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Nodal-line semimetal
A topological nodal-line semimetal is a 3D crystalline material whose conduction and valence bands cross along a symmetry-protected line in momentum space, unlike the point nodes of Weyl semimetals.
Nodding (astronomy)
Nodding is an infrared and submillimeter observing technique in which the telescope repeatedly moves between a source and nearby blank sky, subtracting the bright atmospheric and telescope background, usually paired with chopping.
Node (physics)
In physics, a node is a point on a standing wave where the wave disturbance is zero at every instant, formed by two opposite waves canceling.
Nodes of wave functions
A node of a wave function is a point, line, or surface where the wave function vanishes exactly; in one dimension, the nth bound state has exactly n−1 nodes.
Noel A. Clark
Noel A. Clark, born 1940 in Cleveland, Ohio, is an American condensed matter physicist at the University of Colorado Boulder known for work on ferroelectric liquid crystals.
Noel Hush
Noel Hush (1924–2019) was an Australian theoretical chemist known for electron-transfer theory, often paired with Marcus's work as Marcus–Hush theory, and for a founding role in molecular electronics.
Noel Slater
Noel Bryan Slater (1912–1973) was a British mathematician and physicist known for his statistical theory of unimolecular gas reactions, summarized in his 1959 monograph Theory of Unimolecular Reactions.
Noelle E. Selin
Noelle E. Selin is an atmospheric chemist and policy researcher at MIT, a professor in IDSS and EAPS, known for global mercury modeling that informed the Minamata Convention.
Noether normalization lemma
The Noether normalization lemma, a result in commutative algebra proved by Emmy Noether in 1926, states that every finitely generated algebra over a field is a finite extension of a polynomial subring.
Noether's theorem
Noether's theorem is the statement, proved by Emmy Noether in 1918, that every continuous symmetry of a physical system yields a conservation law, such as energy conservation from time-translation symmetry.
Noetherian ring
A Noetherian ring is a ring in which every ideal can be generated by finitely many elements, named after Emmy Noether, who studied them in 1921.
Noga Alon
Noga Alon is an Israeli mathematician at Princeton University and Tel Aviv University known for the Combinatorial Nullstellensatz, the book The Probabilistic Method, and the 2024 Wolf Prize in Mathematics.
Noise
Noise is any unwanted electrical, acoustic, or radio-frequency disturbance added to a wanted signal, degrading it; its impact is measured by the signal-to-noise ratio, usually in decibels.
Noise barrier
A noise barrier is a wall or earth berm built alongside a highway to reduce traffic noise reaching nearby homes, typically by 5 to 10 decibels.
Noise dosimeter
A noise dosimeter, or sound dosimeter, is a body-worn instrument that measures a worker's noise exposure over a full shift as a dose or time-weighted average.
Noise figure
Noise figure, or noise factor, measures how much a two-port device like an amplifier degrades a signal's signal-to-noise ratio; it was introduced by Harald Friis in 1944.
Noise kernel
The noise kernel is the symmetrized two-point correlation function of a quantum field's stress-energy tensor, serving as the stochastic source in the Einstein-Langevin equation of stochastic gravity.
Noise measurement
Noise measurement is the quantitative determination of sound levels and human exposure to them, using sound level meters and dosimeters and reported as sound level, time-weighted average, and noise dose.
Noise pollution
Noise pollution, also known as sound pollution or community noise, is unwanted or harmful environmental sound, dominated by transportation, that is linked to cardiovascular disease and sleep disturbance.
Noise reduction
Noise reduction, also known as noise removal, is the set of techniques for suppressing background noise in a signal, usually audio, to improve the signal-to-noise ratio and intelligibility.
Noise reduction coefficient
The noise reduction coefficient (NRC) is a single-number rating of how much sound a material absorbs, averaging four octave bands under the ASTM C423 test method.
Noise spectroscopy
Noise spectroscopy is a measurement technique that characterizes materials and devices by analyzing their intrinsic electrical fluctuations, most commonly low-frequency resistance noise, to study charge-carrier dynamics and defects.
Noise-cancelling headphones
Noise-canceling headphones, or ANC headphones, are headphones or earbuds that reduce ambient sound by playing an inverse-phase anti-noise signal, best at canceling steady low-frequency noise like airplane cabins.
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is an ultrasensitive laser absorption technique for detecting gas-phase molecules, combining frequency modulation spectroscopy with a high-finesse optical cavity.
Noise, vibration, and harshness
Noise, vibration, and harshness (NVH) is the engineering field that measures and modifies the noise and vibration characteristics of vehicles, where it is treated as a customer-facing quality attribute.
Noisy intermediate-scale quantum era
The noisy intermediate-scale quantum (NISQ) era is the current stage of quantum computing, named by John Preskill in 2018, with 50 to a few hundred noisy, uncorrected qubits.
Noisy-channel coding theorem
The noisy-channel coding theorem, proved by Claude Shannon in 1948, states that data can be transmitted with arbitrarily few errors at any rate below a channel's capacity, but not above it.
Nomenclature of Organic Chemistry
Nomenclature of Organic Chemistry, commonly called the IUPAC Blue Book, is the official IUPAC rulebook for naming organic chemical compounds; its 2013 recommendations introduced preferred IUPAC names.
Nomex
Nomex is a DuPont trademark for a meta-aramid fiber and paper that is inherently flame-resistant, does not melt, and is used in firefighter garments, protective apparel, and electrical insulation since 1967.
Nominalism
Nominalism, also called via moderna, is the metaphysical view that only particular, concrete things exist, denying universals and abstract entities; William of Ockham was its key medieval exponent.