The Wigner's friend paradox is a thought experiment in quantum mechanics that raises questions about the nature of measurement, observer-dependent reality, and the role of consciousness.
In the Wigner's friend scenario, Wigner, a physicist, has a friend inside a closed laboratory conducting a quantum measurement. According to the Copenhagen interpretation of quantum mechanics, the friend's measurement collapses the quantum superposition into a definite outcome. However, when Wigner opens the laboratory and becomes aware of the measurement, he can observe a superposition of possible outcomes, which seems to contradict the earlier collapse of the wavefunction.
Several interpretations and proposed resolutions have been put forth to address this paradox:
Many-worlds interpretation: According to this interpretation, when the friend makes a measurement, the universe branches into multiple parallel realities, one for each possible outcome. Each observer (Wigner and his friend) experiences their respective branches independently, leading to the appearance of contradictory observations. In this view, there is no paradox since each observer's perception aligns with their own branch of reality.
Consistent histories interpretation: This interpretation emphasizes the importance of the consistent history of the entire experimental setup, including both Wigner and his friend. It suggests that inconsistencies arise when the observer fails to consider the complete history. By considering the complete set of measurements and their outcomes, the paradox can be resolved.
Epistemic approaches: Some interpretations argue that the paradox arises due to the assumption of objective reality independent of observation. They propose that the quantum state describes an observer's knowledge or information rather than a complete description of reality. In this view, the paradox is resolved by considering subjective probabilities and updating knowledge upon obtaining new information.
It's important to note that the Wigner's friend paradox remains a topic of debate and discussion within the field of quantum foundations. Different interpretations offer distinct philosophical viewpoints and ways of resolving the apparent contradiction. Ultimately, the choice of interpretation depends on one's philosophical stance and preferred approach to understanding quantum mechanics.