Theory of General and Special Relativity are both immensely well known theories that discuss gravity in some aspect or another.&nbsp;The chief difference that can be made between them is the level of completeness. But what exactly does each theory talk about?General Relativity discusses the laws of gravitation and its relation to other forces of nature.Special Relativity discusses certain phenomena in the absence of gravity.General Relativity also explores the astrophysical realm more closely than Special Relativity does.Both theories are strangely intriguing to learn about.

General Relativity V.S. Special Relativity 

Space

The theory of relativity usually encompasses two interrelated theories by <a dir="ltr" href="https://en.wikipedia.org/wiki/Albert_Einstein" title="Albert Einstein">Albert Einstein </a>: <a dir="ltr" href="https://en.wikipedia.org/wiki/Special_relativity" title="Special relativity">special relativity </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/General_relativity" title="General relativity">general relativity </a>, proposed and published in 1905 and 1915, respectively.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-1">[1] </a> Special relativity applies to all physical phenomena in the absence of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravity" title="Gravity">gravity </a>. General relativity explains the law of gravitation and its relation to other forces of nature.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-londontimes-2">[2] </a> It applies to the <a dir="ltr" href="https://en.wikipedia.org/wiki/Cosmological" title="Cosmological">cosmological </a>and astrophysical realm, including astronomy.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a>

The theory transformed <a dir="ltr" href="https://en.wikipedia.org/wiki/Theoretical_physics" title="Theoretical physics">theoretical physics </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Astronomy" title="Astronomy">astronomy </a>during the 20th century, superseding a 200-year-old <a dir="ltr" href="https://en.wikipedia.org/wiki/Classical_mechanics" title="Classical mechanics">theory of mechanics </a>created primarily by <a dir="ltr" href="https://en.wikipedia.org/wiki/Isaac_Newton" title="Isaac Newton">Isaac Newton </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-spacetime-4">[4] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-fitz-loren-5">[5] </a> It introduced concepts including <a dir="ltr" href="https://en.wikipedia.org/wiki/Spacetime" title="Spacetime">spacetime </a>as a unified entity of <a dir="ltr" href="https://en.wikipedia.org/wiki/Space" title="Space">space </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Time_in_physics" title="Time in physics">time </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Relativity_of_simultaneity" title="Relativity of simultaneity">relativity of simultaneity </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Kinematics" title="Kinematics">kinematic </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravity" title="Gravity">gravitational </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Time_dilation" title="Time dilation">time dilation </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Length_contraction" title="Length contraction">length contraction </a>. In the field of physics, relativity improved the science of <a dir="ltr" href="https://en.wikipedia.org/wiki/Elementary_particles" title="Elementary particles">elementary particles </a>and their fundamental interactions, along with ushering in the <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_age" title="Atomic age">nuclear age </a>. With relativity, <a dir="ltr" href="https://en.wikipedia.org/wiki/Physical_cosmology" title="Physical cosmology">cosmology </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Astrophysics" title="Astrophysics">astrophysics </a>predicted extraordinary <a dir="ltr" href="https://en.wikipedia.org/wiki/Astronomy" title="Astronomy">astronomical phenomena </a>such as <a dir="ltr" href="https://en.wikipedia.org/wiki/Neutron_stars" title="Neutron stars">neutron stars </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Black_holes" title="Black holes">black holes </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_waves" title="Gravitational waves">gravitational waves </a>.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-spacetime-4">[4] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-fitz-loren-5">[5] </a>

<a dir="ltr" href="https://en.wikipedia.org/wiki/Albert_Einstein" title="Albert Einstein">Albert Einstein </a>published the theory of <a dir="ltr" href="https://en.wikipedia.org/wiki/Special_relativity" title="Special relativity">special relativity </a>in 1905, building on many theoretical results and empirical findings obtained by <a dir="ltr" href="https://en.wikipedia.org/wiki/Albert_A._Michelson" title="Albert A. Michelson">Albert A. Michelson </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Hendrik_Lorentz" title="Hendrik Lorentz">Hendrik Lorentz </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Henri_Poincar%C3%A9" title="Henri Poincaré">Henri Poincaré </a>and others. <a dir="ltr" href="https://en.wikipedia.org/wiki/Max_Planck" title="Max Planck">Max Planck </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Hermann_Minkowski" title="Hermann Minkowski">Hermann Minkowski </a>and others did subsequent work.

