Physics Glossary

What this covers

The glossary currently lists 144 terms drawn from every published concept and equation entry, across classical mechanics, thermodynamics, electromagnetism, quantum mechanics, relativity, nuclear and particle physics, condensed matter, and astrophysics and cosmology. Each term shows its home domain.

A

Accretion Discs · Astrophysics & Cosmology

An accretion disc is a rotating disc of matter spiralling inward toward a central massive object, releasing gravitational energy as heat and radiation via viscous friction.

Active Galactic Nuclei · Astrophysics & Cosmology

An active galactic nucleus is a compact, highly luminous region at a galaxy's centre, powered by accretion onto a central supermassive black hole.

Adiabatic Process · Thermodynamics

An adiabatic process is a thermodynamic process in which no heat is exchanged with the surroundings, so any work done changes the internal energy; for a reversible ideal-gas process, pV^γ stays constant.

Age of the Universe · Astrophysics & Cosmology

The age of the universe is the time elapsed since the Big Bang, currently estimated at about 13.8 billion years from fits of the standard ΛCDM model to cosmic microwave background and other data.

Alpha Decay · Nuclear & Particle

Alpha decay is a type of radioactive decay in which an unstable nucleus emits an alpha particle (a helium-4 nucleus), reducing its atomic number by 2 and mass number by 4.

Atomic Nucleus · Nuclear & Particle

The atomic nucleus is the dense central core of an atom, composed of protons and neutrons bound together by the strong nuclear force.

B

Band Theory · Condensed Matter

Band theory describes how electron energy levels in a solid form continuous bands separated by forbidden gaps, explaining conductors, insulators, and semiconductors.

BCS Theory · Condensed Matter

BCS theory is the microscopic quantum theory of conventional superconductivity, showing that Cooper-paired electrons condense into a coherent quantum state.

Beta Decay · Nuclear & Particle

Beta decay is a type of radioactive decay in which a neutron converts to a proton (or vice versa) within a nucleus, emitting a beta particle and a neutrino.

Big Bang Theory · Astrophysics & Cosmology

The Big Bang theory describes the observable universe as having expanded from an extremely hot, dense initial state approximately 13.8 billion years ago.

Biot-Savart Law · Electromagnetism

The Biot-Savart law gives the magnetic field produced by a small segment of steady current, analogous to Coulomb's law for electric fields.

Black Holes · Relativity

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape once inside the event horizon.

Black Holes (Astrophysical) · Astrophysics & Cosmology

An astrophysical black hole is a collapsed region of matter whose gravity prevents light from escaping, formed and observed under real astrophysical conditions.

Bloch's Theorem · Condensed Matter

Bloch's theorem states that an electron wavefunction in a periodic potential can be written as a plane wave modulated by a function with the lattice's periodicity.

Born Interpretation · Quantum Mechanics

The Born interpretation states that the squared magnitude of a quantum system's wavefunction gives the probability density of finding the system at a given position.

Bose-Einstein Condensation · Condensed Matter

Bose-Einstein condensation is a state of matter formed when a large fraction of bosons occupy the same lowest-energy quantum state.

Bravais Lattices · Condensed Matter

A Bravais lattice is one of 14 distinct three-dimensional lattice types that classify all possible periodic crystal geometries.

Brillouin Zone · Condensed Matter

The first Brillouin zone is the Wigner-Seitz cell of the reciprocal lattice: the set of wavevectors closer to the origin than to any other reciprocal lattice point, within which every distinct electron or phonon state of a crystal can be labelled.

C

Capacitance · Electromagnetism

Capacitance is a measure of a system's ability to store electric charge per unit of applied potential difference.

Carnot Cycle · Thermodynamics

The Carnot cycle is an idealised reversible thermodynamic cycle consisting of two isothermal and two adiabatic processes; it achieves the maximum possible efficiency, 1 − Tc/Th, for any engine operating between two temperatures.

