Standard physics glossary

cosmology · intermediate

Lyman-alpha Forest

A dense series of absorption lines observed in the spectra of distant quasars, produced when light passing through numerous clouds of intervening neutral hydrogen gas at different distances and redshifts absorbs specific wavelengths corresponding to the Lyman-alpha electron transition. The resulting forest-like pattern of absorption lines allows astronomers to map the distribution of diffuse intergalactic gas and probe the large-scale structure of the universe at intermediate redshifts.

mechanics · intermediate

Mach Number

A dimensionless ratio comparing an object's speed to the local speed of sound in the surrounding medium, used especially in aerodynamics. A Mach number less than one indicates subsonic speed, exactly one indicates the speed of sound, and greater than one indicates supersonic speed. Because the speed of sound depends on air temperature and density, the same physical speed corresponds to different Mach numbers at different altitudes.

em · intro

Magnetic Field

A region of space around a magnet, current-carrying wire, or moving charge in which another moving charge or magnetic material would experience a force. Magnetic field lines run from a magnet's north pole to its south pole outside the magnet, and the strength and direction of the field determine how strongly it influences nearby charges and materials. Magnetic fields are inseparably linked to electric fields and moving charges through the theory of electromagnetism.

em · intermediate

Magnetic Flux

A measure of the total magnetic field passing through a given area, calculated as the product of the magnetic field strength and the area, adjusted for the angle at which field lines cross that area. Magnetic flux is central to Faraday's law of electromagnetic induction, since it is the rate of change of magnetic flux through a loop that determines the voltage induced in it. It is measured in webers.

particle · intermediate

Magnetic Monopole

A hypothetical particle that would carry an isolated north or south magnetic pole, unlike all magnets observed so far, which always have both a north and south pole together. Several theories beyond the Standard Model predict that magnetic monopoles should exist, but despite extensive searches, none have ever been detected. Their absence, or extreme rarity, remains an open question connecting particle physics and cosmology.

em · intro

Magnetism

A physical phenomenon produced by the motion of electric charge, resulting in attractive or repulsive forces between certain materials and objects. At the atomic level, magnetism arises from the motion and intrinsic spin of electrons, which act like tiny current loops. Depending on how these atomic magnetic moments interact, materials can display ferromagnetism, paramagnetism, or diamagnetism, producing widely varying magnetic behavior.

optics · intro

Magnification

The ratio of the size of an image produced by an optical system, such as a lens or mirror, to the actual size of the object being viewed, indicating how much larger or smaller the image appears. A magnification greater than one produces an enlarged image, while a value less than one produces a reduced image, and a negative value indicates an inverted image. Magnification depends on the focal length of the optical elements and the distances involved.

mechanics · intro

Mass

A fundamental physical property representing the amount of matter in an object, which determines both its resistance to acceleration under a force, known as inertial mass, and the strength of the gravitational force it exerts and experiences, known as gravitational mass. These two concepts of mass have been measured to be equivalent to extraordinary precision, a fact that underlies Einstein's general theory of relativity. Mass is measured in kilograms and, unlike weight, does not depend on the local strength of gravity.

nuclear · intermediate

Mass Defect

The difference between the total mass of an atomic nucleus's individual protons and neutrons when separate and the actual, slightly smaller mass of the assembled nucleus. This missing mass has been converted into the nuclear binding energy that holds the nucleus together, according to Einstein's mass-energy equivalence. The mass defect explains why nuclear reactions, which rearrange nucleons into more or less tightly bound configurations, can release or require enormous amounts of energy.

nuclear · intro

Mass Number

The total number of protons and neutrons, collectively called nucleons, in an atomic nucleus, denoted by the symbol A. It is used together with the atomic number to identify specific isotopes, written for example as carbon-12 or uranium-235, where the number indicates the mass number. Unlike atomic mass, which accounts for the precise measured mass including binding energy effects, mass number is simply a whole-number count of nucleons.

relativity · intro

Mass-energy Equivalence

The principle, expressed in Einstein's famous equation relating energy, mass, and the speed of light, stating that mass and energy are interchangeable forms of the same underlying physical quantity. A small amount of mass corresponds to an enormous amount of energy, because the conversion factor involves the speed of light squared. This principle explains the vast energy released in nuclear reactions and confirms that even a stationary object has a rest energy associated with its mass.

em · intermediate

Maxwell's Equations

A set of four equations, unified by James Clerk Maxwell in the 1860s, that together describe all classical electric and magnetic phenomena and predict the existence of electromagnetic waves traveling at the speed of light. They describe how electric charges create electric fields, how magnetic fields have no isolated poles, how changing magnetic fields create electric fields, and how electric currents and changing electric fields create magnetic fields. These equations underpin nearly all classical electrical engineering and optics.

