## Glossary

A selection of unusual terms found on this site.

## Acceleration

Any change in the speed with which an object moves, or the direction in which it moves.

## Active Galactic Nucleus

The central portion of a galaxy which gives off unusually large amounts of energy. These are thought to be powered by supermassive Black Holes.

## Amplitude

The height of the peak of a wave, measured relative to its center. Equivalently, the depth of the trough of a wave.

## Antiparticle

Essentially, the "opposite" of a particle. Every type of matter has a corresponding antiparticle, with the same mass but opposite charge, for example. Other numbers describing the particle will be reversed for the antiparticle.

## Atomic Nucleus

The central part of an atom, which contains Neutrons and Protons. Electrons are usually found around the Nucleus. Strictly speaking, this is the only part of an atom involved in Nuclear Reactions (Fission or Fusion).

## Big Bang

An astrophysical theory of the beginning of the Universe. It suggests that the Universe began in a very tiny region of space, and exploded outward. Astrophysicists believe that this occurred roughly 14 billion years ago. Other astrophysical theories for the beginning of the Universe — like the Braneworld theory — exist, though none is as thoroughly studied and supported by the data as the Big Bang model. Scientists have no idea what came before the Big Bang.

## Big Crunch

Essentially the opposite of the Big Bang, the Big Crunch is one possible fate of the Universe. If the matter and energy of the Universe are not moving outward quickly enough, gravity could pull the Universe in on itself, collapsing it in a final Big Crunch. It is not yet known whether this will happen to our Universe.

## Black Hole

A region of spacetime where the warpage of both space and time (gravity) is so intense that nothing — even light — can ever escape. Objects may fall in to the Black Hole, but once they pass the Event Horizon, they can never escape again. Most Black Holes believed to exist are thought to be formed in the collapse of very large stars, or the collision of stars or other Black Holes.

## Boson

A type of particle with "integral angular momentum" — a spin of 0, 1, 2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are photons (which are the particles which give us light) and gravitons (which give us gravity). The other type of particle is the fermion.

## Boundary

A division between two regions. Physicists frequently analyze only a part of a larger system, so that they do not need to keep track of everything. This usually simplifies the analysis, but requires an understanding of what happens at the boundary. Interesting computer simulations usually require a boundary.

## Boundary Conditions

The state of a physical system at a boundary. Interesting computer simulations usually require boundary conditions.

## Brane

Objects which arise in string theory. They can have any number of dimensions, and are usually imagined as existing in a space with more dimensions than the brane itself has.

## Braneworld

A four-dimensional surface — a "brane" — in a spacetime with more than four dimensions.

## Causality

The relationship between cause and effect. Typically, we assume that a given event is a result of events that came before it in time.

## Compact Binary

A specific type of Binary system in which both members are compact (meaning they are White Dwarfs, Neutron Stars, or Black Holes) and have roughly equal mass.

## Cosmic String

A long, heavy object from Quantum Field Theory or String Theory, which is very thin. They may have been created in the early life of the Universe, and would now stretch across the entire Universe. No cosmic string has ever been observed. They may or may not exist.

## Cosmological Constant

A mathematical device used by Einstein to keep the Universe from falling in on itself. He later called this his "greatest blunder", because it kept him from predicting the expansion of the Universe. Astrophysicists now believe there may be a use for the cosmological constant.

## Dark Matter

A type of matter which is found near other matter. It cannot be observed directly (it is "dark"), but can be noticed as a result of the pull of gravity from the dark matter. Astrophysicists believe that more than half of the Universe could be in the form of dark matter.

## Differential Geometry

The branch of geometry which deals with curved surfaces and spaces. Calculus is used to analyze the shape of these surfaces and spaces. Differential geometry is a key tool used in the study of General Relativity.

## Einstein's Equations

A set of "tensor" equations Einstein devised to describe how mass warps spacetime. The set of equations may be written as $$G = 8 π T$$, where both $$G$$ and $$T$$ each represent a set of ten quantities. The $$G$$ quantities represent the warping of spacetime, while the $$T$$ quantities — the "Stress-Energy tensor" represent the mass.

## Electromagnetic Wave

An electric and magnetic disturbance that travels through space like a wave. What we experience as light is an electromagnetic wave. Electromagnetic waves therefore travel at the Speed of Light. Other types of electromagnetic wave range from Radio Waves and Microwaves, through to X-Rays and Gamma Rays.

