Glossary items by topic: Physics

Absolute Zero

The coldest possible temperature, at which all molecular motion stops. On the Kelvin temperature scale, this temperature is the zero point (0 K), which is equivalent to –273° C and –460° F.

Alpha Process

A process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements. For example, when a carbon nucleus captures an alpha particle, a heavier oxygen nucleus is formed.


The size of a wave from the top of a wave crest to its midpoint.

Angular Momentum

A property that an object, such as a planet revolving around the Sun, possesses by virtue of its rotation or circular motion. An object’s angular momentum cannot change unless some force acts to speed up or slow down its circular motion. This principle, known as conservation of angular momentum, is why an object can indefinitely maintain a circular motion around an axis of revolution or rotation.


Matter made up of elementary particles whose masses are identical to their normal-matter counterparts but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron, with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart, both particles are annihilated and energy is released.


The smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds to a unique chemical element determined by the number of protons in its nucleus.

Atomic Nucleus

The positively charged core of an atom consisting of protons and (except for hydrogen) neutrons, and around which electrons orbit.

Celsius (Centigrade) Temperature Scale

A temperature scale on which the freezing point of water is 0° C and the boiling point is 100° C.

Chemical Compound

A pure substance consisting of atoms or ions of two or more different elements. The elements are in definite proportions. A chemical compound usually possesses properties unlike those of its constituent elements. For example, table salt (the common name for sodium chloride) is a chemical compound made up of the elements chlorine and sodium.

Chemical Evolution

The chemical (i.e., pre-biological) changes that transformed simple atoms and molecules into the more complex chemicals needed for the origin of life. For example, hydrogen atoms in the cores of stars combine through nuclear fusion to form the heavier element helium.

Collisional Process

An event involving a collision of objects; for example, the excitation of a hydrogen atom when it is hit by an electron.


The visual perception of light that enables human eyes to differentiate between wavelengths of the visible spectrum, with the longest wavelengths appearing red and the shortest appearing blue or violet.

Conservation of Energy And Mass

A fundamental law of physics, which states that the total amount of mass and energy in the universe remains unchanged. However, mass can be converted to energy, and vice versa.


The transfer of heat through a liquid or gas caused by the physical upwelling of hot matter. The heat transfer results in the circulation of currents from lower, hotter regions to higher, cooler regions. An everyday example of this process is boiling water. Convection occurs in the Sun and other stars.


The ratio of the mass of an object to its volume. For example, water has a density of one gram of mass for every milliliter of volume.


A special form of hydrogen (an isotope called “heavy hydrogen”) that has a neutron as well as a proton in its nucleus.

Doppler Effect

The change in the wavelength of sound or light waves caused when the object emitting the waves moves toward or away from the observer; also called Doppler Shift. In sound, the Doppler Effect causes a shift in sound frequency or pitch (for example, the change in pitch noted as an ambulance passes). In light, an object’s visible color is altered and its spectrum is shifted toward the blue region of the spectrum for objects moving toward the observer and toward the red for objects moving away.

Electromagnetic Force

A fundamental force that governs all interactions among electrical charges and magnetism. Essentially, all charged particles attract oppositely charged particles and repel identically charged particles. Similarly, opposite poles of magnets attract and like magnetic poles repel.


The science dealing with the physical relationship between electricity and magnetism. The principle of an electromagnet, a magnet generated by electrical current flow, is based on this phenomenon.


A negatively charge elementary particle that typically resides outside the nucleus of an atom but is bound to it by electromagnetic forces. An electron’s mass is tiny: 1,836 electrons equals the mass of one proton.

Electron Volt (eV)

A unit of energy that is equal to the energy that an electron gains as it moves through a potential difference of one volt. This very small amount of energy is equal to 1.602 * 10–19 joules. Because an electron volt is so small, engineers and scientists sometimes use the terms MeV (mega-million) and GeV (giga-billion) electron volts.


A substance composed of a particular kind of atom. All atoms with the same number of protons (atomic numbers) in the nucleus are examples of the same element and have identical chemical properties. For example, gold (with 79 protons) and iron (with 26 protons) are both elements, but table salt is not because it is made from two different elements: sodium and chlorine. The atoms of a particular element have the same number of protons in the nucleus and exhibit a unique set of chemical properties. There are about 90 naturally occurring elements on Earth.

Elementary Particles

Particles smaller than atoms that are the basic building blocks of the universe. The most prominent examples are photons, electrons, and quarks.

Escape Velocity

The minimum velocity required for an object to escape the gravity of a massive object.

