In day-to-day usage, the term particle is used to describe very small objects. Physicists use the term particle in a more precise way: they use it to describe the behavior of an object without reference to any internal structure. Thus astronomers might refer to the Earth or the Sun as a particle if they are studying its motion in a crude enough way such that, say, tidal forces are not important.
Originally, scientists thought that atoms were elementary objects with no structure. As such, they would have been the ultimate particles. Indeed, the word "atom" means uncuttable. However, shortly after the existence of atoms was firmly established (perhaps most convincingly by Einstein's explanation of the Brownian motion in 1905), Ernest Rutherford, bombarding gold atoms with alpha particles, discovered that the atom is largely empty space, with electrons revolving around a tiny nucleus. Over the next decades, the proton and neutron were discovered and established as the basic entities making up the nucleus.
With the development of quantum mechanics, the notion of particle took on a new aspect. Particles such as the electron were seen to exhibit characteristics of waves. Electrons exhibited interference phenomena, much like light, and could undergo diffraction. On the other hand, as first postulated by Albert Einstein, light often exhibited particle characteristics. For example, light comes in discrete packets of energy and momentum. Electromagnetic radiation can be thought of as consisting of large numbers of particles, called "photons." This discrete character of light currently underlies much of electronics technology.
Traditionally, the objects that make up the atom, the electron, proton and neutron, as well as the photon, are referred to as "elementary particles." The electron and photon are, as far as we know, without structure. They are completely described by their mass, charge, and spin angular momentum. The electron has nonzero mass and charge, as well as ½ unit of spin angular momentum; the photon has neither mass nor charge and carries one unit of spin angular momentum. The proton has mass nearly 2,000 times larger than that of the electron, the same spin, and opposite electric charge. To ask whether particles such as electrons or protons have structure requires microscopes capable of resolving extremely short distances. Particle accelerators are such microscopes.
The most energetic accelerators today can resolve structures as small as 10-17 cm. On this scale, neither the electron nor the photon exhibits structure. The proton and neutron, however, have a size of about 10-13 cm. Experiments in the late 1960s, similar to Rutherford's in spirit, but involving high-energy electrons scattered off nuclei, demonstrated that nuclei are made of smaller entities called quarks. While there have occasionally been suggestions that the quarks themselves might have structure, there is so far no evidence for this, and some theoretical arguments have been put forward that they do not.
Many other particles have been discovered in cosmic rays and accelerators. Most of these are known as "hadrons" and are strongly interacting like the proton and neutron. They are composed of quarks and can be put together in tables similar to the periodic table. Six others, known as "leptons," are more similar to the electron. These include the muon and tau particles. These have the same electric charge as the electron, and, like the electron, they experience the electromagnetic and weak force (responsible for beta decay) but not the strong force. The muon is about 200 times as massive as the electron; the tau particle about 3,000 times. The other three are the neutrinos. The neutrinos are electrically neutral and extremely light (recent experiments show that the neutrinos have mass less than one millionth that of the electron). They experience only the weak force and thus don't readily stop in matter.
For every known particle, there is also an antiparticle. This is a particle of the same mass but opposite charge. For example, the antiparticle of the electron is the positron, which has been well studied experimentally. The antiparticle of the proton, the antiproton, was discovered in the early 1950s (when the proton was still widely believed to be an elementary particle). More recently, antihydrogen has been created in the laboratory. Some particles, like the photon, are their own antiparticles (neutrinos are not).
The fact that light has both particle and wave aspects emerges immediately if one applies the rules of quantum mechanics to Maxwell's theory of electricity and magnetism. The resulting theory is known as quantum electrodynamics. Just as the photon is described in terms of electric and magnetic fields, the electron is also described by a field. In this theory, Einstein's principle of relativity, and knowledge of the charge and spin of the electron fully determine its other properties. For example, the fact that electrons obey the Pauli exclusion principle is automatic. In this theory it is possible to calculate the properties of the electron and photon, as well as of simple atoms, to extraordinary precision. The magnetic moment of the electron can be calculated to twelve significant figures and measured with comparable accuracy. Beginning in the 1970s, quantum electrodynamics was generalized to a theory that includes the weak and strong interactions. The equations of this theory are similar to Maxwell's equations (Maxwell's equations are a special case). The quantization of this theory leads to a theory called the Standard Model. In addition to the quarks and leptons and the photon, the Standard Model predicts three other particles analogous to the photon, called the W± and the Z . Like the photon, these particles carry spin one, but they are massive, and the W bosons are charged. Just as the photon is responsible for the electromagnetic force, theW and Z particles are responsible for the weak force. The model predicts the masses of these particles to be approximately eighty and ninety times the mass of the proton, respectively. It also predicts their half-lives (these particles are very radioactive, with half-lives of order 10-23 seconds). Both the masses and lifetimes of these particles have been measured in particle accelerators (LEP at the European Laboratory for Particle Physics [CERN] in Geneva and the SLC at the Stanford Linear Accelerator Center) to the level of parts per thousand and agree closely with the theory.
There is one other particle predicted by the Standard Model, known as the Higgs boson, which has yet to be observed. Within the theory, this particle is responsible for the masses of the other elementary particles. From the precision measurements of the properties of the W and Z bosons, the mass of this particle is predicted to lie between approximately 80 and 230 times the mass of the proton. Searches for this particle have so far excluded a Higgs particle with masses about 115 times that of the proton. Accelerators at Fermilab and CERN will search for this crucial particle over the next few years.
Einstein's theory of general relativity predicts another particle, the graviton. In much the same way that the photon is responsible for the electromagnetic force, the graviton is responsible for the gravitational force. This particle interacts so weakly with matter that it is not possible to count gravitons one at a time, as one can photons. However, with very sensitive instruments it should be possible to detect gravitational waves emitted by violent events in the universe. This is the goal of the LIGO project.
Is this the sum total of possible particles? Probably not. Experiments are searching for a variety of particles. Among these are a particle called the axion and one called the neutralino. These have been proposed as possible solutions to puzzles in the Standard Model. Either of these could well comprise the dark matter which astronomers believe constitutes most of the mass of the universe. Other particles, predicted by supersymmetry, are subjects of search at large particle accelerators.
See also:Antiproton, Discovery of; Axion; Boson, Gauge; Boson, Higgs; Lepton; Muon, Discovery of; Neutrino; Neutrino, Discovery of; Neutrino, Solar; Neutron, Discovery of; Positron, Discovery of; Quarks
Kane, G. L. The Particle Garden: Our Universe as Understood by Particle Physicists (Addison-Wesley, Reading, MA, 1995).
par·ti·cle / ˈpärtikəl/ • n. 1. a minute portion of matter: tiny particles of dust. ∎ the least possible amount: he agrees without hearing the least particle of evidence. ∎ Physics another term for elementary particle. ∎ Physics another term for subatomic particle. ∎ Math. a hypothetical object having mass but no physical size. 2. Gram. a minor function word that has comparatively little meaning and does not inflect, in particular: ∎ (in English) any of the class of words such as in, up, off, over, used with verbs to make phrasal verbs. ∎ (in ancient Greek) any of the class of words such as de and ge, used for contrast and emphasis.