Einstein developed <a dir="ltr" href="https://en.wikipedia.org/wiki/General_relativity" title="General relativity">general relativity </a>between 1907 and 1915, with contributions by many others after 1915. The final form of general relativity was published in 1916.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a>

The term "theory of relativity" was based on the expression "relative theory" (<a dir="ltr" href="https://en.wikipedia.org/wiki/German_language" title="German language">German </a>: Relativtheorie ) used in 1906 by Planck, who emphasized how the theory uses the <a dir="ltr" href="https://en.wikipedia.org/wiki/Principle_of_relativity" title="Principle of relativity">principle of relativity </a>. In the discussion section of the same paper, <a dir="ltr" href="https://en.wikipedia.org/wiki/Alfred_Bucherer" title="Alfred Bucherer">Alfred Bucherer </a>used for the first time the expression "theory of relativity" (<a dir="ltr" href="https://en.wikipedia.org/wiki/German_language" title="German language">German </a>: Relativitätstheorie ).<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-6">[6] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-7">[7] </a>

By the 1920s, the physics community understood and accepted special relativity.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-8">[8] </a> It rapidly became a significant and necessary tool for theorists and experimentalists in the new fields of <a dir="ltr" href="https://en.wikipedia.org/wiki/Atomic_physics" title="Atomic physics">atomic physics </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/Nuclear_physics" title="Nuclear physics">nuclear physics </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Quantum_mechanics" title="Quantum mechanics">quantum mechanics </a>.

By comparison, general relativity did not appear to be as useful, beyond making minor corrections to predictions of Newtonian gravitation theory.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a> It seemed to offer little potential for experimental test, as most of its assertions were on an astronomical scale. Its <a dir="ltr" href="https://en.wikipedia.org/wiki/Tensor_analysis" title="Tensor analysis">mathematics </a>seemed difficult and fully understandable only by a small number of people. Around 1960, general relativity became central to physics and astronomy. New mathematical techniques to apply to general relativity streamlined calculations and made its concepts more easily visualized. As astronomical <a dir="ltr" href="https://en.wikipedia.org/wiki/Phenomena" title="Phenomena">phenomena </a>were discovered, such as <a dir="ltr" href="https://en.wikipedia.org/wiki/Quasars" title="Quasars">quasars </a>(1963), the 3-kelvin <a dir="ltr" href="https://en.wikipedia.org/wiki/Microwave_background_radiation" title="Microwave background radiation">microwave background radiation </a>(1965), <a dir="ltr" href="https://en.wikipedia.org/wiki/Pulsar" title="Pulsar">pulsars </a>(1967), and the first <a dir="ltr" href="https://en.wikipedia.org/wiki/Black_hole" title="Black hole">black hole </a>candidates (1981),<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-relativity-3">[3] </a> the theory explained their attributes, and measurement of them further confirmed the theory.

Special relativity is a theory of the structure of <a dir="ltr" href="https://en.wikipedia.org/wiki/Spacetime" title="Spacetime">spacetime </a>. It was introduced in Einstein's 1905 paper "<a dir="ltr" href="https://en.wikipedia.org/wiki/On_the_Electrodynamics_of_Moving_Bodies" title="On the Electrodynamics of Moving Bodies">On the Electrodynamics of Moving Bodies </a>" (for the contributions of many other physicists see <a dir="ltr" href="https://en.wikipedia.org/wiki/History_of_special_relativity" title="History of special relativity">History of special relativity </a>). Special relativity is based on two postulates which are contradictory in <a dir="ltr" href="https://en.wikipedia.org/wiki/Classical_mechanics" title="Classical mechanics">classical mechanics </a>:

