Physics Glossary
Working definitions for the vocabulary you'll keep meeting.
A working glossary of physics vocabulary. Each term gets a one-sentence working definition and, where we cover it in depth, a link to the article that goes further.
A
Acceleration — The rate at which an object's velocity changes. Measured in m/s². When you press the gas pedal, you accelerate; when you brake, you also accelerate (just in the opposite direction).
Antimatter — Particles with the same mass as ordinary matter but opposite charge. When matter meets antimatter, both annihilate into pure energy.
Astrophysics — The application of physics to objects in space — stars, planets, black holes, galaxies.
Atom — The smallest unit of an element that retains its chemical properties. Made of a nucleus (protons and neutrons) surrounded by electrons.
B
Big Bang — The cosmological model in which the universe began as an extremely hot, dense state about 13.8 billion years ago and has been expanding ever since.
Black Hole — A region of spacetime where gravity is so strong that nothing — not even light — can escape. See: Top 10 Mind-Boggling Physics Phenomena.
Boson — A class of particles that includes the force carriers (photons, W and Z bosons, gluons) and the Higgs.
C
Conservation Law — A physical quantity (energy, momentum, charge) that stays constant in an isolated system. The most powerful tools in physics.
Cosmic Microwave Background (CMB) — The faint glow of radiation left over from the Big Bang, observable today as microwaves coming from every direction. See: Top 10 Mind-Boggling Physics Phenomena.
Cosmology — The study of the universe as a whole — its origin, evolution, and large-scale structure.
D
Dark Energy — The unknown form of energy thought to be causing the accelerating expansion of the universe. About 68% of the universe by mass-energy.
Dark Matter — Invisible matter that doesn't interact with light but exerts gravitational pull. About 27% of the universe. See: Top 10 Mind-Boggling Physics Phenomena.
Doppler Effect — The change in frequency of a wave as the source and observer move relative to each other. Why an ambulance siren sounds higher when it's coming toward you and lower when it's moving away.
E
Electromagnetic Spectrum — The full range of electromagnetic radiation, from radio waves (long wavelength, low frequency) through microwaves, infrared, visible light, ultraviolet, X-rays, to gamma rays.
Electron — A subatomic particle with negative charge that orbits the nucleus of an atom. Mass is about 1/1836 that of a proton.
Energy — The capacity to do work. Comes in many forms (kinetic, potential, thermal, chemical, electromagnetic, nuclear) which can be converted into each other but never destroyed.
Entanglement — A quantum effect in which two particles become correlated such that measuring one instantly determines the state of the other, no matter the distance. See: Quantum Mechanics Made Simple.
Entropy — A measure of disorder or the number of microscopic arrangements consistent with a macroscopic state. The Second Law of Thermodynamics says it never decreases in an isolated system.
Escape Velocity — The minimum speed needed to escape a gravitational field. Earth's is about 11.2 km/s. See: The Science Behind Space Exploration.
Event Horizon — The boundary around a black hole inside which nothing can escape.
F
Field — A physical quantity (electric, magnetic, gravitational, quantum) defined at every point in space. The modern view: particles are excitations of fields.
Force — Anything that causes an object to change its motion. Measured in newtons. The four fundamental forces are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
Frequency — How many oscillation cycles occur per second. Measured in hertz (Hz).
G
General Relativity — Einstein's 1915 theory that gravity is the curvature of spacetime caused by mass and energy. Predicts black holes, gravitational waves, and the expansion of the universe.
Gravitational Waves — Ripples in spacetime produced by accelerating massive objects (especially merging black holes or neutron stars). First directly detected in 2015.
Gravity — The force of attraction between any two objects with mass. Vastly weaker than the other three fundamental forces, but it dominates at astronomical scales because it always attracts and never cancels.
H
Heat — Thermal energy in transit from a hotter region to a colder one. Distinct from temperature.
Higgs Boson — The particle associated with the Higgs field; gives other particles their mass. Discovered at CERN in 2012. See: Top 10 Mind-Boggling Physics Phenomena.
