Gravity from positivity: Single massive spin-3/2 particle makes gravity logically inevitable, study claims

Researchers at IPhT (CEA, CNRS) and the Universitat Autònoma de Barcelona have shown that gravity—and with it, supersymmetry—emerge as logical necessities whenever a massive spin-3/2 particle exists in nature. Two principles are enough: causality, the fact that no signal can travel faster than light, and unitarity, the requirement that probabilities are conserved in quantum mechanics. The structure of supergravity is not assumed: it bootstraps itself.