The great breakthrough in accelerator technology came in 1920 with Ernest O. Lawrence's invention of the cyclotron. In the cyclotron, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. Soon unprecedented energies were achieved, and the steady improvement of Lawrence's simple machine has led to today's proton synchrotrons, whose endless circular flight paths allow protons to gain huge energies by passing millions of times through the electric fields that accelerate them. But early in the 1960's physicists had gained enough experience in accelerator technology to be able to build colliders, in which two carefully controlled beams are made to collide with each other at a chosen point. Several colliders exist around the world today, and the technology for them is by now well established.
Colliders are more demanding to build, but the effort pays off handsomely.
In a fixed-target machine, most of the projectile particle is locked up, after impact on the target, in continued forward motion of the debris. In a collider, on the other hand, two particles of equal energy coming together have no net motion, and collision makes all their energy available for new reactions and the creation of new particles. But to the non-scientist, the analogy may be puzzling. Most of us have looked down a microscope, perhaps seeing grains of pollen or the cells of a living organism. We can see objects in the world around us because light bounces off them and enters our eyes.