A stone falls into a pond, and ripples spread across the water. But if the stone is very small and the water is not quite ordinary, the ripples pass right through it, as if it weren't there. That's roughly how sound vibrations behave in superfluid helium: at extremely low energies, they seep through obstacles as if those obstacles didn't exist.
Physicists call this anomalous tunneling. Usually, tunneling is when a particle slips through a barrier that, by classical standards, it shouldn't be able to overcome. But here it's even more surprising: as energy decreases, transmission gets better, not worse, and in the limit reaches one hundred percent. For a long time, this was considered a curiosity unique to certain exotic media. The authors of the new work proved that the reason runs deeper: it lies in the very symmetry of the system.
In a superfluid, there is a special mode of vibration — the phonon, the quantum of sound. Its behavior follows the laws dictated by spontaneous symmetry breaking. Imagine a pencil balancing on its point on a perfectly flat table. While it stands upright, the system is symmetric under rotation. But the slightest breath, and the pencil falls, choosing some particular direction. Superfluidity is like that: symmetry exists, but the ground state 'breaks' it by choosing a specific phase. This gives rise to Nambu–Goldstone modes — the very phonons that, at low energies, do not feel smooth obstacles.
The work showed that if the potential barrier does not touch the broken symmetry, it becomes invisible to long waves. This is a universal rule that works for magnons (magnetization waves) and other systems as well. But if the potential does break the symmetry, the wave starts to reflect — ordinary, not anomalous, tunneling. So physicists now have a simple criterion: preserve the symmetry, and the barrier disappears. And then the stone thrown into the pond ceases to be an obstacle for the ripples on the water.
In superfluid helium, sound can pass through a porous partition that would stop an ordinary liquid, and do so without any loss of energy.
In Peter Watts's novel Blindsight, alien beings use quantum effects for instantaneous communication — anomalous tunneling could be the basis for such technologies.