mechanics · intro

Mechanical Advantage

The factor by which a simple machine, such as a lever, pulley, or inclined plane, multiplies an input force to produce a larger output force, allowing a smaller effort to move or lift a heavier load. Mechanical advantage comes at the cost of moving the input force over a proportionally greater distance, since energy input must still equal energy output in an ideal, frictionless machine. Real machines have somewhat lower mechanical advantage due to friction losses.

particle · intermediate

Meson

A type of hadron composed of one quark and one antiquark bound together by the strong force, making mesons generally less massive and shorter-lived than baryons, which contain three quarks. Pions and kaons are common examples of mesons. Unlike baryons, mesons are bosons, since combining a quark and an antiquark results in an integer total spin.

relativity · intro

Michelson-Morley Experiment

A famous experiment performed in 1887 by Albert Michelson and Edward Morley, designed to detect the motion of Earth through a hypothesized medium for light called the luminiferous ether by measuring tiny differences in the speed of light traveling in different directions. The experiment consistently found no such difference, a null result that helped pave the way for Einstein's special theory of relativity, which does away with the need for an ether entirely.

em · intro

Microwave

A form of electromagnetic radiation with wavelengths between roughly one millimeter and one meter, longer than infrared but shorter than radio waves. Microwaves are used in radar systems, satellite and mobile phone communication, and microwave ovens, which heat food by causing water molecules to rotate rapidly in response to the oscillating microwave field. The faint cosmic microwave background radiation, a relic of the early universe, also falls within this range.

astro · intro

Milky Way

The barred spiral galaxy that contains our solar system, home to an estimated one hundred to four hundred billion stars along with vast amounts of gas, dust, and dark matter. From Earth, the Milky Way appears as a faint band of light stretching across the night sky, formed by the combined glow of countless distant stars in the galaxy's disk. It is part of the Local Group of galaxies, which includes the Andromeda galaxy.

mechanics · intermediate

Moment of Inertia

A measure of an object's resistance to changes in its rotational motion, analogous to mass in linear motion, depending on both the object's mass and how that mass is distributed relative to the axis of rotation. Mass located farther from the rotation axis contributes more to the moment of inertia than mass located closer to it, which is why a figure skater spins faster when pulling their arms inward. It appears directly in the equation relating torque, moment of inertia, and angular acceleration.

mechanics · intro

Momentum

The product of an object's mass and its velocity, a vector quantity that describes the quantity of motion an object possesses. In the absence of external forces, the total momentum of a system remains constant, a principle known as conservation of momentum, which is fundamental to analyzing collisions and explosions. Momentum is distinct from kinetic energy, since it depends linearly on velocity rather than on velocity squared.

particle · intermediate

Muon

A subatomic particle similar to the electron but roughly 207 times more massive, classified as a lepton and carrying the same electric charge as the electron. Muons are unstable, decaying into an electron and two neutrinos after an average lifetime of about 2.2 microseconds, though time dilation allows muons created by cosmic rays high in the atmosphere to reach Earth's surface before decaying. They are studied extensively in particle physics experiments.

waves · intermediate

Natural Frequency

The frequency at which a physical system tends to oscillate freely when disturbed and left alone, without any continuous external driving force, determined by the system's mass, stiffness, or other physical properties. When an external force is applied at or near a system's natural frequency, the resulting oscillations can grow dramatically in amplitude, a phenomenon known as resonance. Understanding natural frequency is essential in designing structures such as bridges and buildings to avoid dangerous resonant vibrations.

particle · intro

Neutrino

An extremely light, electrically neutral subatomic particle classified as a lepton, produced abundantly in nuclear reactions such as those in the Sun, nuclear reactors, and radioactive decay. Neutrinos interact only through the weak nuclear force and gravity, making them extraordinarily difficult to detect, since they can pass through entire planets with only a tiny chance of interacting. There are three known types, or flavors, of neutrino, and they have been shown to oscillate between these types as they travel.

nuclear · intro

Neutron

A subatomic particle with no net electric charge and a mass slightly greater than that of a proton, found together with protons in the nucleus of atoms. Neutrons are composed of three quarks bound together by the strong force and are classified as baryons. Free neutrons outside a nucleus are unstable and decay into a proton, an electron, and an antineutrino within an average lifetime of about 15 minutes.

astro · intro

Neutron Star

An extremely dense stellar remnant formed when a massive star's core collapses during a supernova explosion, composed almost entirely of tightly packed neutrons and supported against further collapse by neutron degeneracy pressure. A typical neutron star packs more mass than the Sun into a sphere only about 20 kilometers across, giving it an almost unimaginable density. Many neutron stars are observed as rapidly rotating pulsars, emitting regular beams of radiation as they spin.

