# Glossary

### Annihilation

When corresponding particles of matter and antimatter (for example, an electron and a positron) contact each other, they annihilate each other, converting their masses into energy in the form of high-energy photons. The total energy produced can be calculated by using the equation $$E = mc^2$$ .

### Antimatter

Every particle of matter in the Standard Model has a corresponding particle of antimatter. Anti-particles can be thought of as equal and opposite “reflections” of their matter counterparts. Matter and antimatter can annihilate with each other, producing large amounts of energy. Surprisingly, matter and antimatter are not exact reflections of each other; there are some slight asymmetries between them.

### Big Bang

Cosmologists believe that the universe started much smaller than it is now — a nearly infinitesimal point. The unfathomably rapid expansion during the first moments of our universe’s existence is called the Big Bang.

### Big Crunch

According to the Big Crunch theory for the ultimate end of the universe, the expansion of the universe will reverse, leading to a massive collapse. This would effectively be like an inverted Big Bang.

### Big Freeze

A likely outcome for our universe is that as stars continue to die, the universe will slowly cool down, eventually decaying into a cloud of particles. This cosmological outcome is called the Big Freeze.

### Big Rip

A dramatic theory for the end of the universe, called the Big Rip, predicts that the universe will expand at an ever increasing rate. This would cause every atom in the universe to be violently torn apart.

### Black Hole

One fascinating consequence of General Relativity is the fact that some objects have such intense gravity that nothing can escape them — not even light! These extraordinary occurrences are called black holes. The extreme warping of spacetime around black holes leads to some bizarre phenomena.

### Boson

Bosons are the fundamental particles responsible for mediating the interactions (forces) between the fermions. They include the photon, gluon, and weak bosons, as well as the Higgs boson. Some physicists have speculated the existence of the graviton; however, it is unknown if such a particle exists.

Telescopes that are sensitive to electromagnetic radiation in the microwave range can detect a faint signal from all directions in outer space. This signal, called the Cosmic Microwave Background Radiation (or CMBR for short), is a faint remnant from the early history of the universe. It is significant evidence for the Big Bang.

### Cosmology

Cosmology is a field of physics devoted to understanding questions about the large-scale structure and evolution of the universe. The generally accepted theory for the origin of the universe is the Big Bang. Three possible outcomes for the future of the universe are the Big Crunch, the Big Rip, and the Big Freeze. Cosmology also encompasses the study of dark matter and dark energy.

### Dark Energy

For some reason, despite the predictions of our current theories, the universe is expanding faster and faster in all directions. This strange fact has been attributed to dark energy, an elusive substance currently at the cutting edge of physics research.

### Dark Matter

Physicists have measured unmistakable gravitational effects in the universe where there isn’t any matter to cause it. This invisible substance, called dark matter, can be found in galaxies in large quantities, and has an unknown constitution. One theory proposes that dark matter is composed of weakly interacting massive particles (WIMPs).

### Double Slit Experiment

The double slit experiment is a simple experiment used to demonstrate the foundations of quantum mechanics. By shining a beam of light through a pair of thin slits, physicists can directly observe wave-particle duality and wavefunction collapse.

### Electromagnetic Force

The electromagnetic force is one of the four fundamental forces. It governs the interactions between charged particles (like charges repel, opposites attract) and is mediated by the photon. All of the quarks, plus the electron, muon, and tauon, all interact with the electromagnetic force. The weak bosons are also charged, and therefore themselves also interact with the electromagnetic force.

### Escape Velocity

Escape velocity is a measure of the speed necessary for a projectile to escape the gravitational pull of an object. For example, the earth’s escape velocity is roughly 11 kilometers per second or about 7 miles per second; therefore (discounting air resistance) an object traveling at that speed from the surface of the earth would be able to escape the earth’s gravity and never fall back again.

### Event Horizon

If you come too close to a black hole, there comes a point when it is physically impossible to return. The imaginary sphere around a black hole that marks this boundary is called the event horizon. Nothing that crosses the event horizon can ever return, including light; therefore, the event horizon marks the region of space that we perceive as black around a black hole.

### Fermion

Fermions are the fundamental particles that make up all of the matter in the universe. Fermions are themselves divided into two categories: quarks and leptons.

### Fundamental Forces

In classical mechanics, forces are thought of as a push or a pull on an object. But in modern physics, forces are thought of more generally as interactions between particles. Although we experience many different forces in our everyday lives, these can all ultimately be explained by just four fundamental interactions between the particles making up everything around us: the weak force, the strong force, electromagnetism, and gravity. The first three are incorporated into the Standard Model of particle physics, while gravity is explained by Einstein’s theory of General Relativity.

### General Relativity

Albert Einstein’s theory of General Relativity (GR) replaced Isaac Newton’s theory of Classical Gravity as the most accurate description of gravitation. Specifically, General Gravity transforms the concept of gravity from a force between objects to a property of spacetime. Black holes are one of the many fascinating consequences of General Relativity. (See also Special Relativity.)

### Gluon

The gluon is a boson that mediates the strong interaction. Gluons are hypothesized to interact with each other to form transient composite structures called glueballs.

### Graviton

The graviton is a theorized boson intended to include the gravitational force in the Standard Model. No graviton has ever been experimentally observed and its theoretical underpinnings remain uncertain.

