THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Navier-Stokes equations
The PDEs governing viscous fluid flow. Nonlinear, exact, and generally unsolved — smoothness in 3D is a Millennium Problem.
Near-field zone
The region r ≪ λ surrounding an oscillating source where the field resembles a time-varying instantaneous quasi-static field (amplitude ∝ 1/r³ for a dipole) that stores and returns energy rather than radiating it. Also called the induction zone or reactive zone.
Nernst's heat theorem
As T → 0, entropy stops depending on anything you can change — the statement that made absolute entropy possible.
Neutron star
The collapsed, city-sized remnant of a massive star, just shy of becoming a black hole.
Newton's laws of motion
The three laws — inertia, F = ma, and equal-and-opposite reaction — that launched classical mechanics in 1687.
Newtonian fluid
A fluid whose shear stress is strictly proportional to its velocity gradient, with viscosity independent of shear rate.
Newtonian limit
The regime where Einstein's field equations reduce to Newton's: weak gravity (g_{μν} = η_{μν} + h_{μν}, |h| ≪ 1), slow sources (v ≪ c), static configuration. In this limit G_{00} ≈ −2∇²Φ/c² and T_{00} ≈ ρc², so EFE becomes ∇²Φ = 4πGρ — the Poisson equation of Newtonian gravity.
No-hair theorem
The statement that a stationary black hole is fully described by just three numbers: its mass, angular momentum, and electric charge.
Node
A point on a standing wave that never moves.
Noether's theorem
Every continuous symmetry of a physical system's action gives rise to a conserved quantity.
Non-abelian gauge theory
A gauge theory whose gauge group is non-commutative, so the gauge fields themselves carry charge under the group and the field strength tensor acquires a self-interaction term. Yang-Mills 1954 introduced the construction; QCD and the weak force are non-abelian; QED is the abelian exception.
nonlinear dynamics
Study of systems where output is not proportional to input; chaos, solitons, turbulence.
normal modes
Independent oscillation patterns of a coupled system; any motion is their superposition.
Null interval
A separation between two events with invariant interval s² = 0 — meaning a light signal exactly connects them. The world-lines of photons are null curves; the light-cone of any event is the locus of null-separated points; null separations sit on the boundary between timelike and spacelike.
Numerical aperture
NA = n sin θ_max. For a fibre, NA = √(n_core² − n_cladding²) gives the sine of the maximum acceptance half-angle. For a microscope objective, NA determines the diffraction-limited resolution λ/(2·NA).
nutation
Small oscillations of a spinning body's axis superimposed on its steady precession.
Obliquity
The tilt angle between a planet's rotation axis and the perpendicular to its orbital plane.
Ohm's law
V = IR. For a metallic conductor at fixed temperature, the current through it is proportional to the voltage across it, with the proportionality constant R being the resistance.
Optical dispersion
The wavelength-dependence of refractive index, n(λ). Causes prism splitting of white light into its colours and chromatic aberration in lenses. Quantified by the Abbe number or the Sellmeier dispersion equation.
Optical fiber
A thin glass fibre of core + cladding structure that guides light by total internal reflection along its length. Loss < 0.2 dB/km at 1.55 μm in silica single-mode fibre; the backbone of global telecommunications.
Order parameter
The quantity that is non-zero in the ordered phase and zero in the disordered one — whose vanishing is the transition.
p-polarization
An EM wave incident on an interface with its electric field parallel to the plane of incidence (German parallel). Also called TM (transverse magnetic) polarisation. Reflection coefficient passes through zero at Brewster's angle.
Page curve
The rise-and-fall of radiation entropy that a unitary black-hole evaporation must produce.
Pair production
The conversion of energy into a matter-antimatter particle pair, most commonly a high-energy photon converting to an electron-positron pair near a nucleus: γ + nucleus → e⁺ + e⁻ + nucleus. Threshold ≥ 2m_e c² = 1.022 MeV; first observed by Carl Anderson in 1932 cloud-chamber tracks, confirming Dirac's 1928 antimatter prediction.
parabola
Conic section given by a quadratic in one variable; the trajectory of a projectile under gravity alone.
Parallel transport
The operation that moves a vector along a curve while keeping it as parallel as possible — its covariant derivative along the curve vanishes: D V^μ/dλ = 0. On flat space this returns the same vector; on a curved space it generally rotates. The rotation around a closed loop is the holonomy and is a direct measure of curvature.
parallel-axis theorem
For any axis parallel to one through the centre of mass, I = I_CM + M·d² (also called Steiner's theorem).
Paramagnetism
The weak alignment of permanent atomic magnetic moments with an applied field, competing against thermal randomisation. Susceptibility is positive and follows Curie's law χ = C/T.
Partition function
Z = Σ exp(−E_i/k_BT), the sum over states from which every thermodynamic quantity is obtained by differentiation.
Pascal
The SI unit of pressure: 1 Pa = 1 N/m². Named for Blaise Pascal.