While quantum mechanics is the best-tested theory within physics to date, it is well-known to fail in describing at least two instances of the dynamics of objects consisting of a macroscopic number of particles: Schrodinger's equation is incompatible with the dynamical emergence of a symmetry-broken state during a phase transition, and it does not by itself describe the observation of any particular outcome in a single quantum measurement. It has been suggested that these two seemingly disparate dynamical processes may both arise from a spontaneous breakdown of the unitarity of quantum dynamics, in close analogy to the spontaneous breakdown of spatial symmetries in equilibrium.
I will first show that quantum dynamics has a diverging susceptibility to non-unitary perturbations, making spontaneous unitarity violation unavoidable in the thermodynamic limit as long as there is any source of arbitrarily weak non-unitarity in nature. I will then argue that such sources should exist, and that our observation of phase transitions in everyday life is proof of their existence. Next, I will show that spontaneous unitarity violations also give a possible route to resolving the quantum measurement problem, and I will briefly contrast it to other approaches addressing this well-known and persistent foundational problem.
Finally, I will highlight some of the constraints imposed on theories of Spontaneous Unitarity Violation by existing experimental results, as well as some of the new experimental predictions to which they give rise.
