Carrollian Quantum Mechanics: Time-like, Space-like and Hybrid Sectors

Abstract

We develop a comprehensive theoretical framework for Carrollian quantum mechanics by performing systematic ultra-relativistic contractions of the Klein--Gordon equation in the limit $c \to 0$. This limiting process uncovers three distinct sectors---time-like, space-like, and hybrid---each governed by a Carroll-invariant wave equation and accompanied by a consistent probabilistic interpretation. The time-like sector exhibits a novel temporal tunneling phenomenon, characterized by the relation $|\mathcal{T}|^2 = 1 + |\mathcal{R}|^2$, which reflects the indefinite character of the Klein--Gordon norm. In the space-like sector, the presence of spatial propagation necessitates a tachyonic dispersion relation, which forces the density to vanish for energy eigenstates, yielding zero-norm (null) states that gain physical relevance within Carrollian physics due to the underlying null geometric structure. The hybrid sector combines features of both sectors and admits two distinct formulations, which may be either tachyonic or non-tachyonic, and can be interpreted as an inhomogeneous Klein--Gordon equation. For all three sectors, we derive the corresponding continuity equations, probability densities, and currents, and investigate canonical quantum systems---including the particle in a box and tunneling phenomena---within the Carrollian regime.