In 1961 Eugene Wigner created a thought experiment in theoretical quantum physics. The idea was developed further by David Deutsch in 1985. The thought experiment is as follows: A friend of Wigner’s is in a laboratory. The friend performs a quantum measurement on a physical system, such as a spin system or something analogous to Schrödinger’s Cat. Assume this physical system is in a superposition of two distinct states at once, for example, state 0 and state 1 (or an alive cat and dead cat if we’re using Schrödinger’s Cat). From this 0 or 1 state basis, Wigner’s friend will measure the system, according to quantum mechanics their measurement will equal a state of 0 or 1, and the system will collapse into that corresponding state.
Now imagine Wigner himself is outside of this laboratory. Wigner knows his friend will perform the 0/1 measurement on the physical system. Wigner will assign a superposition state to the whole laboratory his friend is inside, because the lab is in a linear combination of 1) system is in the state 0 and friend has measured 0, and 2) system is in the state of 1 and friend has measured 1.
Wigner now asks his friend for the result of the measurement. Whatever answer the friend gives, 0 or 1, Wigner assigns the state to the laboratory: either system is in the state 0 and friend has measured 0, or system is in the state of 1 and friend has measured 1. Therefore, it is only at the time when he learns about his friend’s state that the superposition of the laboratory collapses.
Unless Wigner is considered to be in a privileged position as an ultimate observer, the friend’s state of view must be considered valid. This is the paradox: From Wigner’s friend’s point of view, the measurement result was determined long before Wigner asked about it. The state of the friend’s physical reality had all ready collapsed into 0 or 1. When did the collapse objectively occur? Was it when the friend made their measurement, or when Wigner found out about the result?