Chandrasekhar Limit · Astrophysics & Cosmology

The Chandrasekhar limit is the maximum mass, about 1.4 solar masses, that a stable non-rotating white dwarf can have, above which electron degeneracy pressure can no longer support it against gravity.

Conduction · Thermodynamics

Thermal conduction is the transfer of heat through a material from hotter to colder regions by microscopic interactions, such as atomic vibrations and free electrons, without bulk motion of the material; its rate is given by Fourier's law, q = -k∇T.

Conservation of Energy · Classical Mechanics

The law of conservation of energy states that the total energy of an isolated system remains constant over time; energy can be transformed from one form to another but cannot be created or destroyed.

Convection · Thermodynamics

Convection is the transfer of heat by the bulk movement of a fluid, either natural convection driven by buoyancy differences from temperature variations, or forced convection driven by external means such as fans or pumps.

Cooper Pairs · Condensed Matter

A Cooper pair is a loosely bound pair of electrons formed via phonon-mediated attraction, behaving as a composite boson underlying conventional superconductivity.

Cosmic Microwave Background · Astrophysics & Cosmology

The cosmic microwave background is the remnant thermal radiation from approximately 380,000 years after the Big Bang, when the universe first became transparent to light.

Cosmological Constant · Astrophysics & Cosmology

The cosmological constant (Lambda, Λ) is a constant term in Einstein's field equations of general relativity that is equivalent to a uniform, unchanging energy density filling all of space, and that acts as a repulsive influence on cosmic scales.

Coulomb's Law · Electromagnetism

Coulomb's law states that the electrostatic force between two point charges is proportional to the product of their magnitudes and inversely proportional to the square of the distance between them.

Crystal Structure · Condensed Matter

Crystal structure is the ordered, repeating three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid.

Curved Spacetime · Relativity

Curved spacetime is the concept that mass and energy curve the geometry of spacetime, and that this curvature is what is perceived as gravity.

D

Dark Energy · Astrophysics & Cosmology

Dark energy is a form of energy uniformly permeating space, driving the observed accelerating expansion of the universe.

Dark Matter · Astrophysics & Cosmology

Dark matter is a form of matter that does not emit or absorb light, inferred from its gravitational effects on visible matter and the universe's large-scale structure.

De Broglie Hypothesis · Quantum Mechanics

The de Broglie hypothesis proposes that all matter has an associated wavelength inversely proportional to its momentum.

Density of States · Condensed Matter

The density of states g(E) is the number of quantum states available per unit energy interval (usually per unit volume) at energy E, a quantity that determines how a system fills its states and responds to heat, fields and light.

Double-Slit Experiment · Quantum Mechanics

The double-slit experiment demonstrates that particles sent one at a time through two narrow slits accumulate into a wave-like interference pattern, evidence of wave-particle duality and quantum superposition.

E

Effective Mass · Condensed Matter

The effective mass is the mass that an electron or hole appears to have when it accelerates under an external force inside a crystal, determined by the curvature of the energy band: 1/m* = (1/hbar^2) d^2E/dk^2.

Electric Charge · Electromagnetism

Electric charge is a fundamental, conserved property of matter that causes it to experience and exert electromagnetic forces, quantised in integer multiples of the elementary charge.

Electric Field · Electromagnetism

The electric field is a vector field representing the force per unit positive test charge at each point in space, produced by electric charges.

Electric Potential · Electromagnetism

Electric potential is the electric potential energy per unit charge at a point in an electric field, measured in volts.

Energy Bands · Condensed Matter

Energy bands are continuous ranges of allowed electron energies in a crystalline solid, formed when the discrete energy levels of many interacting atoms broaden, and separated by forbidden ranges called band gaps.

Enthalpy · Thermodynamics

Enthalpy is the thermodynamic state function H = U + pV, equal to internal energy plus the product of pressure and volume; its change equals the heat absorbed by a system in a process at constant pressure with only pressure-volume work.

