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• ## a

• absolute constant
An absolute constant is a number that has the same value wherever it appears. Examples:

π (pi) has the value 3.14159…(...) - Read More

• absolute space
(Absolute space has recently been re-conceptualized and re-named as space that has "background-independence.")

• absolute time
The concept of absolute time was a foundational concept of physics as articulated by Isaac Newton in the 1600’s. It was(...) - Read More
• acceleration
Abbreviation: a

In physics, acceleration is speeding up, slowing down, or changing direction. Acceleration contrasts(...) - Read More

• action-at-a-distance

Action-at-a-distance is the creation of an effect without physically touching. An example is magnetism: a magnet pulls(...) - Read More

• algebraic constant
An algebraic constant is a symbol that represents an unchanging number or is simply a number in an algebra equation. The(...) - Read More
• alpha
In quantum physics, the ancient Greek letter α, alpha, represents an important constant of nature. Alpha is .00729735256…(...) - Read More
• amplitude of a wave
The amplitude of a wave is its height, that is, half the distance from trough to crest.

Carbon atoms on the surface of a crystal of graphite.(...) - Read More

• atomic number
The atomic number of an atom is the number of protons in its nucleus. For example, an atom of carbon has six protons and has(...) - Read More
• ## b

• B
B is the symbol that represents the strength and direction, in any particular location, of the magnetic field due to the(...) - Read More
• bare number
For “bare number,” see “dimensionless number.” - Read More
• baryon
Baryons are a category of subatomic particles in the nucleus of the atom. Protons and neutrons are examples of baryons.(...) - Read More
• baryonic matter
All the matter that we ordinarily deal with in everyday life is baryonic matter. Baryonic matter is composed of atoms which(...) - Read More
• Bohr atom
Even though it’s now considered obsolete, Niels Bohr took a huge step forward in quantum physics, when in 1913, he proposed(...) - Read More
• ## c

• complex number
This is an example of a complex number: 3 + 4i. It means take 3 and add 4 times i. The letter i is the symbol for the square(...) - Read More
• conservation laws
Physics includes several conservation laws. The most well-known is the Law of Conservation of Energy. This law states that(...) - Read More
• conservation of energy
The Law of Conservation of Energy says that in our universe the amount of energy remains constant. Energy cannot be created(...) - Read More
• conservation of matter
The Law of Conservation of Matter was abandoned as inaccurate by physicists early in the 1900’s. For more information, see(...) - Read More
• conserved quantity
A conserved quantity is something that remains constant in amount over time and cannot be created nor destroyed. Conserved(...) - Read More
• constant

A constant is a number or symbol that always has the same value. There are four types of constants:

• constant of nature
A constant of nature is another way of saying a "physical constant." - Read More
• Coulomb's Law
Coulomb’s Law is a formula that allows us to calculate the electric force generated between two electrical charges.(...) - Read More
• ## d

• dark energy
Dark energy is the name given to an unexplained aspect of the universe. Dark energy has been postulated as an explanation(...) - Read More
• dark matter
"Dark matter" is a name that’s been given to a substance which many scientists believe may exist. If it exists, this(...) - Read More
• dimensionless number
Pi is an example of a dimensionless number. A dimensionless number doesn’t imply any measurement units. Pi is just 3.14, not(...) - Read More
• ## e

• e
e is the symbol for Euler's number. Similarly to pi, e is a constant which occurs repeatedly throughout nature. It has a(...) - Read More
• Euler's number (e)
Euler's number, similarly to pi, is a constant which occurs repeatedly throughout nature. It is symbolized as e. It has a(...) - Read More
• ## f

• fine-structure constant
See alpha, , α.  - Read More
• frequency
Frequency is the number of repeated motions in a period of time. These motions can be up and down, like a hammer, or back(...) - Read More
• frequency and wavelength
Frequency and wavelength are both properties of waves. They are closely related, so this article discusses the two(...) - Read More
• function
The term “function” is important in the field of quantum mechanics because is it the basis of the key term “wave function.”(...) - Read More
• ## i

• i
The letter i is the symbol for the square root of -1. In other words i = √-1. The symbol i often appears in the equations of(...) - Read More
• imaginary number
An imaginary number is one that includes the square root of -1, written √(-1). While one might think that such a number(...) - Read More
• ion
An ion is formed when an atom gains or loses an electron. An atom which has lost one or more electrons has a positive(...) - Read More
• ionize
To ionize is to convert a substance from atomic or molecular form to ions.

