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Quasi-geostrophic theory
Quasi-geostrophic (QG) theory is a simplified system of equations for large-scale midlatitude atmospheric and oceanic flow, developed mainly by J. G. Charney in the late 1940s and still central to weather forecasting.
Quasi-harmonic approximation
The quasi-harmonic approximation (QHA) is a lattice dynamics method that treats a crystal as harmonic at each volume, letting phonon frequencies vary with volume to obtain thermal expansion and heat capacity.
Quasi-isometry
A quasi-isometry is a map between metric spaces that preserves distances up to a multiplicative factor and additive constant, the central notion of geometric group theory.
Quasi-likelihood
Quasi-likelihood is a statistical method for estimating and testing model parameters from only the mean and variance of the data, without assuming a full probability distribution.
Quasi-maximum likelihood estimation
Quasi-maximum likelihood estimation (QML, or pseudo maximum likelihood, PML) is a statistical method that maximizes a convenient likelihood even when the assumed distribution is wrong.
Quasi-Monte Carlo method
The quasi-Monte Carlo (QMC) method is a numerical integration technique that replaces random sample points with deterministic, evenly spread low-discrepancy points, so the integration error shrinks faster with sample size.
Quasi-Newton method
A quasi-Newton method, also called a variable metric method, is an optimization algorithm that minimizes a differentiable function by approximating the Hessian from successive gradients.
Quasi-periodic eruption
A quasi-periodic eruption (QPE) is a repeating burst of soft X-ray radiation, lasting about an hour and recurring every few hours, produced near a galaxy's central massive black hole.
Quasi-periodic oscillation
A quasi-periodic oscillation (QPO) is a nearly periodic flickering of X-ray brightness from an accreting neutron star or black hole, first seen in early 1996.
Quasi-satellite
A quasi-satellite, or quasi-moon, is a small body that orbits the Sun in step with a planet, appearing to circle the planet without being bound to it.
Quasi-star
A quasi-star, or black hole star, is a hypothetical early-universe object with a black hole inside a massive gas envelope, proposed as a seed of supermassive black holes.
Quasi-stationarity in branching processes
Quasi-stationarity in branching processes studies quasi-stationary distributions on the non-extinct states of a branching process conditioned on survival, including the Yaglom limit for Galton–Watson processes and applications in ecology and epidemiology.
Quasicrystal
A quasicrystal, or quasiperiodic crystal, is an ordered solid whose atoms lack periodic repetition, giving sharp diffraction patterns with forbidden fivefold symmetries, first observed by Dan Shechtman in 1982.
Quasielastic neutron scattering
Quasielastic neutron scattering (QENS) is a neutron spectroscopy technique in which neutrons exchange tiny amounts of energy with a sample, revealing diffusive molecular motions on picosecond to nanosecond timescales.
Quasienergy
Quasienergy is the quantity that plays the role of energy for a quantum system with a Hamiltonian periodic in time, as described by Floquet theory.
Quasigroup
A quasigroup is a set with a binary operation in which division is always possible in both directions, like a group without needing associativity or an identity.
Quasineutrality
Quasineutrality is the property of a plasma by which ion and electron densities are nearly equal, so the bulk carries no net charge except in thin sheaths near walls.
Quasinormal modes
Quasinormal modes (QNMs) are the characteristic damped oscillations of a perturbed black hole or compact star, like the tones of a struck bell, underlying black-hole spectroscopy.
Quasinormal modes of black holes
Quasinormal modes of black holes are damped oscillations of a perturbed black hole that dominate the ringdown after a merger, with frequencies encoding the hole's mass and spin.
Quasiparticle
A quasiparticle is an excitation of an interacting many-particle system that behaves like a single particle with well-defined energy and momentum, introduced by Lev Landau.
Quasiparticle interference
Quasiparticle interference (QPI) is the standing-wave pattern formed when electronic quasiparticles scatter from impurities or defects and interfere, letting a scanning tunneling microscope recover a material's electronic structure.
Quasiparticle poisoning
Quasiparticle poisoning is the degradation of quantum information in superconducting qubits, caused by quasiparticles tunneling across Josephson junctions, causing decoherence in transmons and flipping the encoded parity of topological qubits.
Quasistatic approximation
The quasistatic approximation is a geophysics modeling simplification that neglects inertial terms in the momentum balance, underpinning poroelastic consolidation, crustal deformation, ice-sheet flow, and atmospheric dynamics.
Quasistatic process
A quasistatic process, also called a quasi-equilibrium process, is an idealized thermodynamic process in which the system stays in equilibrium at each instant, so its work can be computed.
Quaternary
The Quaternary is the current geological period, covering the past 2.58 million years and defined by glacial cycles, human evolution, and the extinction of large mammals.
Quaternary ammonium and amine N-oxide chemistry
Quaternary ammonium and amine N-oxide chemistry covers two nitrogen functional groups whose nitrogen carries a full or near-full positive charge, comparing their bonding, preparation, properties, and applications.
Quaternary ammonium cation
The quaternary ammonium cation is a permanently positive polyatomic ion with a central nitrogen bonded to four organic groups; its salts, called quats, serve as disinfectants and surfactants.
Quaternary ammonium compounds
Quaternary ammonium compounds (QACs, also called quats) are organically substituted ammonium salts used since the 1930s as disinfectants, antiseptics, and surfactants in many products.
Quaternary fatty ammonium salts and esterquats
Quaternary fatty ammonium salts and esterquats are quaternary ammonium compounds with long fatty alkyl chains, used mainly as fabric softeners, hair conditioners, and antistatic agents.
Quaternary glaciation
Quaternary glaciation: the ongoing ice age of the last 2.58 million years, marked by 41-kyr then ~100-kyr glacial–interglacial cycles, Northern Hemisphere ice sheets, the Mid-Pleistocene Transition at ~900 ka, and the current Holocene interglacial.