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
Symmetry in quantum mechanics act on any four vector A = ("A", "A", "A", "A") and mix the time-like and the space-like components, according to: In relativistic quantum mechanics, wavefunctions are no longer single-component scalar fields, but now 2(2"s" + 1) component spinor fields, where "s" is the spin of the particle. The transformations of these functions in spacetime are given below. Under a proper orthochronous Lorentz transformation in Minkowski space, all one-particle quantum states locally transform under some representation of the Lorentz group: where is a finite-dimensional representation, in other words a dimensional square matrix, and is thought of as a column vector