Energy Diffusion in Gases

In the air surrounding us, how does a particle diffuse? Thanks to Einstein and other pioneers,it has been well known that generally the particle will undergo the Brownian motion, and in the last century this insight has been corroborated by numerous experiments and applications. Another fundamental question is how the energy carried by a particle diffuses. The conventional transport theories assumed the Brownian motion as the underlying energy transporting mechanism, but however, it should be noticed that in fact this assumption has never been tested and verified directly in experiments. Here we show that in clear contrast to the prediction based on the Brownian motion, in equilibrium gases the energy diffuses ballistically instead, spreading in a way analogous to a tsunami wave. This finding suggests a conceptually new perspective for revisiting the existing energy transport theories of gases, and provides a chance to solve some important application problems having challenged these theories for decades.