Entropy · Thermodynamics

Entropy is a state function that measures the number of microscopic configurations consistent with a system's macroscopic state; thermodynamically, its change is the reversible heat transferred divided by temperature, and statistically S = kB ln W.

Equipartition Theorem · Thermodynamics

The equipartition theorem states that in classical thermal equilibrium, every independent quadratic term in a system's energy (such as a component of kinetic energy or a harmonic potential) has an average value of one-half kBT.

Equivalence Principle · Relativity

The equivalence principle states that the effects of a uniform gravitational field are locally indistinguishable from the effects of uniform acceleration.

Exchange Interaction · Condensed Matter

The exchange interaction is a quantum mechanical effect, from the Pauli exclusion principle and electrostatic repulsion, determining parallel or antiparallel spin alignment.

Exoplanets · Astrophysics & Cosmology

An exoplanet is a planet that orbits a star other than the Sun; more than six thousand have been confirmed, mostly through transit and radial-velocity measurements.

F

Faraday's Law · Electromagnetism

Faraday's law of induction states that a changing magnetic flux through a circuit induces an electromotive force proportional to the rate of change of that flux.

Fate of the Universe · Astrophysics & Cosmology

The fate of the universe refers to the possible long-term outcomes of cosmic evolution, determined mainly by the nature of dark energy; current data favour continued accelerating expansion, but the outcome is not certain.

Fermi Energy · Condensed Matter

The Fermi energy is the energy of the highest occupied electron quantum state in a system of fermions at absolute zero.

Ferromagnetism · Condensed Matter

Ferromagnetism is a form of magnetism in which neighbouring atomic magnetic moments spontaneously align in parallel below a critical (Curie) temperature.

First Law of Thermodynamics · Thermodynamics

The first law of thermodynamics states that energy is conserved: the change in a system's internal energy equals the heat added to the system minus the work done by the system, ΔU = Q − W.

Fundamental Forces · Nuclear & Particle

The four fundamental forces are gravity, electromagnetism, the strong force, and the weak force — the basic interactions through which all particles influence one another.

G

Galaxy Clusters · Astrophysics & Cosmology

A galaxy cluster is a gravitationally bound system of hundreds to thousands of galaxies embedded in hot gas and a dominant dark matter halo, with total masses of about 10^14 to 10^15 solar masses.

Galaxy Formation · Astrophysics & Cosmology

Galaxy formation is the process by which galaxies formed from early universe density fluctuations, driven primarily by dark matter's gravitational collapse.

Gamma Radiation · Nuclear & Particle

Gamma radiation is high-energy electromagnetic radiation emitted by an atomic nucleus transitioning from an excited state to a lower energy state.

Gauss's Law · Electromagnetism

Gauss's law states that the net electric flux through any closed surface is proportional to the total electric charge enclosed by that surface.

Gibbs Free Energy · Thermodynamics

The Gibbs free energy G = H − TS is the thermodynamic potential that, at constant temperature and pressure, decreases in every spontaneous process and is minimised at equilibrium; its change also gives the maximum non-expansion work available, such as electrical work.

Gravitational Lensing (Astronomy) · Astrophysics & Cosmology

Gravitational lensing is the bending of light by the gravity of a massive object, as predicted by general relativity, which astronomers use to measure masses, magnify distant sources and detect unseen objects.

Gravitational Time Dilation · Relativity

Gravitational time dilation is the phenomenon in which time passes more slowly for a clock deeper within a gravitational field, as measured relative to a clock further away.

Gravitational Waves (Astronomy) · Astrophysics & Cosmology

Gravitational wave astronomy is the observational discipline of detecting spacetime ripples produced by accelerating massive objects such as merging black holes and neutron stars.

H

Half-Life · Nuclear & Particle

Half-life is the time required for half of a sample of a radioactive substance to decay.

Heat · Thermodynamics

Heat is energy transferred from one system to another because of a temperature difference between them, by conduction, convection or radiation; it is a process quantity, not something a body contains.

