page_title: History of subatomic physics
section_title: Quarks, leptons, and four fundamental forces
hierarchical_section_title: History of subatomic physics / Physics goes to high energies / Quarks, leptons, and four fundamental forces
context_page_description: The idea that matter consists of smaller particles and that there exists a limited number of sorts of primary, smallest particles in nature has existed in natural philosophy at least since the 6th century BC. Such ideas gained physical credibility beginning in the 19th century, but the concept of "elementary particle" underwent some changes in its meaning: notably, modern physics no longer deems elementary particles indestructible. Even elementary particles can decay or collide destructively; they can cease to exist and create particles in result.
  Increasingly small particles have been discovered and researched: they include molecules, which are constructed of atoms, that in turn consist of subatomic particles, namely atomic nuclei and electrons. Many more types of subatomic particles have been found. Most such particles were eventually found to be composed of even smaller particles such as quarks. Particle physics studies these smallest particles and their behaviour under high energies, whereas nuclear physics studies atomic nuclei and their constituents: protons and neutrons.
context_section_description: Because the quantum field theory (see above) postulates no difference between particles and interactions, classification of elementary particles allowed also to classify interactions and fields.
  Now a large number of particles and (non-fundamental) interactions is explained as combinations of a (relatively) small number of fundamental substances, thought to be fundamental interactions (incarnated in fundamental bosons), quarks (including antiparticles), and leptons (including antiparticles). As the theory distinguished several fundamental interactions, it became possible to see which elementary particles participate in which interaction. Namely:
  All particles participate in gravitation.
  All charged elementary particles participate in electromagnetic interaction.
  As a consequence, neutron participates in it with its magnetic dipole in spite of zero electric charge. This is because it is composed of charged quarks whose charges sum to zero.
  All fermions participate in the weak interaction.
  Quarks participate in the strong interaction, along gluons (its own quanta), but not leptons nor any fundamental bosons other than gluons.
  The next step was a reduction in number of fundamental interactions, envisaged by early 20th century physicists as the "united field theory". The first successful modern unified theory was the electroweak theory, developed by Abdus Salam, Steven Weinberg and, subsequently, Sheldon Glashow. This development culminated in the completion of the theory called the Standard Model in the 1970s, that included also the strong interaction, thus covering three fundamental forces. After the discovery, made at CERN, of the existence of neutral weak currents, mediated by the Z boson foreseen in the standard model, the physicists Salam, Glashow and Weinberg received the 1979 Nobel Prize in Physics for their electroweak theory.
  The discovery of the weak gauge bosons (quanta of the weak interaction) through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics.
  While accelerators have confirmed most aspects of the Standard Model by detecting expected particle interactions at various collision energies, no theory reconciling general relativity with the Standard Model has yet been found, although supersymmetry and string theory were believed by many theorists to be a promising avenue forward. The Large Hadron Collider, however, which began operating in 2008, has failed to find any evidence whatsoever that is supportive of supersymmetry and string theory, and appears unlikely to do so, meaning "the current situation in fundamental theory is one of a serious lack of any new ideas at all." This state of affairs should not be viewed as a crisis in physics, but rather, as David Gross has said, "the kind of acceptable scientific confusion that discovery eventually transcends."
  The fourth fundamental force, gravitation, is not yet integrated into particle physics in a consistent way.
caption: The Standard Model