The consensus problem in quantum physics

Is the reality we experience an objective fact? Could it be that I experience a different reality than my friends? Who’s right? Or are all right? Consider conflicting witness statements, which are a regular occurrence in trials. Was that car red or blue?

Wigners’s Friend

One of the problems in the interpretation of quantum physics is the consensus problem. Nobel laureate Eugene Wigner did already point this out with his thought experiment Wigner’s Friend.

In Wigner’s thought experiment, a person we’ll call Wigner’s friend is doing experiments in a laboratory. The friend measures the state of a particle that is in a superposition of two states, say 0 and 1. The measurement collapses the quantum state of the particle to 0 or 1 – the quantum collapse – and the result is recorded by the friend.

Wigner himself is standing at the closed door outside the lab. From his perspective, the lab and its friend — assuming they are completely isolated from all environmental influences — continue to evolve quantum mechanically together. After all, quantum mechanics makes no statements about the size of the system to which the theory applies. Basically it applies to elementary particles, to the sun and moon, and also to humans.

If quantum mechanics is universally applicable, Wigner argued, then both the particle and Wigner’s friend are now entangled and in a quantum superposition, although the friend’s measurement has apparently already collapsed the particle’s superposition.

Wigner originally believed that the observer’s consciousness played an important role in the quantum collapse, but because of this thought experiment and the contradictions it produced, he backtracked on it. The consensus that observers have about the observed reality has been broken here. Every observer perceives his own reality. This was apparently unacceptable enough for Wigner to abandon his idea that the consciousness of the observer played a role.

Alternative realities?

In a similar fashion as which Bell’s theorem, which does make a statement about the validity of the assumption that reality is permanent and governed by the laws of relativity – the so-called local hidden variables hypothesis, researchers at the Center for Quantum Dynamics at Griffith University in Australia concluded that the Wigner’s Friend experiment offers the possibility to answer the question of whether quantum manifestations are objective reality – observable and equal for all observers. If not, then each observer experiences his own alternative facts, as Kellyanne Conway apparently believes. Then Trump also lives in his own alternate reality in which he has indeed been deprived of his rightful majority of votes.

Alternative facts are real?

Alternative realities experiment 2019

This experiment was carried out in 2019. See figure. The boxes represent the laboratory with Wigner’s friend. Wigner is standing next to the box. The human observers have been replaced by so-called photonic memories. The researchers themselves say:

This, however, would require a convincing revision of our minimal definition of what qualifies as an observer, which typically comes at the cost of introducing new physics that is not described by standard quantum theory.

Similar to a Bell test, this Wigner’s Friend experiment examined whether an inequality would be violated, the so-called Clause-Horne-Shimony-Holt inequality. For the sake of completeness:

S =〈A1B1〉+〈A1B0〉+〈A0B1〉-〈A0B0〉≤ 2

A0, A1, B0 en B1 are the results of the measurements of the two independent researchers, say Alice and Bob.

There are three conditions for the inequality – F, L and OIF:

  • Free choice: Alice and Bob have free choice in what they are going to measure.
  • Locality: The choices made by Alice and Bob do not influence each others outcomes.
  • Observer Independent Facts: All observers are able to compare their outcomes with each other.

If the Clause-Horne-Shimony-Holt inequality is violated, then at least one of the above assumptions about reality is false.

In the 2019 experiment the inequality was violated with a significance of: S = 2,416 ± 0,075.

In the discussion, the researchers present the necessary caveats. It was not possible in this version to close all possible loopholes, and it is questionable to consider the ‘photonic memories’ as valid observers. But the result is nevertheless intriguing. If we accept the result and assume that F is true, then it is also true that:

  • either observer independent facts do not exist
  • or our choices do influence the observations of others.

Now let us wait for more results of the experiments by the Australian Griffith University team.

If you would like to know my opinion: There is only one mind. And we are all fragments of that. But then the results of this experiment still raise the question of whether the fragments of the one mind – we – can experience different realities.