## Electron

A tiny particle usually found swirling around an atomic nucleus, the electron carries the standard unit of negative charge, which balances the positive charges in a nucleus. The interaction between electrons and nuclei is responsible for chemistry. Electrons can become detached from the nucleus, when given enough energy, and become free. Electrons are members of the particle class called fermions, and are roughly 2,000 times lighter than neutrons and protons.

## Elementary Particle

A particle which is not made up of smaller particles. The neutron and proton are both made up of smaller particles — called quarks. On the other hand, quarks are not believed to be made of anything smaller. Like quarks, electrons and photons are also believed to be elementary particles.

## EMRI

Extreme Mass-Ratio Inspiral — A particular type of binary in which there is a very large difference in the masses of the two objects. Generally, this will involve a super-massive Black Hole with a mass millions of times that of our Sun, and a Neutron Star or Black Hole with a mass roughly the same as our Sun.

## Epicycle

A secondary circle centered on another, usually larger, circle. Its center moves along the circumference of the main circle.

## Event Horizon

A surface — like the one surrounding a Black Hole — enclosing a region of space from which nothing (even light) can ever escape.

## Exclusion Principle

A rule in Physics which says that no two identical particles can be in the same state (position, for instance) at the same time. This principle only applies to fermions, not to bosons. It is usually referred to as the "Pauli Exclusion Principle", after its inventor Wolfgang Pauli.

## Extreme Mass-Ratio Inspiral

A particular type of binary in which there is a very large difference in the masses of the two objects. Generally, this will involve a super-massive Black Hole with a mass millions of times that of our Sun, and a Neutron Star or Black Hole with a mass roughly the same as our Sun.

## Failure of Simultaneity

Simultaneous events are events in different places which happen at the same time. It turns out that this concept depends on how quickly one is moving. That is, if two observers are moving relative to each other, they will not be able to agree on the simultaneity of events. This is the Failure of Simultaneity.

## Fermion

A type of particle with "odd half-integral angular momentum" — a spin of 1/2, 3/2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are electrons, neutrons, and protons. The other type of particle is the boson.

## First Law of Motion

The first of Newton's Laws of Motion, which says that moving objects move in a straight line. Specifically, the Law says, "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

## Fission

or Nuclear Fission. A physical process which takes a nucleus of a heavy element (like Uranium or Plutonium, for example) and breaks them down into two or more smaller nuclei. This process releases large amounts of energy. It is sometimes called "Splitting the Atom". This is used in many modern power plants to generate electricity by heating water with the energy released. The two or more smaller nuclei which are produced are frequently toxic, and nearly always radioactive, which makes this method of producing electricity controversial. Fission is also the method used in simple nuclear weapons.

## Flatness Problem

The unexpected result that the Universe is not expanding so slowly that it will clearly collapse back on itself in a Big Crunch, nor expanding so quickly that it will clearly keep expanding forever. Instead, measurements show that the Universe is treading a fine line between the two — the Universe is referred to as "flat". Astrophysicists would not expect this to be the case unless there were some cause for the Universe to tread such a fine line. Finding this cause is the "flatness problem".

## Frequency

The number of occurrences of something in a given period of time. For a wave, this is might be the number of times the wave peaks in one second.

## Fusion

or Nuclear Fusion. A physical process which takes light elements and combines them into a heavier element. An example is the fusion of two Hydrogen atoms to form a Helium atom. This is the process which gives stars (including our Sun) heat and energy so that they shine. Though fusion is not yet used to produce electricity, scientists are working on this method, which will provide a nearly-inexhaustible source of cheap electrical energy, with little of the pollution or danger of nuclear fission. Fusion is also the force behind some very powerful nuclear weapons — the "H-Bomb".

## General Theory of Relativity

Einstein's version of the laws of physics, when there is gravity. Building on the Special Theory of Relativity, this theory generalizes Einstein's work so that the laws of physics must be the same for all observers, even in gravity. Einstein showed that gravity is best understood as a warping of the geometry of spacetime, rather than as a pulling of objects on each other. The crucial idea is that objects move along geodesics — which are determined by the warping of spacetime — while spacetime is warped by massive objects according to the formula $$G = 8 π T$$.

