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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 Quantum mind/body problem.
The thought experiment 
The Wigner's Friend thought experiment posits a friend of Wigner who performs the Schrödinger's cat experiment after Wigner leaves the laboratory. Only when he returns does Wigner learn the result of the experiment from his friend, that is, whether the cat is alive or dead. The question is raised: was the state of the system a superposition of "dead cat/sad friend" and "live cat/happy friend," only determined when Wigner learned the result of the experiment, or was it determined at some previous point?
Consciousness and measurement 
Wigner designed the experiment to illustrate his belief that 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 discusses this scenario in "Remarks on the mind-body question", one in his collection of essays, Symmetries and Reflections, 1967. The idea has become known as the consciousness causes collapse interpretation.
Consciousness and Superposition 
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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.
The state of the observer's perception is considered to be entangled with the state of the cat. The perception state 'I perceive a live cat' accompanies the 'live-cat' state and the perception state 'I perceive a dead cat' accompanies the 'dead-cat' state. [..] It is then assumed that a perceiving being always finds his/her perception state to be in one of these two; accordingly, the cat is, in the perceived world, either alive or dead.[..] I wish to make clear that, as it stands, this is far from a resolution of the cat paradox. For there is nothing in the formalism of quantum mechanics that demands that a state of consciousness cannot involve the simultaneous perception of a live and a dead cat.
Alternative interpretations 
Wigner's friend in Many Worlds 
The Many worlds interpretation avoids the need to postulate that consciousness causes collapse — indeed, that collapse occurs at all. Unconscious systems split (decohere) when there is an irreversible difference between their state in the world where the cat survived or will survive and their Counterpart in the other case. In the example below this happens with the cyanide device and the telephone. Similarly, conscious systems split when there is an irreversible difference between their state in the world where the cat survived (or will survive) and their Counterpart in the other case. In the example below, this happens with the cat, Wigner's friend and Eugene Wigner.
According to “Many worlds”, when Wigner's friend (the investigator) finds out the result of the Schrödinger's cat experiment, the part of the world where the friend is 'decoheres irreversibly' or splits.  In one split world the friend observes a live cat. In the other the friend observes a dead cat.
According to many-worlds the device was split into two states—cyanide released or not (…) As the cyanide/no-cyanide interacts with the cat the cat is split into two states (dead or alive). From the surviving cat's point of view it occupies a different world from its deceased copy. The onlooker is split into two copies only when the box is opened and they are altered by the states of the cat. The cat splits when the device is triggered, irreversibly. The investigator splits when they open the box. The alive cat has no idea that the investigator has split, any more than it is aware that there is a dead cat in the neighbouring split-off world. The investigator can deduce, after the event, by examining the cyanide mechanism, or the cat's memory, that the cat split prior to opening the box. —
So, it is maintained that Eugene Wigner splits when there is an irreversible difference between Wigner in the world where the cat survived and Wigner’s counterpart in the world where the cat died. In the original thought experiment Wigner postulated that he would find out when he returned to the laboratory. There can be other cases.
For example, perhaps in the world where the cat survived Wigner's friend may telephone at once with the good news. In the world where the cat died Wigner may find out later. In that case when Wigner's friend makes the telephone call in one world Eugene Wigner splits into two. One counterpart knows the result. The other counterpart does not know.
Objective Collapse Theories 
According to objective collapse theories, wave function collapse occurs when a superposed systems reaches a certain objective threshold of size, complexity etc. Objective collapse proponents would expect a system as macroscopic as a cat to have collapsed before the box was opened, so the question of observation-of-observers does not arise for them.
Wigner's original remarks about his friend appeared in his article "Remarks on the Mind-Body Question", published in the book "The Scientist Speculates", edited by I. J. Good. The article is reprinted in Wigner's own book Symmetries and Reflections.
See also 
- R. Penrose, Road to Reality, section 29.8