In the early 1900s, scientists began to investigate electrons with energies much higher than those in chemical reactions or even in radioactive decay. These so-called “high energy” electrons posed a number of mysteries. For example, how could they be produced? What was their exact nature? And what were they doing inside atoms?
In the 1930s, two types of high energy electrons were discovered: cosmic rays and X-rays. Cosmic rays are energetic particles that rain down on Earth from outer space. They are thought to be produced by supernovae, which are exploding stars. X-rays are electromagnetic waves with very short wavelengths. They are produced when high energy electrons collide with matter.
The study of high energy electrons continued in the 1940s with the development of particle accelerators. These devices use electric fields to accelerate charged particles to extremely high energies. The first particle accelerator was built in 1930, but it wasn’t until the 1950s that accelerators capable of reaching energies approaching those of cosmic rays were constructed.
Particle accelerators have allowed scientists to probe the structure of matter at ever smaller length scales and to study the interactions of high energy particles with matter in unprecedented detail. In addition, they have provided a powerful tool for studying the basic laws of physics and for searching for new particles beyond those already known.