Light in a dense atomic vapor doesn't diffuse normally. Photons are absorbed and re-emitted by atoms, and the step lengths between scattering events follow a heavy-tailed distribution — a Lévy flight with exponent α = 1. The resulting transport is superdiffusive: photons spread faster than a random walk would predict, because occasional long steps dominate the statistics.
Álvarez Troncoso et al. (arXiv:2603.18967) measure this Lévy parameter using backward-scattered photons for the first time. Forward transmission measurements have established α = 1 for rubidium vapor at various densities. The backward measurement confirms the same value, but reveals an unexpected asymmetry in the scattering statistics.
Forward-traveling photons, even at moderate densities, undergo predominantly multiple scattering. By the time light exits the front of the vapor cell, each photon has been absorbed and re-emitted many times. The Lévy statistics emerge from the aggregate of these events. But backward-scattered photons — those reflected back toward the source — retain a significant fraction of single-scattering events even at high density.
The asymmetry is geometric. A photon scattered backward on its first interaction escapes immediately; it doesn't need to traverse the full depth of the vapor. Forward photons must run the gauntlet. The survival probability for single-scattering events is therefore direction-dependent: backward light preserves the first interaction, while forward light erases it under layers of subsequent scattering.
The Lévy exponent is the same in both directions — the heavy-tailed step distribution is a property of the medium, not the observation angle. But the composition of the signal differs. Forward light is purely collective; backward light is a mixture of individual and collective events. The same statistical law, built from different microscopic histories.