An ion is a type of atom that has an imbalance(...) - Read More

• isotope
Isotopes are a special kind of atom. Atoms with the same number of protons but different numbers of neutrons are isotopes of(...) - Read More
• ## j

• Joule
A Joule is a unit for measuring energy. It's the amount of energy needed to lift a medium tomato one meter off the surface(...) - Read More
• ## l

• L
(Symbol is L. Also called “rotational momentum” or “moment of momentum.”) Angular momentum is the momentum or oomph which an(...) - Read More
• Law of Conservation of Energy
The Law of Conservation of Energy says that the amount of energy in an isolated system remains constant. Energy cannot be(...) - Read More
• Law of Conservation of Matter
The Law of Conservation of Matter was abandoned as inaccurate by physicists early in the 1900’s. For more information, see(...) - Read More
• linear momentum
Linear momentum is the oomph something has due to having mass that’s in motion. If a football player were to ram into a(...) - Read More
• lines of force
“Lines of force” can best be explained with an illustration. The photo shows the lines of magnetic force created with a(...) - Read More
• ## m

• mathematical constant
Pi and e are examples of mathematical constants. A mathematical constant has the same value no matter which mathematical(...) - Read More
• metal
A metal is a kind of matter, usually derived from rocks found in nature. Examples of metals include iron, aluminum, and(...) - Read More
• moment
“Moment,” as used in physics, means the amount of force experienced by an object due to the amount of force applied at a(...) - Read More
• moment of force
Moment of force is also called torque. “Moment,” as used in physics, means the amount of force experienced by an object due(...) - Read More
• moment of momentum
In classical physics, moment of momentum is the momentum or oomph which an object has as it rotates about an axis. It is the(...) - Read More
• momentum
(Abbreviated p. Momentum is the oomph something has due to having mass that’s in motion. Let’s say you just happen to be(...) - Read More
• ## n

• normal matter
Scientists call the everyday matter of our world, such as tables and chairs, “normal matter.” Normal matter is made up of(...) - Read More
• ## o

• operator
In mathematics, “operator” has two main definitions. 1) An operator is a mathematical symbol, for example +, that represents(...) - Read More
• orbital angular momentum
(Symbol: ɭ or L). Electrons have two types of rotational motion: orbital angular momentum and spin. Orbital angular momentum(...) - Read More
• ordinary matter
Scientists call the everyday matter of our world, such as tables and chairs, “ordinary matter.” Ordinary matter is made up(...) - Read More
• oscillation
Definition (1): An oscillation is a repetitive back-and-forth motion. A pendulum swinging back and forth is an everyday(...) - Read More
• oscillator
An oscillator is something that oscillates. Something that vibrates. See oscillation for more information.

Billiard balls epitomize particles that follow Newton's Laws of(...) - Read More

• period
If you were standing on a dock and water waves were coming at you, the period would be the time interval between waves. In(...) - Read More
• Periodic Table of the Elements
The Periodic Table of the Elements lists all the elements, over 100 of them, in a specific sequence. This includes the(...) - Read More
• physical constant
The constant c, the speed of light in a vacuum, is an example of a physical constant. It is a constant of nature, always(...) - Read More
• pi
Pi is symbolized π and pronounced like "pie." It is the mathematical symbol for the number 3.14159…, which goes on(...) - Read More
• probability amplitude
“Probability amplitude” is a term used in quantum physics. It’s a number that appears in Schrodinger’s Wave Equation. Let’s(...) - Read More
• ## q

• QFT
See Quantum Field Theory. - Read More
• quantities of dimension one
For “quantities of dimension one,” see “dimensionless number.” - Read More
• quantum
One definition of “quantum” is: a tiny packet of energy at the atomic level. A quantum is the smallest possible unit of(...) - Read More
• quantum angular momentum
For quantum angular momentum, see angular momentum. - Read More
• Quantum Field Theory
Quantum Field Theory (QFT) is the current theory of how atomic and subatomic particles behave. It is the most up-to-date(...) - Read More
• quantum number
Electrons have a few handfuls of properties. Four have been selected as the electron’s “quantum numbers.” The quantum(...) - Read More
• quantum spin
• ## r

• renormalization
Renormalization is a technique for achieving greater precision in certain physics theories.* In quantum mechanics,(...) - Read More
• rotational momentum
Rotational momentum is the same thing as “angular momentum.” It is the momentum or oomph that an object has when it rotates(...) - Read More
• ## s

• scanning tunneling microscope
A scanning tunneling microscope (STM) is a type of electron microscope. It is much more powerful than an ordinary microscope(...) - Read More
• singularity
“Singularity” has a physics definition and a mathematical definition. Both definitions are given here. They fit together(...) - Read More
• Sommerfeld's Constant
See alpha, α. - Read More
• spin
Spin is a property of subatomic and atomic particles. While spin was originally thought of as a particle twirling on its(...) - Read More
• Stern-Gerlach device
(Also called a “Stern-Gerlach machine.”) The Stern-Gerlach (SG) device detects the “quantum spin” of atoms and subatomic(...) - Read More
• ## t

• Tesla
A Tesla is a unit for measuring the strength of magnetism in any particular location. A Tesla is approximately the strength(...) - Read More
• torque
For torque, see “moment of force.” - Read More
• ## w

• wave
A wave is a movement that propagates through a medium. The accompanying animation demonstrates the motion of a seismic wave,(...) - Read More
• wave equation
This article starts with the nature of wave equations in classical physics and moves into a brief description of the(...) - Read More
• waveform
The waveform is the shape of a wave. Below is a graph on paper of a waveform representing the sound waves created by blowing(...) - Read More
• wavelength
Wavelength is a property of waves. It is the distance from wave crest to wave crest. Or from trough to trough—same distance.(...) - Read More
• ## α

• α
In quantum physics, the ancient Greek letter α, alpha, represents an important constant of nature. Alpha is .00729735256…(...) - Read More
• ## π

• π
See entry for pi. - Read More