Wigner's Friend
Wigner’s friend is a famous quantum physics thought experiment proposed by
Eugene Wigner in 1961. It is a variation on
Schrödinger's cat made by assuming a conscious observer (the friend) inside a sealed lab. The friend measures a quantum particle and sees a clear result, while Wigner, outside the lab, must treat the entire closed room as being in a massive, unresolved quantum superposition (until he observes or learns the new information).
The "Paradox" Explained
Inside the Lab: The friend measures a qubit and sees a definite state (like 0 or 1). To the friend, the wave function has collapsed.
Outside the Lab: Wigner has no information yet. According to standard quantum mechanics, Wigner must describe the whole lab—including the friend and the particle—as entangled in a joint superposition.
The Conflict: Both observers use valid quantum rules, but they arrive at contradicting descriptions of reality.
This is the considered the deep puzzle of the
measurement problem: does a wave function collapse for a conscious observer, or only when information reaches the ultimate outside observer?
The Information Philosophy Explanation: The wave function collapses when information is registered in the measurement device for any observer to see, the friend has seen it now and Wigner can see it later, or be told by his friend.
Until Wigner learns the new information, his best estimate of the system state is to treat the entire closed room as being in the quantum superposition. This is not really a paradox, Wigner and his friend just have different amounts of
knowledge.
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