This post is part of a collaborative series with FYP on pilot-wave hydrodynamics. Previous posts: 1) Introduction; 2) Chladni patterns; 3) Faraday instability; 4) Walking droplets; 5) Droplet lattices; 6) Quantum double-slit experiments; 7) Hydro single- and double-slit experiments; 8) Quantum tunneling
Quantum tunneling is a strange subatomic behavior that was first described to explain how alpha particles escape a nucleus during radioactive decay. Classically, a particle trapped in a well can only escape if its energy is sufficiently high, but in quantum mechanics, even a particle with lower-than-necessary energy can occasionally “tunnel” out.
To test whether hydrodynamic walkers can tunnel, researchers built corrals. In the central region, the pool on which the walker moves is relatively deep. Over the walls, the pool is much shallower. In this shallow area, the wave from the droplet’s bouncing decays quickly, creating a partially reflective barrier. For most collisions, the walker reflects off the barrier. Other times, apparently at random, a collision results in the walker crossing the wall and tunneling out of its well.
Over many experiments, researchers were able to construct a probabilistic view of walker tunneling. In quantum mechanics, a particle’s likelihood of tunneling out of a well depends on the particle’s energy and the well’s thickness. The analogs for a walker are velocity and barrier thickness. The thicker the barrier, the harder it is for a walker to tunnel through. Conversely, a faster walker has a higher probability of tunneling through a barrier of a given thickness. As the authors themselves observe:
“Although our experiment is foreign to the quantum world, the similarity of the observed behaviors is intriguing.” #
As we wrap up our series tomorrow, we’ll consider some of those similarities more deeply.