Heat Engines · Thermodynamics

A heat engine is a device that converts part of the heat flowing from a high-temperature source to a low-temperature sink into mechanical work by taking a working substance through a repeated thermodynamic cycle.

Heat Transfer · Thermodynamics

Heat transfer is the movement of thermal energy from regions of higher temperature to regions of lower temperature by three mechanisms: conduction, convection and thermal radiation.

Helmholtz Free Energy · Thermodynamics

The Helmholtz free energy F = U − TS is the thermodynamic potential that measures the maximum work obtainable from a system at constant temperature; at fixed temperature and volume, a system in equilibrium minimises F.

Hertzsprung-Russell Diagram · Astrophysics & Cosmology

The Hertzsprung-Russell diagram plots stellar luminosity against surface temperature, revealing distinct evolutionary groupings including the main sequence.

Hubble Constant · Astrophysics & Cosmology

The Hubble constant (H0) is the present-day rate at which the universe expands, expressed as the recession speed per unit distance, with current measurements falling between about 67 and 73 km/s per megaparsec.

I

Ideal Gas Law · Thermodynamics

The ideal gas law, pV = nRT, is the equation of state of a hypothetical gas of non-interacting point particles, relating pressure, volume, amount of substance and absolute temperature; real gases follow it closely at low pressure and high temperature.

Inflation · Astrophysics & Cosmology

Cosmic inflation is a theoretical period of extremely rapid exponential expansion proposed to have occurred a fraction of a second after the Big Bang.

Insulators · Condensed Matter

An electrical insulator is a material with very high electrical resistivity, typically because its valence band is completely full and separated from the empty conduction band by a large band gap, leaving almost no mobile charge carriers at ordinary temperatures.

Internal Energy · Thermodynamics

Internal energy is the total energy contained in a thermodynamic system due to the random motion and interactions of its particles, excluding the kinetic and potential energy of the system as a whole; it is a state function whose change is given by the first law.

Isobaric Process · Thermodynamics

An isobaric process is a thermodynamic process at constant pressure; the heat added equals the change in enthalpy, part of it doing expansion work, and for an ideal gas volume is proportional to absolute temperature.

Isochoric Process · Thermodynamics

An isochoric process is a thermodynamic process at constant volume; no pressure-volume work is done, so all heat added changes the internal energy, and for an ideal gas pressure is proportional to absolute temperature.

Isothermal Process · Thermodynamics

An isothermal process is a thermodynamic process in which the temperature of the system stays constant; for an ideal gas the internal energy is then unchanged, so the heat absorbed equals the work done.

K

Kinetic Theory of Gases · Thermodynamics

The kinetic theory of gases explains the macroscopic properties of gases, such as pressure, temperature, viscosity and diffusion, as consequences of the random motion and collisions of very large numbers of molecules.

L

Large-Scale Structure · Astrophysics & Cosmology

Large-scale structure is the largest-scale organisation of matter in the universe, forming a filamentary cosmic web of galaxies separated by voids.

Latent Heat · Thermodynamics

Latent heat is the heat absorbed or released by a substance during a change of phase, such as melting, freezing, boiling or condensation, which occurs at constant temperature and pressure.

Length Contraction · Relativity

Length contraction is the relativistic effect in which the measured length of an object is shorter along its direction of motion, as measured by a relatively moving observer.

LIGO/Virgo · Astrophysics & Cosmology

LIGO and Virgo are laser interferometer gravitational-wave observatories that directly detect spacetime distortions from passing gravitational waves.

Linear Momentum · Classical Mechanics

Linear momentum is the product of an object's mass and its velocity, p = mv — a vector quantity whose total is conserved in any isolated system.

Lorentz Transformations · Relativity

The Lorentz transformations are equations relating the space and time coordinates of an event as measured in two inertial frames moving at constant relative velocity.

M

Magnetic Field · Electromagnetism

The magnetic field is a vector field produced by moving electric charges and intrinsic magnetic moments, exerting force on other moving charges.

