Wignera: Wigner's Friend
Quantum Foundations arc, page 10 of 193. Back to The Pixelated Physics Universe
Who they are
Wignera's friend measured the qubit inside the lab. To Wignera, outside, the friend is still in superposition. Who is right?
Wignera is a character of the Quantum Foundations arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
A friend measures a qubit inside a sealed lab. Treating the whole lab with quantum mechanics, an outside observer assigns an entangled state of qubit and friend, while the friend has seen a definite result. Recent no-go theorems show that some reasonable assumptions about observers cannot all hold together. Eugene Wigner posed it in 1961; Frauchiger and Renner sharpened it in 2018, and Bong et al. (2020) proved a stronger no-go theorem with a proof-of-principle photon experiment.
$$ \begin{gathered} \text{Friend: } 0 \text{ or } 1 \\ \text{Wigner: } |\Psi\rangle = \frac{|0\rangle|F_0\rangle + |1\rangle|F_1\rangle}{\sqrt{2}} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
Honest caveat: Which assumption fails is still debated, and neither observer is simply "right". In the 2020 experiment a photon's path stood in for the observer.
Status: part of this topic is unsolved, unconfirmed or debated. The text above says which part; treat it as an open question, not settled fact.
Sources
- Frauchiger & Renner (2018) Quantum theory cannot consistently describe the use of itself, Nat. Commun. 9, 3711
- Bong et al. (2020) A strong no-go theorem on the Wigner's friend paradox, Nat. Phys. 16, 1199
Crossovers
Wignera meets Decoherex (Interference / Quantum Decoherence). Wignera, outside the lab, treats her friend and the qubit as one big superposition. Decoherex warns her that any real lab is constantly entangling with its air and walls, which makes interference between the friend's outcomes practically impossible to see. That explains why the friend looks classical; it does not settle the deeper question of who is right.
Wignera meets Kochenspek (Kochen–Specker Theorem). Wignera asks whose measurement result counts as fact. Kochenspek adds that the results cannot all be pre-written, independent of which other measurements go with them. Together they make the observer's role harder to ignore, though neither theorem picks an interpretation.
Wear the lore
Concept keywords: Wigner's Friend, Wignera, Quantum Foundations, physics lore, Pixelated Physics