Standard physics glossary

cosmology · intermediate

S8 Parameter

A cosmological parameter that quantifies the amplitude of matter clustering, or how clumpy the distribution of matter is, in the universe on a specific reference scale, combining the overall matter density with the strength of density fluctuations. Similar to the Hubble tension, some large-scale structure surveys measuring S8 directly from weak lensing and galaxy clustering find a value that differs somewhat from the value predicted by fitting the standard cosmological model to the cosmic microwave background.

math-methods · intro

Scalar

A physical quantity that is fully described by a single numerical value and a unit, with no associated direction, as opposed to a vector, which has both magnitude and direction. Examples of scalar quantities include mass, temperature, energy, and speed. Scalars can be added, subtracted, and multiplied using ordinary arithmetic, which makes them generally simpler to work with mathematically than vector quantities.

cosmology · intermediate

Scale Factor

A quantity in cosmology that describes how relative distances between objects in the universe change over time due to the expansion of space, conventionally set to a value of one at the present day. A scale factor of one half, for instance, indicates a time in the past when the universe was half its current size, and comparing the scale factor at the time light was emitted to its value today directly relates to the observed cosmological redshift of that light.

waves · intermediate

Scattering

The deflection of particles or waves from their original path of travel due to interactions with other particles, irregularities, or a medium they pass through. Scattering can occur elastically, where kinetic energy is conserved, or inelastically, where some energy is transferred to the scattering target. Rayleigh scattering of sunlight by air molecules, for instance, is responsible for the blue color of the daytime sky.

quantum · intermediate

Schrodinger Equation

A fundamental equation of quantum mechanics, formulated by Erwin Schrodinger in 1926, that describes how the quantum state, represented by a mathematical function called the wavefunction, of a physical system evolves over time. Solving the Schrodinger equation for a given system, such as an electron in an atom, reveals the allowed energy levels and the probability distribution of finding the particle at different locations. It plays a role in quantum mechanics comparable to that of Newton's laws in classical mechanics.

relativity · intermediate

Schwarzschild Radius

The radius that a given mass would need to be compressed within for its escape velocity to equal the speed of light, effectively defining the size of the event horizon for a non-rotating black hole of that mass. Named after German physicist Karl Schwarzschild, who found this exact solution to Einstein's field equations shortly after general relativity was published. For an object with the Sun's mass, the Schwarzschild radius is only about three kilometers.

thermo · intro

Second Law of Thermodynamics

A fundamental law of physics stating that the total entropy of an isolated system can never decrease over time, only increase or stay the same in idealized reversible processes. This law explains why heat flows spontaneously from hot objects to cold ones and not the reverse, why perpetual motion machines that produce more energy than they consume are impossible, and why natural processes tend to move toward greater disorder.

quantum · intro

Semiconductor

A material whose electrical conductivity lies between that of a good conductor and a good insulator, and which can be precisely controlled by adding small amounts of impurities, a process called doping, or by applying an electric field. Silicon is the most widely used semiconductor material, forming the basis of transistors, diodes, and integrated circuits that underlie virtually all modern electronics.

cosmology · intermediate

SH0ES Collaboration

Also called: SH0ES

A research program, whose name stands for Supernovae and H0 for the Equation of State, that measures the present-day expansion rate of the universe using a chain of distance measurements anchored by Cepheid variable stars and calibrated with Type Ia supernovae. Results from the SH0ES collaboration have consistently produced a measured value for the Hubble constant that is higher than the value inferred from cosmic microwave background observations, contributing significantly to the ongoing Hubble tension.

waves · intro

Simple Harmonic Motion

A type of periodic motion in which the restoring force acting on an object is directly proportional to its displacement from equilibrium and always directed back toward that equilibrium position. A mass on an ideal spring and a pendulum swinging through small angles both approximate simple harmonic motion, producing smooth, sinusoidal oscillation over time. It is the simplest and most mathematically tractable model of oscillatory behavior in physics.

relativity · intermediate

Singularity

A point or region in spacetime where certain physical quantities, such as density or spacetime curvature, become infinite according to the equations of general relativity, most notably predicted to exist at the center of a black hole. At a singularity, the known laws of physics, including general relativity itself, are believed to break down, suggesting that a more complete theory combining gravity and quantum mechanics is needed to properly describe what actually happens there.

optics · intro

Snell's Law

A law of optics describing the relationship between the angles of incidence and refraction when a wave, typically light, passes between two media with different refractive indices. It states that the refractive index of the first medium multiplied by the sine of the incidence angle equals the refractive index of the second medium multiplied by the sine of the refraction angle. Named after Dutch astronomer Willebrord Snellius, it is the foundational equation for designing lenses and other refractive optical components.

astro · intro

Solar Wind

A continuous stream of charged particles, mainly protons and electrons, ejected from the Sun's outer atmosphere, the corona, and traveling outward through the solar system at speeds typically between 300 and 800 kilometers per second. The solar wind interacts with planetary magnetic fields, compressing and shaping them, and its interaction with Earth's magnetosphere produces auroras near the polar regions. Strong bursts of solar wind can also disrupt satellites and power grids.

