What might physics be like if it was not quantum? In research published in the Journal of Physics A: Mathematical and Theoretical, Valerio Scarani and two collaborators have examined a world of possible theories beyond quantum physics, classifying for the first time a set of scenarios for interactions involving three objects. They provide a nice overview of the work in an accompanying news article, "At the end of the quantum world".
Physicists would like to be able to explain why quantum theory is singled out from other theories that could describe our Universe, and CQT researchers have previously made important steps towards answering this question—see CQT's past news highlights on information causality and the connection between the uncertainty principle and non-locality. Rather than addressing the ‘how’, this new work reveals details of the possibilities that are ruled out.
Valerio, a Principal Investigator at CQT and Associate Professor at NUS, and his co-authors, Stefano Pironio from the Université Libre de Bruxelles, Belgium, and Jean-Daniel Bancal from the University of Geneva in Switzerland, start from the assumption that any physics theory must be no-signaling, meaning that it doesn't allow information to be transmitted faster than light. They then map mathematically the correlations that can exist between three parties under this condition, finding unexpected complexity in the resulting mathematical description, which is known as a ‘polytope’. The correlations allowed in quantum physics are just a subset of these. "This complexity means also richness: many of the features of this construction are still to be explored. It’s almost sad that nature uses only part of this world!" Valerio and his co-authors write in the news story that accompanies the article.
The paper "Extremal correlations of the tripartite no-signaling polytope" appears in the 11 February edition of the journal: S. Pironio et al., J. Phys. A: Math. Theor. 44 065303 (2011); arXiv: 1101.2477.