Topics, N

N-Body Problem > see Many-Body Systems.

n-Plectic Structures > see symplectic structures.

N-Point Correlation Functions > s.a. correlations; green functions, in quantum field theory; Normal Order.
* In quantum field theory: They are important in the study of the short-distance structure of quantum field theory; For small |xy| and the right function spaces one gets behaviors like \(\langle\)0| φ(x) φ(y) |0\(\rangle\) = c |x y|d; > s.a. quantum field theory effects.
@ In quantum field theory: Lu ht/05 [1-point functions, perturbative]; Mestre & Oeckl JMP(06) [relationships, Hopf-algebra approach]; Guerra et al EPJB(07)-a0704 [calculation, and non-perturbative renormalization group].

n-Poset > see generalizations of posets.

Nambu Algebras / Brackets / Mechanics > s.a. poisson brackets; deformation quantization.
@ References: Nambu PRD(73); Fecko JMP(13)-a1306 [symmetries and conserved quantities]; Müller & Névir JPA(14) [geometric application].

Nambu Tensor > see killing fields.

Nambu-Goldstone Boson > see symmetry breaking.

Nambu-Jona-Lasinio Model
@ General references: Battistel et al PRD(08)-a0803 [strategy to handle divergences]; Novello IJMPA(11)-a1004 [from gravitational interaction and extended Mach principle].
@ In curved spacetime: Elizalde et al PRD(94)ht/93 [phase structure and chiral symmetry breaking].
> Online resources: see Wikipedia page.

Nanoscale Systems > see generalized thermodynamics.

Narcowich-Wigner Spectrum
@ References: Dias & Prata RPMP(09)-a0812 [of a pure state].

Nariai Metric > see schwarzschild-de sitter spacetime.

Narratability of a Theory
* Idea: A theory is narratable if specifying the state of the system on every spacelike hypersurface in a given foliation of spacetime is sufficient to determine the states on other hypersurfaces, so the history of the system can be told as a narration of states at successive times.
@ References: Judes a1002 [and cluster decomposition].

Naturalness of a Physical Theory > s.a. Fine Tuning; physical theories [criteria].
* Types: Structural and numerical naturalness (no fine tuning); The former is more subjective, and has generated many wrong theories.
* Rem: This criterion has been for many years a guiding principle in the search for physics beyond the Standard Model, but it has been called into question; It might be rejected at some deep scales (see, e.g. Linde's smörgåsbord picture of many universes created after inflation), and needs to be treated with care.
@ References: Dine a1501-ARNPS [rev, conjectures, challenges and alternatives]; Pivovarov a1508-proc [formal criterion, and the Standard Model].

Navier-Stokes Equation > s.a. fluid; variational principles.
$ Navier-Stokes equation: The non-linear set of pde's describing the time evolution of the fluid velocity and pressure of an incompressible viscous homogeneous Newtonian fluid in terms of given initial velocity and external body forces; It expresses momentum conservation, and is obtained applying Newton's laws to the flow of the fluid, and adding a term that accounts for energy lost through viscosity

t u + (u · ∇) u = –ρ–1p+ ν2u ,

where u satisfies ∇ · u = 0, p = pressure, ρ = density (constant), and ν = (kinematic molecular) viscosity; > s.a. mathematics.
@ References: Succi 01 [lattice Boltzmann equation; r PT(02)dec]; Streater mp/01 [corrections]; Brüger et al JCP(04) [high-order numerical solution]; Gill & Zachary NLA(10)mp/07, mp/07 [initial data for global solutions]; Romeo a1007-conf [aspects of the initial-value problem]; news sf(14)feb [Terence Tao's new approach]; Dlotko a1501 [new approach]; news Quanta(15)jul [and the Boltzmann equation, incompleteness]; Gutiérrez-Santacreu a1508 [existence of smooth solutions]; > s.a. Boltzmann Equation.
@ Variations: Lichtenfelz a1502 [on negatively curved Riemannian manifolds, non-uniqueness of solutions].
> Related topics: see solutions of einstein's equation.
> Online resources: see MathWorld page; Wikipedia page.

Nebulae > see interstellar matter.

Negative Probabilities > see probability in physics.

Nekhoroshev Theorem > see types of integrable systems.

Nernst Theorem > see thermodynamics [3rd law].