Magnetic Force · Electromagnetism

The magnetic force is the force exerted by a magnetic field on a moving charge or current-carrying conductor.

Magnetism · Condensed Matter

Magnetism in solids arises from electron magnetic moments and their collective alignment in response to, or independent of, an applied field.

Main Sequence · Astrophysics & Cosmology

The main sequence is the long, stable phase of a star's life during which it fuses hydrogen into helium in its core, forming a diagonal band on the Hertzsprung-Russell diagram in which a star's position is set mainly by its mass.

Mass Defect · Nuclear & Particle

Mass defect is the difference between the mass of an assembled nucleus and the sum of the masses of its individual protons and neutrons.

Mass-Energy Equivalence · Relativity

Mass-energy equivalence is the principle that mass and energy are equivalent and mutually convertible, expressed by E = mc².

Maxwell Relations · Thermodynamics

The Maxwell relations are equalities between partial derivatives of thermodynamic quantities, derived from the equality of mixed second derivatives of the thermodynamic potentials; they allow hard-to-measure quantities such as entropy changes to be found from measurable ones such as pressure, volume and temperature.

Maxwell's Equations · Equations Library

Maxwell's equations are four coupled partial differential equations describing how electric and magnetic fields are generated by charges, currents, and each other.

Maxwell-Boltzmann Distribution · Thermodynamics

The Maxwell-Boltzmann distribution gives the probability that a molecule in a classical ideal gas at thermal equilibrium has a given speed; it depends only on the molecular mass and the temperature.

Meissner Effect · Condensed Matter

The Meissner effect is the expulsion of magnetic field lines from the interior of a superconductor below its critical temperature.

Metals · Condensed Matter

A metal is a material with a partially filled or overlapping electron band, allowing efficient electrical and thermal conduction.

Milky Way · Astrophysics & Cosmology

The Milky Way is the barred spiral galaxy containing the Solar System, comprising roughly 100-400 billion stars and an extended dark matter halo.

N

Neutron Stars · Astrophysics & Cosmology

A neutron star is an extremely dense stellar remnant composed almost entirely of neutrons, supported by neutron degeneracy pressure.

Newton's First Law · Classical Mechanics

Newton's first law states that an object remains at rest, or continues moving in a straight line at constant velocity, unless acted upon by a net external force.

Newton's Second Law · Classical Mechanics

Newton's second law states that the net force acting on an object equals the rate of change of its momentum with time; for an object of constant mass, this reduces to F = ma.

Newton's Second Law (F=ma) · Equations Library

Newton's second law equation states that the net force F acting on an object of constant mass m equals the product of that mass and the resulting acceleration a: F = ma.

Newton's Third Law · Classical Mechanics

Newton's third law states that when one body exerts a force on a second body, the second body simultaneously exerts a force of equal magnitude and opposite direction on the first.

Nuclear Binding Energy · Nuclear & Particle

Nuclear binding energy is the energy required to completely separate a nucleus into its individual protons and neutrons, equal via E=mc² to the nucleus's mass defect.

Nuclear Fission · Nuclear & Particle

Nuclear fission is the process in which a heavy atomic nucleus splits into two or more lighter nuclei, releasing energy and typically several free neutrons.

Nuclear Fusion · Nuclear & Particle

Nuclear fusion is the process in which two light atomic nuclei combine to form a heavier nucleus, releasing energy due to increased binding energy per nucleon.

O

Ohm's Law · Electromagnetism

Ohm's law states that the voltage across a conductor is directly proportional to the current flowing through it, V = IR.

P

P-N Junction · Condensed Matter

A p-n junction is the interface between p-type and n-type regions of a semiconductor, where diffusion of carriers creates a depletion region and built-in electric field that allow current to flow easily in one direction but not the other.

Particle in a Box · Quantum Mechanics

The particle in a box is a model of a particle confined to a region of space with infinitely high potential walls, producing quantised energy levels.

Photoelectric Effect · Quantum Mechanics

The photoelectric effect is the ejection of electrons from a material's surface when struck by light of sufficiently high frequency.

