Standard physics glossary

thermo · intro

Phase Change

A transition of a substance between different states of matter, such as solid, liquid, and gas, occurring at specific temperatures and pressures and typically involving the absorption or release of latent heat without a change in temperature during the transition itself. Common phase changes include melting, freezing, boiling, condensation, sublimation, and deposition, each governed by the balance between thermal energy and the intermolecular forces holding the substance together.

waves · intermediate

Phase Velocity

The speed at which a single point of constant phase, such as a wave crest, moves through space, calculated as the wave's frequency multiplied by its wavelength. For simple waves traveling through a uniform medium, phase velocity is constant, but in dispersive media, where different frequencies travel at different speeds, phase velocity can differ from the group velocity of an overall wave packet.

quantum · intro

Phosphorescence

A type of light emission similar to fluorescence, in which a substance absorbs energy and later re-emits it as light, but with a much longer delay, ranging from fractions of a second to many hours, because the excited electrons become temporarily trapped in a metastable energy state before eventually releasing their energy. Glow-in-the-dark materials rely on phosphorescence to continue emitting light well after the original light source is removed.

quantum · intro

Photoelectric Effect

The emission of electrons from a metal surface when light of sufficiently high frequency shines on it, a phenomenon that could not be explained by classical wave theories of light. Albert Einstein explained the effect in 1905 by proposing that light consists of discrete packets of energy called photons, and that an electron is emitted only if a single photon carries enough energy to overcome the metal's binding energy, regardless of the light's overall intensity.

quantum · intro

Photon

The elementary particle that carries the electromagnetic force and constitutes light and all other forms of electromagnetic radiation. Photons have no rest mass, always travel at the speed of light in a vacuum, and carry energy proportional to their frequency, as described by Planck's relation. Photons exhibit both particle-like and wave-like behavior, a duality central to quantum mechanics.

particle · intermediate

Pion

The lightest type of meson, composed of an up or down quark paired with an up or down antiquark, playing a historically important role as the particle proposed by Hideki Yukawa to explain the strong force that holds atomic nuclei together at short range. Pions come in three varieties, distinguished by their electric charge, and are unstable, decaying quickly into lighter particles such as muons and neutrinos.

cosmology · intro

Planck Satellite

A European Space Agency space observatory launched in 2009 that mapped the cosmic microwave background across the full sky with unprecedented precision, measuring tiny temperature and polarization fluctuations left over from the early universe. Data from the Planck satellite has provided some of the most precise measurements to date of key cosmological parameters within the standard Lambda-CDM model, including the universe's composition, geometry, and expansion history.

quantum · intro

Planck's Constant

A fundamental physical constant that sets the scale of quantum effects, relating the energy of a photon to its frequency, with an extremely small value of approximately 6.626 times ten to the minus thirty-four joule-seconds. Introduced by Max Planck in 1900 to explain blackbody radiation, this constant appears throughout quantum mechanics, including in the Heisenberg uncertainty principle and the de Broglie wavelength relation.

thermo · intermediate

Planck's Law

A formula, derived by Max Planck in 1900, that precisely describes the intensity of electromagnetic radiation emitted by a blackbody at each wavelength as a function of its temperature. By assuming that energy could only be emitted or absorbed in discrete packets, later called quanta, Planck's law successfully matched experimental observations where earlier classical theories had failed, marking the historical birth of quantum theory.

astro · intro

Planet

A celestial body that orbits a star, is massive enough for its own gravity to pull it into a roughly spherical shape, and has cleared the neighborhood around its orbit of other significant debris, according to the definition adopted by the International Astronomical Union in 2006. Planets are distinguished from smaller bodies like asteroids and dwarf planets, such as Pluto, which fails to meet the orbital-clearing criterion.

thermo · intro

Plasma

A state of matter consisting of a gas that has been heated or energized enough that a significant fraction of its atoms have been ionized, separating electrons from nuclei and creating a mix of free-moving charged particles. Plasma conducts electricity and responds strongly to electric and magnetic fields, unlike ordinary gases. It is often called the fourth state of matter and makes up the vast majority of visible matter in the universe, including the interiors and atmospheres of stars.

waves · intermediate

Polarization

A property of transverse waves, especially light, describing the orientation of the wave's oscillation relative to its direction of travel. Unpolarized light contains waves oscillating in all directions perpendicular to its motion, while polarized light has its oscillations restricted to a single plane or a specific rotating pattern. Polarizing filters, used in sunglasses and photography, block light oscillating in unwanted directions to reduce glare.

particle · intro

Positron

The antiparticle of the electron, having the same mass as an electron but a positive electric charge instead of a negative one. Positrons are produced in certain types of radioactive decay, in pair production from high-energy photons, and are used in medical imaging through positron emission tomography, commonly known as a PET scan. When a positron meets an electron, the pair annihilates into gamma-ray photons.

