The modern view is that light needs no medium 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 the luminiferous aether , at rest relative to the "fixed stars" and through which the Earth moves.
The Michelson—Morley 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 interferometer to accomplish this. The apparatus was more than accurate enough to detect the expected effects, but he obtained a null result when the first experiment was conducted in ,  and again in The interpretation of the null result of the Michelson—Morley 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 special relativity.
While the Michelson—Morley experiment showed that the velocity of light is isotropic, it said nothing about how the magnitude of the velocity changed if at all in different inertial frames. The Kennedy—Thorndike experiment was designed to do that, and was first performed in by Roy Kennedy and Edward Thorndike. Stilwell first in  and with better accuracy in The strategy was to compare observed Doppler shifts with what was predicted by classical theory, and look for a Lorentz factor correction.
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Such a correction was observed, from which was concluded that the frequency of a moving atomic clock is altered according to special relativity. Those classic experiments have been repeated many times with increased precision. Other experiments include, for instance, relativistic energy and momentum increase at high velocities, experimental testing of time dilation , and modern searches for Lorentz violations.
General relativity has also been confirmed many times, the classic experiments being the perihelion precession of Mercury 's orbit, the deflection of light by the Sun , and the gravitational redshift of light. Other tests confirmed the equivalence principle and frame dragging. Far from being simply of theoretical interest, relativitistic effects are important practical engineering concerns. Satellite-based measurement needs to take into account relativistic effects, as each satellite is in motion relative to an Earth-bound user and is thus in a different frame of reference under the theory of relativity.
From Wikipedia, the free encyclopedia. This article is about the scientific concept. For philosophical or ontological theories about relativity, see Relativism. For the silent film, see The Einstein Theory of Relativity. Introduction History. Fundamental concepts. Principle of relativity Theory of relativity Frame of reference Inertial frame of reference Rest frame Center-of-momentum frame Equivalence principle Mass—energy equivalence Special relativity Doubly special relativity de Sitter invariant special relativity World line Riemannian geometry.
Equations Formalisms. Main articles: History of special relativity and History of general relativity. Main article: Special relativity. Main articles: General relativity and Introduction to general relativity.
Main article: Tests of special relativity. Main article: Tests of general relativity.
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Relativity: The Special and General Theory
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What's So Special About Special Relativity?
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Oxford: Oxford Univ. American Journal of Science. Bibcode : AmJS July Reviews of Modern Physics. Bibcode : RvMP Spacetime physics: Introduction to Special Relativity 2nd ed. It's a fact of life: Some things are absolute, and some are relative. For me, the teapot on the table is to the left of my cup.
From the point of view of an observer sitting directly opposite, it's the other way around: My cup is to the left of the teapot. Whether or not an object is located to the left or to the right of another depends on the observer. On the other hand, if the cup is filled to the brim with coffee, all observers should agree to the fact, regardless of where they sit. That, it would seem, is an absolute statement, independent of who makes the observation.
Einstein's special theory of relativity special relativity is all about what's relative and what's absolute about time, space, and motion. Some of Einstein's conclusions are rather surprising. They are nonetheless correct, as numerous physics experiments have shown. The clock which traveled recorded marginally less passage of time than the other as predicted by the theory , although the difference was of course minimal due to the relatively slow speeds involved.
At very high speeds, however, the effect is much more noticeable. Experiments have demonstrated that an ultra-short-lived muon particle, which habitually travels at Particles traveling at speeds up to So, traveling at close to the speed of light would theoretically allow time travel into the future, as time slows down for the speeding object in order to "protect" the cosmic speed limit of the speed of light. A corollary of all this is that, if it were possible to exceed the speed of light , then it would also be possible to go back in time, which raises the possibility of time-travel paradoxes where a person goes back in time and interferes in their own past or kills their own grandparents, etc , although some scientists believe that some as yet undiscovered law of physics may intervene to prevent such paradoxes.
Actually, special relativity does not specifically forbid the existence of particles that travel faster than light , and there is a hypothetical sub-atomic particle called a tachyon, which would indeed spend its entire life traveling faster than the speed of light , but it is currently still hypothetical. This is sometimes considered a paradox in that each twin sees the other twin as traveling, and so, it is argued, each should see the other aging more slowly.
But in fact this is based on a misunderstanding of relativity , because in reality only one twin experiences acceleration and deceleration, and so only one twin ages less. An equivalent paradox concerning the related phenomenon of length contraction is often referred to as the "tunnel paradox", whereby a hypothetical train approaching a tunnel at near- light speed sees the tunnel as much shorter than it really is, whereas someone in the tunnel sees the approaching train as short.
Essentially, then, the Special Theory of Relativity can be boiled down to its two main postulates: firstly, that physical laws have the same mathematical form when expressed in any inertial system so that all motion, and the forces that result from it, is relative ; and secondly that the speed of light is independent of the motion of its source and of the observer, and so it is NOT relative to anything else and will always have the same value when measured by observers moving with constant velocity with respect to each other.
Not such a scary proposition at first glance, perhaps, but it does lead to some rather interesting implications, which we will begin to consider in subsequent sections. How fast are we traveling through space??