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 in Quantum mechanics.
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," that was only determined when Wigner learned the result of the experiment, or was it determined at some previous point?
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 material. Wigner's 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.
Just as in any Many-worlds interpretation, a bad world-count can lead to different probabilities.
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 happen 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 happen 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.
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.
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 open, so the question of observation-of-observers does not arise for them.