Postulates of Special Relativity · Relativity

Special relativity rests on two postulates: the equivalence of inertial frames, and the constancy of the speed of light for all observers.

Pulsars · Astrophysics & Cosmology

A pulsar is a rapidly rotating, highly magnetised neutron star that emits beams of radiation from its magnetic poles, observed from Earth as extremely regular pulses when a beam sweeps across the line of sight.

Q

Q-Value · Nuclear & Particle

The Q-value of a nuclear reaction is the energy released or absorbed, calculated from the mass difference between initial and final particles.

Quantum Entanglement · Quantum Mechanics

Quantum entanglement is a correlation between two or more quantum systems such that their combined state cannot be described as a product of individual states.

Quantum Harmonic Oscillator · Quantum Mechanics

The quantum harmonic oscillator is the quantum mechanical treatment of a particle in a parabolic potential, yielding equally spaced quantised energy levels and a non-zero ground state energy.

Quantum Superposition · Quantum Mechanics

Quantum superposition is the principle that a quantum system can exist in a linear combination of two or more distinct states simultaneously until measured.

Quantum Tunnelling · Quantum Mechanics

Quantum tunnelling is the phenomenon in which a quantum particle has a nonzero probability of passing through a potential energy barrier higher than its own energy.

Quarks · Nuclear & Particle

Quarks are fundamental particles that combine to form composite particles such as protons and neutrons, and are never observed in isolation.

Quasars · Astrophysics & Cosmology

A quasar is an extremely luminous active galactic nucleus in which gas falling onto a supermassive black hole radiates enough energy to outshine its entire host galaxy.

R

Radioactive Decay · Nuclear & Particle

Radioactive decay is the spontaneous process by which an unstable atomic nucleus transforms into a more stable configuration, emitting radiation.

Radioactive Decay Law · Nuclear & Particle

The radioactive decay law is the first-order differential equation dN/dt=-λN and its exponential solution describing the rate at which radioactive nuclei decay.

Red Giants · Astrophysics & Cosmology

A red giant is an evolved star of low or intermediate mass that has exhausted hydrogen in its core and swollen to tens or hundreds of times its former radius, with a cool, reddish surface and a luminosity far above its main-sequence value.

Refrigerators and Heat Pumps · Thermodynamics

A refrigerator or heat pump is a device that uses work to transfer heat from a lower-temperature region to a higher-temperature region, the reverse of a heat engine; its performance is measured by the coefficient of performance, which can exceed 1.

S

Schrödinger Equation · Equations Library

The Schrödinger equation describes how the quantum state (wavefunction) of a physical system evolves over time in response to the system's Hamiltonian.

Schwarzschild Metric · Relativity

The Schwarzschild metric is the exact solution to the Einstein field equations describing spacetime around a spherically symmetric, non-rotating mass.

Second Law of Thermodynamics · Thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system never decreases; equivalently, heat does not flow spontaneously from a colder to a hotter body, and no cyclic engine can convert heat entirely into work.

Semiconductors · Condensed Matter

A semiconductor is a material with electrical conductivity between conductors and insulators, controllable by doping with impurity atoms.

Specific Heat Capacity · Thermodynamics

Specific heat capacity is the amount of heat required to raise the temperature of one kilogram of a substance by one kelvin; it depends on the substance, its phase, temperature and whether pressure or volume is held constant.

Spin · Quantum Mechanics

Spin is an intrinsic form of angular momentum carried by elementary particles, quantised in half-integer or integer multiples of the reduced Planck constant, with no classical counterpart.

Standard Model · Nuclear & Particle

The Standard Model is the theory describing all known fundamental particles and the electromagnetic, weak, and strong forces governing their interactions.

Stellar Black Holes · Astrophysics & Cosmology

A stellar black hole is a black hole formed by the gravitational collapse of a massive star, with a mass from a few to roughly one hundred solar masses.

