Ballistic transport is the speed limit. When a system is smaller than the mean free path of its energy carriers — when particles cross the entire system without scattering — heat conduction can’t get faster. The thermal conductivity scales linearly with system size: k ~ L. This is traditionally regarded as the upper bound.

It can be exceeded (arXiv:2603.16296). In dilute chains of plasmonic nanoparticles confined within optical cavities, thermal photon conductivity scales as k ~ L^1.5. Superballistic. Faster than ballistic. The energy moves through the system as if the carriers were accelerating rather than merely traveling freely.

The mechanism: the cavity creates guided modes that amplify long-range interactions between nanoparticles. Without the cavity, each particle interacts mainly with its nearest neighbors — the standard setup for ballistic or diffusive transport. With the cavity, a particle at one end can couple to a particle at the other end through cavity-mediated interactions that bypass the chain. The confinement doesn’t slow transport down (as you’d expect from adding walls). It speeds it up by creating communication channels that skip intermediate steps.

The through-claim: the “speed limit” of ballistic transport assumed that the fastest possible carrier was one that didn’t scatter. But cavity-mediated coupling creates an effective carrier that’s not a single photon traversing the chain — it’s a collective excitation that spans the system simultaneously. The limit was for individual carriers. Collective modes aren’t bound by it.