### Gravity

Gravity is classically considered a force between massive objects governed by the equation F = Gm1m2/r^2 first discovered by Isaac Newton. However, General Relativity reframes gravity as a property of spacetime. Gravity is one of the four fundamental forces.

### Higgs Boson

The Higgs boson is a boson in the Standard Model that is responsible for giving the fundamental particles their characteristic masses.

### Lepton

Leptons are the collection of fermions that don’t interact with the Strong force. There are six leptons: electron, muon, tauon, electron neutrino, muon neutrino, and tauon neutrino.

### Photon

The photon is a boson in the Standard Model responsible for mediating the electromagnetic force. Photons can have different energy levels ranging from low-energy x-rays to high-energy gamma rays. A particular range of photons within the visible light spectrum can be seen by humans as the colors of the rainbow.

### Quantum Entanglement

One fascinating consequence of quantum mechanics is the strange phenomenon of entanglement, in which observing the state of one entangled particle determines the state of another in a completely different location. This effect can be harnessed by quantum computers to perform powerful calculations.

### Quantum Mechanics

The theory of quantum mechanics was constructed in response to the surprising results of many experiments, including the famous double slit experiment. Quantum mechanics is currently the most accurate theory for describing the motions and interactions of fundamental particles. Among the many surprising predictions of quantum mechanics are wave-particle duality and quantum entanglement.

### Quark

Quarks are particles in the standard model that interact with the Strong Force (gluons). There are six types of quarks: up, down, charm, strange, top, and bottom.

### Reductionism

Our everyday experiences are varied and complex, and yet we would like to understand them all in terms of simpler components. Everything we see and interact with on a daily basis is constructed of atoms from the periodic table; atoms are themselves made of smaller particles.

### Relative Motion

The principle of relative motion is the intuitive idea, originally proposed by Galileo Galilei, that there is no “universal reference frame.” It is meaningless to describe an object as traveling at some speed without reference to some background frame of reference.

The Schwarzschild radius for an object is, simply put, the radius the object would have to be squeezed into to form a black hole. This can be calculated by determining the radius at which the escape velocity exceeds the speed of light. The formula for the Schwarzschild radius is $$r=2GM/c^2$$ . It is named after physicist Karl Schwarzschild.

### Spacetime

Spacetime is, quite simply, space and time put together. We can think of spacetime as a four-dimensional continuum with three spatial dimensions and one time dimension.

### Special Relativity

Albert Einstein’s theory of Special Relativity replaced Isaac Newton’s classical laws of motion as the most accurate description of moving objects. Specifically, it helps us understand how objects behave when they are traveling near the speed of light. You can learn about some of the many fascinating consequences of Special Relativity in this video. (See also General Relativity.)

### Speed Of Light

The speed of light is exactly 299,792,458 meters per second (roughly $$3.00 \times 10^8 m/s$$ ). It is abbreviated with the letter c for the Latin word “celer” (meaning speed). According to Special Relativity, nothing can travel faster than the speed of light.

### Standard Model

The Standard Model of particle physics records the properties of all of the fundamental particles that make up everything we see around us. The Standard Model organizes all particles into two categories: fermions and bosons.

### String Theory

String Theory is a field of study at the cutting edge of theoretical physics research. It proposes that all fundamental particles are actually composed of unimaginably tiny vibrating strings. The theory proposes that the possible vibrations of these strings are what cause the properties of different particles.

### Strong Force

The strong force (or strong interaction) is a fundamental interaction mediated by the gluon. It affects all particles with a strong charge, also called a color charge. There are three color charges: red, green, and blue (plus their anti-equivalents, antired, antigreen, and antiblue). All quarks can take color charges and thus interact with the strong force.

### Theory Of Everything

An enticing goal of physics is to construct a single theory underlying all of our experiences. Such a “Theory of Everything” has not yet been developed, but with every new insight into the way our world works, physicists grow closer step by step. One central theme in physicists’ efforts to build a theory of everything is unification.

### Unification

By unifying separate theories into more general conglomerates, we may eventually combine all of our different models of the universe into a single universal theory. For example, James Clerk Maxwell unified the separate theories of electricity and magnetism into a single powerful theory of electromagnetism.

### Wave Particle Duality

Wave-particle duality is the name for the fact that tiny particles of matter and energy behave like waves in some ways and like particles in others. This behavior is modeled in quantum mechanics by a mathematical construct called a wavefunction.

### Wavefunction

In quantum mechanics, wavefunction is a mathematical representation of a particle. Taking the square modulus of the wavefunction produces a probabilistic prediction about the location of a particle, but does not indicate a single definite position.

### Weak Boson

The weak bosons are the mediators of the weak force. They come in three varieties: W+, W-, and Z0.

### Weak Force

The weak force (or weak interaction) is a fundamental interaction between all particles in the standard model. The weak interaction dictates the way fundamental particles can decay into other fundamental particles (for example, the phenomenon of radioactive beta decay). The weak force is mediated by the weak bosons.

### Weakly Interacting Massive Particle Wimp

Weakly Interacting Massive Particles (or WIMPs for short) are theorized particles that may explain the strange phenomenon of dark matter. If they exist, then the only fundamental forces they could interact with would be the gravitational force and the weak force.