Nerve > s.a. cover [nerve of a covering].
$ Of a category: The simplicial set in which the simplices in degree k are the chains of morphisms a0, a0, ..., ak in the category; In other words: A vertex for each object, and a k-simplex for each k-tuple of composable morphisms.
> Online resources: see Webb ln(99).

Nester Form > see Metric-Affine Gravity.

Net > s.a. Conformal Net; Filter Base; geodesic net.
$ Def: A function from a directed set X to some other set; A "sequence" whose index set is not totally ordered.
* Examples: A sequence, which is obtained for X = \(\mathbb N\); Observable nets in quantum field theory (> see C*-Algebras; observable algebras).
@ References: Mustafa & Ray CG(10) [basis for weak ε-nets for finite sets of points in Euclidean space].


Neumann Functions > see bessel functions.

Neumann Problem > s.a. green functions.
* Idea: A boundary-value problem for second-order elliptic partial differential equations.
* Result: ∃ 1! solution to ∇2φ = –ρ/ε0 inside a region V, with ∂φ/∂n fixed on ∂V, up to an additive constant.
> Specific theories: see action for general relativity [covariant notion of Neumann boundary conditions].

Neural Network > see network.

Neutral Manifold / Metric > see types of metrics.

Neutralino > s.a. dark-matter phenomenology.
* Idea: A particle predicted by supersymmetric extensions of the Standard Model that is a candidate for dark matter.
@ References: Abbasi et IceCube PRL(09) [bounds from muon flux in Antarctic ice]; Grothaus et al JHEP(13)-a1207 [parameter space and naturalness]; Fan & Reese JHEP(13)-a1307 [searches for light, nonthermal wino dark matter].

Neutrino > s.a. in cosmology and astrophysics; mixing and oscillations.

Neutrix Calculus > s.a. regularization; renormalization.
* Idea: Neutrices are additive groups of negligible functions that do not contain any constants except 0; Their calculus was developed by van der Corput and Hadamard in connection with asymptotic series and divergent integrals.


Neutron Stars > s.a. pulsars.

Newman-Janis Algorithm > see solution-generating methods for the einstein equation.

Newman-Penrose Formalism > see spin coefficients.

Newman-Tamburino Metrics
@ References: Steele gq/04 [Killing vector]; De Groote & Van den Bergh CQG(08), CQG(08) [with Maxwell field].

News Tensor > see asymptotic flatness at null infinity.

Newton-Cartan Theory

Newton-Cotes Methods > see integration.

Newton-Schrödinger Equations
@ References: Anastopoulos & Hu NJP(14)-a1403 [problems].

Newton-Wigner Localization > see localization; quantum particles.

Newton's Bucket Argument
* Idea: The argument by Newton for the existence of absolute space, based on the fact that one can tell whether a reference frame is rotating with respect to absolute space by looking at whether the surface of the water inside a bucket is flat or concave (once the water is at rest with respect to the bucket); The argument was criticized by Mach.
@ References: in Barbour & Pfister 95; Kubiak v1110, Sekhar a1305 [and the liquid-mirror telescope]; Staley PT(13)dec [Mach's position]; Shuler PhysEd(15) [and Mach's principle].
> Online resources: see Wikipedia page.

Newton's Cradle
* Idea: A device that demonstrates conservation of momentum and energy using a set of swinging metal spheres.
@ References: news pw(14)may [quantum analog].
> Online resources: see Wikipedia page.

