equation in the c.m. frame in which Comparison of the resultant form with the time independent Schr\"{o}dinger equation makes this similarity explicit. A plausible structure for the quasipotential formula_83 can be found by observing that the one-body Klein-Gordon equation formula_127 takes the form formula_128 when one introduces a scalar interaction and timelike vector interaction via formula_129and formula_130. In the two-body case, separate classical and quantum field theory arguments show that