<ol><li>The laws of <a dir="ltr" href="https://en.wikipedia.org/wiki/Physics" title="Physics">physics </a>are the same for all observers in any <a dir="ltr" href="https://en.wikipedia.org/wiki/Inertial_frame_of_reference" title="Inertial frame of reference">inertial frame of reference </a>relative to one another (<a dir="ltr" href="https://en.wikipedia.org/wiki/Principle_of_relativity" title="Principle of relativity">principle of relativity </a>). </li><li>The <a dir="ltr" href="https://en.wikipedia.org/wiki/Speed_of_light" title="Speed of light">speed of light </a>in a <a dir="ltr" href="https://en.wikipedia.org/wiki/Vacuum" title="Vacuum">vacuum </a>is the same for all observers, regardless of their relative motion or of the motion of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Light" title="Light">light </a>source. </li> </ol>

The resultant theory copes with experiment better than classical mechanics. For instance, postulate 2 explains the results of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment" title="Michelson–Morley experiment">Michelson–Morley experiment </a>. Moreover, the theory has many surprising and counterintuitive consequences. Some of these are:

<ul><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Relativity_of_simultaneity" title="Relativity of simultaneity">Relativity of simultaneity </a>: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Time_dilation" title="Time dilation">Time dilation </a>: Moving <a dir="ltr" href="https://en.wikipedia.org/wiki/Clock" title="Clock">clocks </a>are measured to tick more slowly than an observer's "stationary" clock. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Length_contraction" title="Length contraction">Length contraction </a>: Objects are measured to be shortened in the direction that they are moving with respect to the observer. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Speed_of_light#Upper_limit_on_speeds" title="Speed of light">Maximum speed is finite </a>: No physical object, message or field line can travel faster than the speed of light in a vacuum. <ul><li>The effect of gravity can only travel through space at the speed of light, not faster or instantaneously. </li> </ul> </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence" title="Mass–energy equivalence">Mass–energy equivalence </a>: E = mc 2 , energy and mass are equivalent and transmutable. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Mass_in_special_relativity" title="Mass in special relativity">Relativistic mass </a>, idea used by some researchers.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-:0-9">[9] </a> </li> </ul>

The defining feature of special relativity is the replacement of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Galilean_transformation" title="Galilean transformation">Galilean transformations </a>of classical mechanics by the <a dir="ltr" href="https://en.wikipedia.org/wiki/Lorentz_transformation" title="Lorentz transformation">Lorentz transformations </a>. (See <a dir="ltr" href="https://en.wikipedia.org/wiki/Maxwell%27s_equations" title="Maxwell's equations">Maxwell's equations </a>of <a dir="ltr" href="https://en.wikipedia.org/wiki/Electromagnetism" title="Electromagnetism">electromagnetism </a>.)

General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the <a dir="ltr" href="https://en.wikipedia.org/wiki/Equivalence_principle" title="Equivalence principle">equivalence principle </a>, under which the states of <a dir="ltr" href="https://en.wikipedia.org/wiki/Accelerated_motion" title="Accelerated motion">accelerated motion </a>and being at rest in a <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravity" title="Gravity">gravitational field </a>(for example, when standing on the surface of the Earth) are physically identical. The upshot of this is that <a dir="ltr" href="https://en.wikipedia.org/wiki/Free_fall" title="Free fall">free fall </a>is <a dir="ltr" href="https://en.wikipedia.org/wiki/Inertia" title="Inertia">inertial motion </a>: an object in free fall is falling because that is how objects move when there is no <a dir="ltr" href="https://en.wikipedia.org/wiki/Force" title="Force">force </a>being exerted on them, instead of this being due to the force of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravity" title="Gravity">gravity </a>as is the case in <a dir="ltr" href="https://en.wikipedia.org/wiki/Classical_mechanics" title="Classical mechanics">classical mechanics </a>. This is incompatible with classical mechanics and <a dir="ltr" href="https://en.wikipedia.org/wiki/Special_relativity" title="Special relativity">special relativity </a>because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is <a dir="ltr" href="https://en.wikipedia.org/wiki/Curvature" title="Curvature">curved </a>. In 1915, he devised the <a dir="ltr" href="https://en.wikipedia.org/wiki/Einstein_field_equations" title="Einstein field equations">Einstein field equations </a>which relate the curvature of spacetime with the mass, energy, and any momentum within it.