mechanics · intro

Potential Energy

Stored energy that an object possesses because of its position, configuration, or state, which has the potential to be converted into kinetic energy or another form of energy. Gravitational potential energy depends on an object's height within a gravitational field, while elastic potential energy depends on the deformation of a spring or similar material. Potential energy is always defined relative to a chosen reference point.

mechanics · intro

Power (Physics)

The rate at which energy is transferred, converted, or work is done, measured in watts, where one watt equals one joule of energy per second. Power is distinct from energy itself; a small amount of energy delivered very quickly, or a large amount delivered slowly, can both correspond to significant or modest power depending on the timescale involved. Power ratings are used to compare the performance of engines, electrical appliances, and biological systems alike.

mechanics · intro

Pressure

The amount of force applied perpendicular to a surface per unit area, measured in pascals, where one pascal equals one newton per square meter. Pressure explains why a sharp knife cuts more easily than a dull one, since the same force is concentrated over a much smaller area, and why deep-sea creatures can withstand the immense hydrostatic pressure found at great ocean depths. Pressure plays a central role in fluid mechanics, thermodynamics, and atmospheric science.

nuclear · intro

Proton

A subatomic particle with a positive electric charge equal in magnitude to that of the electron, found in the nucleus of every atom alongside neutrons, and defining the chemical identity of an element through its count, the atomic number. Protons are composed of three quarks, two up quarks and one down quark, bound together by the strong nuclear force via gluons, and are classified as baryons.

astro · intro

Pulsar

A rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles, which sweep across space like a lighthouse beam as the star spins. If one of these beams happens to point toward Earth periodically, the pulsar appears to flash, or pulse, at extremely regular intervals, sometimes hundreds of times per second. Pulsars were first discovered in 1967 and have since been used as extraordinarily precise natural clocks for testing general relativity.

quantum · intro

Quantum

The smallest discrete amount, or fundamental unit, of a given physical property or interaction, such as a single photon representing a quantum of light or electromagnetic energy. The term comes from the recognition, foundational to quantum mechanics, that many physical quantities, such as energy in bound systems, are not continuous but instead come only in specific discrete amounts. This quantization contrasts sharply with the smooth, continuous quantities assumed in classical physics.

quantum · intermediate

Quantum Electrodynamics

Also called: QED

The quantum field theory describing how light and matter interact, specifically how photons mediate the electromagnetic force between charged particles like electrons. Developed in the late 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, it is one of the most precisely tested theories in all of physics, with predictions confirmed by experiment to more than ten significant figures in some cases.

quantum · intermediate

Quantum Entanglement

A quantum mechanical phenomenon in which two or more particles become linked such that the quantum state of each particle cannot be described independently of the others, even when the particles are separated by large distances. Measuring a property of one entangled particle instantly determines the corresponding property of its partner, a correlation that Einstein famously described as spooky action at a distance, though it does not allow faster-than-light communication of information.

quantum · intermediate

Quantum Field Theory

A theoretical framework that combines quantum mechanics with special relativity, describing particles as excitations, or ripples, in underlying fields that permeate all of space. Every fundamental particle, such as the electron or photon, corresponds to a specific quantum field, and interactions between particles arise from these fields coupling to one another. Quantum field theory forms the mathematical foundation of the Standard Model of particle physics.

quantum · intro

Quantum Mechanics

The branch of physics describing the behavior of matter and energy at the smallest scales, such as atoms and subatomic particles, where classical physics breaks down. Quantum mechanics introduces concepts such as wave-particle duality, quantized energy levels, and inherent probabilistic uncertainty, replacing the deterministic predictions of classical mechanics with probabilities described by a mathematical object called the wavefunction. It underlies the operation of lasers, transistors, and much of modern chemistry.

quantum · intermediate

Quantum Number

A number that specifies the value of a quantized, discrete property of a quantum system, such as the energy, angular momentum, or spin of an electron within an atom. A set of quantum numbers uniquely identifies the state of a particle within a system, and the allowed combinations are restricted by rules such as the Pauli exclusion principle. Quantum numbers explain the structure of the periodic table and the arrangement of electrons in atomic orbitals.

quantum · intermediate

Quantum Tunneling

A quantum mechanical phenomenon in which a particle has a nonzero probability of passing through an energy barrier that it would not have enough energy to overcome according to classical physics. Tunneling arises from the wave-like nature of matter described by quantum mechanics and is essential to processes such as nuclear fusion in stars, alpha decay of radioactive nuclei, and the operation of devices like scanning tunneling microscopes and certain transistors.

particle · intermediate

Quark

A fundamental particle that combines with other quarks to form composite particles called hadrons, such as protons and neutrons, and is never observed alone due to a property of the strong force called confinement. There are six types, or flavors, of quark, ranging from the light up and down quarks that make up ordinary matter to the much heavier charm, strange, top, and bottom quarks. Quarks carry fractional electric charge and interact through the strong force via gluons.