Newton's Gravitational Constant

Newton's Laws of Dynamics > s.a. classical mechanics [including variations and generalizations].
* First law / Law of inertia: For a free particle in an inertial frame, du/dt = 0, where u = velocity and in the relativistic version t is usually proper time; In words, unless a net force is acting on an object, the object will keep moving with constant speed in the same direction; As opposed to the Aristotelian view that any object moving at a constant speed must be continually pushed if it is to maintain its motion.
* Second law: In an inertial frame, m du/dt = f, where f is the force acting on the particle; Has been considered as a definition rather than a true law of nature, based on the argument that force cannot be measured directly but only through the acceleration it produces on a given mass.
* Third law: The "principle of action and reaction," for objects A and B, the force FAB of A on B is equal and opposite to the force FBA of B on A; It holds unless there are accelerated charges (magnetic forces, or self-forces from radiation reaction); The laws of conservation of energy-momentum and angular momentum are based on it; It can be violated when the interacting particles are embedded in a non-equilibrium environment.
* Limitations, tests: Newton's second law is expected to break down for subatomic scales; At macroscopic ones, one way to test its validity is to check that the frequency of a pendulum is independent of amplitude (if small).
@ General references: Eisenbud AJP(58)mar [objections and formulation]; Brehme AJP(85)oct [laws as definitions]; Anderson AJP(90)dec [not definitions]; Grabowska & Urbański mp/04 [frame-independent]; Kokarev a0905 [three lectures]; Zimba 09 [illustrated guide]; Lee FP(11) [and infinite systems]; Stocklmayer et al TPT(12) [introduction of the laws in the classroom].
@ First law: Pfister FPL(04); Rabinowitz IJTP(08) [and quantum mechanics]; Weatherall a1206-ch [inertial motion and its explanation by the geodesic principle]; Hecht TPT(15)#2 [history, origins].
@ Second law: Hacyan AJP(09)jul [meaning of modifications or tests]; Pourciau AJP(11)oct [what did Newton say in the Principia?]; Stefanini & Reali a1502 [new interpretation proposed].
@ Third law: Anandan & Brown FP(95) [and pilot-wave theory]; Fraser SHPSA(05) [and argument for second law]; Dykstra AJP(09)aug; Pinheiro PS(11) [possible violations]; Ivlev et al PRX(15) + news PhysOrg(15) [violation, and statistical mechanics]; > s.a. Action-Reaction Principle; photon phenomenology [negative effective mass].
@ Bounds on violations: Abramovici & Vager PRD(86) [ok down to 3 × 10–9 cm/s2]; Gundlach et al PRL(07) + pn(07)apr [ok down to 5 × 10–14 cm/s2].
@ Modifications: Milton & Willis PRS(07) [second, continuum elastodynamics]; Unzicker gq/07 [and gravity]; Sprott AJP(09)sep [modified third law]; > s.a. force; MOND.
> Related concepts: see force; inertia and inertial frame; mass.

Newton's (Shell / "Superb") Theorem
* Idea: A spherically symmetric mass distribution attracts a body outside of it as if the entire mass were concentrated at the center; In Newtonian physics it is also true that a hollow, spherically symmetric mass distribution produces no gravitational effect inside it; In general relativity, while a corresponding statement about points outside a spherically symmeric object is still true (mutatis mutandis), a spherical shell does have an effect at locations inside.
@ References: Schmid AJP(11)may [elementary geometric proof]; Zang & Yi IJMPCS(12)-a1203-conf [in general relativity]; > s.a. schwarzschild solution [interior metric].

Newtonian Limit of General Relativity > see phenomenology of gravity.

Newtonian Physics > see cosmological models [Newtonian cosmology]; newtonian gravity and tests of newtonian gravity; physical theories [frameworks].

* Idea: Particles with only gravitational interactions.
@ References: Ronga JPCS(16)-a1511 [detection].

Nicolai Map
* Idea: A change of variables for a gauge theory from the gauge connection in a fixed gauge to the anti-selfdual part of the curvature.
@ References: Bochicchio & Pilloni JHEP(13)-a1304.

Nieh-Yan Form
@ General references: Guo et al CTP(99)ht/98; Li JPA(99)ht.
@ And physics: Soo PRD(99)ht/98 [and the Adler-Bell-Jackiw anomaly]; Mielke PRD(09), Banerjee CQG(10)-a1002 [and gravity]; > s.a. theories of gravity.

Nightmare Scenario
* Idea: The scenario in which the Large Hadron Collider (LHC) fails to discover physics Beyond the Standard Model (BSM).
@ References: Bertone et al JCAP(12) [consequences for dark matter, in cMSSM].

Nijenhuis Torsion Tensor > see types of symplectic structures.

9j Symbol > see SU(2).

No-Cloning Theorem > s.a. quantum technology.
@ References: Hari Dass a1005 [for harmonic-oscillator coherent states]; Gauthier RPMP(13) [for an infinite multiverse]; Ortigoso a1707 [1970 proof].

No-Hair Theorems > see black-hole hair; Cosmic No-Hair Conjecture.