Some of the consequences of general relativity are:

<ul><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_time_dilation" title="Gravitational time dilation">Gravitational time dilation </a>: Clocks run slower in deeper gravitational wells.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-10">[10] </a> </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Precession#Relativistic_(Einsteinian)" title="Precession">Precession </a>: Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of <a dir="ltr" href="https://en.wikipedia.org/wiki/Mercury_(planet)" title="Mercury (planet)">Mercury </a>and in <a dir="ltr" href="https://en.wikipedia.org/wiki/Binary_pulsar" title="Binary pulsar">binary pulsars </a>). </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/General_relativity#Light_deflection_and_gravitational_time_delay" title="General relativity">Light deflection </a>: Rays of <a dir="ltr" href="https://en.wikipedia.org/wiki/Light" title="Light">light </a>bend in the presence of a gravitational field. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Frame-dragging" title="Frame-dragging">Frame-dragging </a>: Rotating masses "drag along" the <a dir="ltr" href="https://en.wikipedia.org/wiki/Spacetime" title="Spacetime">spacetime </a>around them. </li><li><a dir="ltr" href="https://en.wikipedia.org/wiki/Metric_expansion_of_space" title="Metric expansion of space">Metric expansion of space </a>: the universe is expanding, and the far parts of it are moving away from us <a dir="ltr" href="https://en.wikipedia.org/wiki/Faster-than-light#Universal_expansion" title="Faster-than-light">faster than the speed of light </a>. </li> </ul>

Technically, general relativity is a theory of <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitation" title="Gravitation">gravitation </a>whose defining feature is its use of the <a dir="ltr" href="https://en.wikipedia.org/wiki/Einstein_field_equations" title="Einstein field equations">Einstein field equations </a>. The solutions of the field equations are <a dir="ltr" href="https://en.wikipedia.org/wiki/Metric_tensor_(general_relativity)" title="Metric tensor (general relativity)">metric tensors </a>which define the <a dir="ltr" href="https://en.wikipedia.org/wiki/Topology" title="Topology">topology </a>of the spacetime and how objects move inertially.

Einstein stated that the theory of relativity belongs to a class of "principle-theories". As such, it employs an analytic method, which means that the elements of this theory are not based on hypothesis but on empirical discovery. By observing natural processes, we understand their general characteristics, devise mathematical models to describe what we observed, and by analytical means we deduce the necessary conditions that have to be satisfied. Measurement of separate events must satisfy these conditions and match the theory's conclusions.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-londontimes-2">[2] </a>

Relativity is a <a dir="ltr" href="https://en.wikipedia.org/wiki/Falsifiability" title="Falsifiability">falsifiable </a>theory: It makes predictions that can be tested by experiment. In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-faq-11">[11] </a> The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 1881 and 1938 were critical to its validation. These are the <a dir="ltr" href="https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment" title="Michelson–Morley experiment">Michelson–Morley experiment </a>, the <a dir="ltr" href="https://en.wikipedia.org/wiki/Kennedy%E2%80%93Thorndike_experiment" title="Kennedy–Thorndike experiment">Kennedy–Thorndike experiment </a>, and the <a dir="ltr" href="https://en.wikipedia.org/wiki/Ives%E2%80%93Stilwell_experiment" title="Ives–Stilwell experiment">Ives–Stilwell experiment </a>. Einstein derived the <a dir="ltr" href="https://en.wikipedia.org/wiki/Lorentz_transformation" title="Lorentz transformation">Lorentz transformations </a>from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.

<a dir="ltr" href="https://en.wikipedia.org/wiki/Maxwell%27s_equations" title="Maxwell's equations">Maxwell's equations </a>—the foundation of classical electromagnetism—describe light as a wave that moves with a characteristic velocity. 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 <a dir="ltr" href="https://en.wikipedia.org/wiki/Luminiferous_aether" title="Luminiferous aether">luminiferous aether </a>, at rest relative to the "fixed stars" and through which the Earth moves. Fresnel's <a dir="ltr" href="https://en.wikipedia.org/wiki/Aether_drag_hypothesis#Partial_aether_dragging" title="Aether drag hypothesis">partial ether dragging hypothesis </a>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.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-maxb-12">[12] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-Pais_1982_111%E2%80%93113-13">[13] </a>

