Psi-Epistemic Theories: The Role of Symmetry

TL;DR

This paper proves that in dimensions three and higher, symmetric psi-epistemic theories cannot reproduce quantum predictions, but without symmetry, such theories can exist, highlighting fundamental limitations and possibilities in quantum foundations.

Contribution

The paper establishes a new no-go theorem for symmetric psi-epistemic theories in dimensions d≥3, extending previous results without relying on Kochen-Specker or PBR assumptions.

Findings

No symmetric psi-epistemic theories in dimensions d≥3 satisfy maximum nontriviality.

Existence of maximally-nontrivial psi-epistemic theories without symmetry in all finite dimensions.

The proof employs measure theory and differential geometry techniques.

Abstract

Formalizing an old desire of Einstein, "psi-epistemic theories" try to reproduce the predictions of quantum mechanics, while viewing quantum states as ordinary probability distributions over underlying objects called "ontic states." Regardless of one's philosophical views about such theories, the question arises of whether one can cleanly rule them out, by proving no-go theorems analogous to the Bell Inequality. In the 1960s, Kochen and Specker (who first studied these theories) constructed an elegant psi-epistemic theory for Hilbert space dimension d=2, but also showed that any deterministic psi-epistemic theory must be "measurement contextual" in dimensions 3 and higher. Last year, the topic attracted renewed attention, when Pusey, Barrett, and Rudolph (PBR) showed that any psi-epistemic theory must "behave badly under tensor product." In this paper, we prove that even without the Kochen-Specker or PBR assumptions, there are no psi-epistemic theories in dimensions d>=3 that satisfy two reasonable conditions: (1) symmetry under unitary transformations, and (2) "maximum nontriviality" (meaning that the probability distributions corresponding to any two non-orthogonal states overlap). This no-go theorem holds if the ontic space is either the set of quantum states or the set of unitaries. The proof of this result, in the general case, uses some measure theory and differential geometry. On the other hand, we also show the surprising result that without the symmetry restriction, one can construct maximally-nontrivial psi-epistemic theories in every finite dimension d.