Standard physics glossary

quantum · intro

Quantum

The smallest discrete amount, or fundamental unit, of a given physical property or interaction, such as a single photon representing a quantum of light or electromagnetic energy. The term comes from the recognition, foundational to quantum mechanics, that many physical quantities, such as energy in bound systems, are not continuous but instead come only in specific discrete amounts. This quantization contrasts sharply with the smooth, continuous quantities assumed in classical physics.

quantum · intermediate

Quantum Electrodynamics

Also called: QED

The quantum field theory describing how light and matter interact, specifically how photons mediate the electromagnetic force between charged particles like electrons. Developed in the late 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, it is one of the most precisely tested theories in all of physics, with predictions confirmed by experiment to more than ten significant figures in some cases.

quantum · intermediate

Quantum Entanglement

A quantum mechanical phenomenon in which two or more particles become linked such that the quantum state of each particle cannot be described independently of the others, even when the particles are separated by large distances. Measuring a property of one entangled particle instantly determines the corresponding property of its partner, a correlation that Einstein famously described as spooky action at a distance, though it does not allow faster-than-light communication of information.

quantum · intermediate

Quantum Field Theory

A theoretical framework that combines quantum mechanics with special relativity, describing particles as excitations, or ripples, in underlying fields that permeate all of space. Every fundamental particle, such as the electron or photon, corresponds to a specific quantum field, and interactions between particles arise from these fields coupling to one another. Quantum field theory forms the mathematical foundation of the Standard Model of particle physics.

quantum · intro

Quantum Mechanics

The branch of physics describing the behavior of matter and energy at the smallest scales, such as atoms and subatomic particles, where classical physics breaks down. Quantum mechanics introduces concepts such as wave-particle duality, quantized energy levels, and inherent probabilistic uncertainty, replacing the deterministic predictions of classical mechanics with probabilities described by a mathematical object called the wavefunction. It underlies the operation of lasers, transistors, and much of modern chemistry.

quantum · intermediate

Quantum Number

A number that specifies the value of a quantized, discrete property of a quantum system, such as the energy, angular momentum, or spin of an electron within an atom. A set of quantum numbers uniquely identifies the state of a particle within a system, and the allowed combinations are restricted by rules such as the Pauli exclusion principle. Quantum numbers explain the structure of the periodic table and the arrangement of electrons in atomic orbitals.

quantum · intermediate

Quantum Tunneling

A quantum mechanical phenomenon in which a particle has a nonzero probability of passing through an energy barrier that it would not have enough energy to overcome according to classical physics. Tunneling arises from the wave-like nature of matter described by quantum mechanics and is essential to processes such as nuclear fusion in stars, alpha decay of radioactive nuclei, and the operation of devices like scanning tunneling microscopes and certain transistors.

particle · intermediate

Quark

A fundamental particle that combines with other quarks to form composite particles called hadrons, such as protons and neutrons, and is never observed alone due to a property of the strong force called confinement. There are six types, or flavors, of quark, ranging from the light up and down quarks that make up ordinary matter to the much heavier charm, strange, top, and bottom quarks. Quarks carry fractional electric charge and interact through the strong force via gluons.

astro · intro

Quasar

An extremely luminous, compact region at the center of a distant galaxy, powered by a supermassive black hole actively accreting surrounding gas and dust, which heats up and radiates enormous amounts of energy across the electromagnetic spectrum as it spirals inward. Quasars can outshine the combined light of every star in their host galaxy and are among the most distant and energetic objects observable, offering a glimpse into the early universe.