Particle physics

Particle physics is the study of the most fundamental particles in the universe—that is, extremely small particles that cannot be seen with the naked eye. There are two basic kinds of particles in the universe: bosons and fermions. The interactions of these particles form the basis for what is called the standard model of particle physics, which essentially describes how the universe works.

Curiosity about invisible particles dates as far back as the days of the ancient Greeks, but it was not until the late twentieth century that science gained a deeper understanding of subatomic particles and their characteristics. Particle physics overlaps another branch of science called quantum physics, in that both aim to further understand the known particles while searching for new ones that might still be undiscovered in the universe. Particle physics is what helps scientists categorize the building blocks of the universe as well as the various forces that hold them together, such as gravity and electromagnetism.

Brief History

The discovery of subatomic particles resulted from the collective effort of dozens of scientists over the course of several hundred years to reach a fundamental understanding of the makeup of the universe. One scientist in particular who made a huge contribution to this field was English physicist J. J. Thomson (1856–1940), whose 1897 experiments with cathode rays led to his discovery of the electron and opened the door to a world full of yet-to-be-discovered subatomic matter.

Although the idea of the electron had originally been proposed in 1874, it was not until 1900 that its existence was formally accepted within the scientific community. John Dalton (1766–1844), an English chemist and physicist who proposed the theory of atoms, was another major contributor to this field. Among the tenets of his theory was the fundamental statement that all chemical elements are composed of atoms. He also claimed that atoms could not be subdivided into smaller particles, an assertion that was later disproved.

Overview

By dividing various subatomic particles up into different groups, physicists are able to classify them much more easily. The two main types of particles are fermions, so called because they behave according to a set of rules known as Fermi-Dirac statistics, and bosons, which obey a different set of rules known as Bose-Einstein statistics. All particles, elementary and composite alike, are either fermions or bosons.

Elementary particles are among the smallest forms of energy and matter and are not composed of smaller particles. There are two types of elementary fermions, called quarks and leptons. The elementary bosons include gauge bosons like photons, gluons, W bosons, Z bosons, and scalar bosons like Higgs bosons. The elementary fermions make up all the matter in the universe, while each of the elementary bosons carries a fundamental force of the universe: electromagnetism (photons), the strong nuclear force (gluons), the weak nuclear force (W and Z bosons), and the generation of particle masses through the Higgs field (Higgs bosons). Another elementary boson has been posited—the graviton, representing gravitational force—but its existence is unconfirmed.

In contrast to elementary particles are the composite particles, which are composed of various combinations of quarks and leptons. The known composite particles can be broadly classified as either hadrons or atomic particles, such as atoms themselves, their nuclei, and the molecules they form when they bond together. Conventional hadrons can be further subdivided into either baryons, which are composed of three quarks, or mesons, which are composed of one quark and one antiquark. Primarily since the early twenty-first century, experiments have also identified exotic hadron candidates, including tetraquarks and pentaquarks, which do not fit the simplest three-quark or quark-antiquark patterns. Protons and neutrons are both baryons.

Electrons, on the other hand, are leptons, of a type known as charged leptons. Another type of lepton is the neutral leptons, more commonly called neutrinos. Charged leptons can combine to form composite particles, while neutrinos rarely do. Leptons exist in three different variations, also known as generations: electronic leptons, the first generation, which consist of electrons (charged) and electron neutrinos (neutral); muonic leptons, the second generation, consisting of muons (charged) and muon neutrinos (neutral); and tauonic leptons, the third generation, consisting of taus (charged) and tau neutrinos (neutral). Muons and taus are highly unstable and will quickly decay into electrons.

When studying particle physics, it is important to understand how antimatter works. In the most basic of terms, antimatter is simply the “opposite” of regular matter. Most known particles have their corresponding antiparticles: quarks and antiquarks, neutrinos and antineutrinos, electrons and positrons, and so on. When antimatter collides with regular matter, the particles of both literally destroy themselves by canceling each other out, the process called annihilation. The energy of the collision is then converted into elementary bosons, typically photons.

An important concept in particle physics is that of wave-particle duality, a paradox that was first observed in electromagnetic radiation. The debate over whether light consists of particles or waves dates to the seventeenth century, with Isaac Newton (1642–1727) championing the particle theory and Christiaan Huygens (1629–95) arguing for waves. Newton’s particle (or “corpuscular”) theory prevailed until the beginning of the nineteenth century, when English physicist Thomas Young (1773–1829) conducted what later became known as the double-slit experiment. Under the conditions of the experiment, light was shown to behave as a wave rather than as a particle, and the wave theory became widely accepted. In the late nineteenth and early twentieth centuries, however, the discovery of the photoelectric effect, in which directing light of a certain frequency at a metal surface causes electrons to be ejected from the metal, cast doubt on the wave theory of light.

In 1905, German-born physicist Albert Einstein (1879–1955) published an explanation of the photoelectric effect that described light as being quantized in discrete particles, later called photons. This led to the development of the concept of wave-particle duality, in which the behavior of light sometimes conforms to a particle model and sometimes to a wave model, depending on the context. The concept was later expanded to apply to all matter in varying degrees, not just photons. This is related to the uncertainty principle, first proposed by German physicist Werner Heisenberg (1901–76) in 1927. In simplified terms, the uncertainty principle states that one can determine either the exact position or the exact momentum of a given object at one time, but not both. When determining position, the object is treated as, and thus exhibits the behavior of, a particle; when determining momentum, however, it is treated, and thus behaves, as a wave.

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