Why Do We Collide Particles?

Around 100 years ago, Ernest Rutherford was the first to recognize that much could be learned from having atomic-scale particles collide with each other, and studying details of what happened as a result of the collisions. Since he did not have particle accelerators, he used atoms of helium (minus their electrons) to bombard atoms from a wide variety of materials. The helium atoms are produced at what were then high energies (a few MeV) in radioactive decays of heavy elements.

He then observed the directions in which the helium atoms were deflected by the target atoms, and found, to his surprise, that a few were deflected through very large angles. He then came to the unexpected conclusion that almost all of the atom's mass is concentrated in a region about 1/100,000 of the atom's radius. This region is what we now call the atomic nucleus.