42,772 articles
Free module
A free module, also called a free vector space, is a module with a basis, so every element is uniquely a linear combination of basis elements.
Free monoid
The free monoid on a set is the monoid of all finite words over it, with concatenation as the operation and the empty word as identity.
Free neutron decay
Free neutron decay is the radioactive decay of a neutron outside an atomic nucleus into a proton, an electron, and an antineutrino, with a mean life of about 15 minutes.
Free object
A free object is an algebraic structure built from a set of generators with no relations beyond those the structure type forces, so maps of the generators extend uniquely.
Free particle
A free particle is a quantum-mechanical particle subject to no force, described by plane waves and localized wave packets that spread in time, with a continuous energy spectrum rather than discrete levels.
Free probability
Free probability is a branch of probability theory in which random variables are noncommuting operators and independence is modeled on free products, introduced by Dan-Virgil Voiculescu in the 1980s.
Free product
The free product of groups is a group built from them by identifying only their identity elements, so elements of different factors do not commute; it is the coproduct in the category of groups.
Free shear turbulence
Free shear turbulence is turbulence in flows unbounded by solid walls, the class comprising jets, wakes, and mixing layers, where viscous effects can usually be ignored at high Reynolds number.
Free spectral range
The free spectral range, also called axial mode spacing, is the frequency spacing between adjacent modes of an optical resonator, given for length L by c/(2L).
Free variables and bound variables
In formal logic, a free variable is not bound by a quantifier and a bound variable is; the distinction applies to occurrences, and formulas without free variables are sentences.
Free volume theory
Free volume theory is a family of models in polymer and glass physics explaining viscosity, diffusion, and the glass transition through the empty space between molecules or chain segments.
Free-electron quantum optics
Free-electron quantum optics is an emerging field of physics studying coherent quantum-mechanical interactions between free-electron wave packets and light and material excitations, with milestones including electron-photon entanglement demonstrated in 2025.
Free-electron–photon entanglement
Free-electron–photon entanglement is a quantum state in which a free electron and photons share correlations no classical description can reproduce, first demonstrated in April 2025 by researchers in Göttingen.
Free-energy relationship
A free-energy relationship, or linear free-energy relationship (LFER), is a linear correlation between rate or equilibrium constants of related reaction series in physical organic chemistry.
Free-fall time
The free-fall time is the time a self-gravitating gas cloud would take to collapse if gravity acted alone; in star formation it serves as the fundamental clock.
Free-flow electrophoresis
Free-flow electrophoresis (FFE) is a continuous separation technique in which an electric field deflects charged molecules, cells, and organelles into distinct streams, developed since the late 1950s.
FreeFEM
FreeFEM is free, open-source software that solves partial differential equations by the finite element method, with users writing the variational form in its own scripting language.
Freeman Dyson
Freeman John Dyson was a British-born theoretical physicist and writer who spent his career at Princeton's Institute for Advanced Study and unified quantum electrodynamics in 1949.
Freeze-cast nacre-mimetic ceramics
Freeze-cast nacre-mimetic ceramics are ceramic composites whose layered, brick-and-mortar microstructure, imitating mollusk-shell nacre, is produced by directional freezing of a ceramic slurry and infiltration with a polymer or metal.
Freeze-in
Freeze-in is a mechanism for producing particle dark matter in the early Universe, in which a feebly interacting particle is gradually populated without reaching thermal equilibrium.
Freezing
Freezing is the phase transition in which a liquid becomes a solid when its temperature falls to the freezing point, where solid and liquid are in equilibrium.
Freezing rain
Freezing rain is liquid rain that freezes into a glaze of ice on contact with any surface at or below freezing, coating roads, trees, and power lines.
Freezing-point depression
Freezing-point depression, described by Blagden's law, is the lowering of a liquid's freezing point when a solute dissolves in it, as in road de-icing and antifreeze.
Frege system
A Frege system, also called a Frege proof system, is a standard axiomatic proof system for propositional logic, named after Gottlob Frege and central to proof complexity.
Frege's propositional calculus
Frege's propositional calculus is the axiomatic system for propositional logic in Gottlob Frege's 1879 Begriffsschrift, using only negation and the conditional, with six key axioms and modus ponens.
Freiman's theorem
Freiman's theorem is a result of additive combinatorics, proved by Gregory Freiman, stating that sets of integers with a small sumset lie in generalized arithmetic progressions.
French units of measurement
French units of measurement, also called units of measurement in France, were the traditional weights and measures of France, replaced by the metric system between 1795 and 1840.
Frenet–Serret formulas
The Frenet–Serret formulas, or Frenet formulas, are differential geometry equations describing how the tangent, normal, and binormal frame of a space curve changes along the curve.
Frenkel defect
A Frenkel defect is a point defect in a crystal where an ion, usually a cation, moves from its lattice site to an interstitial site, forming a vacancy-interstitial pair.
Frenkel polariton
A Frenkel polariton is a hybrid light–matter quasiparticle formed when a Frenkel exciton, a tightly bound electronic excitation localized on a molecule, couples strongly to a cavity photon.