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The theory of relativity transformed theoretical physics and astronomy during

the 20th century. When first published, relativity superseded a 200-year-old

theory of mechanics created primarily by Isaac Newton.[4][5][6]

In the field of physics, relativity catalyzed and added an essential depth of

knowledge to the science of elementary particles and their fundamental

interactions, along with ushering in the nuclear age. With relativity,

cosmology and astrophysics predicted extraordinary astronomical

phenomena such as neutron stars, black holes, and gravitational waves.[4][5]

[6]

Two-theory view[edit]The theory of relativity was representative of more than

a single new physical theory. There are some explanations for this. First,

special relativity was published in 1905, and the final form of general

relativity was published in 1916.[4]

Second, special relativity applies to elementary particles and their

interactions, whereas general relativity applies to the cosmological and

astrophysical realm, including astronomy.[4]

Third, special relativity was widely accepted in the physics community by1920. This theory rapidly became a significant and necessary tool for

theorists and experimentalists in the new fields of atomic physics, nuclear

physics, and quantum mechanics. Conversely, general relativity did not

appear to be as useful. There appeared to be little applicability for

experimentalists as most applications were for astronomical scales. It

seemed limited to only making minor corrections to predictions of Newtonian

gravitation theory.[4]

Finally, the mathematics of general relativity appeared to be very difficult.

Consequently, it was thought that a small number of people in the world, at

that time, could fully understand the theory in detail, but this has been

discredited by Richard Feynman. Then, at around 1960 a critical resurgence

in interest occurred which has resulted in making general relativity central to

physics and astronomy. New mathematical techniques applicable to the

study of general relativity substantially streamlined calculations. From this,

physically discernible concepts were isolated from the mathematical

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complexity. Also, the discovery of exotic astronomical phenomena in which

general relativity was crucially relevant, helped to catalyze this resurgence.

The astronomical phenomena included quasars (1963), the 3-kelvin

microwave background radiation (1965), pulsars (1967), and the discovery of

the first black hole candidates (1971).[4]

On the theory of relativity[edit]Einstein stated that the theory of relativity

belongs to a class of "principle-theories". As such it employs an analytic

method. This means that the elements which comprise this theory are not

based on hypothesis but on empirical discovery. The empirical discovery

leads to understanding the general characteristics of natural processes.

Mathematical models are then developed which separate the natural

processes into theoretical-mathematical descriptions. Therefore, by analytical

means the necessary conditions that have to be satisfied are deduced.

Separate events must satisfy these conditions. Experience should then matchthe conclusions.[7]

The special theory of relativity and the general theory of relativity are

connected. As stated below, special theory of relativity applies to all physical

phenomena except gravity. The general theory provides the law of

gravitation, and its relation to other forces of nature.[7]

include the principle of relativity, the constancy of the speed of light, and time dilation.[9]

The predictions of special relativity have been confirmed in numerous tests since Einsteinpublished his paper in 1905, but three experiments conducted between 1881 and 1938

were critical to its validation. These are the MichelsonMorley experiment, the

KennedyThorndike experiment, and the IvesStilwell experiment. Einstein derived the

Lorentz transformations from first principles in 1905, but these three experiments allowthe transformations to be induced from experimental evidence.

Maxwell's equations the foundation of classical electromagnetism describe light as a

wave which moves with a characteristic velocity. The modern view is that light needs nomedium of transmission, but Maxwell and his contemporaries were convinced that light

waves were propagated in a medium, analogous to sound propagating in air, and ripples

propagating on the surface of a pond. This hypothetical medium was called theluminiferous aether, at rest relative to the "fixed stars" and through which the Earthmoves. Fresnel'spartial ether dragging hypothesis ruled out the measurement of first-

order (v/c) effects, and although observations of second-order effects (v2/c2) were

possible in principle, Maxwell thought they were too small to be detected with then-current technology.[10][11]

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The MichelsonMorley experiment was designed to detect second order effects of the

"aether wind" the motion of the aether relative to the earth. Michelson designed an

instrument called the Michelson interferometerto accomplish this. The apparatus wasmore than accurate enough to detect the expected effects, but he obtained a null result

when the first experiment was conducted in 1881,[12] and again in 1887.[13] Although the

failure to detect an aether wind was a disappointment, the results were accepted by thescientific community.[11]In an attempt to salvage the aether paradigm, Fitzgerald and

Lorentz independently created an ad hoc hypothesis in which the length of material

bodies changes according to their motion through the aether.[14] This was the origin ofFitzgerald-Lorentz contraction, and their hypothesis had no theoretical basis. The

interpretation of the null result of the MichelsonMorley experiment is that the round-trip

travel time for light is isotropic (independent of direction), but the result alone is not

enough to discount the theory of the aether or validate the predictions of specialrelativity.[15][16]

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