Around the middle of the 19th century, experiments on electricity transmitted through rarefied gases lead to the discovery of invisible rays deflectable by magnetic forces. It emerges that such rays, and indeed even normal light, behave mechanically and are able to drive a suitably constructed mill.

At the end of the century, it is accidentally discovered that the element uranium emits rays which can penetrate through bodies. In uranium ore, new elements are discovered that are detected from their radiation.

Einstein and his contemporaries are confronted with the problem of finding theoretical explanations for the different types of radiation and their effects.