 If we place an isolated concentrated homogeneous gas in the middle of a large empty volume, we would expect the particles to spread out in an expanding sphere about the center of the gas, much as waves of radiation spread out from concentrated charge sources. It is therefore tempting to think that there is a relationship between the thermodynamic and electromagnetic arrows of time. In a debate in 1909, Albert Einstein and Walther Ritz apparently disagreed about the nature of this relationship, although the exact points of dispute remain a bit unclear. The common story told is that Ritz took the position that the asymmetry of radiation had to be judged lawlike and that the thermodynamic asymmetry could be derived from this law. Einstein’s position is instead that “irreversibility is exclusively based on reasons of probability” (Ritz and Einstein 1909, English translation from Zeh 1989: 13). It is unclear whether Einstein meant probability plus the right boundary conditions, or simply probability alone. In any case, Ritz is said to believe that the radiation arrow causes the thermodynamic one, whereas Einstein is said to hold something closer to the opposite position. The real story is far more complicated, as Ritz had a particle-based ontology in mind as well as many additional considerations (see Frisch and Pietsch 2016 for subtleties of the actual historical debate).