relativity

1. The quality or state of being relative.
2. A state of dependence in which the existence or significance of one entity is solely dependent on that of another.
3. Physics.
   1. Special relativity.
   2. General relativity.

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A theory concerning time, space, and the motion of objects, proposed first in 1905 by Albert Einstein in his special theory of relativity.

The “special theory of relativity” is based on the principle of special relativity, which states that all observers moving at constant velocities with respect to each other should find the same laws of nature operating in their frames of reference. It follows from this principle that the speed of light would have to appear to be the same to every observer. The theory predicts that moving clocks will appear to run slower than stationary ones (see time dilation), that moving objects will appear shorter and heavier than stationary ones, and that energy and mass are equivalent (see E = mc²). There is abundant experimental confirmation of these predictions.

The general theory of relativity is the modern theory of gravitation, proposed in 1915, also by Albert Einstein. The central point of the theory is the principle of general relativity, which states that all observers, regardless of their state of motion, will see the same laws of physics operating in the universe. The most famous prediction of the theory is that light rays passing near the sun will be bent — a prediction that has been well verified.

Until the end of the 19th century, physicists believed that all physical phenomena, ranging from the motion of atoms to that of celestial bodies, were governed by one set of laws: the laws of motion formulated by Newton in the Principia. Newton's theory implied that these laws were also valid for systems that move at constant speed relative to each other. That laws stay the same for such systems is known as the principle of relativity. It is impossible to find from within if a system is uniformly moving or not by performing mechanical experiments. A fundamental concept of Newtonian physics is the existence of absolute space. During the 19th century, when Thomas Young showed that light is a wave phenomenon, an invisible substance called ether, linked to absolute space, was believed to be the medium that carried these waves.

During the 1880s Albert A. Michelson and Edward Williams Morley attempted to measure the velocity of Earth relative to the ether by measuring the velocity of light. Their experiments showed that the velocity of light is exactly the same in every direction and thus does not depend on the proper motion of Earth. Some physicists argued that this result showed that the principle of relativity does not apply to electromagnetic radiation. Dutch physicist Hendrik Antoon Lorentz and Irish physicist George FitzGerald tried to explain that the velocity of light is independent from the motion of Earth by assuming that everything contracts in the direction in which one is moving. They argued that the instrument that Michelson and Morley had used contracted imperceptibly in the direction of Earth's motion, thus falsifying the measurement of the velocity of light. Perhaps the most important aspect of their theory is that they held on to the idea of an ether.

In 1905 Einstein published a theory based on the notion that it is impossible to determine the absolute motion of a moving object. Einstein's concern, however, was not the failure of Michelson and Morley to measure the motion of Earth relative to the ether, but the validity of James Clerk Maxwell's electromagnetic theory in systems that move at speeds close to the velocity of light. Einstein's theory did not require the presence of an ether and was based on the following assumptions: (1) absolute speed cannot be measured, only speed relative to some other object; (2) the measured value of the speed of light in a vacuum is always the same no matter how fast the observer or light source is moving; and (3) the maximum velocity that can be attained in the universe is that of light.

Einstein's theory is called the "special" theory of relativity because it applies the principle of relativity only to systems in uniform motion relative to each other. Because of the principle of relativity, passengers who are traveling smoothly in a train cannot tell whether they are moving or not unless they look out of a window. The situation becomes different if the train is uniformly accelerated. The passengers will feel a slight push in the direction opposite to that in which the train is moving. This is termed acceleration force: Because of this extra force it appears that the laws of physics would be different for bodies accelerated with respect to each other.

In 1916 Einstein published his general theory of relativity, which he based on the assumption that the laws of physics are also the same in systems that are accelerated relative to each other. To formulate this theory, he introduced the principle of equivalence: Acceleration forces and gravitational forces are not distinguishable from each other. Einstein argued that if one were in a closed elevator that is uniformly accelerated upward, one would perceive a force that is indistinguishable from gravitation. The principle of equivalence can also be expressed by saying that the inertia of an object (its reluctance to be set in motion) is proportional to its mass. The principle of equivalence was already known to Galileo, who had deduced it from his experiments with wooden balls rolling down sloping planes. In 1891 Hungarian scientist Roland Eötvös made precise measurements and established that inertial and gravitational mass are equivalent to an extremely high degree. Because acceleration forces and gravitational forces are equivalent, they should not be distinguishable, but viewed as a property of space.

