Wigner's friend
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Wigner's friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödinger's cat experiment designed as a point of departure for discussing the mind-body problem as viewed by the Copenhagen interpretation of quantum mechanics.
In the Copenhagen interpretation, the collapse of the wavefunction is said to take place when a quantum system is measured. Essentially, the Wigner's friend experiment asks the question: at what stage does a "measurement" take place? It posits a friend of Wigner who performs the Schrödinger's cat experiment while Wigner is out of the room. Only when Wigner comes into the room does he himself know the result of the experiment: until this point, was the state of the system a superposition of "dead cat/sad friend" and "alive cat/happy friend," or was it determined at some previous point?
Wigner designed the experiment to highlight how he believed consciousness is necessary to the quantum mechanical measurement process. If a material device is substituted for the conscious friend, the linearity of the wave function implies that the state of the system is in a linear sum of possible states. It is simply a larger indeterminate system.
However, a conscious observer (according to his reasoning) must be in either one state or the other, hence conscious observations are different, hence consciousness is not material. Wigner's essay with discussing this scenario is "Remarks on the mind-body question" in his collection of essays Reflections and Symmetries, 1967. The idea has become known as the consciousness causes collapse interpretation, or less flatteringly, the spiritual interpretation.
A counterargument is that the superimposition of two conscious states is not paradoxical — just as there is no interaction between the multiple quantum states of a particle, so the superimposed consciousnesses need not be aware of each other.