Noether Symmetries, Theorem

Noise > s.a. partial differential equations [stochastic].
* Shot noise: Fluctuations in a counting rate that are Poissonian, i.e., if the mean number of counts in a certain time interval or spatial volume is N, then the variance (δN)2 of that number is also equal to N.
@ 1/f noise: in Kaplan & Glass 95 [II]; Kuzovlev PU(15)-a1504 [origin and significance].
@ Shot noise: Beenakker & Schönenberger PT(03)may.
@ Quantum noise: Crow & Joynt PRA(14)-a1309 [classical simulation]

Noiseless Subsystems > see generalized coherent states.

Non-Archimedean Structures / Numbers > s.a. Archimedean Property / Adelic and p-Adic Structures; types of numbers; geometry.
* Idea: A structure which has a pair of non-zero elements, one of which is infinitesimal with respect to the other; In other words, one with infinitely large and/or small elements; For example, hyperreal numbers, surreal numbers, p-adic numbers.
@ In physics: El Naschie CSF(04) [fundamental length and all that]; Rosinger m.HO/05 [use in general].
@ And elementary particles: Dimitrijevic et al FU(04)ht; Varadarajan & Virtanen a1002.
@ And cosmology: Avinash & Rvachev FP(00); Djordjević & Nešić PAOB-a1011 [p-adic and adelic quantum cosmology and tachyonic inflation]; > s.a. quantum particles.
> Online resources: see Wikipedia page.

Non-Associative Algebras
* Motivation: They appear in some exotic systems such as magnetic monopoles.
@ In physics: Bojowald et al PRL(15)-a1510 [quantum mechanics, potentially testable effects]; Kupriyanov a1603-PoS [in physics]; > s.a. observable algebras [in quantum theory]; Supersymmetry.

Non-Associative Geometry > s.a. Octonions \ types of manifolds.
@ General references: Wulkenhaar ht/96, ht/96, PLB(97)ht/96 [standard model], ht/96 [grand unification]; Nesterov & Sabinin CMUC(00)ht-conf, PRD(00)ht [and spacetime]; Beggs & Majid JPCS(10); > s.a. particle physics.
@ And gravity / spacetime: Nesterov & Sabinin ht/00-conf, Sabinin IJTP(01) [and discrete spacetime]; Nesterov & Sabinin PRD(00)ht [de Sitter spacetime], IJGMP(06)ht/04 [FLRW models]; Harikumar & Rivelles CQG(06)ht [and non-commutative gravity]; Farsnworth & Boyle JHEP-a1303 [spectral action]; Blumenhagen & Fuchs JHEP(16)-a1604.
@ Other physics: Sbitneva IJTP(01) [special relativistic kinematics]; Blumenhagen & Plauschinn JPA(11)-a1010 [from string theory]; Mylonas & Szabo FdP(14)-a1404-proc [on non-geometric spaces]; Barnes et al PoS-a1601 [field theory]; Bakas a1605-proc [in Dirac's generalization of Maxwell theory]; > s.a. Supersymmetry.
> Other physics: see modified quantum mechanics; particle physics beyond the standard model; phase space [quantum]; types of gauge theories.

Non-Commutativity > s.a. non-commutative geometry and spacetime; in physical theories, in field theory and gravitation.
@ References: MacKenzie ThSc(97)may [general notion].

Non-Conservative System > see classical systems.

Non-Degenerate Bilinear Form > see Bilinear Form.

Non-Demolition Principle > s.a. experiments in quantum mechanics.
@ References: Marcovitch & Reznik a1002-wd [and quantum predictions].

Non-Equilibrium Systems > see in statistical mechanics and thermodynamics; states in quantum field theory; temperature.

Non-Euclidean Geometry > see geometry.

Non-Extensive Statistical Mechanics

Non-Gaussianity > see under Gaussianity.

Non-Hermitian Quantum Theory > see modified quantum mechanics [PT-symmetric].

Non-Holonomic Systems > see types of constrained systems; quantum systems.

Non-Imprisonment Conditions on Spacetime
@ References: Minguzzi JMP(08) [and distinction property].

Non-Linear Analysis > see analysis.

Non-Linear Systems / Field Theory > see classical systems; sigma-model; types of quantum field theories.