mechanics · intro

Newton's First Law

Also called: law of inertia

One of Isaac Newton's three laws of motion, stating that an object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by a net external force. Also called the law of inertia, it establishes that force is required to change motion, not to sustain it, correcting the earlier Aristotelian view that a continuous force was needed to keep objects moving.

mechanics · intro

Newton's Law of Gravitation

A law formulated by Isaac Newton stating that every pair of objects with mass attracts each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This single law successfully explained planetary orbits, the tides, and falling objects on Earth under one unified framework, and it remained the best description of gravity until superseded in extreme cases by Einstein's general relativity.

mechanics · intro

Newton's Second Law

One of Isaac Newton's three laws of motion, stating that the net force acting on an object equals the product of its mass and its acceleration, commonly written as force equals mass times acceleration. This law quantitatively links force, mass, and motion, allowing the prediction of an object's future motion if the forces acting on it are known. It forms the mathematical foundation of classical mechanics.

mechanics · intro

Newton's Third Law

One of Isaac Newton's three laws of motion, stating that for every action force one object exerts on a second object, the second object exerts an equal and opposite force back on the first. This pairing of forces always acts on two different objects, never canceling each other out for a single object, and explains phenomena such as rocket propulsion, walking, and the recoil of a fired gun.

mechanics · intro

Normal Force

The contact force exerted by a surface on an object resting on or pressing against it, directed perpendicular to the surface, preventing the object from passing through it. On a flat, horizontal surface, the normal force typically balances the object's weight, but on an inclined or curved surface, its magnitude and direction adjust to match the geometry and any other forces involved. Normal force is a key component in calculating friction, since friction is often proportional to it.

astro · intro

Nova

A sudden, bright outburst of light from a white dwarf star in a binary system, caused by the rapid, explosive nuclear fusion of hydrogen gas that has accumulated on the white dwarf's surface after being pulled from a companion star. Unlike a supernova, a nova does not destroy the star, and the same white dwarf can undergo repeated nova outbursts over time as more material accumulates and ignites.

nuclear · intro

Nuclear Fission

Also called: fission

The splitting of a heavy atomic nucleus, such as uranium-235 or plutonium-239, into two or more smaller nuclei, usually triggered by the absorption of a neutron, and releasing a large amount of energy along with additional neutrons. These released neutrons can trigger further fission events, creating a chain reaction that powers nuclear reactors and, when uncontrolled, nuclear weapons. Fission releases energy because the resulting smaller nuclei are more tightly bound than the original heavy nucleus.

nuclear · intro

Nuclear Fusion

Also called: fusion

The combination of two light atomic nuclei into a single heavier nucleus, releasing a large amount of energy because the resulting nucleus is more tightly bound than the two original nuclei. Fusion powers the Sun and other stars, where immense pressure and temperature force hydrogen nuclei together to form helium. Achieving controlled, net-positive fusion on Earth as a practical energy source remains an active area of research.

nuclear · intro

Nuclear Reactor

A device designed to initiate and control a sustained nuclear fission chain reaction, releasing energy in a regulated manner, primarily used to generate electricity or to produce isotopes for research and medicine. Reactors use control rods to absorb excess neutrons and regulate the reaction rate, and a coolant to transfer the released heat to a turbine system. Reactor designs vary widely, using different fuels, moderators, and coolant systems.

nuclear · intro

Nucleon

A collective term for the particles found in an atomic nucleus, referring to both protons and neutrons, which are structurally similar particles held together by the strong nuclear force. The total number of nucleons in a nucleus is given by the mass number. Nucleons themselves are composite particles, each made of three quarks bound together by gluons.

astro · intermediate

Nucleosynthesis

The process by which new atomic nuclei are formed from pre-existing protons and neutrons, occurring in several distinct settings, including the early hot phase of cosmic expansion, which produced the lightest elements like hydrogen and helium, and the interiors of stars, which fuse progressively heavier elements up to iron. Elements heavier than iron are primarily produced in violent events such as supernovae and neutron star mergers, through rapid neutron capture processes.

nuclear · intro

Nucleus

The small, extremely dense central region of an atom, containing protons and neutrons bound tightly together by the strong nuclear force, and accounting for nearly all of an atom's mass despite occupying only a tiny fraction of its volume. The number of protons in the nucleus determines the chemical element, while the number of neutrons can vary, producing different isotopes. The nucleus was first identified experimentally by Ernest Rutherford in 1911.

em · intro

Ohm's Law

A fundamental relationship in electrical circuits stating that the current flowing through a conductor between two points is directly proportional to the voltage across those two points, with resistance as the constant of proportionality, commonly written as voltage equals current multiplied by resistance. Named after German physicist Georg Ohm, it applies well to many conductors and resistors under normal conditions, though some materials and components deviate from this simple linear relationship.

