## Copy of `NCSA - Relativity Glossary`

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 NCSA - Relativity Glossary Category: Meteorology and astronomy > Relativity Date & country: 13/09/2007, USAWords: 32

Accretion Disk
In a binary system containing a star and a compact object (white dwarf, neutron star, or black hole) gas may flow from the star to the compact object. According to the theoretical model, the gas will spiral in and fall to the surface of the compact object creating a flow of matter in the shape of a disk. It is generally believed that this model explains many features of X-ray pulsars

Apparent Horizon
When matter falls inward to form a black hole it is not always easy to see where the event horizon might be. It might appear at one time that a light ray is capable of escaping but infalling matter might eventually prevent it from doing so. The apparent horizon is a surface on which outgoing light rays are just trapped, and cannot expand outward. It is a stronger condition than the event horizon, and the apparent horizon always lies inside the event horizon, or coincides with it.This situation iâ€¦

Arc Second
The size of a celestial object expressed in terms of the angle that it covers (or 'subtends') when viewed from Earth. For example, the moon subtends an angle of 1/2 a degree. One degree of arc is defined as equivalent to 60 minutes of arc (or 'arc minutes'). Arc minutes are further divided into arc seconds, such that there 60 x 60 or 3600 arc seconds per degree. So the moon's apparent size can also be expressed as 1/2 degree x 3600 = 1800 arc seconds. If the the distance to an object is also knoâ€¦

Axisymmetry
An axisymmetric system looks the same if we change our point of view by rotating our position about an axis. Since a symmetry in physics is an operation that leaves our system unchanged, an object that does not look different after a rotation about an axis has 'axis-symmetry.'If we are looking at a soup can within our imaginary sphere and put our imginary axis through the center of its two flat faces we will find that it is axisymmetric if we take the label off of it, but is not axisymmetric if â€¦

Big Bang
The 'fireball' of cosmic creation. Modern cosmology is founded on the 'Big Bang' model in which all the known universe is thought have have emerged some 13-20 billion years ago from an unimaginably hot, dense state born of a singularity. See also Naked Singularity.

Binary Pulsar
A source that pulsates in the radio or x-ray spectrum is called a 'pulsar' and it is generally believed that a pulsar is a neutron star (although some of the pulsars with longer periods might be white dwarfs). A binary pulsar is a binary star system (a system where two stars orbit each other), where one of the two is a pulsar.

Black Hole
A black hole is a region of spacetime enclosed by an event horizon. A black hole is formed by the collapse of massive objects. If the heat and pressure supplied by the fusion of the material within the star is less than the gravitational pull inward, the object may collapse to form a white dwarf, a neutron star, or (if it is massive enough) a black hole.A black hole is termed 'black' because nothing can escape from within it, not even light. Everything that passes through the event horizon is goâ€¦

Cosmic Censorship
At the center of a mathematical description of a black hole discovered by the German mathematician, Karl Schwarzschild, is a singularity, a point at which the laws of physics break down and space becomes infinitely curved. Fortunately, the Schwarzschild spacetime has another feature, an event horizon, out through which nothing can pass (although things CAN pass IN through the horizon). Because the singularity is 'clothed' by the event horizon it cannot affect the exterior spacetime and we do notâ€¦

Coupled Equations
An example of a coupled equation may be found in the example of state and federal tax. The state (in our example) takes 20 percent of the part of your income (after the federal tax is deducted), and the federal government takes 10 percent of your income (after the state tax is deducted). Coupled, Hyperbolic-Elliptic, Nonlinear, Partial Differential Equation This is name is a sequence of adjectives, each with a specific meaning, that tells us something about the type of differential equation we aâ€¦

Critical Circumference
This is the circumference below which an object of given mass would collapse to form a black hole. This circumference depends on the mass of the object in question. For example, a collapsing star equal to 10 suns will have a critical circumference of 198 kilometers or 118 miles. See also Schwarschild Radius.

Electromagnetic Field
An electromagnetic field consists of energy oscillations associated with electric and magnetic fields initially caused by the motions of electric charges. The resulting waves propagate through space at the speed of light.The famous British Scientist, James Clark Maxwell (1871-79), formulated the mathematical laws governing the propagation of electromagnetic waves in space. These laws provided the underpinning for classical 'electrodynamics.' Certain problems in the manifestation of these laws prâ€¦

Although initially created by moving charges, electromagnetic radiation electromagnetic fields propogates freely through the vacuum requiring no further influence from matter to sustain it. Such radiation is both generated by and indicative of a wide range of phenomena in the universe (since visible light, radio, x-ray and infrared are all manifestations of electromagnetic radiatioan). By measuring the intensity and wavelength of such radiation, scientists can gain insights into the underlying câ€¦

Event horizon
The event horizon defines the boundary of a black hole behind which nothing, not even light, can escape. Consider an event (a given position at a given time) in spacetime. Now imagine that light rays shoot out in all directions from this event. If none of them can escape to an infinite distance then that event is inside the event horizon. If any can escape, that event is outside the event horizon.

General Solution
See Types of Calcuations and their Solutions.

Gravitational Lensing
Consider the example given under spacetime curvature where we describe two dimensional creatures living in a bedsheet. In that example we saw how parallel light rays could be caused to meet by passing on either side of a massive object.This tells us that the curvature of spacetime can focus light rays. In effect, the curvature of spacetime acts on light somewhat like a giant convex lens extending around the massive object.Because of this effect, sensitive devices sometimes see two images of an aâ€¦

The energy that is emitted by strong sources of gravitational waves, for example, certain collapsing or colliding stars.

