THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
separatrix
Phase-space boundary between qualitatively different motions.
Shapiro delay
The extra round-trip travel time a light or radio signal accrues when it passes through the curved spacetime near a mass.
shell theorem
A uniform spherical shell attracts an external particle as if all its mass were at the centre; it exerts zero net force on an internal particle.
simple harmonic oscillator
System with a linear restoring force F = −kx; its solution is a pure sinusoid.
Single-slit diffraction
The intensity pattern I(θ) = I₀ sinc²(πa sin θ/λ) produced when light of wavelength λ passes through a slit of width a. First minimum at sin θ = λ/a; central maximum carries most of the energy.
Singularity
A place where the classical geometry of spacetime breaks down — defined by what is missing, not by what is there.
Skin depth
δ = √(2/(μσω)), the 1/e penetration depth of an EM wave into a conductor. Fields decay exponentially with depth, so high-frequency currents flow only in a thin surface layer — the skin effect.
Snell's law
n₁ sin θ₁ = n₂ sin θ₂. The ratio of the sines of the incidence and refraction angles at an interface equals the inverse ratio of refractive indices. Derived by Snell 1621 (unpublished), Descartes 1637 (published).
Solenoid
A long, tightly wound helical coil of wire. Carrying a current, it produces a uniform magnetic field along its axis: B = μ₀ n I.
Sonic boom
The sharp pressure impulse heard when the Mach cone of a supersonic source sweeps past an observer.
Spacelike
A separation between two events with invariant interval s² < 0 — meaning no signal at or below c can connect them. Spacelike-separated events have a frame-dependent temporal order; the relativity of simultaneity is exactly the freedom to choose any timelike frame and slice spacelike directions as 'now.'
Spacetime
The 4-dimensional pseudo-Euclidean manifold of events introduced by Hermann Minkowski in 1908, in which the three spatial coordinates and time enter on equal footing. The geometric arena of special relativity; the substrate on which world-lines, light-cones, and the invariant interval are defined.
Specific heat capacity
The heat required to raise one unit mass of a substance by one degree — large for water, small for metals.
Speed of light (c)
c = 1/√(μ₀ε₀) = 2.99792458 × 10⁸ m/s exactly, by SI definition since 1983. The invariant propagation speed of electromagnetic waves in vacuum and the universal speed limit of special relativity.
Spin
The intrinsic angular momentum of a particle — and, in statistical physics, the two-valued variable that stands in for it.
Standing wave
A wave pattern locked in place by interference, with fixed nodes and antinodes that don't propagate.
Standing-wave ratio (SWR)
VSWR = V_max/V_min = (1+|Γ|)/(1−|Γ|). A measure of transmission-line reflection. SWR = 1 is a perfect match (no reflection); SWR = ∞ is total reflection (open or short).
State function
A property whose value depends only on the current state of a system, not on the path taken to reach it — internal energy, temperature, pressure, entropy.
static equilibrium
The condition of a body at rest, requiring net force and net torque both to vanish.
static friction
The friction force that resists the onset of sliding; can match applied forces up to a maximum of μ_s · N.
Stationary action
The precise formulation of least action: δS = 0 for every first-order variation of the path vanishing at the endpoints.
Stokes' law
The drag force on a sphere of radius r moving slowly through a fluid of viscosity η: F = 6πηrv.
Strain
The dimensionless fractional length change a gravitational wave imprints on space: ΔL/L.
Streamline
A curve whose tangent at every point coincides with the local fluid velocity. In steady flow, streamlines are also particle paths.
Stress-energy tensor
T_{μν} — the symmetric (0,2) tensor whose components encode energy density (T_{00}), momentum density (T_{0i}), pressure (T_{ii}), and shear stress (T_{ij}, i≠j). Conserved: ∇^μ T_{μν} = 0. The matter side of Einstein's field equations. Perfect fluid: T_{μν} = (ρ + p/c²) u_μ u_ν − p g_{μν}.
Superposition principle
For any linear system, the sum of two solutions is also a solution: waves add, they don't collide.
Surface gravity
The acceleration at a black hole's horizon, which plays the role of its temperature.
symmetry
An operation under which a system is unchanged — in physics, the source of every conservation law via Noether's theorem.
Symplectic
The antisymmetric non-degenerate 2-form dq ∧ dp on phase space. Preserved exactly by Hamiltonian flow.
Synchrotron radiation
The electromagnetic radiation emitted by a relativistic charged particle following a curved trajectory in a magnetic field. Power scales as γ⁴ in circular motion; the spectrum is broad with characteristic frequency ω_c ∝ γ³c/R. Basis of synchrotron light sources and pulsar emission.