Physical world and mathematics

Dot product

The dot product, also called the scalar product, is an operation on two vectors that returns a single number, the product of their lengths and the cosine of the angle between them.

Dou (volume)

The dou (斗) is a traditional Chinese unit of dry capacity for grain, equal to 10 sheng, whose value grew from about 2,000 mL in Han times to about 10,000 mL under the Ming and Qing.

Double beta decay

Double beta decay is a rare nuclear decay in which a nucleus converts two neutrons into two protons, emitting two electrons and two antineutrinos; Maria Goeppert-Mayer predicted it in 1935.

Double bond

A double bond, or double covalent bond, is a covalent bond in which two pairs of electrons are shared between two atoms, giving a bond order of 2.

Double Chooz

Double Chooz was a reactor neutrino experiment in France that measured the mixing angle θ13 with two detectors at different distances from the Chooz reactors.

Sep 29, 2026

Double counting (proof technique)

Double counting is a proof technique in combinatorics that counts the elements of one finite set in two different ways and equates the results, yielding identities, inequalities, and nonexistence claims.

Double detonation

Double detonation is a proposed mechanism for Type Ia supernovae in which a helium shell detonation on a white dwarf triggers a second, core detonation, confirmed in 2025.

Double electron capture

Double electron capture is a nuclear decay process in which a nucleus absorbs two bound atomic electrons, converting two protons into neutrons and lowering the atomic number by two.

Double electron capture of xenon-124

Double electron capture of xenon-124 is a rare radioactive decay in which a xenon-124 nucleus absorbs two atomic electrons and becomes tellurium-124, first observed directly in 2019 with the longest half-life ever measured.

Sep 29, 2026

Double electron–electron resonance

Double electron–electron resonance (DEER), also called PELDOR, is a pulsed EPR technique that measures nanometer-scale distances between unpaired electron spins, most often spin labels on biomolecules.

Double exchange

Double exchange is a magnetic exchange mechanism in which hopping electrons couple ionic spins ferromagnetically, proposed by Clarence Zener in 1951 for mixed-valence manganese oxides.

Double factorial

The double factorial of a nonnegative integer n, written n!!, is the product of all positive integers up to n with the same parity as n.

Double field theory

Double field theory is a T-duality covariant reformulation of the massless sector of closed string theory, introduced by Chris Hull and Barton Zwiebach in 2009.

Double layer (plasma physics)

A double layer is a localized plasma structure of two opposite charge layers producing a strong electric field and a potential jump, seen in auroras and laboratories.

Double layer (surface science)

The electrical double layer, first modeled by Helmholtz in 1879, is the arrangement of charged species and dipoles at a solid-solution interface that stores charge in supercapacitors.

Double layer formation and stability

Double layer formation and stability covers how plasma double layers, localized charge structures that accelerate ions and electrons, form through currents, plasma expansion, or mixed electron populations.

Double layers in space and astrophysical plasmas

Double layers in space and astrophysical plasmas are localized pairs of oppositely charged sheets that accelerate particles without collisions, measured by satellites in Earth's auroral acceleration region and elsewhere.

Double pendulum

The double pendulum is a pendulum with a second pendulum attached to its end, one of the simplest physical systems exhibiting deterministic chaos through strong sensitivity to initial conditions.

Double perovskite

A double perovskite is a crystal of formula A2BB′X6 whose single B cation site is split into two alternating sublattices, including lead-free halide optoelectronic materials.

Double planet

A double planet, also called a binary planet, is a pair of planet-sized bodies orbiting a shared barycenter outside the larger body, with no confirmed example known.

Double resonance spectroscopy

Double resonance spectroscopy is a family of techniques in which two radiation fields, often at different wavelengths, drive two quantum transitions sharing a common level, revealing which lines in a congested spectrum belong together.

Sep 29, 2026

Double sampling (statistics)

Double sampling, also called two-phase sampling, is a survey sampling method that takes cheap auxiliary measurements on a large sample, then measures costly study variables on a subsample.

Double white dwarf

A double white dwarf is a binary star system where both stars are white dwarfs, whose mergers may trigger Type Ia supernovae and emit gravitational waves.

Double-clad fiber

A double-clad fiber is an optical fiber with a small signal core and a larger inner cladding that guides multimode pump light, the basis of cladding-pumped high-power fiber lasers and amplifiers.

Double-crystal monochromator

A double-crystal monochromator is an X-ray optical device in which two crystals diffract a beam twice to select a narrow, tunable energy band, standard at hard X-ray synchrotron beamlines.

Double-diffusive convection

Double-diffusive convection, first called thermohaline convection, arises when a fluid's density depends on two components, like heat and salt, that diffuse at very different rates.

Double-negative acoustic metamaterial

A double-negative acoustic metamaterial is an engineered material in which both the effective mass density and bulk modulus are negative, first demonstrated theoretically by Li and Chan in 2004.

Double-network hydrogel

A double-network hydrogel is a soft material of two interpenetrating polymer networks, one brittle and one stretchable, giving gels with about 90% water and strength far beyond ordinary hydrogels.

Double-precision floating-point format

Double-precision floating-point format, known as binary64, is the 64-bit number format defined by IEEE 754 that offers about 15 to 17 significant decimal digits of precision.