>[!abstract] >Wigner's friend is a thought experiment in theoretical quantum physics, first published by the Hungarian-American physicist Eugene Wigner in 1961, and further developed by David Deutsch in 1985. ("Wigner’s Friend", 2025). ## My interpretation Wigner's friend is an extension of the [[Schrödinger's cat]] thought experiment that aims to address the [[Measurement problem]] in quantum physics. In Schrödinger's cat thought experiment, observer ${\displaystyle A}$ performs a measurement on some physical system in a superposition of states (e.g., a box in which a cat is dead *and* alive, or more generally state is 0 *and* state is 1). The measurement by the conscious observer causes the wave function to collapse, such that the system is no longer in a superposition of states. But, if another observer ${\displaystyle B}$ located outside the lab is subsequently measuring ${\displaystyle A}s knowledge of the state of the system by asking ${\displaystyle A}$ about the initial measurement, then ${\displaystyle B}$ is argued by Wigner to be causing another wave function collapse — that of the entire lab, which is until then in a superposition of states (system is in state 0 and ${\displaystyle A}$ has measured 0, *and* system is in state 1 and ${\displaystyle A}$ has measured 1). This leads to an apparent paradox. From ${\displaystyle A}s point of view, the laboratory was no longer in a superposition state as soon as the initial measurement was done, regardless of how long it took for ${\displaystyle B}$ to become aware of the result. Both ${\displaystyle A}$ and ${\displaystyle B}$ have a claim that it is *their* consciousness that led to the collapse of the wave function. Wigner uses this thought experiment to conclude that the linear time evolution of quantum states (as determined by the Schrödinger equation) cannot depend on consciousness being involved (i.e., that the mind is not special in causing the collapse of the wave function). ## Proposed extension Wigner himself saw his chain of reasoning as a [[reductio ad absurdum]] argument, which leads me to think of this experiment in relation to [[Hilbert's hotel]] as follows. What if, instead of two observers ${\displaystyle A}$ and ${\displaystyle B}$, there was an infinite sequence of observers each in their hotel room, such as ${\displaystyle A_0}$ performs the initial measurement in a room, after which ${\displaystyle A_1}$ in the next room over calls ${\displaystyle A_0}$ to inquire about the result, then ${\displaystyle A_2}$ calls ${\displaystyle A_1}$, and so on *ad infinitum* such that ${\displaystyle A_n}$ always learns about the result from ${\displaystyle A_{n-1}}$, only then causing the collapse of the wave function of a progressively larger system inclusive of all observers from ${\displaystyle A_0}$ up to to ${\displaystyle A_{n-1}}$. I see at least two immediate implications to this extension of the experiment: 1. Perhaps trivially, assuming a non-zero delay between successive observations ${\displaystyle A_{n-1}}$ and ${\displaystyle A_n}$, the experiment never ends. 2. This means that even though there is over time an infinitely increasing number of observers aware of the result, the whole system remains in a perpetual superposition of states as seen from the viewpoint of the yet-unaware observers, of which there will always be infinitely many. I think this just magnifies the apparent paradox which made Wigner reject this thought experiment as convincing evidence that consciousness possesses some special influence over physical processes, including the collapse of the wave function. ## References - Wigner’s Friend. (2025, February 15). In *Wikipedia*. https://en.wikipedia.org/w/index.php?title=Wigner%27s_friend&oldid=1272114083