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Quantum mechanics is famous for its indeterminism, but we can usually use probabilities to quantify our uncertainty about future observations. 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. This work, published in Communications Physics, sheds some light on the debate concerning the interpretation of quantum mechanics.

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. In the latter, a cat is trapped in a box with poison that will be released if a radioactive atom decays. 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. What does the cat experience when it is in the superposition? Wigner sharpened the question by pushing quantum theory to its conceptual limits. He investigated what happens when an observer also has quantum properties.

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