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 <a dir="ltr" href="https://en.wikipedia.org/wiki/Michelson_interferometer" title="Michelson interferometer">Michelson interferometer </a>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 1881,<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-michel1-14">[14] </a> and again in 1887.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-michel2-15">[15] </a> Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-Pais_1982_111%E2%80%93113-13">[13] </a> In an attempt to salvage the aether paradigm, FitzGerald and Lorentz independently created an <a dir="ltr" href="https://en.wikipedia.org/wiki/Ad_hoc_hypothesis" title="Ad hoc hypothesis">ad hoc hypothesis </a>in which the length of material bodies changes according to their motion through the aether.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-16">[16] </a> This was the origin of <a dir="ltr" href="https://en.wikipedia.org/wiki/FitzGerald%E2%80%93Lorentz_contraction" title="FitzGerald–Lorentz contraction">FitzGerald–Lorentz contraction </a>, and their hypothesis had no theoretical basis. The interpretation of the null result of the Michelson–Morley experiment is that the round-trip travel time for light is <a dir="ltr" href="https://en.wikipedia.org/wiki/Isotropic" title="Isotropic">isotropic </a>(independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-robertson-17">[17] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-tw-18">[18] </a>

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 <a dir="ltr" href="https://en.wikipedia.org/wiki/Inertial_frame" title="Inertial frame">inertial frames </a>. The Kennedy–Thorndike experiment was designed to do that, and was first performed in 1932 by Roy Kennedy and Edward Thorndike.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-KT-19">[19] </a> They obtained a null result, and concluded that "there is no effect ... unless the velocity of the solar system in space is no more than about half that of the earth in its orbit".<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-tw-18">[18] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-20">[20] </a> That possibility was thought to be too coincidental to provide an acceptable explanation, so from the null result of their experiment it was concluded that the round-trip time for light is the same in all inertial reference frames.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-robertson-17">[17] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-tw-18">[18] </a>

The Ives–Stilwell experiment was carried out by Herbert Ives and G.R. Stilwell first in 1938<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-21">[21] </a> and with better accuracy in 1941.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-Ives1941-22">[22] </a> It was designed to test the <a dir="ltr" href="https://en.wikipedia.org/wiki/Transverse_Doppler_effect" title="Transverse Doppler effect">transverse Doppler effect </a> – the <a dir="ltr" href="https://en.wikipedia.org/wiki/Redshift" title="Redshift">redshift </a>of light from a moving source in a direction perpendicular to its velocity—which had been predicted by Einstein in 1905. The strategy was to compare observed Doppler shifts with what was predicted by classical theory, and look for a <a dir="ltr" href="https://en.wikipedia.org/wiki/Lorentz_factor" title="Lorentz factor">Lorentz factor </a>correction. Such a correction was observed, from which was concluded that the frequency of a moving atomic clock is altered according to special relativity.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-robertson-17">[17] </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-tw-18">[18] </a>

Those classic experiments have been repeated many times with increased precision. Other experiments include, for instance, <a dir="ltr" href="https://en.wikipedia.org/wiki/Tests_of_relativistic_energy_and_momentum" title="Tests of relativistic energy and momentum">relativistic energy and momentum increase </a>at high velocities, <a dir="ltr" href="https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation" title="Experimental testing of time dilation">experimental testing of time dilation </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation" title="Modern searches for Lorentz violation">modern searches for Lorentz violations </a>.

General relativity has also been confirmed many times, the classic experiments being the perihelion precession of <a dir="ltr" href="https://en.wikipedia.org/wiki/Mercury_(planet)" title="Mercury (planet)">Mercury </a>'s orbit, the <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_lens" title="Gravitational lens">deflection of light </a>by the <a dir="ltr" href="https://en.wikipedia.org/wiki/Sun" title="Sun">Sun </a>, and the <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_redshift" title="Gravitational redshift">gravitational redshift </a>of light. Other tests confirmed the <a dir="ltr" href="https://en.wikipedia.org/wiki/Equivalence_principle" title="Equivalence principle">equivalence principle </a>and <a dir="ltr" href="https://en.wikipedia.org/wiki/Frame_dragging" title="Frame dragging">frame dragging </a>.