## Geodesic

Essentially the "straightest path" in a curved space or curved spacetime. This is the path followed by an object with no forces acting on it. In the curved spacetime of General Relativity, these paths may seem to be very curved — even appearing as circles or ellipses, for example. A geodesic is easily understood by looking at a very small region around the object. Even in highly curved spacetime, a small enough region will seem flat, so there is a natural idea of a "straight path". By following short segments, the whole geodesic is built up into one long path.

## Gravitational Wave

A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose.

## Horizon Problem

A problem with the simplistic Big Bang theory having to do with the smoothness of the Universe. The Early Universe should have been very random in terms of the temperature and density of different parts of space. This randomness should not have had time to distribute itself more evenly. Yet, this is what is observed in the Universe. The theory of Inflation solves this problem.

## Inflation

A brief period shortly after the Big Bang during which the Universe expanded very rapidly. The theory of Inflation is necessary to make the theory of the Big Bang agree with astronomical observations.

## Initial Data

Physicists frequently analyze a physical situation starting at some moment in time, and ignore what happened before that moment in time. To do this, however, they need to understand what was happening at that initial moment. That is, they need Initial Data. This is very similar to using Boundaries, which requires having Boundary Conditions.

## Inspiral

The gradually-shrinking orbit of a binary system. As the pair of stars in the binary orbit each other, they give off energy in the form of gravitational waves. This lost energy draws them closer in their orbit — eventually resulting in a Merger.

## Interference

A phenomenon which can occur any time there is any type of wave, which amounts to two waves canceling each other out. If two different waves meet at the same place, and one would hit its peak while the other would hit its trough, the waves will cancel, and there will be no disturbance. Alternatively, if both waves would hit their peaks at the same time, the waves will boost each other, so that there is a greater disturbance.

## Interferometer

A scientific device which makes use of the Interference of waves — typically, light waves. This type of device can measure changes in length with extraordinary precision, and forms the basis of modern gravitational wave detectors.

## Light Year

The distance traveled by light in one year. This is roughly 1013 kilometers, or 6 × 1012 miles.

## Merger

The portion of the Inspiral of a binary system in which the individual objects are highly distorted, and their orbit is changing rapidly. This portion is not well-understood, and must be simulated using Numerical Relativity.

## Metric

A set of numbers which encodes information about the geometry of space or spacetime. Together with an understanding of how that information is encoded (using Coordinates) everything about the geometry can be understood.

## Neutrino

A type of particle which has no charge and an extremely small mass. It is a Fermion, and is extremely difficult to stop or to detect. Nonetheless, they are produced in large numbers. The Sun, for example, sends 30 million neutrinos through every square inch of the Earth every single second. They are so hard to stop, however, that if a neutrino were sent through a solid light year of lead, it would still have a 50:50 chance of flying right through without stopping.

## Neutron

One of the particles in an atomic nucleus. These particles have no electric charge, but they hold together the protons (positive particles in a nucleus), and account for roughly half of the particles in the nucleus. Neutrons are fermions, and are believed to form the majority of the matter in a neutron star.

## Neutron Star

A type of star which is very old, having cooled off and stopped nuclear fusion reactions. When gravity pulls the star down on itself, the electrons and protons are squeezed together, leaving just neutrons. The star is then supported against gravity by "neutron degeneracy pressure" (no two neutrons can be in the same place at the same time). These are produced when a star is too heavy to be a white dwarf, but not heavy enough to turn into a Black Hole.

## Newton's First Law of Motion

The first of Newton's Laws of Motion, which says that moving objects move in a straight line. Specifically, the Law says, "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

## Nuclear Fission

A physical process which takes a nucleus of a heavy element (like Uranium or Plutonium, for example) and breaks them down into two or more smaller nuclei. This process releases large amounts of energy. It is sometimes called "Splitting the Atom". This is used in many modern power plants to generate electricity by heating water with the energy released. The two or more smaller nuclei which are produced are frequently toxic, and nearly always radioactive, which makes this method of producing electricity controversial. Fission is also the method used in simple nuclear weapons.

## Nuclear Fusion

A physical process which takes light elements and combines them into a heavier element. An example is the fusion of two Hydrogen atoms to form a Helium atom. This is the process which gives stars (including our Sun) heat and energy so that they shine. Though fusion is not yet used to produce electricity, scientists are working on this method, which will provide a nearly-inexhaustible source of cheap electrical energy, with little of the pollution or danger of nuclear fission. Fusion is also the force behind some very powerful nuclear weapons — the "H-Bomb".