cosmology · intermediate

Omega Lambda

A dimensionless cosmological parameter representing the ratio of the energy density associated with the cosmological constant, or dark energy, to the critical density needed for a spatially flat universe. Current measurements within the standard Lambda-CDM model place omega lambda at approximately 0.7, making dark energy the dominant component of the universe's total energy content and the primary driver of its accelerating expansion.

cosmology · intermediate

Omega Matter

A dimensionless cosmological parameter representing the ratio of the actual average matter density in the universe, including both ordinary and dark matter, to the critical density needed for a spatially flat universe. Current measurements within the standard Lambda-CDM model place omega matter at approximately 0.3, meaning matter accounts for about 30 percent of the universe's total energy budget, with dark energy making up most of the remainder.

astro · intro

Orbit

The curved path an object follows around another object due to gravitational attraction, such as a planet orbiting the Sun or a satellite orbiting Earth. Stable orbits are typically elliptical, with circular orbits representing a special case, and are governed by the balance between an object's forward velocity and the inward pull of gravity. Kepler's laws and Newton's law of gravitation together provide the mathematical framework for describing and predicting orbital motion.

astro · intro

Orbital Period

The time it takes for an object to complete one full orbit around another object, such as the time for a planet to orbit its star or a moon to orbit its planet. According to Kepler's third law, the square of a planet's orbital period is proportional to the cube of its average distance from the Sun, meaning planets farther out take proportionally much longer to complete an orbit.

astro · intermediate

Orbital Velocity

The speed an object must maintain to stay in a stable orbit around a massive body, resulting from the balance between the object's tendency to move in a straight line and the inward pull of gravity constantly curving its path. Orbital velocity depends on the mass of the central body and the distance from it; objects in lower orbits must travel faster to avoid falling, while those in higher orbits move more slowly.

waves · intro

Oscillation

A repetitive back-and-forth or up-and-down variation of a physical quantity around some equilibrium value, occurring in systems ranging from a swinging pendulum and a vibrating guitar string to alternating electric current and quantum mechanical wavefunctions. Simple harmonic motion is the most basic and mathematically tractable form of oscillation, in which the restoring force is directly proportional to the displacement from equilibrium.

particle · intermediate

Pair Production

A process in which a high-energy photon converts into a particle and its corresponding antiparticle, most commonly an electron and a positron, typically occurring near an atomic nucleus that absorbs some of the photon's momentum. Pair production requires the photon to carry at least enough energy to account for the combined rest mass of the two created particles, following Einstein's mass-energy equivalence. It is the reverse process of particle-antiparticle annihilation.

astro · intro

Parallax

The apparent shift in the position of a nearby object against a distant background, caused by observing it from two different vantage points. Astronomers use parallax to measure distances to relatively nearby stars, by observing a star's apparent position from opposite sides of Earth's orbit around the Sun six months apart and using the known baseline distance to calculate the star's distance through simple trigonometry.

em · intermediate

Paramagnetism

A weak form of magnetism in which a material becomes weakly magnetized in the same direction as an applied external magnetic field, due to unpaired electrons whose magnetic moments partially align with the field. Unlike ferromagnetic materials, paramagnetic materials lose their magnetization almost immediately once the external field is removed, since thermal motion randomizes the alignment of the electron moments. Aluminum and oxygen are common examples of paramagnetic substances.

particle · intro

Particle Accelerator

A machine that uses electric and magnetic fields to accelerate charged particles, such as protons or electrons, to very high speeds and energies, then often collides them together or with a stationary target. Particle accelerators, including large facilities like the Large Hadron Collider, allow physicists to study the fundamental structure of matter and recreate conditions similar to those shortly after the early hot phase of the universe.

quantum · intermediate

Pauli Exclusion Principle

A fundamental rule of quantum mechanics, formulated by Wolfgang Pauli, stating that no two identical fermions, such as electrons within the same atom, can occupy exactly the same quantum state simultaneously. This principle explains the structure of the periodic table, since electrons must fill successively higher energy levels rather than all crowding into the lowest one, and it underlies the degeneracy pressure that supports white dwarf and neutron stars.

mechanics · intro

Pendulum

A weight suspended from a fixed point that swings freely back and forth under the influence of gravity, exhibiting periodic motion that, for small swing angles, closely approximates simple harmonic motion. The period of a simple pendulum depends only on its length and the local gravitational acceleration, not on the mass of the weight or, for small angles, the amplitude of the swing, a property first studied systematically by Galileo.

waves · intro

Period (Wave)

The time it takes for one complete cycle of a repeating wave or oscillation to occur, measured in seconds, and equal to the inverse of the wave's frequency. A wave with a longer period completes fewer cycles per second and therefore has a lower frequency. Period is used interchangeably across many types of periodic motion, from pendulums and orbits to alternating current and vibrating strings.