Far from being simply of theoretical interest, relativistic 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. Global positioning systems such as <a dir="ltr" href="https://en.wikipedia.org/wiki/GPS" title="GPS">GPS </a>, <a dir="ltr" href="https://en.wikipedia.org/wiki/GLONASS" title="GLONASS">GLONASS </a>, and <a dir="ltr" href="https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)" title="Galileo (satellite navigation)">Galileo </a>, must account for all of the relativistic effects, such as the consequences of Earth's gravitational field, in order to work with precision.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-23">[23] </a> This is also the case in the high-precision measurement of time.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-Francis2002-24">[24] </a> Instruments ranging from electron microscopes to particle accelerators would not work if relativistic considerations were omitted.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-25">[25] </a>

The spacetime symmetry group for Special Relativity is the <a dir="ltr" href="https://en.wikipedia.org/wiki/Poincar%C3%A9_group" title="Poincaré group">Poincaré group </a>, which is a ten-dimensional group of three Lorentz boosts, three rotations, and four spacetime translations. It is logical to ask what symmetries if any might apply in General Relativity. A tractable case may be to consider the symmetries of spacetime as seen by observers located far away from all sources of the gravitational field. The naive expectation for asymptotically flat spacetime symmetries might be simply to extend and reproduce the symmetries of flat spacetime of special relativity, viz. , the Poincaré group.

In 1962 <a dir="ltr" href="https://en.wikipedia.org/wiki/Hermann_Bondi" title="Hermann Bondi">Hermann Bondi </a>, M. G. van der Burg, A. W. Metzner<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-bondi_etal_1962-26">[26] </a> and <a dir="ltr" href="https://en.wikipedia.org/wiki/Rainer_K._Sachs" title="Rainer K. Sachs">Rainer K. Sachs </a><a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-sachs1962-27">[27] </a> addressed this <a dir="ltr" href="https://en.wikipedia.org/wiki/Bondi%E2%80%93Metzner%E2%80%93Sachs_group" title="Bondi–Metzner–Sachs group">asymptotic symmetry </a>problem in order to investigate the flow of energy at infinity due to propagating <a dir="ltr" href="https://en.wikipedia.org/wiki/Gravitational_wave" title="Gravitational wave">gravitational waves </a>. Their first step was to decide on some physically sensible boundary conditions to place on the gravitational field at light-like infinity to characterize what it means to say a metric is asymptotically flat, making no a priori assumptions about the nature of the asymptotic symmetry group — not even the assumption that such a group exists. Then after designing what they considered to be the most sensible boundary conditions, they investigated the nature of the resulting asymptotic symmetry transformations that leave invariant the form of the boundary conditions appropriate for asymptotically flat gravitational fields. What they found was that the asymptotic symmetry transformations actually do form a group and the structure of this group does not depend on the particular gravitational field that happens to be present. This means that, as expected, one can separate the kinematics of spacetime from the dynamics of the gravitational field at least at spatial infinity. The puzzling surprise in 1962 was their discovery of a rich infinite-dimensional group (the so-called BMS group) as the asymptotic symmetry group, instead of the finite-dimensional Poincaré group, which is a subgroup of the BMS group. Not only are the Lorentz transformations asymptotic symmetry transformations, there are also additional transformations that are not Lorentz transformations but are asymptotic symmetry transformations. In fact, they found an additional infinity of transformation generators known as supertranslations . This implies the conclusion that General Relativity does not reduce to special relativity in the case of weak fields at long distances.<a dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_note-strominger2017-28">[28] </a> :35

