Pumping and Ratchet Mechanisms Universally Simulate Many-Body Active Dynamics

Researchers have shown that two simple mechanisms, a many-body Brownian pump and a many-body Brownian ratchet, can universally simulate any local active dynamics in spin systems. This finding is significant because active systems can exhibit phenomena forbidden in equilibrium, and it is often unclear when their behavior, specified by abstract local update rules, can arise from physically natural driving. The study establishes a direct connection between active dynamics and well-defined driving mechanisms. The first mechanism, the many-body Brownian pump, relies on a time-periodic Hamiltonian coupled to a cold bath. The second, the many-body Brownian ratchet, elevates the traditional concept of a Brownian ratchet (a transport mechanism) to a many-body context. This ratchet consists of a static Hamiltonian coupled to a hot bath and a cold bath, where the resulting steady heat current not only drives transport but also generates local active dynamics. Both mechanisms provide pathways to reproduce the complexity of active systems. Using probabilistic cellular automata as an explicit model, the authors prove that for any continuous-time or discrete-time local active dynamics, there is always a many-body Brownian ratchet (or pump) that approximates the dynamics. The inherent noise in this approximation can be made arbitrarily weak by tuning energy scales and other parameters. As a concrete demonstration, they constructed a simple ferromagnetic Ising ratchet on a bilayer lattice. When the two layers are coupled to baths at different temperatures, this model serves as a robust classical memory even under a symmetry-breaking field, something impossible in equilibrium. This work suggests that ratchets can use steady heat currents to autonomously generate and stabilize novel collective behavior, offering a new static setting for nonequilibrium many-body dynamics.

Type IIB Axion-Dilaton Wormholes and BPS Limit Hessian Revisited

A recent theoretical analysis has revisited Type IIB axion-dilaton Euclidean saddles, focusing on a specified axion charge sector. In this context, the solution with energy E=0 corresponds to a BPS instanton, while solutions with E>0 describe non-BPS wormholes with a smooth throat. Although both cases satisfy the same radial equations, their fluctuation problems are distinct, underscoring the complexity of these structures in string theory. For the BPS instanton (E=0), the study details how the quadratic action is reduced to a physical Hessian after considering the Hamiltonian constraint, gauge quotient, charge-sector boundary condition, and the removal of collective zero modes. This Hessian, denoted as H_ν, factorizes into the form Q_ν†Q_ν. This result is interpreted as an endpoint theorem, extending beyond a simple stability theorem for the full E>0 wormhole. This finding provides a firmer foundation for understanding the spectra of wormholes in Type IIB string theory. The work also separates the connected two-ended wormhole throat from its long-distance two-end multipole operator term. Once the coefficient matrix Cij is derived, the different-component and same-component placements of the two end insertions appear as terms in the same quadratic expression. Removing either term requires a genuine projection or explicit cancellation, highlighting the interconnectedness of these theoretical structures and their impact on understanding spacetime geometry in string theory.