Non-Linear Quantum Mechanics > s.a. brownian motion; causality violations; schrödinger equation [including WKB].
* Motivation: Include as part of the dynamical evolution the transformation associated with the wave-function collapse.
* Feature: Superluminal propagation, a generic phenomenon in a large class on non-dissipative quantum theories.
@ Intros, reviews: Goss Levi PT(89)oct; news Nat(90)jul; Svetlichny qp/04 [arXiv bibliography]; Habib et al qp/05-conf [intro].
@ General references: Białynicki-Birula & Mycielski AP(76); Kibble CMP(78) [relativistic]; Giusto et al PhyD(84); Białynicki-Birula in(86); Bon et al NCA(87) [solutions, thermodynamic description]; Weinberg AP(89), PRL(89) + comment Peres PRL(89); Castro JMP(90) [and geometric quantum mechanics]; Jordan PLA(90); Nattermann qp/97; Puszkarz qp/97, qp/97, qp/99, qp/99, qp/99; Strauch PRE(07)-a0707 [propagation scheme]; Rego-Monteiro & Nobre JMP(13) [classical field theory].
@ Derivations, motivation: Parwani qp/06-proc, TMP(07) [information theory-motivated]; Adami et al JSP(07) [from many-body dynamics]; Lochan & Singh Pra-a0912 [and quantum measurement, superpositions, and time]; Wu et al IJTP(10)-a1104 [and Gross-Pitaevskii equation]; Molladavoudi et al PLA(12).
@ Non-polynomial: Parwani BJP(05)qp/04-proc, AP(05) [information-theoretic argument]; Parwani & Tan PLA(07)qp/06 [solutions].
@ Special systems: Gattobigio et al JMP(99) [on half-line]; Gardiner et al PRA(00)qp/99 [with chaotic potential]; Teismann RPMP(05) [with L2 solutions]; Shu & Zhang JMP(06) [harmonic potential]; Doikou et al NPB(08)-a0706 [on an interval]; Carles & Sparber JPA(12)-a1110 [coherent states in periodic potentials].
@ And gravity: Soni Pra(02)gq/00; Singh gq/03; Svetlichny IJTP(05), JGSP(06)qp-conf [at the Planck scale]; > s.a. quantum-gravity phenomenology.
@ Difficulties: Gisin PLA(90) [causality]; Caticha PLA(98)qp, PRA(98)qp; Doebner qp/98-proc; Puszkarz qp/98 [energy non-uniqueness]; Lücke qp/99 [non-locality]; Mielnik PLA(01) [causality]; Kent PRA(05) [consistent with causality]; Burq & Zworski CMP(05) [semiclassical, instability]; Cavalcanti et al a1206 [the preparation problem]; Ho & Hsu IJMPA(15)-a1401 [non-locality].
@ Consequences: Cassidy PRD(95) [and closed timelike curves]; Czachor & Doebner PLA(02)qp/01 [multi-particle correlations]; > s.a. cosmic rays.
@ Tests: Bollinger et al PRL(89); Majumder et al PRL(90) [optically pumped 201Hg].
@ Numerical simulations: Luo a0912; Mocz & Succi a1503/PRE.
@ Related topics: Minelli & Pascolini LNC(80), in(80) [solitons], NCB(85) [conservation laws]; Valentini PRA(90) [and complementarity]; Polchinski PRL(91) [causality and EPR, comment Mielnik qp/00]; Lücke qp/95 [observables]; Bona qp/99-proc [for density matrices], qp/99-proc [symmetries]; Rapedius & Korsch JPA(08) [barrier penetration]; Jordan PRA(10)-a1002 [tests of linearity, conceptual]; Andrianov & Ioffe JPA(12) [supersymmetric, rev]; > s.a. Ehrenfest Theorem.

Non-Local Field Theories > see locality; modified gravity theories; types of quantum field theories.

Non-Metricity > see affine connection.

Non-Perturbative Features of Field Theory > see instantons; solitons.

Non-Renormalization Theorems > see renormalization; supersymmetry in field theory.

Non-Squeezing Theorem > see symplectic geometry.

Non-Standard Analysis

Non-Symmetric Geometry and Gravity > s.a. kaluza-klein theory; unified theories.
* Idea: A geometry and theory of gravity in which the metric tensor is not symmetric; The vacuum field equations are

gμν, σgρν Γρμσgμρ Γρσν = 0 ,    [(–g)1/2 g[μν]],ν = 0 ,    R(αβ) = 0 ,    R[αβ], γ + R[βγ], α + R[γα], β = 0.