Stellar Evolution · Astrophysics & Cosmology

Stellar evolution is the sequence of physical changes a star undergoes over its lifetime, driven primarily by its initial mass.

Stellar Nucleosynthesis · Astrophysics & Cosmology

Stellar nucleosynthesis is the process by which chemical elements are formed through nuclear fusion inside stars and explosive nucleosynthesis in supernovae and neutron star mergers.

Stellar Structure · Astrophysics & Cosmology

Stellar structure is the physics of how a star holds itself together: gravity pulling inward is balanced at every radius by an outward pressure gradient, while energy generated in the core is transported to the surface.

Superconductivity · Condensed Matter

Superconductivity is a phenomenon in which certain materials exhibit zero electrical resistance and expel magnetic fields below a critical temperature.

Supermassive Black Holes · Astrophysics & Cosmology

A supermassive black hole is a black hole with a mass from roughly a hundred thousand to tens of billions of solar masses, found at the centre of most large galaxies.

Supernovae · Astrophysics & Cosmology

A supernova is a powerful stellar explosion occurring from either massive star core collapse or a white dwarf exceeding the Chandrasekhar limit.

T

Temperature · Thermodynamics

Temperature is the physical quantity that is equal for systems in thermal equilibrium and determines the direction of heat flow between them; thermodynamically 1/T = dS/dU, and for an ideal gas it is proportional to the average translational kinetic energy of the particles.

Thermal Expansion · Thermodynamics

Thermal expansion is the tendency of matter to change its size in response to a change in temperature, described for solids by the linear coefficient α through ΔL = αL0ΔT and for volumes by β, approximately 3α for isotropic solids.

Thermal Radiation · Thermodynamics

Thermal radiation is electromagnetic radiation emitted by all matter above absolute zero because of the thermal motion of its charged particles; an ideal black body emits a spectrum fixed only by its temperature, with total power proportional to T^4.

Thermodynamic Efficiency · Thermodynamics

Thermodynamic efficiency describes how effectively a process or device converts energy into a useful form; first-law efficiency compares useful energy output to energy input, while second-law (exergy) efficiency compares performance to the maximum allowed by the second law.

Third Law of Thermodynamics · Thermodynamics

The third law of thermodynamics states that the entropy of a system approaches a constant minimum value as its temperature approaches absolute zero, which for a perfect crystal is zero, and that absolute zero cannot be reached in a finite number of steps.

Time Dilation · Relativity

Time dilation is the relativistic effect in which time is measured to pass more slowly for a clock in relative motion or in a stronger gravitational field.

U

Uncertainty Principle · Quantum Mechanics

The uncertainty principle states that certain pairs of physical properties, most famously position and momentum, cannot both be known to arbitrary precision simultaneously.

V

Van der Waals Equation · Thermodynamics

The van der Waals equation, (p + an²/V²)(V − nb) = nRT, is an equation of state for real fluids that corrects the ideal gas law for intermolecular attraction (a) and finite molecular volume (b), and qualitatively predicts the liquid-gas transition and critical point.

W

Wave Function · Quantum Mechanics

The wave function is a complex-valued function that encodes the complete quantum state of a system; its squared magnitude gives the probability density of measurement outcomes.

Wave-Particle Duality · Quantum Mechanics

Wave-particle duality is the observation that quantum entities such as photons and electrons exhibit both wave-like and particle-like behaviour depending on the experiment performed.

White Dwarfs · Astrophysics & Cosmology

A white dwarf is a compact stellar remnant supported against gravitational collapse by electron degeneracy pressure.

Z

Zeroth Law of Thermodynamics · Thermodynamics

The zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other; it establishes temperature as a well-defined property that thermometers can measure.

About this glossary

Each definition is the canonical definition from the full Codex entry it links to, so the glossary and the entries cannot drift apart. The glossary grows automatically as entries are published. Dedicated long-form glossary pages (300–800 words per term, as set out in the Codex specification) are planned but not yet written; until then, each term links to its full three-level entry.