Event Horizon

The spherical outer boundary of a black hole. Once matter crosses this threshold, the speed required for it to escape the black hole’s gravitational grip is greater than the speed of light.

Excited State

A greater-than-minimum energy state of any atom that is achieved when at least one of its electrons resides at a greater-than-normal distance from its parent nucleus.

Fahrenheit Temperature Scale

A temperature scale on which the freezing point of water is 32° F and the boiling point is 212° F.


A nuclear process that releases energy when heavyweight atomic nuclei break down into lighter nuclei. Fission is the basis of the atomic bomb.


The flow of fluid, particles, or energy through a given area within a certain time. In astronomy, this term is often used to describe the rate at which light flows. For example, the amount of light (photons) striking a single square centimeter of a detector in one second is its flux.


Describes the number of wave crests passing by a fixed point in a given time period (usually one second). Frequency is measured in Hertz (Hz).


A nuclear process that releases energy when light atomic nuclei combine to form heavier nuclei. Fusion is the energy source for stars like our Sun.

Geosynchronous Orbit

Also known as geostationary. An orbit in which an object circles the Earth once every 24 hours, moving at the same speed and direction as the planet’s rotation. The object remains nearly stationary above a particular point, as observed from Earth. The International Ultraviolet Explorer (IUE) and some weather satellites are examples of satellites in geosynchronous orbit.

Gravitational Constant (G)

A value used in the calculation of the gravitational force between objects. In the equation describing the force of gravity, “G” represents the gravitational constant and is equal to 6.672 * 10–11 Nm2/kg2.

Gravitational Instability

A condition that occurs when an object’s inward-pulling gravitational forces exceed the outward-pushing pressure forces, thus causing the object to collapse on itself. For example, when the pressure forces within an interstellar gas cloud cannot resist the gravitational forces that act to compress the cloud, then the cloud collapses upon itself to form a star.

Gravity (Gravitational Force)

The attractive force between all masses in the universe. All objects that have mass possess a gravitational force that attracts all other masses. The more massive the object, the stronger the gravitational force. The closer objects are to each other, the stronger the gravitational attraction.

Ground State

The minimum energy state of an atom that is achieved when all of its electrons have the lowest possible energy and therefore are as close to the nucleus as possible.


The amount, degree, or quantity of energy passing through a point per unit time. For example, the intensity of light that Earth receives from the Sun is far greater than that from any other star because the Sun is the closest star to us.

Inverse Square Law

A law that describes any quantity, such as gravitational force, that decreases with the square of the distance between two objects. For example, if the distance between two objects is doubled, then the gravitational force exerted between them is one-fourth as strong. Likewise, if the distance to a star is doubled, then its apparent brightness is only one-fourth as great.


An atom with one or more electrons removed (or added), giving the atom a positive (or negative) charge.


The process by which ions are produced, typically by collisions with other atoms or electrons, or by absorption of electromagnetic radiation.


An atom of a given element having a particular number of neutrons in the nucleus. Isotopes of a given element differ in the numbers of neutrons within the nucleus. Adding or subtracting a neutron from the nucleus changes an atom’s mass but does not affect its basic chemical properties.

Kelvin Scale

The temperature scale most commonly used in science, on which absolute zero is the lowest possible value. On this scale, water freezes at 273 K and boils at 373 K.

Kilometer (km)

A measure of distance in the metric system equal to 1000 meters or about 0.6 of a mile.

Kinetic Energy

The energy that an object has by virtue of its motion.

Lyman Limit

A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the Lyman limit is exceedingly difficult to detect.

Magnetic Field

A region of space in which magnetic forces may be detected or may affect the motion of an electrically charged particle. As with gravity, magnetism has a long-range effect and magnetic fields are associated with many astronomical objects.


A measure of the total amount of matter contained within an object.

Matter-Antimatter Annihilation

A highly efficient energy-generation process in which equal amounts of matter and antimatter collide and destroy each other, thus producing a burst of energy.

Molecular Velocity

The average speed of the molecules in a gas of a given temperature.


A tightly knit group of two or more atoms bound together by electromagnetic forces among the atoms’ electrons and nuclei. For example, water (H2O) is two hydrogen atoms bound with one oxygen atom. Identical molecules have identical chemical properties.


A neutral, weakly interacting elementary particle having a very tiny mass. Stars like the Sun produce more than 200 trillion trillion trillion neutrinos every second. Neutrinos from the Sun interact so weakly with other matter that they pass straight through the Earth as if it weren’t there.

Neutrino Detector

A device designed to detect neutrinos.


