Particle theory is an incredibly widely believed theory of matter, which holds, essentially, that matter is made up of small particles which are constantly moving. Particle theory is the domain of particle physics, which is also known as high-energy physics, because many of the most fundamental particles don’t generally occur in nature, but can be generated by creating extremely high-energy interactions between particles.
The predecessor to modern particle theory dates back all the way to ancient Greece, when philosophers in the 6th century BCE held that there were fundamental particles of matter. For thousands of years after this, however, particle theory vanished through most of the world, with various other theories springing up instead. In the 19th century, however, it made a comeback, with a scientist named John Dalton proposing a fundamental, indivisible particle that made up all matter. He named this particle the atom, from the Greek word that meant indivisible.
By the end of the 19th century it had become apparent, however, that the atom was not, in fact, indivisible, and that there were even smaller particles that made up the atom. During the 20th century, and into the 21st century, these particles were split, and new high energy particles were discovered. The discovery of these particles helped refine and expand the nature of particle physics, and a great deal of the work done today has to do with generating new particles to observe. This is done by creating very high-energy interactions in particle accelerators, where particles are launched at each other at incredibly high speeds, essentially shattering apart into their constituent bits and releasing energetic particles that quickly dissipate.
The working model of particle theory right now is known as the Standard Model. The Standard Model, although not a true Unifying Theory, comes remarkably close. It addresses three of the four known interactions: the electromagnetic, the strong, and the weak forces. It fails to address the gravitational force, making it incomplete, but it still allows for a great deal of comprehensive understanding of particles and the universe as a whole. The model contains twenty-four fundamental particles, which make up matter, and the gauge bosons that mediate the forces. It also predicts a type of boson, the Higgs boson, which is the only boson that hasn’t yet been observed, expected to be detected by the Large Hadron Collider.
At a basic level, particle theory helps categorize the three main states of matter that we see on a day to day level. Particles are viewed in relation to each other, and the amount of energy they have, which affects how much they move. In a state where particles are strongly attracted to each other, and are held tightly together in a state where they vibrate but remain relatively fixed, a solid exists. When there is some attraction between the particles, and they are held somewhat together, with a relatively-free degree of movement, a liquid exists. And when there is little attraction between the particles, and they can move freely around, a gas exists.