K
Kinetic Energy — Energy associated with motion. KE = ½mv² for a non-relativistic object.
L
Light — Electromagnetic radiation, especially in the wavelength range humans can see. Travels at exactly 299,792,458 m/s in vacuum.
M
Mass — A measure of how much matter is in an object, and how strongly it resists being accelerated. Related to energy via E = mc².
Mass-Energy Equivalence — Einstein's discovery that mass and energy are interchangeable: E = mc². A small amount of mass converts to a huge amount of energy.
Momentum — Mass times velocity. Conserved in isolated systems. p = mv classically; relativistic versions are more complicated.
N
Newton's Laws — The three foundational laws of classical mechanics. (1) An object in motion stays in motion unless acted on by a force. (2) F = ma. (3) For every action there is an equal and opposite reaction.
Nuclear Fusion — The process by which atomic nuclei combine to form heavier nuclei, releasing energy. Powers the sun.
Nuclear Fission — The splitting of an atomic nucleus into smaller pieces, releasing energy. The basis of nuclear reactors and atomic weapons.
O
Orbit — The curved path of one object around another due to gravity. Conic sections — ellipses, parabolas, or hyperbolas depending on energy.
P
Particle — A small localized object. In modern physics, a quantum of a field.
Photon — A particle of light. Also the force carrier for electromagnetism.
Planck Constant — The fundamental constant of quantum mechanics. h ≈ 6.626 × 10⁻³⁴ J·s. Sets the scale at which quantum effects matter.
Potential Energy — Energy stored due to position or configuration — gravitational, elastic, chemical, electrical.
Proton — A positively charged particle in the atomic nucleus. Made of three quarks.
Q
Quantum Mechanics — The physics of matter and energy at atomic and subatomic scales. See: Quantum Mechanics Made Simple.
Quantum Tunneling — A quantum effect in which a particle passes through an energy barrier it classically couldn't cross. Underlies flash memory and nuclear fusion in stars.
Quark — An elementary particle that combines with others to form protons, neutrons, and other composite particles.
R
Radiation — The emission of energy as waves or particles. Includes electromagnetic radiation (light, X-rays) and particle radiation (alpha, beta).
Relativity — Two related theories by Einstein: special (1905, motion at high speeds) and general (1915, gravity as spacetime curvature).
S
Schrödinger's Cat — A thought experiment by Erwin Schrödinger illustrating the strangeness of quantum superposition. A cat in a sealed box is, in some sense, both alive and dead until the box is opened.
Special Relativity — Einstein's 1905 theory that the laws of physics are the same in all non-accelerating frames, and that the speed of light is constant.
Spacetime — The four-dimensional fabric of the universe — three space dimensions plus one time dimension — that general relativity describes as curving in response to mass and energy.
Standard Model — The current theoretical framework describing the known elementary particles and three of the four fundamental forces (excluding gravity).
Superconductivity — The ability of certain materials, cooled to low temperatures, to conduct electricity without resistance. See: Top 10 Mind-Boggling Physics Phenomena.
Superposition — The quantum principle that a particle can be in multiple states simultaneously until measured.
T
Thermodynamics — The physics of heat, energy, and work. Three laws: energy is conserved, entropy never decreases, absolute zero is unreachable.
Time Dilation — The slowing of time for an object moving at high speeds relative to an observer, predicted by special relativity. See: Top 10 Mind-Boggling Physics Phenomena.
U
Uncertainty Principle — Heisenberg's principle that certain pairs of properties (position and momentum, energy and time) cannot both be known with arbitrary precision.
V
Velocity — Speed plus direction. A vector quantity.
W
Wave-Particle Duality — Quantum particles behave like waves in some experiments and like particles in others. See: Quantum Mechanics Made Simple.
Wavelength — The distance between successive crests of a wave.
Work — Force times distance moved in the direction of the force. The mechanical way energy gets transferred.
See Also
- What Is Physics? — Big-picture orientation.
- Branches of Physics — Field-by-field guide.
- Physics resources hub — All articles.