Gravitational Waves
Think about the example described under spacetime curvature in which we have two dimensional creatures living on the surface of a bedsheet.Now imagine that a physics professor grabs one end of the bedsheet and begins to shake it violently up and down. This will cause ripples to travel through the fabric. The imaginary creatures within the bedsheet will not be able to see what is happening, but they they will be able to measure the time variation in the geometry of their space. The wave travellinâ€¦

Light Year
A light year is the distance light can travel in a year. Light travels at 186,282 miles per second, so one can see that this is truly a gigantic distance. Yet in some respects it is still quite small. The nearest star to our sun is over four light years away, and the galaxy itself is about 100,000 light years across.

Naked Singularity
A singularity from which the universe is 'unshielded' because there is no event horizon. The physical consequences of 'naked' singularities are hotly debated among physicists.To quote the renowned mathematician and physicist Roger Penrose:It is sometimes said that if naked singularities do occur, then this would be disastrous for physics.I do not share this view. We already have the example of the big bang singularity in the remote past, which seems not to be avoidable. The 'disaster' to physicsâ€¦

Neutron Star
If a star's mass is too great, its nuclear matter will be compressed beyond the limits given by a white dwarf. The electrons and protons of the star's matter will combine to form neutrons, and the star will in some cases possess regions that are more dense than an atomic nucleus. In a sense, Neutron Stars are like giant atomic nuclei - although the physics of so large an object as a neutron star will have many important differences.

Nonlinear Process
In linear processes the output is directly proportional to its input. For example, the pressure of a gas in a fixed volume is directly proportional to its temperature. In nonlinear processes this direct proportionality is lost.Linear processes can be accounted for by the sum of their parts and are easy to predict. Not so for nonlinear processes: they tend to be complex; their outcomes can be difficult to predict and often display so-called chaotic behavior, and the mathematical equations describâ€¦

Redshift
Redshift is the lengthening of the wavelength of electromagnetic radiation (or, equivalently, the shortening of its frequency). There are three types of redshift.The (Relativistic) Doppler EffectNamed after its discoverer, Christian Doppler (1803-53), this is the change in frequency that results when the emitter is travelling away from the viewer, or when the viewer is travelling away from the emitter (in special relativity this is really the same thing). However, if the emitter and observer areâ€¦

Reference frame
Consider a astronaut travelling in a starship, and someone sitting on the earth. According to the special theory of relativity, each perceives himself or herself to be at rest while the other is perceived to be moving. Each of these observers is said to be in his or her own 'reference frame.' Any two observers travelling with the same velocity (the same speed and direction) are said to be in the same reference frame. Two people travelling with a different speed and direction are said to be in diâ€¦

The Schwarzschild radius is the radius at which the event horizon of a Critical Circumference.

Singularity
In the center of the mathematical model of a black hole is a singularity which has the shape of a point (or a ring if the hole is rotating), at which the curvature of spacetime becomes infinitely large. A singularity represents a great difficulty for theoreticians because it is impossible to predict how a singularity will affect objects in its causal future. If cosmic censorship is true, then this needn't cause any trouble because they will only be found inside event horizons.

Spacetime
Space has three dimensions. However, the theory of relativity predicts that time, like space, is a dimension. In order to describe a four dimensional universe which has three spatial dimensions and one time dimension the word 'spacetime' was coined. Each point in spacetime is called an event.

Spacetime Curvature
Imagine that the universe has two spatial dimensions instead of three, and that there are flat creatures living on its surface. Now imagine that the surface they are living on is subject to deformations, something like a bedsheet. The creatures living on the bedsheet can only see length and depth, they can only see within the bedsheet. They cannot even imagine the concept of height.Now imagine that the bedsheet is draped over a basketball, and the creatures are very small. If the creatures attemâ€¦

Speed of Light
Light travels at a speed of 186,282 miles per second in vacuum from the point of view of a nearby observer. Because of the effects of general relativity the speed of light near a massive object will appear slower to a distant observer, and this effect has been confirmed in experiments.The speed of light is the theoretical limit to the speed of any particle in the universe. More fundamentally, no cause can result in an effect that requires travel faster than light. For example, I cannot affect whâ€¦

Stationary Black Bole
Stationary refers to a time-independent mathematical description of a black hole (not its rotation - a rotating black hole can still be 'stationary.') To understand what this means, consider the physical system represented by a perfectly symmetrical top spinning without friction. Every detail of this system remains the same as time goes by, and thus we can say the system is 'stationary, ' even though it is spinning.By definition, a stationary black hole 'sits' alone in space. It interacts with nâ€¦

Supernova(e)
A supernova is an exploding star. Such an explosion occurs in our galaxy at a rate of about one every 30 years. Its causes are not precisely known, but the violent movement of matter within the star may produce a significant amount of gravitational radiation. It is thought that supernovae produce pulsars.

White Dwarf
When a star has burned most of its nuclear fuel it can no longer provide the heat and pressure necessary to prevent its gravitational collapse. However, there is still another effect which can prevent the forming of a black hole.The effect arises from quantum physics which tells us two things about the electrons in the stellar material. The Pauli Exclusion Principle tells us that no two electrons can exist in the same place in space. Quantum mechanics restricts the number of places that an electâ€¦

Worldline
Imagine that time is like a spatial dimension, and it is plotted on the y-axis of a sheet of graph paper before us. Let the x-axis be one of the three spatial dimensions of our world. A ball travelling to the right would be depicted as a line with a positive slope. A ball at rest would be plotted as a straight vertical line. Because physicists do think of time as a dimension similar to a spatial dimension, they draw diagrams, like the one described above, to illustrate the trajectory of a particâ€¦