Wigner's friend

Wigner's friend is a thought experiment in quantum physics exploring how observation affects reality. Imagine a scientist's friend inside a closed box performing a measurement on a quantum system, like determining a particle’s state. From the friend's viewpoint, the measurement collapses the system into a definite state. However, for an outside observer (Wigner), before opening the box, the combined system (including the friend) might be considered in a superposition—both states simultaneously. This raises questions about the nature of reality and whether observations create definite outcomes or if multiple possibilities coexist until observed, highlighting the peculiar and philosophical aspects of quantum mechanics.