* History: The non-symmetric theory of gravity was proposed by J Moffat; It attracted a lot of attention until the mid-1990s, when it was thought that the theory does not include black holes, but this claim was proved incorrect by work by Burko and Ori.
@ General references: Kunstatter et al JMP(83) [geometrical structure]; Damour et al PRD(93), gq/93 [problems]; Cornish & Moffat gq/94; Moffat JMP(95), JMP(95); Ragusa PRD(97); Jurco et al a1512-MG14 [and generalized geometry].
@ Related topics: Clayton IJMPA(97)gq/95 [Hamiltonian]; Mebarki et al PS(97) [quantization]; Wanas & Kahil GRG(99)gq [quantization of paths].
@ Cosmology: Moffat ap/97 [birefringence]; Prokopec & Valkenburg PLB(06)ap/05 [inflation and cmb].
@ Black holes: Burko & Ori PRL(95)gq, gq/95 [black holes do form].
@ Other phenomenology: Woolgar PRD(90) [lunar orbit]; Legare & Moffat gq/95 [test particles]; Moffat & Sokolov PLB(96)ap/95 [galaxies]; Moffat gq/04, JCAP(05)ap/04 [galaxy rotation curves]; Moffat gq/04 [Gravity Probe-B], CQG(06) [time delay]; Janssen & Prokopec CQG(06)gq [instability]; Prokopec & Valkenburg ap/06, Janssen & Prokopec JPA(07)gq/06-conf [massive, as dark matter]; Hammond IJMPD(13) [non-symmetric part as the potential for the spin field]; Pérez Teruel MPLA(14) [and particle interactions].
> Related topics: see anomalous acceleration [Pioneer 10/11]; bianchi models; equivalence principle.
> Online resources: see Wikipedia page.

Nordström Theory of Gravity > see Scalar Theory of Gravity.

Nordtvedt Effect > s.a. equivalence principle; tests of general relativity with orbits.
* Idea: A (possible) violation of the strong equivalence principle in the Earth-Moon system; It would show up in a departure from geodesic motion, e.g., with a "polarization" of the Moon's orbit.
@ References: Dicke in(64); Nordtvedt PR(68), PR(68); in Misner et al 73, p1128.
> Online resources: see Wikipedia page.

Norm / Normed Spaces

Normal Coordinates on a Lie Group > see coordinates.

Normal Coordinates on a Manifold > see coordinates.

Normal Distribution > see gaussian.

Normal Matrix / Operator > see operator theory.

Normal Modes > see molecular physics.

Normal Product / Ordering > s.a. fock space.
$ Def: The normally-ordered product in the algebra of boson and fermion operators on Fock space is

:A1 A2 ... An: or N(A1 A2 ... An):= (–1)p Ap1 Ap2 ... Apn ,

where {p1, p2, ..., pn} is a permutation of {1, 2, ..., n} such that annihilation operators always appear to the right of creation operators, and p is the number of times two fermion operators have been commuted; In addition, to make this operation well-defined, we require linearity, :A+B: = :A: + :B: and :cA: = c :A:.
* Applications: Resolve operator-ordering ambiguities, and regularize divergent quantities; e.g. for a scalar field

H = \(\frac12\)k (ak ak + ak ak ) ω   diverges,   H = :\(\frac12\)k (ak ak + ak ak ) ω: = k ak ak ω   does not .

* Conditions: It depends on a choice of vacuum, so it is not obvious how to define it in curved spacetime.
@ General references: Wurm & Berg AJP(08)jan [basic ideas and results]; Plimak & Stenholm AP(12)-a1307 [generalization to interacting fields].
@ Combinatorics, formulas: Katriel LNC(74); Katriel JOB(02); Błasiak et al PLA(03), CzJP(05)qp-conf, JMP(05); Solomon et al qp/04-conf; Horzela et al qp/04-conf; Schork PLA(06) [q-deformed bosons]; Błasiak PhD(05)qp; Mansour et al IJTP(08)qp/06 [non-crossing], PLA(07)qp/06 [generalization]; Błasiak et al AJP(07)jul-a0704 [introduction]; Błasiak & Flajolet SLC-a1010.
@ In curved spacetime: Nikolić GRG(05)ht/02 [generalization based on 2-point function].
> Online Resources: see Wikipedia page.

