Wigner's friend

Wigner's friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödinger's cat experiment designed to highlight unanswered questions in 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 the inadequacy of the idea (central to the Copenhagen interpretation) that the measurement process is somehow inherently special.

Compare quantum suicide.

Actually, this was not Wigner's motivation. He was showing how consciousness is quite necessary to the quantum mechanical measurement process. If the friend is substitued by a device, the linearity of the wave function will determine that the state of the system is in a linear sum a|s1> +b|s2>, but this is impossible for our friend. The system is not in either |s1> or |s2>, and cannot be so in the quantum mechanical descriptions. However, the person must be in either |s1> or |s2>, hence conscious observations are different, hence consciousness is not material. This is Wigner's purpose and he seems to do a good job of discussing the mind-body problem from this point of view. His essay with the friend is "Remarks on the mind-body question" in his collection of essays Reflections and Symmetries, 1967. Comments are welcome to robert.paul@eoascientific.com Thanks.