Particle theory is one of the central concepts of modern physics. The structure of matter and many aspects of its behavior, can best be understood by considering it to be made up of small, discrete particles. The same is true of light and other forms of electromagnetic radiation. This idea has emerged gradually over a long period of time, but certain individuals stand out as key figures in the development of the theory.
Democritus and Dalton
The concept of an atom, which comes from the Greek word meaning “indivisible,” was first proposed by the philosopher Democritus, who was born in the 5th century B.C. Democritus believed that all matter was composed of minutely small, indivisible pieces. This proved to be an idea ahead of its time, however, and it was largely ignored by other philosophers. It was only at the start of the 19th century that an English chemist named John Dalton took the idea further, proposing that an atomic theory of matter was the easiest way to explain chemical reactions between different substances.
Planck and Einstein
During the 18th and 19th centuries, there were two competing theories about the nature of light, one viewing it as a stream of particles and the other as a wave. Some experiments seemed to support the latter view because light does indeed propagate like a wave. In 1900, however, Max Planck proposed that when light is emitted by atoms, it takes the form of discrete particles. This “quantum theory” of light was strengthened in 1905 when Albert Einstein showed how it could explain the photoelectric effect.
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Thomson, Rutherford and Bohr
In the late 1890s, a British physicist named J. J. Thomson discovered that atoms were not monolithic and indivisible but must contain smaller particles: negatively charged electrons. The familiar, though not entirely correct, image of electrons orbiting a nucleus like planets around the Sun was proposed by Ernest Rutherford in 1911. The picture was refined two years later when Niels Bohr, using Planck’s quantum theory, showed how the electrons had to be confined to certain fixed energy levels.
Around 1920, Rutherford speculated that the atomic nucleus could be divided into a number of positively charged protons and electrically uncharged neutrons, although it was only in 1932 that Rutherford’s student James Chadwick found experimental evidence for the neutron. Since that time, many further subatomic particles have been discovered using high-energy accelerators, and it is now known that the proton and neutron can be further subdivided into quarks -- a name coined by Murray Gell-Mann in 1964.