This thought experiment thus raises the question: What does it mean for an observer in a quantum superposition to observe the result of a measurement? Can an observer always trust what they see and use this data to make predictions about future measurements?
The fact that quantum theory sets no validity limits for its application leads to a clear tension between the perception of the friend, who sees a specific single result, and the description of Wigner, who observes the friend in a superposition of different perceptions. From the point of view of another observer, called Wigner, the measurement process of the friend can be described as a quantum superposition. In the thought experiment an observer, usually called Wigner's friend, performs a quantum measurement and perceives an outcome. He investigated what happens when an observer also has quantum properties. What does the cat experience when it is in the superposition? Wigner sharpened the question by pushing quantum theory to its conceptual limits. Governed by quantum mechanical laws, the radioactive atom is in a superposition between decaying and not decaying, which also means that the cat is in a superposition between life and death. In the latter, a cat is trapped in a box with poison that will be released if a radioactive atom decays. In 1961, the Nobel prize winning theoretical physicist Eugene Wigner proposed what is now known as the Wigner's friend thought experiment as an extension of the notorious Schroedinger's cat experiment. This work, published in Communications Physics, sheds some light on the debate concerning the interpretation of quantum mechanics. However, a team of researchers at the University of Vienna, the IQOQI Vienna (Austrian Academy of Sciences) and the Perimeter Institute for Theoretical physics have recently shown that in certain extreme quantum scenarios it is not possible to make such probabilistic predictions, provided that certain key assumptions of quantum mechanics hold true.
Quantum mechanics is famous for its indeterminism, but we can usually use probabilities to quantify our uncertainty about future observations.