When the observer is inside the box, who collapses the wave function?
You know SchrΓΆdinger's Cat: a cat in a box exists in a superposition of alive and dead until observed. But physicist Eugene Wigner asked a deeper question in 1961:
What if a conscious observerβWigner's friendβis inside the box, performing the measurement? From the friend's perspective, they see a definite result. But from Wigner's perspective outside, isn't the friend also in superposition?
This seemingly simple extension reveals a profound tension in quantum mechanics about the nature of observation, consciousness, and reality itself.
When did the collapse occur? Was it when the friend measured? Or when Wigner opened the lab?
Both answers lead to problems!
Wigner's Friend exposes the measurement problem: What counts as a "measurement"? What causes wave function collapse? Does consciousness play a special role?
Both outcomes occur in different branches. The friend sees "up" in one world and "down" in another. Wigner and friend split into multiple copies.
Collapse happens when a "classical" system measures. But what makes something "classical"? This just pushes the problem back.
Facts are relative to observers. The friend has her facts, Wigner has his. There's no absolute "view from nowhere."
Collapse happens spontaneously at some scale (GRW theory). Large systems like brains can't be in superposition for long.
Amazingly, physicists have tested extended versions of Wigner's Friend in the lab!
In 2018, Daniela Frauchiger and Renato Renner proposed an extended scenario with two Wigner's Friend setups. They showed that if quantum mechanics applies universally, observers can reach contradictory conclusions about the same events.
Experiments by teams in Vienna and Brisbane have confirmed that:
SchrΓΆdinger proposes his cat thought experiment
Eugene Wigner publishes "Remarks on the Mind-Body Question" introducing the Friend scenario
David Deutsch refines the thought experiment
Frauchiger and Renner publish their extended paradox
Experimental tests confirm quantum mechanics' predictions about observer-dependent facts
Wigner's Friend forces us to confront a startling possibility: perhaps there is no single, objective reality that all observers share. Perhaps facts about the world are irreducibly perspectival.
Or perhaps quantum mechanics breaks down at the scale of conscious observers. Or perhaps the friend really is in superposition until Wigner looks.
The next time you observe something, ask: who's observing you? π¬