waves · intro

Sound Wave

A longitudinal mechanical wave created by vibrating objects, traveling through a medium such as air, water, or a solid, as alternating regions of compression and rarefaction of the medium's particles. Sound waves require a physical medium to travel through and cannot propagate through a vacuum, unlike electromagnetic waves. The frequency of a sound wave determines its pitch, while its amplitude determines its loudness.

relativity · intro

Spacetime

A unified four-dimensional framework combining the three familiar spatial dimensions with time into a single continuum, introduced formally in special relativity and central to general relativity. In spacetime, events are located not just by where they occur but also by when, and the separation between two events depends on the observer's motion in a way that mixes space and time coordinates. Massive objects curve spacetime, and this curvature is what we perceive as gravity.

relativity · intermediate

Spacetime Curvature

The warping of the four-dimensional fabric of spacetime caused by the presence of mass and energy, as described by Einstein's general theory of relativity. Rather than gravity being a force pulling objects together, general relativity explains that massive objects like stars and planets curve the spacetime around them, and other objects simply follow the straightest possible paths, called geodesics, through this curved geometry, which appears to us as gravitational attraction.

cosmology · intermediate

SPARC Database

Also called: SPARC

A curated collection of high-quality rotation curve data, whose name stands for Spitzer Photometry and Accurate Rotation Curves, for a large sample of nearby spiral and irregular galaxies, combining infrared photometry with radio observations of gas rotation speeds. The SPARC database is widely used by researchers testing predictions of dark matter distribution models and alternative gravitational theories against observed galaxy rotation curves.

relativity · intro

Special Relativity

A theory developed by Albert Einstein in 1905 that describes the physics of objects moving at constant velocity, particularly at speeds approaching the speed of light, based on two postulates: that the laws of physics are the same in all inertial reference frames, and that the speed of light in a vacuum is constant for all observers regardless of their motion. Special relativity leads to phenomena including time dilation, length contraction, and the equivalence of mass and energy.

thermo · intro

Specific Heat

The amount of heat required to raise the temperature of one unit of mass of a substance by one degree, a characteristic property that varies from one material to another. Water has an unusually high specific heat compared to most substances, meaning it absorbs a large amount of heat with only a small temperature change, which is why it is effective at moderating temperature in cooling systems and coastal climates.

quantum · intermediate

Spectral Line

A narrow, distinct feature appearing at a specific wavelength within the spectrum of light emitted or absorbed by a substance, corresponding to a transition between two discrete quantum energy levels in an atom or molecule. Emission lines appear bright against a darker background when atoms release light, while absorption lines appear as dark gaps when atoms absorb light at those same wavelengths. Spectral lines act as a unique fingerprint that allows astronomers to identify the chemical composition of distant stars and galaxies.

mechanics · intro

Speed

The rate at which an object covers distance, calculated by dividing distance traveled by the time taken, and expressed as a scalar quantity with magnitude but no specified direction. Speed differs from velocity, which is the corresponding vector quantity that also specifies the direction of motion. Average speed considers total distance over total time, while instantaneous speed describes how fast an object is moving at a specific moment.

relativity · intro

Speed of Light

The speed at which light and all other massless particles or fields travel through a vacuum, exactly 299,792,458 meters per second, a fundamental constant of nature denoted by the letter c. According to special relativity, this speed represents an absolute cosmic speed limit; no object with mass can be accelerated to reach it, since doing so would require infinite energy. The speed of light also appears in Einstein's mass-energy equivalence relation.

waves · intro

Speed of Sound

The speed at which sound waves travel through a given medium, dependent primarily on the medium's stiffness and density; sound generally travels fastest through solids, slower through liquids, and slowest through gases. In dry air at room temperature, sound travels at approximately 343 meters per second, though this speed increases with air temperature. The speed of sound serves as the reference value for calculating an object's Mach number.

quantum · intermediate

Spin (Quantum)

An intrinsic form of angular momentum carried by elementary particles, distinct from any actual physical spinning motion, that is quantized to take only specific discrete values. Electrons and other fermions have half-integer spin, while force-carrying bosons like the photon have integer spin, a distinction that determines fundamentally different statistical behavior between the two particle categories. Spin also underlies phenomena such as magnetic resonance imaging and the intrinsic magnetism of electrons.

particle · intro

Standard Model

The current, well-tested theoretical framework in particle physics describing all known elementary particles, including quarks, leptons, and force-carrying bosons, along with three of the four fundamental forces: electromagnetism, the weak force, and the strong force. Developed throughout the latter half of the twentieth century, the Standard Model has been confirmed by decades of experimental results, including the 2012 discovery of the Higgs boson, though it notably does not incorporate gravity or explain dark matter.

waves · intro

Standing Wave

A wave pattern that appears stationary in space, formed when two waves of the same frequency and amplitude travel in opposite directions and interfere with each other, typically arising from a wave reflecting back along the same path it came from. Standing waves have fixed points called nodes, where the amplitude is always zero, and antinodes, where the amplitude oscillates at its maximum. Standing waves on a guitar string determine the specific musical notes it can produce.