<ol><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-21" title="Jump up">^ </a> <cite dir="ltr">Ives, H.E.; Stilwell, G.R. (1938). "An experimental study of the rate of a moving atomic clock". Journal of the Optical Society of America . 28 (7): 215. <a dir="ltr" href="https://en.wikipedia.org/wiki/Bibcode_(identifier)" title="Bibcode (identifier)">Bibcode </a>:<a dir="ltr" href="https://ui.adsabs.harvard.edu/abs/1938JOSA...28..215I" rel="nofollow">1938JOSA...28..215I </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/Doi_(identifier)" title="Doi (identifier)">doi </a>:<a dir="ltr" href="https://doi.org/10.1364%2FJOSA.28.000215" rel="nofollow">10.1364/JOSA.28.000215 </a>. </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-Ives1941_22-0" title="Jump up">^ </a> <cite dir="ltr">Ives, H.E.; Stilwell, G.R. (1941). "An experimental study of the rate of a moving atomic clock. II". Journal of the Optical Society of America . 31 (5): 369. <a dir="ltr" href="https://en.wikipedia.org/wiki/Bibcode_(identifier)" title="Bibcode (identifier)">Bibcode </a>:<a dir="ltr" href="https://ui.adsabs.harvard.edu/abs/1941JOSA...31..369I" rel="nofollow">1941JOSA...31..369I </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/Doi_(identifier)" title="Doi (identifier)">doi </a>:<a dir="ltr" href="https://doi.org/10.1364%2FJOSA.31.000369" rel="nofollow">10.1364/JOSA.31.000369 </a>. </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-23" title="Jump up">^ </a> <cite dir="ltr"><a dir="ltr" href="https://web.archive.org/web/20151105155910/http://relativity.livingreviews.org/Articles/lrr-2003-1/download/lrr-2003-1Color.pdf" rel="nofollow">"Archived copy" </a>(PDF) . Archived from <a dir="ltr" href="http://relativity.livingreviews.org/Articles/lrr-2003-1/download/lrr-2003-1Color.pdf" rel="nofollow">the original </a>(PDF) on 2015-11-05. Retrieved 2015-12-09 . </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-Francis2002_24-0" title="Jump up">^ </a> <cite dir="ltr">Francis, S.; B. Ramsey; S. Stein; Leitner, J.; Moreau, J.M.; Burns, R.; Nelson, R.A.; Bartholomew, T.R.; Gifford, A. (2002). <a dir="ltr" href="https://web.archive.org/web/20130217211012/http://tycho.usno.navy.mil/ptti/ptti2002/paper20.pdf" rel="nofollow">"Timekeeping and Time Dissemination in a Distributed Space-Based Clock Ensemble" </a>(PDF) . Proceedings 34th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting : 201–214. Archived from <a dir="ltr" href="http://tycho.usno.navy.mil/ptti/ptti2002/paper20.pdf" rel="nofollow">the original </a>(PDF) on 17 February 2013. Retrieved 14 April 2013 . </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-25" title="Jump up">^ </a> <cite dir="ltr">Hey, Tony; Hey, Anthony J. G.; Walters, Patrick (1997). <a dir="ltr" href="https://archive.org/details/isbn_9780521435321" rel="nofollow">Einstein's Mirror </a> (illustrated ed.). Cambridge University Press. p. x (preface). <a dir="ltr" href="https://en.wikipedia.org/wiki/ISBN_(identifier)" title="ISBN (identifier)">ISBN </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Special:BookSources/978-0-521-43532-1" title="Special:BookSources/978-0-521-43532-1"><bdi dir="ltr">978-0-521-43532-1 </bdi> </a>. </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-bondi_etal_1962_26-0" title="Jump up">^ </a> <cite dir="ltr">Bondi, H.; Van der Burg, M.G.J.; Metzner, A. (1962). "Gravitational waves in general relativity: VII. Waves from axisymmetric isolated systems". Proceedings of the Royal Society of London A . A269 (1336): 21–52. <a dir="ltr" href="https://en.wikipedia.org/wiki/Bibcode_(identifier)" title="Bibcode (identifier)">Bibcode </a>:<a dir="ltr" href="https://ui.adsabs.harvard.edu/abs/1962RSPSA.269...21B" rel="nofollow">1962RSPSA.269...21B </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/Doi_(identifier)" title="Doi (identifier)">doi </a>:<a dir="ltr" href="https://doi.org/10.1098%2Frspa.1962.0161" rel="nofollow">10.1098/rspa.1962.0161 </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/S2CID_(identifier)" title="S2CID (identifier)">S2CID </a> <a dir="ltr" href="https://api.semanticscholar.org/CorpusID:120125096" rel="nofollow">120125096 </a>. </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-sachs1962_27-0" title="Jump up">^ </a> <cite dir="ltr">Sachs, R. (1962). "Asymptotic symmetries in gravitational theory". Physical Review . 128 (6): 2851–2864. <a dir="ltr" href="https://en.wikipedia.org/wiki/Bibcode_(identifier)" title="Bibcode (identifier)">Bibcode </a>:<a dir="ltr" href="https://ui.adsabs.harvard.edu/abs/1962PhRv..128.2851S" rel="nofollow">1962PhRv..128.2851S </a>. <a dir="ltr" href="https://en.wikipedia.org/wiki/Doi_(identifier)" title="Doi (identifier)">doi </a>:<a dir="ltr" href="https://doi.org/10.1103%2FPhysRev.128.2851" rel="nofollow">10.1103/PhysRev.128.2851 </a>. </cite> </li><li><a aria-label="Jump up" dir="ltr" href="https://en.wikipedia.org/wiki/Theory_of_relativity#cite_ref-strominger2017_28-0" title="Jump up">^ </a> <cite dir="ltr">Strominger, Andrew (2017). "Lectures on the Infrared Structure of Gravity and Gauge Theory". <a dir="ltr" href="https://en.wikipedia.org/wiki/ArXiv_(identifier)" title="ArXiv (identifier)">arXiv </a>:<a dir="ltr" href="https://arxiv.org/abs/1703.05448" rel="nofollow">1703.05448 </a> . <q dir="ltr">...redacted transcript of a course given by the author at Harvard in spring semester 2016. It contains a pedagogical overview of recent developments connecting the subjects of soft theorems, the memory effect and asymptotic symmetries in four-dimensional QED, nonabelian gauge theory and gravity with applications to black holes. To be published Princeton University Press, 158 pages. </q> </cite> </li> </ol>

