Physical interpretation of non-normalizable harmonic oscillator states and relaxation to pilot-wave equilibrium

Abstract Non-normalizable states are difficult to interpret in the orthodox quantum formalism but often occur as solutions to physical constraints in quantum gravity. We argue that pilot-wave theory gives a straightforward physical interpretation of non-normalizable quantum states, as the theory requires only a normalized density of configurations to generate statistical predictions. In order to better understand such states, we conduct the first study of non-normalizable solutions of the harmonic oscillator from a pilot-wave perspective. We show that, contrary to intuitions from orthodox quantum mechanics, the non-normalizable eigenstates and their superpositions are bound states in the sense that the velocity field $$v_y \rightarrow 0$$ v y → 0 at large $$\pm y$$ ± y . We argue that defining a physically meaningful equilibrium density for such states requires a new notion of equilibrium, named pilot-wave equilibrium, which is a generalisation of the notion of quantum equilibrium. We define a new H-function $$H_{pw}$$ H pw , and prove that a density in pilot-wave equilibrium minimises $$H_{pw}$$ H pw , is equivariant, and remains in equilibrium with time. We prove an H-theorem for the coarse-grained $$H_{pw}$$ H pw , under assumptions similar to those for relaxation to quantum equilibrium. We give an explanation of the emergence of quantization in pilot-wave theory in terms of instability of non-normalizable states due to perturbations and environmental interactions. Lastly, we discuss applications in quantum field theory and quantum gravity, and implications for pilot-wave theory and quantum foundations in general.