Physical world and mathematics

Reactive Black 5

Reactive Black 5, also sold as Remazol Black B, is a vinyl sulfone diazo reactive dye used to dye cellulosic fibers such as cotton in deep black shades.

Reactive Blue 19

Reactive Blue 19, sold as Remazol Brilliant Blue R, is an anthraquinone-based vinyl sulfone reactive dye that gives brilliant royal blue shades on cellulosic fibers such as cotton.

Reactive crystallization is a crystallization method in which a chemical reaction between dissolved species generates a solid crystalline product, creating the supersaturation that drives nucleation and growth.

Reactive distillation

Reactive distillation is a chemical process method combining reaction and distillation in a single column, so products are removed as they form and equilibrium-limited reactions reach high conversion.

Reactive extraction

Reactive extraction is a liquid–liquid separation method in which a reactive extractant chemically complexes a solute, such as an acid or metal ion, into an organic phase.

Reactive molecular dynamics

Reactive molecular dynamics is a molecular simulation method where chemical bonds break and form during the trajectory, so reaction pathways and intermediate species emerge directly.

Reactive oxygen species

Reactive oxygen species, also called reactive oxygen intermediates, are reactive oxygen-containing chemicals formed as oxygen is reduced to water, ranging from superoxide and hydrogen peroxide to the hydroxyl radical.

Oct 22, 2026

Reactive oxygen species generation by nanoparticles

Reactive oxygen species generation by nanoparticles is the production of reactive oxygen derivatives such as superoxide and hydroxyl radicals at nanoparticle surfaces, driving nanotoxicity and experimental cancer and antimicrobial therapies.

Reactive scattering

Reactive scattering is the branch of collision physics in which colliding molecules exchange atoms, as in A + BC → AB + C, making it the microscopic event underlying every elementary chemical reaction.

Reactive transport model

A reactive transport model is a numerical model that couples fluid flow, solute transport, and chemical reactions in porous or fractured subsurface media, applied to CO₂ storage and remediation.

Reactivity (chemistry)

Chemical reactivity is the tendency of a chemical species to undergo reaction, quantified by the rate constant for a specified elementary reaction under stated conditions.

Reactivity ratio

The reactivity ratio is a parameter in copolymerization kinetics measuring how readily a growing polymer chain end adds its own monomer rather than the comonomer, introduced by Mayo and Lewis in 1944.

Reactivity series

The reactivity series, also called the activity series of metals, is an ordered list ranking metals from most to least reactive, used to predict displacement reactions and extraction methods.

Oct 22, 2026

Reactor antineutrino anomaly

The reactor antineutrino anomaly is a 6 percent deficit in measured reactor antineutrino rates at short baselines, identified in 2011 against the Huber–Mueller flux prediction.

Reactor neutrinos

Reactor neutrinos are antineutrinos produced by nuclear reactors, first detected at the Savannah River Plant in 1956; reactor experiments have since measured key neutrino oscillation parameters.

Reagent

A reagent, also called a chemical reagent, is a substance added to a chemical system to bring about a reaction or test whether one occurs, as in analytical testing.

Real analysis

Real analysis, also called real variable analysis, is the rigorous branch of mathematics underlying calculus, built on the completeness of the real numbers and covering limits, continuity, differentiation, and integration.

Real closed field

A real closed field is an ordered field where every non-negative element is a square and every odd-degree polynomial has a root, introduced by Artin and Schreier in the 1920s.

Real gas

A real gas, or imperfect gas, is a gas whose molecules have finite size and exert forces on each other, deviating from the ideal gas law.

Real image

A real image is an image formed where light rays from an object actually cross, so it can be projected onto a screen, film, or the retina.

Real number

A real number is an element of the real number system, written ℝ, which contains the rational numbers and fills the gaps between them with irrationals such as √2 and π.

Real projective plane

The real projective plane is the space of lines through the origin in three-dimensional real space, a closed non-orientable surface that cannot be embedded in ordinary space.

Real rank zero

Real rank zero is a property of C-algebras introduced by Brown and Pedersen in 1991, meaning self-adjoint elements can be approximated by ones with finite spectrum.

Real-time clock

A real-time clock (RTC) is a computer clock, usually an integrated circuit, that tracks the time and date and keeps running on battery backup when the system is off.

Oct 22, 2026

Real-time quantum error correction feedback

Real-time quantum error correction feedback is the closed-loop process where a quantum processor measures error syndromes, a decoder interprets them in real time, and corrections are applied.

Realgar

Realgar is a red to orange-red arsenic sulfide mineral and a principal ore of arsenic, occurring in low-temperature hydrothermal veins and historically used as a pigment.

Realist synthesis

Realist synthesis is a theory-driven method of evidence synthesis, described by Ray Pawson in 2002, that explains how and why complex health and social interventions work, for whom, and under what circumstances.

Realizability

Realizability is a family of semantic techniques in mathematical logic, introduced by Stephen Cole Kleene in 1945, where a number or program counts as evidence for a statement.

Realization of the SI base units

Realization of the SI base units means the laboratory experiments, known as mises en pratique, that physically produce the SI units from the defining constants fixed in 2019.