What’s hard about that? You might guess that this gets difficult. Here are a few of the problems: Just a space the size of Earth contains around 1027 particles (a 1 followed by 27 zeroes). This presents a massive computing problem: The more particles you add, the slower your simulation. So you can only run a fraction of the actual number of particles, and you can only model small volumes of space. It’s hard to model both electrons and protons. You can get away with a bigger time step for protons: they’re heavier and move more slowly. But electrons are 1800 times lighter, and you need a correspondingly smaller time step. The fact that the particles generate electric currents, which generate fields, which affect the particles, makes things difficult. If you can assume that the particles don’t modify the fields, things are much easier.