Physical world and mathematics

Radiation reaction and energy balance in post-Newtonian theory

Radiation reaction in post-Newtonian theory is a dissipative force appearing at the 2.5PN order, linking lost energy and angular momentum to gravitational wave fluxes, observed in binary pulsars.

Radiation shielding

Radiation shielding is the placement of attenuating material between a source of ionizing radiation and people or equipment, designed so dose rates beyond the shield are acceptable.

Sep 29, 2026

Radiation thermometry measures an object's temperature without physical contact by analyzing its emitted thermal radiation; converting radiance temperature to true temperature requires knowing the surface's emissivity, the field's central problem.

Radiation trapping

Radiation trapping, also called resonance imprisonment, is the process in which an emitted photon is repeatedly absorbed and re-emitted by atoms of the same gas before escaping.

Radiation treatment planning

Radiation treatment planning, also called forward planning, is the process of designing a radiation therapy course: imaging the patient, outlining targets, and optimizing beam arrangements and doses.

Radiation-dominated era

The radiation-dominated era is the period of cosmic history, from the Big Bang until roughly 50,000 years after it, when radiation's energy density exceeded matter's and controlled the universe's expansion, ending at matter-radiation equality.

Radiative cooling

Radiative cooling is a passive cooling method in which an object dumps heat to outer space by emitting infrared radiation through the atmosphere's 8–13 µm transparency window.

Radiative efficiency

Radiative efficiency is the fraction of accreted matter's rest-mass energy that a black hole converts into radiation, rising from about 0.06 for non-spinning holes to 0.42 for maximally spinning ones.

Radiative equilibrium

Radiative equilibrium is the state of a planetary atmosphere in which net radiative heating is zero at every level, first formulated by Karl Schwarzschild in 1906.

Radiative forcing

Radiative forcing, also called climate forcing, is the change in Earth's energy balance from an external driver such as carbon dioxide, measured in watts per square meter.

Radiative heat transfer

Radiative heat transfer is the exchange of energy by electromagnetic radiation, analyzed through geometry-based view factors for transparent media and the radiative transfer equation for participating media.

Radiative improved mode

The radiative improved mode (RI mode) is a tokamak plasma regime, discovered on ISX-B and studied on TEXTOR-94, where noble gas impurity injection improves energy confinement.

Radiative recombination

Radiative recombination is the capture of a free electron by an atomic ion into a bound state, with a photon carrying away the excess energy; it is the inverse of photoionization.

Radiative transfer

Radiative transfer, also called radiation transport, describes how radiation propagates through a medium that absorbs, emits, and scatters it; its governing equation underlies stellar atmospheres, atmospheric physics, and remote sensing.

Radiative zone

The radiative zone is the layer of the Sun's interior between the core and the convection zone, where energy travels outward as photons over more than 170,000 years.

Radical (chemistry)

A radical, often called a free radical or chemical radical, is a molecular entity with an unpaired electron, typically highly reactive and driving polymerization, atmospheric oxidation, and biological signaling.

Radical 1,2-metalate rearrangement

The radical 1,2-metalate rearrangement is a reaction in which an organic radical combines with a boron ate complex, causing a carbon substituent to migrate and form a new carbon-carbon bond.

Radical addition

Radical addition is an organic reaction in which a free radical adds to an unsaturated molecule such as an alkene, forming a new sigma bond and a new radical.

Radical alkylation

Radical alkylation is the formation of a carbon–carbon bond by adding a carbon-centered alkyl radical to a substrate, most often electron-deficient heteroarenes such as pyridines and pyrimidines.

Radical borylation

Radical borylation forms carbon–boron bonds through radical intermediates, typically via metal-free photochemical, electrochemical, or thermal activation of alkyl halides, carboxylic acid derivatives, and C–H bonds for late-stage functionalization.

Radical center (geometry)

The radical center, also called the power center, of three circles is the point where the three pairwise radical axes meet, existing whenever the circle centers are not collinear.

Radical conjugate addition

Radical conjugate addition is a carbon-carbon bond-forming reaction in which a carbon-centered radical adds to the β-carbon of an electron-deficient alkene; the intermolecular version is known as the Giese reaction.

Radical copolymerization

Radical copolymerization is a copolymerization reaction in which two or more monomers join a growing chain by free-radical means, supplying about 45% of manufactured plastics worldwide.

Sep 29, 2026

Radical cyclization

Radical cyclization is an organic reaction in which a radical intermediate adds to a multiple bond within the same molecule, forming a ring used to build carbocycles and heterocycles.

Radical hydroamination

Radical hydroamination is the addition of an amine across a carbon–carbon double or triple bond via radical intermediates, prized for anti-Markovnikov addition to unactivated alkenes that polar catalysis handles poorly.

Radical hydrofunctionalization

Metalloradical hydrogen atom transfer hydrofunctionalization, usually called MHAT, is a family of alkene reactions in which a cobalt, manganese, or iron hydride transfers a hydrogen atom to form a radical and a new bond.

Radical methylation

Radical methylation is the attachment of a methyl group via radical intermediates rather than polar SN2 transfer, used by radical SAM enzymes and synthetic chemists.

Radical of an ideal

The radical of an ideal I in a commutative ring, written √I, is the set of elements some positive power of which lies in I; it is itself an ideal containing I.

Radical pair mechanism

The radical pair mechanism is a physical process in which a weak magnetic field like Earth's alters chemical reactions involving radical pairs, proposed to underlie birds' magnetic compass.