## Nucleus

The central part of an atom, which contains Neutrons and Protons. Electrons are usually found around the Nucleus. Strictly speaking, this is the only part of an atom involved in Nuclear Reactions (Fission or Fusion).

## Numerical Relativity

The branch of Relativity research which deals with simulating the development of Spacetime, using computers. This is believed to be the only possible way to understand things like the merger of two Black Holes.

## Observer

A person or piece of equipment that measures something in physics. Frequently, we speak of an observer measuring time or a distance in a particular place.

## Origin

A particular point with respect to which positions are measured.  For example, positions on the Earth are frequently measured relative to the point on the Equator which is due South of Greenwich, England.  In GPS coordinates, that point is given as 0,0 (its latitude is 0, and its longitude is 0).  Or we might say something like “10 miles southeast of town hall.”  In that case, town hall would be the origin.

## Parsec

A measure of distance which is roughly 3.26 Light Years.

## Pauli Exclusion Principle

A rule in Physics which says that no two identical particles can be in the same state (position, for instance) at the same time. This principle only applies to fermions, not to bosons. It is usually referred to as the "Pauli Exclusion Principle", after its inventor Wolfgang Pauli.

## Period

The length of time between two events. For a wave, this is usually the length of time it takes two successive peaks to pass a given point. This number is simply 1 divided by the Frequency of the wave.

## Phase

1. For a wave, the position of any particular feature of the wave.
2. For matter, a distinct form of a substance, such as solid, liquid, or vapor.

## Phase Transition

A change of the state of matter from one phase to another, such as the transition from liquid water to solid ice.

## Photon

An Elementary Particle which carries the energy of light. The photon is a Boson, and has no mass. It always moves at the Speed of Light.

## pi

or π (Pronounced as "pie".) An important number in geometry. This is defined to be the ratio of the circumference of a circle to the diameter of that circle, in flat space. π is an irrational number, which means that its exact value cannot be written down, though it can be calculated as precisely as necessary. Its value is approximately 3.14159265358979323...

## Powers of 10

In order to write very large, or very small numbers efficiently, scientists use a system of writing in powers of 10. For example, the number 108 represents 10 multiplied by itself 8 times, which is a 1 with 8 zeroes after it: 100,000,000. Thus, rather than writing 300,000,000, a scientist will write 3 × 108. Similarly, a number like 10-21 represents 0.1 multiplied by itself 21 times, which is a 1 with 21 zeroes in front of it (including the one before the decimal point): 0.000000000000000000001.

## Proton

One of the particles in an atomic nucleus. These are electrically positive particles which attract electrons to the atom. Protons are fermions, and are very similar to neutrons, except that they have electric charge, and a slightly higher mass.

## Pulsar

A neutron star with a very high rate of spin, and very intense magnetic fields. The pulsar gives off beams of radiation along its magnetic poles. If these poles are not aligned with the spin poles, the beam will sweep around like the beam of a lighthouse.

## Quantum Field Theory in Curved Spacetime

A theory which attempts to incorporate ideas from Quantum Mechanics and General Relativity. This theory is needed when the quantum behavior of objects is important, and there is extreme curvature of spacetime.

## Quantum Mechanics

A modern physical theory which is vital to describing extremely small objects, like electrons around an atom. Though this theory also applies to larger objects, its effects become very similar to those of Newton's theory — which is typically much easier to use and understand. One of the most important ideas in Quantum Mechanics is the Uncertainty Principle.

## Quark

An Elementary Particle which makes up the Neutron and the Proton, as well as many more exotic particles. These particles are Fermions, and have charges of either 2/3 as much as an electron's charge, or -1/3. They come in "flavors" of up, down, strange, charmed, bottom, and top, as well is in "colors" of red green and blue. The "flavor" and "color" are just fanciful names given to describe intrinsic properties of these particles — similar to charge, mass, or spin.

## Quasar

A very dense and very bright object seen in the distant Universe. Quasars are believed to be powered by Black Holes.

## Randall-Sundrum Model

A theory of the Universe which postulates that we live in a five-dimensional spacetime, though we are confined to a four-dimensional Slice of it. This model is still being explored as an accurate description of the Universe in which we live.

## Ringdown

The portion of an Inspiral, following the Merger, when the two objects have combined into one. During this brief period, the combined object will settle down by giving off gravitational waves.

