THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
equatorial bulge
The excess of the Earth's equatorial radius over its polar radius (about 21 km), caused by the centrifugal deformation of rotation.
Ferroelectricity
The presence of a spontaneous electric polarization in certain crystals that can be reversed by an applied field — the electric analogue of ferromagnetism.
Ferromagnet
A material that holds a magnetisation with no external field, up to a sharp critical temperature at which it loses it.
Ferromagnetism
The spontaneous parallel alignment of atomic magnetic moments via quantum exchange interaction, producing permanent magnetization below a critical Curie temperature. The origin of magnetism in iron, nickel, cobalt, and everyday magnets.
Fluctuation
The spontaneous deviation of a quantity from its mean in a system at thermal equilibrium.
Frame Dragging
A rotating mass twists the spacetime around it, forcing nearby objects to co-rotate.
free fall
Motion under gravity alone, with no air resistance or other forces — every object accelerates at the same rate.
Gravitational Lensing
The bending of light by mass, turning galaxies and clusters into natural telescopes that magnify and multiply whatever lies behind them.
Gravitational redshift
Photons climbing out of a gravitational potential lose energy and frequency: Δν/ν = gh/c² for a tower of height h on Earth's surface; equivalently, clocks at lower potential tick slower than clocks at higher potential. A direct consequence of the equivalence principle, derivable without the field equations.
Gravitational wave
A propagating ripple in the curvature of spacetime, traveling at the speed of light.
Hawking radiation
The thermal glow emitted by a black hole's horizon, which makes black holes evaporate.
Johnson noise
The thermal voltage noise of any resistor, V² = 4k_BTRΔf, independent of the resistor's material.
Joule–Thomson effect
Squeeze a gas through a plug and it changes temperature — but only cools if it is already cold enough.
Kolmogorov spectrum
E(k) ∝ ε^(2/3) k^(−5/3) — the universal inertial-range energy spectrum of fully developed turbulence.
Length contraction of current
The relativistic effect that a current-carrying lattice, viewed in the rest frame of its drift electrons, has its inter-ion spacing contracted by the Lorentz factor γ. Produces the net + charge density that explains magnetic attraction as relativistic electrostatics.
Meißner effect
The active expulsion of magnetic flux from the interior of a superconductor on cooling below T_c in an applied field. The signature that superconductivity is a distinct thermodynamic phase, not mere zero resistance.
Milankovitch cycles
The three astronomical cycles — eccentricity, obliquity, precession — that drive Earth's ice ages on 10,000- to 100,000-year timescales.
monkey and the hunter
Classic thought experiment: a dart fired straight at a monkey that falls at the trigger still hits the monkey, because both are in the same free fall.
Mössbauer effect
Recoilless gamma-ray emission and absorption by nuclei bound in a solid lattice. The lattice absorbs the recoil momentum collectively, leaving the gamma-ray energy unbroadened by Doppler — producing emission lines narrow enough to resolve frequency shifts at parts-per-quadrillion precision. The technique that made the Pound-Rebka 1960 gravitational redshift measurement feasible.
Neutron star
The collapsed, city-sized remnant of a massive star, just shy of becoming a black hole.
nutation
Small oscillations of a spinning body's axis superimposed on its steady precession.
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.
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.
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.
Penrose Process
Extracting energy from a spinning black hole by feeding it negative energy.
Perihelion Precession
The slow rotation of an elliptical orbit's long axis, whose general-relativistic part first confirmed Einstein's theory through Mercury.
Phase transition
A change from one phase to another — melting, boiling, sublimation — that occurs at a definite temperature and pressure and absorbs or releases latent heat.
Piezoelectricity
The appearance of an electric voltage across certain crystals when they are mechanically squeezed — and the converse: the same crystals deform when a voltage is applied.
Poincaré recurrence
In a bounded Hamiltonian system, almost every trajectory returns arbitrarily close to its starting state — given enough time.