In 1961, the Nobel Prize-winning physicist Eugene Wigner conceptualized a thought experiment revealing a little-known paradox of quantum mechanics. Wigner’s thought experiment demonstrated a strange quirk of the universe. It allows for two observers to experience two different realities from the same event.

It’s based on a quantum measurement of a physical system. In our example, we’ll use the polarization of a single photon. Polarization is a property of photons that when measured, can be described linearly as being either horizontal or vertical. However, in quantum mechanics, until that measurement is made, the photon exists in both polarization states at the same time. This is known as a superposition.

The collapse of a superposition into a known state is a fundamental quantum principle illustrated famously by the double slit experiment. In the double slit experiment, an electron beam is projected through two slits, creating a wavelike interference pattern over time. Once a measurement device is placed in front of one of the slits, probing the electron, the interaction of the measurement device with the electron causes the wave to collapse into a defined electron path.

In Wigner’s thought experiment, he envisions a friend working in a separate lab from his own. His friend is tasked with measuring the polarization state of a single photon and recording the result. To an observer outside of the lab, the friend’s measurement is a unitary interaction that leaves the photon and friends record in an entangled state. The recorded state and the polarization state of the photon are in effect the same information.

Wigner himself can observe the experiment, though he is provided no information about his friend’s measurement or the recorded outcome. Without this information, and with no means of interacting with the photon, quantum mechanics forces him to assume that the photon’s polarization is in a superposition of all possible states.

When Wigner finally asks his friend to read him the recorded measurement, from his perspective the superposition state of the photon now collapses into a single polarization state. Wigner and his friend’s record now both share the same polarization state information as that of his friend's measurement.
This creates a curious implication. During the period of time when Wigner is forced to view the photon as being in a superposition, his friend views the same photon as having a defined state. But since both points of view must be regarded as equally valid, this is where an apparent paradox comes into play. When now exactly did the collapse of the photon’s superposition occur? Was it when the friend had finished his measurement, or when the information of its result entered Wigner's consciousness?


This paradox potentially creates a rift in the foundation of science itself, calling into question the nature of measurement. Can objective facts even exist? Scientists carry out experiments to establish objective facts, but if they experience different realities how can they agree on what these facts might be?


Wigner’s thought experiment was put to the test by a team of physicists at Heriot-Watt University in Edinburgh with the results being published in February 2019. The experiment tested for the validity of observer-independence at the quantum level, similarly using photon polarization. Using an extended variant of Wigner's friend scenario, the results of that experiment lends considerable strength to Wigner’s thought experiment and interpretations of quantum theory that are observer-dependent.

This view of reality questions if observers have the freedom to make whatever observations they want. It also brings locality into question. Locality limits an object to only be directly influenced by its immediate surrounding. It prevents interactions at a distance.


Several counter-arguments have been proposed against the paradox created by Wigner’s experiment. The most obvious counterpoints are those of flaws in assumptions made within the experiment itself. Some have proposed that a privileged position as an ultimate observer, encompassing the entire experiment would be able to reconcile the paradox by viewing the experiment from a larger world view. Others have suggested that conscious observation without interaction do not count as observation, therefore no paradox is created. Some interpretations have even speculated that the observation is personalized due to the information available to the observer and that there is no paradox created, just missing information.

To date, no framework of quantum mechanics offers a full explanation for the implications of Wigner’s thought experiments. And with experimental evidence edging closer to undermining the idea of observer-independent objective reality, the fundamental assumptions of science itself may be in danger.

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