Browse All 842 Physics Terms A–Z

Special Relativity Astrophysics, Cosmology & Relativity Theoretical Framework Albert Einstein's 1905 theory describing the physical behavior of space and time for observers moving in uniform non-accelerated inertial frames. E² = (p c)² + (m₀ c²)², γ = 1 / √(1 - v²/c²)

Specific Humidity Geophysics & Environmental Physics g/kg (Grams of Water Vapor per kg of Air) The mass ratio of water vapor contained within an air parcel relative to the total mass of the moist air parcel. q = m_v / (m_v + m_d) ≈ 0.622 e / P

SPECT Scan Biophysics & Medical Physics MBq (Activity) & Hounsfield-like Functional Units Single-Photon Emission Computed Tomography (SPECT), a nuclear medicine 3D functional imaging technique using gamma-emitting radioisotopes. Gamma Camera 3D Tomographic Reconstruction

Speed of Light Foundations of Physics m/s The maximum speed at which all massless particles, electromagnetic waves, and information travel through a vacuum. c = 299,792,458 m/s

Speed of Sound Waves, Oscillations & Acoustics m/s The speed at which a pressure wave travels through an elastic medium. v = √(K / ρ) (Fluids), v = √(γ R T / M) (Ideal Gas)

Spherical Aberration Optics m An image blur defect in spherical lenses and mirrors where marginal rays focus closer to the element than paraxial central rays. Δf_spherical = f_marginal - f_paraxial

Spherical Harmonics Mathematical & Computational Physics Spherical Surface Functions A set of special orthogonal functions defined on the 2D sphere surface serving as angular solutions to Laplace's equation in 3D spherical coordinates. Y_l,m(θ, φ) = √(((2l+1)/(4π)) ((l-m)!/(l+m)!)) P_l^m(cos θ) e^(i m φ)

Spherical Mirror Optics m A curved mirror having a reflective surface shaped like a section of a sphere, classified as concave (converging) or convex (diverging). 1/f = 1/v + 1/u, f = R/2

Spin Glass Condensed Matter & Materials Physics Disordered Magnetic Phase A disordered magnetic state characterized by random, competing ferromagnetic and antiferromagnetic interactions that freeze atomic spins into a frustrated, non-periodic spatial orientation. Frustrated Exchange Interaction J_ij

Spin-Orbit Coupling Quantum & Atomic Physics eV (Energy Shift) A relativistic quantum interaction between an electron's intrinsic spin magnetic moment and the internal magnetic field created by its orbital motion around the nucleus. H_SO = ξ(r) L · S

Spontaneous Emission Quantum & Atomic Physics s⁻¹ (Transition Rate) The process by which an excited atom or quantum system spontaneously decays to a lower energy state, emitting a photon of random phase and direction. Rate = A₂₁ N₂

Spontaneous Symmetry Breaking Nuclear & Particle Physics Energy VEV (GeV) A physics phenomenon where the underlying mathematical equations of a system possess symmetry, but the ground state (vacuum) drops into a non-symmetric state. Vacuum Expectation Value ⟨0|φ|0⟩ = v ≠ 0

Standard Model Nuclear & Particle Physics Theoretical Framework The theoretical physics framework classifying all known elementary particles and describing three fundamental forces: electromagnetism, weak force, and strong force. SU(3)_C × SU(2)_L × U(1)_Y Gauge Symmetry

Standing Waves Waves, Oscillations & Acoustics m A stationary wave pattern formed by the superposition of two identical waves traveling in opposite directions along the same medium. y(x,t) = [2 A sin(k x)] cos(ω t)

Stark Effect Quantum & Atomic Physics eV (Energy Shift) The shifting and splitting of atomic spectral lines caused by the presence of an external static electric field. ΔE = -p · E_ext (First-order: ΔE ∝ E_ext)

Static Equilibrium Classical Mechanics N, N·m The state of a rigid body where both net external force and net external torque are zero, keeping the body completely at rest. ∑ F = 0, ∑ τ = 0

Statistics and Probability Mathematical & Computational Physics Probability (0 to 1) The mathematical frameworks for analyzing random phenomena, quantifying measurement uncertainty, and modeling ensemble behavior in statistical mechanics. P(A|B) = P(B|A) P(A) / P(B), μ = E[X], σ² = Var(X)

Stefan-Boltzmann Law Thermodynamics & Statistical Physics W/m² The physical law asserting that total radiant power emitted per unit surface area of a blackbody is proportional to the fourth power of its absolute thermodynamic temperature. E = σ T⁴, σ = 5.670374 × 10⁻⁸ W/(m²·K⁴)

Stellar Nucleosynthesis Astrophysics, Cosmology & Relativity Nuclear Energy & Abundances The collective nuclear fusion reactions inside stars that synthesize heavier chemical elements from lighter primordial hydrogen and helium. 3 α → ¹²C (Triple-Alpha Process at T ~ 10⁸ K)

Stellarator Plasma Physics Dimensionless Magnetic Transform A magnetic confinement fusion device that produces a helical magnetic field using complex, 3D non-planar external electromagnetic coils without requiring net plasma current. Rotational Transform ι = 1 / q (Generated entirely by 3D external coils)

Stern-Gerlach Experiment Quantum & Atomic Physics N (Force), J/T (Magnetic Moment) A landmark 1922 experiment that proved space quantization and the intrinsic angular momentum (spin) of subatomic particles. F_z = μ_z (∂B_z / ∂z), S_z = ± ℏ / 2

Stimulated Emission Quantum & Atomic Physics s⁻¹ (Transition Rate) The process in which an incoming photon stimulates an excited atom to drop to a lower state, emitting a second identical photon. Rate = B₂₁ ρ(ν) N₂

Stochastic vs Deterministic Radiation Effects Biophysics & Medical Physics Risk Factor / Dose Threshold (Gy) The classification of biological radiation damage into probability-driven stochastic risks (cancer) and threshold-driven deterministic tissue reactions (cataracts, erythema). Stochastic Probability P ∝ Dose; Deterministic Severity ∝ Dose (Threshold D_th)

Stokes' Law Classical Mechanics N A mathematical law expressing viscous drag force experienced by small spherical particles moving at low speeds through a viscous fluid. Fd = 6 π η r v

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