Normal Space > see types of topologies.

Normal Subgroup of a Group > see group.

Normalizer of a Subset of a Group > see group.

Nothing > s.a. boundary conditions in quantum cosmology [universe-from-nothing proposals].
* Idea: The absence of spacetime.
@ References: Brown & Dahlen PRD(12)-a1111 [as an infinitely-negatively-curved spacetime, and tunneling in cosmology].

* Idea: A particle whose helicity properties are opposite to those of the photon; A (3+1)-dimensional free Abelian 2-form, described by a gauge theory; Proposed in the 1960s by Ogievetskii and Polubarinov, but the theory goes under the names of Kalb & Ramond, who developed the theoretical concept.
@ References: Dvoeglazov PS(01)phy/98; Malik ht/03-proc [as Hodge theory], PPNL(11)-a0912-conf [superfield formalism, BRST transformations]; Dvoeglazov JPCS(15)-a1308 [notoph-graviton-photon coupling]; Bandos & Ortín PRD(15)-a1502 [in N = 8 supergravity].

Nova > see star types.

Nowhere Dense Subset
* Idea: AX is nowhere dense if every ball UX contains another ball VX which has no points in A.

NP-Completeness > see computational complexity; mathematics [Millennium Problems].

NP Formalism > stands for Newman-Penrose.

Nuclear Operator > same as Trace-Class.

Nuclear Physics > s.a. nuclear technology.

Nucleosynthesis > see nuclear physics; astrophysics; early-universe nucleosynthesis.

Null Cone > see Light Cone.

Null Coordinates > see coordinates.

Null Curve, Surface > see spacetime subsets.

Nullity (Spinor) > see types of spinors.

Numbers > s.a. Mathematical Constants; physical constants; types .
* Special numbers: All numbers are special, but some are more special than others; Large numbers studied are the Sandreckoner's number 1063, Googol and Googolplex; Other special integers include 142,857 (see what happens to it when multiplied by 2, 6, 4, 5, and by 7), 26 (the only one between a square and a cube); > for more see number theory [including prime numbers, factoring, ...].
* Notation: We use a "geometric" modulus 10 notation, with a set of 10 "Arabic" symbols {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} that can be used to write more than 10 integers by position-dependent convention, in which the one placed in the n-th position from the right is to be interpreted as multiplied by 10n–1; Babylonians used to count in modulus 60; Alternatives are the Roman numerals, or the factorial number system [> see MathPages page].
@ General references: Johnson & Jedrzejewski 14 [patterns and drawings].
@ Special numbers: Adrian 06; Flannery 06 [21/2]; Posamentier & Lehmann 09.
> Examples: see Catalan Numbers; e; Euler(-Mascheroni) Constant; Feigenbaum Number; Golden Ratio; Googol; Googolplex; Googolplexian; i; Infinite; Omega Number; π; Silver Mean; Zero.
> Online Resources: see Internet Encyclopedia of Science pages.

Number Operator > see fock space.

Numerical Methods > see computational physics; numerical general relativity.

Numerical Relativity > s.a. models.

NUT Space > s.a. maxwell fields in curved spacetime; null infinity; schwarzschild spacetime [modified solution, and NUT parameter].
* Idea: A solution of Einstein's equation that can be interpreted as describing the exterior field of two counter-rotating semi-infinite sources possessing negative masses and infinite angular momenta, which are attached to the poles of a static finite rod of positive mass.
* NUT 4-momentum and charges: The NUT 4-momentum is the magnetic dual of the Bondi-Sachs 4-momentum at null infinity, and it is absolutely conserved, even if there is gravitational radiation; Gravitational fields with non-vanishing NUT 4-momenta are not physically significant in classical general relativity, but may play a role in quantum gravity [@ Ashtekar & Sen JMP(82)].
@ References: Krori & Bhattacharjee PLA(81) [in Brans-Dicke theory]; Nouri-Zonoz et al CQG(99)gq/98 [dual]; Dadhich & Patel gq/02 [G → 0 limit]; Manko & Ruíz CQG(05)gq [interpretation].

Nyquist Theorem
* Idea: One of the few general results known in non-equilibrium thermodynamics; It relates tiny equilibrium voltage fluctuations across a conductor with its resistance.
@ References: Singh a1409 [non-linear Nyquist theorem].

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