In "Our Mathematical Universe" by Max Tegmark, in the footnote at p. 208, Tegmark writes "..the well known fact that things are more likely to bump into each other when you lower the temperature, just as slow neutrons are more likely than fast ones to strike targets in a nuclear reactor" Is this really true? It seems completely wrong to me. Suppose that we lower the speeds of all particles in a container by 1/2. The effect will be that the particles move in slow motion compared to before. Hence, all the collisions between the particles which occured in a time interval of length t before, now occur in a time interval of length 2t, so that fewer collisions occur in a time interval of length t than before, not more collisions. What is the truth?