In a formulation of the special theory, mathematician Hermann Minkowski had introduced a four-dimensional space in which the fourth dimension is time. In this space-time continuum as adapted to the general theory, gravitation corresponds to the amount of curvature in a non-Euclidean, four-dimensional space. Near a large mass, space becomes more curved, and objects moving near that mass will follow the curvature of space.

One of the interesting consequences of the equivalence of gravitation and acceleration force is the bending of light rays by the presence of a large mass, such as a star or planet. For example, if light enters through a small hole on one side of a spaceship that is accelerated, the light ray will reach the other side after the spaceship has moved. The same effect would exist if the spaceship were to come close to a massive planet or star. To an observer in the spaceship, the light ray will appear curved in either case. But the observer cannot tell whether the spaceship is being accelerated or is near a planet or star.

In 1919, during a solar eclipse, Arthur Eddington showed that a star whose light passed close to the Sun appeared to be displaced by a minute amount that corresponded to the value calculated by Einstein. This was the first experimental proof of the general theory of relativity. Several other experiments since then have eliminated most doubts about both theories of relativity among physicists.

Quotes:

"Each of us is incomplete compared to someone else -- an animal's incomplete compared to a person... and a person compared to God, who is complete only to be imaginary." - Georges Bataille

"When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity." - Albert Einstein

Relativity

Artist	M. C. Escher
Year	1953
Type	lithograph
Dimensions	27.7 cm × 29.2 cm (10.9 in × 11.5 in)

Relativity is a famous lithograph print by the Dutch artist M. C. Escher, first printed in December 1953.

It depicts a world in which the normal laws of gravity do not apply. The architectural structure seems to be the centre of an idyllic community, with most of its inhabitants casually going about their ordinary business, such as dining. There are windows and doorways leading to park-like outdoor settings. Yet all the figures are dressed in identical attire and have featureless bulb-shaped heads. Identical characters such as these can be found in many other Escher works.

In the world of Relativity, there are actually three sources of gravity, each being orthogonal to the two others. Each inhabitant lives in one of the gravity wells, where normal physical laws apply. There are sixteen characters, spread between each gravity source. The apparent confusion of the lithograph print comes from the fact that the three gravity sources are depicted in the same space.

The structure has three stairways, and each stairway can be used by people who belong to two different gravity sources. This creates interesting phenomena, such as in the top stairway, where two inhabitants use the same stairway in the same direction and on the same side, but each using a different face of each step; thus, one descends the stairway as the other climbs it, even while moving in the same direction nearly side-by-side. In the other stairways, inhabitants are depicted as climbing the stairways upside-down, but based on their own gravity source, they are climbing normally.

Another interesting fact is that each of the three parks belongs to one of the gravity wells. All but one of the doors seem to lead to basements below the parks. Though physically possible, such basements are certainly unusual and add to the surreal effect of the picture.

This is one of Escher’s most popular works and has been used in a variety of ways, as it can be appreciated both artistically and scientifically. Interrogations about perspective and the representation of three-dimensional images in a two-dimensional picture are at the core of Escher's work, and Relativity represents one of his greatest achievements in this domain.

See also

• M. C. Escher in popular culture

Sources

• Locher, J. L. (2000). The Magic of M. C. Escher. Harry N. Abrams, Inc. ISBN 0-8109-6720-0.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - relativitet

Nederlands (Dutch)
betrekkelijkheid, relativiteit

Français (French)
n. - (gén, Phys, Ling) relativité

Deutsch (German)
n. - Relativität, Abhängigkeit

Ελληνική (Greek)
n. - σχετικότητα

Italiano (Italian)
relativitý

Português (Portuguese)
n. - relatividade (f)

Русский (Russian)
относительность

Español (Spanish)
n. - relatividad

Svenska (Swedish)
n. - (vet) relativitet

中文（简体）(Chinese (Simplified))
有关系, 比较性, 相关性

中文（繁體）(Chinese (Traditional))
n. - 有關係, 比較性, 相關性

한국어 (Korean)
n. - 관계 있음, 상호의존, 임금의 상대적 격차

日本語 (Japanese)
n. - 関係のあること, 関連性, 相関性, 相対性原理, 相対性理論

العربيه (Arabic)
‏(الاسم) النسبيه‏

עברית (Hebrew)
n. - ‮יחסות, תורת היחסות‬

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