astro · intro

Quasar

An extremely luminous, compact region at the center of a distant galaxy, powered by a supermassive black hole actively accreting surrounding gas and dust, which heats up and radiates enormous amounts of energy across the electromagnetic spectrum as it spirals inward. Quasars can outshine the combined light of every star in their host galaxy and are among the most distant and energetic objects observable, offering a glimpse into the early universe.

em · intro

Radio Wave

The lowest-energy, longest-wavelength form of electromagnetic radiation, with wavelengths ranging from about a millimeter to hundreds of kilometers. Radio waves are widely used for communication, including broadcast radio, television, mobile phones, and Wi-Fi, because they travel long distances, pass through many materials, and can be generated and detected relatively easily using antennas. Astronomers also use radio telescopes to study emissions from distant galaxies, pulsars, and the cosmic microwave background.

nuclear · intro

Radioactive Decay

The spontaneous transformation of an unstable atomic nucleus into a different, more stable configuration, releasing energy and particles in the process, such as alpha particles, beta particles, or gamma rays. Each radioactive isotope decays at a statistically predictable rate, characterized by its half-life, though the exact moment any individual nucleus decays is fundamentally random and cannot be predicted. Radioactive decay is used in applications ranging from power generation to medical treatment and dating ancient materials.

thermo · intermediate

Random Walk

A mathematical model describing a path made up of a series of random steps, in which each step's direction is independent of the previous ones, used to describe processes such as the diffusion of particles, stock price fluctuations, and the motion of a molecule undergoing Brownian motion. Over many steps, a random walk tends to spread out gradually from its starting point in a statistically predictable way, even though each individual step is unpredictable.

astro · intro

Redshift

A shift of light or other electromagnetic radiation toward longer, redder wavelengths, caused by the relative motion of a source moving away from an observer, by the expansion of space itself over cosmic distances, or by light climbing out of a strong gravitational field. Cosmological redshift, observed in the light from distant galaxies, provides key evidence for the ongoing expansion of the universe, with more distant galaxies generally showing greater redshift.

optics · intro

Reflection

The bouncing back of a wave, such as light or sound, when it encounters a boundary between two different media instead of passing through. For light hitting a smooth, flat surface, the angle at which the wave reflects equals the angle at which it strikes the surface, a relationship known as the law of reflection. Mirrors rely on highly efficient, regular reflection, while rough surfaces produce diffuse reflection, scattering light in many directions.

optics · intro

Refraction

The bending of a wave, most commonly light, as it passes from one medium into another with a different density or optical property, caused by a change in the wave's speed at the boundary. The amount of bending depends on the angle of incidence and the relative refractive indices of the two media, as described by Snell's law. Refraction is responsible for phenomena such as a straw appearing bent in a glass of water and the focusing action of lenses.

optics · intro

Refractive Index

A dimensionless number describing how much a material slows down and bends light compared to its speed in a vacuum, calculated as the speed of light in a vacuum divided by the speed of light within the material. A higher refractive index means light travels more slowly through the material and bends more sharply when entering or leaving it. Refractive index generally varies slightly with wavelength, a phenomenon responsible for the dispersion of light into colors by a prism.

cosmology · intermediate

Reionization

A major transition in the universe's history during which the neutral hydrogen gas that filled intergalactic space became ionized once again, as ultraviolet radiation from the first generations of stars and galaxies stripped electrons from hydrogen atoms throughout the cosmos. This process is believed to have occurred gradually, completing roughly one billion years after the standard hot early expansion phase, and it fundamentally changed how light could travel through intergalactic space.

em · intro

Resistance (Electrical)

A measure of how strongly a material opposes the flow of electric current through it, measured in ohms, arising from collisions between moving charge carriers and the atoms of the material. Resistance depends on a material's inherent resistivity, as well as its length, cross-sectional area, and temperature. According to Ohm's law, resistance relates the voltage across a component to the current flowing through it.

waves · intermediate

Resonance

A phenomenon in which a system experiences dramatically increased amplitude of oscillation when it is driven by an external force at, or very near, its natural frequency. Resonance explains why a wine glass can shatter when exposed to a sound at exactly its resonant frequency, and why soldiers historically break step when crossing bridges, to avoid inadvertently driving the structure at a dangerous resonant frequency. It is exploited deliberately in musical instruments and radio tuning circuits.

relativity · intermediate

Rest Mass

Also called: invariant mass

The mass of an object as measured in a reference frame where the object is at rest, also called invariant mass, since it remains the same value regardless of the observer's relative motion, unlike an object's total relativistic energy, which does depend on relative velocity. Rest mass is the quantity that appears in Einstein's mass-energy equivalence relation for a stationary object, and it represents the intrinsic mass a particle has independent of its motion.

nuclear · intro

Rutherford Scattering

A landmark experiment conducted in 1909 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford, in which alpha particles were fired at a thin sheet of gold foil. Most particles passed through with little deflection, but a small fraction bounced back at large angles, an unexpected result that led Rutherford to conclude that atoms contain a small, dense, positively charged nucleus, replacing the earlier plum pudding model of atomic structure.

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.