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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. 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. 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.
See also
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