A neutral (no electric charge) elementary particle having slightly more mass than a proton and residing in the nucleus of all atoms other than hydrogen.

Non-Thermal Radiation

Radiation that is not produced from heat energy — for example, radiation released when a very fast-moving charged particle (such as an electron) interacts with a magnetic force field. Because the electron’s velocity in this case is not related to the gas temperature, this process has nothing to do with heat.

Nuclear Transformation

The process by which an atomic nucleus is transformed into another type of atomic nucleus. For example, by removing an alpha particle from the nucleus, the element radium is transformed into the element radon.


The degree to which light is prevented from passing through an object or a substance. Opacity is the opposite of transparency. As an object’s opacity increases, the amount of light passing through it decreases. Glass, for example, is transparent and most clouds are opaque.

Periodic Table (of the Elements)

A chart of all the known chemical elements arranged according to the number of protons in the nucleus (also known as the atomic number). Elements with similar properties are grouped together in the same column.

Photoelectric Effect

The release of electrons from a solid material when it is struck by radiant energy, such as visible or ultraviolet light, X-rays, or gamma rays.


A packet of electromagnetic energy, such as light. A photon is regarded as a charge-less, mass-less particle having an indefinitely long lifetime.

Planck Curve

The graphical representation of the mathematical relationship between the frequency (or wavelength) and intensity of radiation emitted from an object by virtue of its heat energy.


A substance composed of charged particles, like ions and electrons, and possibly some neutral particles. Our Sun is made of plasma. Overall, the charge of a plasma is electrically neutral. Plasma is regarded as an additional state of matter because its properties are different from those of solids, liquids, and normal gases.

Potential Energy

The energy of an object owing to its position in a force field or its internal condition, as opposed to kinetic energy, which depends on its motion. Examples of objects with potential energy include a diver on a diving board and a coiled spring.


A positively charged elementary particle that resides in the nucleus of every atom.

Proton-Proton Chain

A series of nuclear events occurring in the core of a star whereby hydrogen nuclei (protons) are converted into helium nuclei. This process releases energy.


A basic building block of protons, neutrons, and other elementary particles.

RADAR (Radio Detection and Ranging)

A method of detecting, locating, or tracking an object by using beamed, reflected, and timed radio waves. RADAR also refers to the electronic equipment that uses radio waves to detect, locate, and track objects.

Radiative Process

An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light converts the energy of that radiation into electrical potential energy.


The spontaneous decay of certain rare, unstable, atomic nuclei into more stable atomic nuclei. A natural by-product of this process is the release of energy.


A theory of physics that describes the dynamical behavior of matter and energy. The consequences of relativity can be quite strange at very high velocities and very high densities. A direct result of the theory of relativity is the equation E = mc2, which expresses a relationship between mass (m), energy (E), and the speed of light (c).


The orbital motion of one object around another. The Earth revolves around the Sun in one year. The moon revolves around the Earth in approximately 28 days.


The spin of an object around its central axis. Earth rotates about its axis every 24 hours. A spinning top rotates about its center shaft.

Shock Wave

A high-pressure wave that travels at supersonic speeds. Shock waves are usually produced by an explosion.


The four-dimensional coordinate system (three dimensions of space and one of time) in which physical events are located.

Speed Of Light (c)

The speed at which light (photons) travels through empty space is roughly 3 * 108 meters per second or 300 million meters per second.

Strong Force

The force that binds protons and neutrons within atomic nuclei and is effective only at distances less than 10—13 centimeters.


A measure of the amount of heat energy in a substance, such as air, a star, or the human body. Because heat energy corresponds to motions and vibrations of molecules, temperature provides information about the amount of molecular motion occurring in a substance.

Thermal Radiation

Radiation released by virtue of an object’s heat, namely, the transfer of heat energy into the radiative energy of electromagnetic waves. Examples of thermal radiation are sunlight, the orange glow of an electric range, and the light from in incandescent light bulb.


Unstable and disorderly motion, as when a smooth, flowing stream becomes a churning rapid.


The speed of an object moving in a specific direction. A car traveling at 35 miles per hour is a measurement of speed. Observing that a car is traveling 35 miles per hour due north is a measurement of velocity.


A vibration in some media that transfers energy from one place to another. Sound waves are vibrations passing in air. Light waves are vibrations in electromagnetic fields.


The distance between two wave crests. Radio waves can have lengths of several feet; the wavelengths of X-rays are roughly the size of atoms.

Weak Force

The force that governs the change of one kind of elementary particle into another. This force is associated with radioactive processes that involve neutrons.

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