astro · intro

Star

A massive, self-luminous sphere of plasma held together by its own gravity, generating energy primarily through nuclear fusion reactions in its core, which fuse lighter elements into heavier ones and release the light and heat that make stars shine. Stars form from collapsing clouds of gas and dust and evolve through predictable stages over millions to billions of years, ultimately ending their lives as white dwarfs, neutron stars, or black holes, depending on their initial mass.

em · intro

Static Electricity

The buildup of electric charge on the surface of an object, typically caused by friction between two materials that transfers electrons from one surface to the other, leaving one object with excess positive charge and the other with excess negative charge. This accumulated charge remains stationary until it finds a path to discharge, sometimes producing a visible spark, as commonly experienced after walking across a carpet and then touching a metal doorknob.

thermo · intermediate

Statistical Mechanics

The branch of physics that uses probability and statistics to connect the microscopic behavior of a system's individual particles to its observable macroscopic properties, such as temperature, pressure, and entropy. Rather than tracking every individual particle in a gas containing billions of billions of molecules, statistical mechanics uses averages and probability distributions to derive reliable predictions about the system's overall behavior, providing the theoretical foundation for classical thermodynamics.

thermo · intermediate

Stefan-Boltzmann Law

A law of thermodynamics stating that the total energy radiated per unit surface area by a blackbody is proportional to the fourth power of its absolute temperature. This steep dependence on temperature means that even a modest increase in an object's temperature causes a dramatic increase in the amount of energy it radiates, which is why hotter stars are vastly more luminous per unit surface area than cooler ones.

quantum · intermediate

Stimulated Emission

A process in which a photon interacting with an atom or molecule already in an excited energy state triggers the emission of a second photon that is identical in wavelength, phase, and direction to the first. This process, first described theoretically by Einstein in 1917, is the fundamental physical mechanism that makes lasers possible, since it allows a single photon to trigger a cascading amplification of identical, coherent photons.

particle · intro

Strong Force

Also called: strong nuclear force

One of the four fundamental forces of nature, responsible for binding quarks together to form protons, neutrons, and other hadrons, and for holding protons and neutrons together within an atomic nucleus despite the electrical repulsion between positively charged protons. The strong force is the most powerful of the fundamental forces at short range but decreases rapidly beyond the scale of an atomic nucleus, and it is mediated by particles called gluons.

astro · intro

Sun

The star at the center of our solar system, a massive sphere of mostly hydrogen and helium plasma that generates energy through nuclear fusion in its core, converting hydrogen into helium and releasing the light and heat that sustain life on Earth. Classified as a main-sequence star of average size and temperature compared to other stars, the Sun accounts for over ninety-nine percent of the total mass in the solar system.

cosmology · intermediate

Sunyaev-Zeldovich Effect

A distortion in the observed spectrum of the cosmic microwave background caused when its photons pass through the hot, ionized gas found within massive galaxy clusters and gain a small amount of energy through interactions with high-energy electrons. Because this effect depends on the gas's properties rather than on distance in the usual sense, it provides astronomers with a distance-independent method for detecting massive galaxy clusters throughout the observable universe.

quantum · intermediate

Superconductivity

A phenomenon in which certain materials, when cooled below a characteristic critical temperature, lose all electrical resistance entirely and can conduct electric current indefinitely without any energy loss. Superconductors also expel magnetic fields from their interior, an effect called the Meissner effect, allowing them to levitate above magnets. Superconductivity has practical applications in powerful electromagnets used in MRI machines and particle accelerators, though most known superconductors require extremely low temperatures to function.

astro · intro

Supernova

A powerful and extremely luminous stellar explosion, occurring either when a massive star exhausts its nuclear fuel and its core collapses catastrophically, or when a white dwarf in a binary system accumulates too much mass and undergoes a runaway nuclear reaction. A supernova can briefly outshine an entire galaxy and scatters heavy elements forged within the star into surrounding space, seeding future generations of stars and planets with the material needed to form them.

waves · intro

Superposition

The principle stating that when two or more waves overlap in the same region of space, the resulting displacement at any point is simply the sum of the displacements each individual wave would have produced on its own. Superposition underlies the phenomena of constructive and destructive interference. In quantum mechanics, superposition takes on a deeper meaning, describing how a quantum system can exist in a combination of multiple possible states simultaneously until measured.

mechanics · intro

Surface Tension

The tendency of the surface of a liquid to behave like a stretched elastic membrane, arising because molecules at the surface experience a net inward pull from neighboring molecules, unlike molecules within the bulk liquid, which are pulled equally in all directions. Surface tension allows small insects to walk on water and causes water droplets to form roughly spherical shapes, since a sphere minimizes surface area for a given volume.

math-methods · intermediate

Symmetry (Physics)

A property of a physical system that remains unchanged, or invariant, under a specific transformation, such as rotation, reflection, or translation in space or time. Symmetries play a profound role in physics, since, according to Noether's theorem, every continuous symmetry of a physical system corresponds to a conserved quantity; for example, symmetry under time translation corresponds to conservation of energy, and symmetry under spatial translation corresponds to conservation of momentum.