Wigner’s friend

Introduction: What is Wigner’s Friend?

Wigner’s Friend is a thought experiment proposed by the Hungarian physicist Eugene Wigner in 1961 to investigate the foundational issues of quantum mechanics, specifically the measurement problem. It explores the idea that the act of measurement alters the state of a system, leading to different outcomes depending on the observer’s perspective. The experiment involves two observers, one inside and one outside of the laboratory, measuring the same quantum system but with different outcomes.

The Wigner’s Friend Thought Experiment

The Wigner’s Friend experiment involves a laboratory with a single photon that can be in either a vertical or horizontal polarization state. Wigner’s Friend is outside the laboratory, while his friend is inside and performs a measurement on the photon. According to the principles of quantum mechanics, the photon exists in a superposition of both vertical and horizontal polarization until it is measured.

When Wigner’s Friend observes the laboratory from outside, he sees the photon in a superposition of states, but once his friend makes a measurement, she observes it either in a vertical or horizontal polarization state. Wigner’s Friend and his friend both have different observations on the same system, leading to a paradoxical situation where two contradictory realities exist simultaneously.

Controversies and Criticisms of the Experiment

The Wigner’s Friend experiment has sparked many debates and discussions among physicists, philosophers, and scientists. One criticism of the experiment is that it might not be a realistic scenario, as it requires the possibility of two observers with different perspectives measuring the same system. Others argue that the experiment is not a legitimate way to address the measurement problem in quantum mechanics, as it relies on a subjective observer’s interpretation of reality.

Moreover, some argue that the experiment does not resolve the measurement problem, as it only presents a paradoxical situation without explaining how the observer’s perspective affects the system. Critics also argue that the experiment implies that human consciousness is essential in the measurement process, which is a problematic and controversial idea.

Implications for the Interpretation of Quantum Mechanics

The Wigner’s Friend experiment challenges the traditional interpretation of quantum mechanics, which assumes that a measurement outcome leads to a definite state of a system. The experiment suggests that the act of measurement depends on the observer’s perspective, leading to different realities for each observer. It also implies that the observer’s measurement interacts with the measured system, making it difficult to distinguish the observer from the system.

The experiment has prompted many interpretations of quantum mechanics, including the many-worlds interpretation, the decoherence theory, and the relational interpretation. It highlights the fundamental conceptual issues in quantum mechanics and the need for new ways to understand the nature of reality.