Background
The Standard Model is the most comprehensive theory of particle physics. It describes all known particles in the universe and all forces governing their interactions, except gravity.
Instead of everything being made of protons, neutrons, and electrons, the Standard Model states that all visible matter is composed of particles thousands of times smaller: quarks and leptons. These particles exert forces and interact by emitting and absorbing five additional particles (explore the particles).
This framework provides fundamental explanations for most phenomena involving electromagnetism, radioactive decay, and nuclear reactions, though it remains incomplete (learn why).
Incomplete Puzzle
The model's history began in 1897 with the discovery of electrons, followed by the proton (1919) and the neutron (1932). These discoveries showed that atoms were not the basic building blocks of matter.
However, two scientific models suggested more to uncover.
Conservation laws state that specific properties of an isolated system, like its energy, remain constant. When exploring radioactive decay, physicists noticed discrepancies in these properties over time, hinting at the existence of unseen particles, like missing pieces from a puzzle.
Similarly, quantum mechanics predicted the existence of antimatter—the oppositely charged version of normal matter—in much the same way x² = 4 means that x = 2 and -2 (1440 Topics: Quantum Mechanics).
Particle Zoo
The prediction was confirmed in 1932 when scientists analyzed cosmic rays—high-energy particles from space. Like scattered glass and metal debris from a car crash, cosmic rays produced showers of particles and antiparticles when they struck atoms in the atmosphere.
Using cloud chambers, scientists determined the properties of these new particles (watch explainer). They also used newly constructed particle accelerators to discover particles created in the collisions of protons and electrons (learn how). By the 1960s, over 100 unique particles had been observed.
Collision data revealed that most particles were not indivisible. Instead, like unique dishes made from a subset of common ingredients, most were made of elementary particles called quarks.
Originally named “aces,” quarks come in six different flavors—up, down, strange, charm, top, and bottom—and combinations of these quarks and their antimatter versions form much of our world’s matter (e.g., a proton is made of two up quarks and one down quark). Particles not made of quarks, such as the electron, make up a second category called leptons.
Force Unification
While matter particles were organized into leptons or combinations of quarks, physicists used quantum mechanics to model three of the four fundamental forces as gauge bosons—particles that transmitted force between matter particles.
In other words, everything in the universe is made up of quarks and leptons that interact by exchanging gauge bosons. For example, magnets attract one another by exchanging photons, the gauge bosons of the electromagnetic force.
Experiments show that the gauge bosons for the weak nuclear force, which is responsible for radioactive decay, and the electromagnetic force become identical at high energies, much like water and ice both become steam at high temperatures.
The Standard Model explains that this unified electroweak force existed in the high-energy environment of the early universe. However, expansion and cooling caused it to transition into two forces, with the Higgs field giving mass to weak nuclear force particles (learn more).
Challenges
To help scientists better understand the early moments after the Big Bang, researchers continue to look for experimental evidence of the strong nuclear force combining with the electroweak force at extremely high energies (1440 Topics: Big Bang).
The development of a theory of quantum gravity to model gravitons—the gauge bosons of the gravitational force—may lead to further unification toward a theory of everything (watch explainer).
The Standard Model cannot explain why matter and antimatter are imbalanced in the universe or what dark matter and energy are (1440: Dark Universe).
