According to Einstein's famous equation, all matter is made of energy, but all matter is also made of fundamental particles - little, discrete, bundles of energy.
The idea that all matter was composed of tiny, similar pieces was first put forth by Greek philosophers Democritus and Leucippus thousands of years ago, but the idea remained theory until the 20th century. Some particles were discovered in the 1930s using cloud and bubble chambers that indirectly tracked the movements of otherwise invisible particles onto photographs. It was in the 1950s and 1960s that scientists developed the equipment necessary to explore the realm of the very small more completely, in order to decipher the inner workings of atoms. The tools they developed included particle accelerators that mapped particle interactions in three dimensions using computers.
What these scientists found was somewhat surprising at the time. They discovered a wide variety of particles which were initially dubbed "The Particle Zoo". As scientists worked out how all the particles interacted to make matter, this zoo was more formally called "The Standard Model".
The Standard Model defines the interactions of the particles and groups particles according to their properties. Two of these properties are mass (measured in MeV, mega electron volt, a unit of energy), and electric charge. Another more exotic property is spin. Spin is essentially the quantum angular momentum of a particle, but it cannot be defined as a result of rotation in the conventional definition of angular momentum. Spin is the ultimate unit of angular momentum from which angular momentum in larger bodies arises. Spin is measured in integer and half integer values for elementary particles (0, 1/2, 1, 3/2, 2 ...).
Next week we will begin to look at the elementary particles: fermions and leptons.
