An undulator, or wiggler, used in a free electron laser. (credit: UCLA Particle Beam Physics Lab) Lots of interesting stuff happens really fast. Think about a chemical reaction, for instance. The rate of reactions might be slow, but each individual reaction proceeds quickly. This is because a chemical reaction is, essentially, the shuffling of electrons between different atoms, and electrons are fleet of foot. Generally, if you want to watch something this fast happen, you use what is called pump-probe spectroscopy, in which one short pulse of light initiates an action while another measures the result.