## Singularity

A point at which the curvature of spacetime becomes infinite. Singularities can form — for example — when too much matter is squeezed into a region which is too small. Singularities are found at the center of a Black Hole, at the beginning of the Universe in the Big Bang, and at the end of the Universe (if it ever comes) in the Big Crunch. Many scientists believe — though there is no solid evidence — that all singularities in the present Universe lie hidden behind Event Horizons. This is the "Cosmic Censorship Conjecture".

## Singularity Theorem

A theorem (a mathematically proven fact) which shows that a singularity must exist under certain circumstances. For example, physicists have shown that the Universe must have begun with a singularity.

## Spacetime

A concept in physics which merges our usual notion of space with our usual notion of time.

## Special Theory of Relativity

Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity becomes very nearly the same as Newton's theory, which is much easier to use.

## Speed

For a wave, the speed of a particular point (such as its crest).

## Speed of Light

A constant of Nature. This speed is precisely 299,792,458 meters per second, or roughly 670,616,629 miles per hour. One of the most unusual discoveries of science has been the fact that all Observers measure light as moving at exactly this speed, even if those observers are moving relative to each other. This fact is one of the basic ingredients in Einstein's Special Theory of Relativity.

## Spin

An intrinsic property of particles. (That is, a property which does not change. Mass and electric charge are examples of intrinsic properties.) Spin is related to the usual notion of spin, though it is a little more difficult to understand. Spin comes in units of 1/2, so that a particle may have a spin of 0, 1/2, 1, 3/2, and so on. A particle's spin determines whether it is a Fermion or a Boson.

## Stochastic

Pertaining to a process involving a randomly-determined sequence of observations.

## String

The fundamental object in String Theory, which replaces the notion of a particle in standard Quantum Mechanics. Rather than being a simple point-like object, fundamental particles become tiny strings or loops. The vibrations of these strings result in various properties like spin.

## String Theory

A theory of physics taking the String as its fundamental object. This theory attempts to solve problems in standard Quantum Mechanics and Quantum Field Theory. It actually predicts the existence of gravity.

## Supernova

Violently exploding stars which shine very brightly for days or weeks. They occur when the fuel for nuclear reactions is used up, and a star cools. Gravity pulls all the matter down toward the star's center. If this happens quickly, nuclear reactions may suddenly begin again, detonating the star in a nuclear explosion.

## Superstring

Another name for a "Supersymmetric String", which is usually referred to simply as a String.

## Sympathetic Resonance

A phenomenon in which an object with a natural resonant frequency is excited by the vibrations of another object vibrating at the same frequency. For example, a singer breaking a wine glass with her voice demonstrates an extreme sympathetic vibration of the glass, excited by the vibrations of the singer's voice.

## Uncertainty Principle

The principle of Quantum Mechanics — as well as Quantum Field Theory and String Theory — which says that an observer can never know both the position and velocity of a particle with perfect precision. Specifically, the more certain an observer is of the position, the less certain that observer must be of the velocity, and vice-versa.

## Vacuum Energy

The energy that is present even in otherwise empty space. This energy has been measured to exist (in the "Casimir Effect"). Whereas matter causes the expansion of the Universe to slow down, vacuum energy actually causes the expansion to speed up.

## Virtual Particle

A particle which cannot be directly detected, but is assumed to exist due to its indirect effects on real particles. Virtual particles can form in pairs from the vacuum of space.

## Wavelength

The distance between two neighboring peaks or troughs of a wave.

## White Dwarf

A type of star which is very old, having cooled off and stopped nuclear fusion reactions. A white dwarf is supported by "electron degeneracy pressure" (no two electrons can be in the same place at the same time). These are produced when a star is not heavy enough to turn into a Neutron Star or a Black Hole.

## Worm Hole

A tube in spacetime which connects two widely separated places in the Universe. Wormholes could provide the possibility of time travel, though they would probably be very unstable, and only exist for a short period of time.

## X-Ray

A type of light — or electromagnetic wave — which is invisible to the naked eye. X-Rays are much more energetic than the light we see. They can penetrate skin very easily, for example. In the doctor's or dentist's office, X-Rays are detected on a photographic plate, allowing us to see inside the body. Like all other forms of electromagnetic radiation, X-Rays travel at roughly 300,000,000 meters per second (186,000 miles per second).

## Inspiration

Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.

The Red Queen in Lewis Carroll's Through the Looking Glass