<ul><li><cite dir="ltr">Einstein, Albert (2005). Relativity: The Special and General Theory . Translated by Robert W. Lawson (The masterpiece science ed.). New York: Pi Press. <a dir="ltr" href="https://en.wikipedia.org/wiki/ISBN_(identifier)" title="ISBN (identifier)">ISBN </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Special:BookSources/978-0-13-186261-6" title="Special:BookSources/978-0-13-186261-6"><bdi dir="ltr">978-0-13-186261-6 </bdi> </a>. </cite> </li><li><cite dir="ltr">Einstein, Albert (1920). <a dir="ltr" href="https://www.ibiblio.org/ebooks/Einstein/Einstein_Relativity.pdf" rel="nofollow">Relativity: The Special and General Theory </a>(PDF) . Henry Holt and Company. </cite> </li><li><cite dir="ltr">Einstein, Albert; trans. Schilpp; Paul Arthur (1979). <a dir="ltr" href="https://archive.org/details/autobiographical1979eins" rel="nofollow">Albert Einstein, Autobiographical Notes </a> (A Centennial ed.). La Salle, IL: Open Court Publishing Co. <a dir="ltr" href="https://en.wikipedia.org/wiki/ISBN_(identifier)" title="ISBN (identifier)">ISBN </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Special:BookSources/978-0-87548-352-8" title="Special:BookSources/978-0-87548-352-8"><bdi dir="ltr">978-0-87548-352-8 </bdi> </a>. </cite> </li><li><cite dir="ltr">Einstein, Albert (2009). Einstein's Essays in Science . Translated by Alan Harris (Dover ed.). Mineola, NY: Dover Publications. <a dir="ltr" href="https://en.wikipedia.org/wiki/ISBN_(identifier)" title="ISBN (identifier)">ISBN </a> <a dir="ltr" href="https://en.wikipedia.org/wiki/Special:BookSources/978-0-486-47011-5" title="Special:BookSources/978-0-486-47011-5"><bdi dir="ltr">978-0-486-47011-5 </bdi> </a>. </cite> </li><li><cite dir="ltr">Einstein, Albert (1956) [1922]. <a dir="ltr" href="https://en.wikipedia.org/wiki/The_Meaning_of_Relativity" title="The Meaning of Relativity">The Meaning of Relativity </a> (5 ed.). Princeton University Press. </cite> </li><li><a dir="ltr" href="http://www.gutenberg.org/files/36276/36276-pdf.pdf?session_id=c6fd49e1b7f6dd81790ed141e999cf2bce859386" rel="nofollow">The Meaning of Relativity </a>Albert Einstein: Four lectures delivered at Princeton University, May 1921 </li> </ul>

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