Two interrelated physics theories by Albert Einstein
This article is about the scientific concept. For philosophical or ontological theories about relativity, seeRelativism. For the silent film, seeThe Einstein Theory of Relativity.
Simulation of the mergerGW150914, showingspacetime distortion from gravity as the black holes orbit and merge
Thetheory of relativity usually encompasses two interrelatedphysics theories byAlbert Einstein:special relativity andgeneral relativity, proposed and published in 1905 and 1915, respectively.[1] Special relativity applies to all physical phenomena in the absence ofgravity. General relativity explains the law of gravitation and its relation to the forces of nature.[2] It applies to thecosmological and astrophysical realm, including astronomy.[3]
Einstein developedgeneral relativity between 1907 and 1915, with contributions by many others after 1915. The final form of general relativity was published in 1916.[3]
The term "theory of relativity" was based on the expression "relative theory" (German:Relativtheorie) used in 1906 by Planck, who emphasized how the theory uses theprinciple of relativity. In the discussion section of the same paper,Alfred Bucherer used for the first time the expression "theory of relativity" (German:Relativitätstheorie).[6][7]
By the 1920s, the physics community understood and accepted special relativity.[8] It rapidly became a significant and necessary tool for theorists and experimentalists in the new fields ofatomic physics,nuclear physics, andquantum mechanics.
By comparison, general relativity did not appear to be as useful, beyond making minor corrections to predictions of Newtonian gravitation theory.[3] It seemed to offer little potential for experimental test, as most of its assertions were on an astronomical scale. Itsmathematics 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 astronomicalphenomena were discovered, such asquasars (1963), the 3-kelvinmicrowave background radiation (1965),pulsars (1967), and the firstblack hole candidates (1981),[3] the theory explained their attributes, and measurement of them further confirmed the theory.
Thespeed of light invacuum is the same for all observers, regardless of their relative motion or of the motion of thelight source.
The resultant theory copes with experiment better than classical mechanics. For instance, postulate 2 explains the results of theMichelson–Morley experiment. Moreover, the theory has many surprising and counterintuitive consequences. Some of these are:
Relativity of simultaneity: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
Time dilation: Movingclocks are measured to tick more slowly than an observer's "stationary" clock.
Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in vacuum.
The effect of gravity can only travel through space at the speed of light, not faster or instantaneously.
General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with theequivalence principle, under which the states ofaccelerated motion and being at rest in agravitational field (for example, when standing on the surface of the Earth) are physically identical. The upshot of this is thatfree fall isinertial motion: an object in free fall is falling because that is how objects move when there is noforce being exerted on them, instead of this being due to the force ofgravity as is the case inclassical mechanics. This is incompatible with classical mechanics andspecial relativity 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 thatspacetime is curved. Einstein discussed his idea with mathematicianMarcel Grossmann and they concluded that general relativity could be formulated in the context ofRiemannian geometry which had been developed in the 1800s.[10]In 1915, he devised theEinstein field equations which relate the curvature of spacetime with the mass, energy, and any momentum within it.
Some of the consequences of general relativity are:
Technically, general relativity is a theory ofgravitation whose defining feature is its use of theEinstein field equations. The solutions of the field equations aremetric tensors which define thetopology of the spacetime and how objects move inertially.
Experimental evidence
Einstein explained that the theory of relativity falls under a category of scientific frameworks known as "principle-theories"—theories that start not from speculative constructs or imagined mechanisms, but from well-established empirical facts and observed regularities in nature. Unlike constructive theories, which attempt to build models of phenomena from assumed underlying processes, principle-theories, such as relativity, adopt an analytic approach: they begin with experimentally verified principles and work deductively to uncover the logical consequences and constraints that any physical process must obey. 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.[2]
Relativity is afalsifiable 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.[12] 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 theMichelson–Morley experiment, theKennedy–Thorndike experiment, and theIves–Stilwell experiment. Einstein derived theLorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.
Maxwell's equations—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 theluminiferous aether, at rest relative to the "fixed stars" and through which the Earth moves. 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.[13][14]
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 theMichelson interferometer to accomplish this. The apparatus was sufficiently accurate to detect the expected effects, but he obtained a null result when the first experiment was conducted in 1881,[15] and again in 1887.[16] Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community.[14] In an attempt to salvage the aether paradigm, FitzGerald and Lorentz independently created anad hoc hypothesis in which the length of material bodies changes according to their motion through the aether.[17] This was the origin ofFitzGerald–Lorentz contraction, 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 isisotropic (independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity.[18][19]
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 differentinertial frames. The Kennedy–Thorndike experiment was designed to do that, and was first performed in 1932 by Roy Kennedy and Edward Thorndike.[20] 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".[19][21] 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.[18][19]
The Ives–Stilwell experiment was carried out by Herbert Ives and G.R. Stilwell first in 1938[22] and with better accuracy in 1941.[23] It was designed to test thetransverse Doppler effect – theredshift 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 aLorentz factor correction. Such a correction was observed, from which was concluded that the frequency of a moving atomic clock is altered according to special relativity.[18][19]
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 asGPS,GLONASS, andGalileo, must account for all of the relativistic effects in order to work with precision, such as the consequences of the Earth's gravitational field.[24] This is also the case in the high-precision measurement of time.[25] Instruments ranging from electron microscopes to particle accelerators would not work if relativistic considerations were omitted.[26]
^Miller, Arthur I. (1981),Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911), Reading: Addison–Wesley,ISBN978-0-201-04679-3
^Einstein, A.;Grossmann, M. (1913). "Entwurf einer verallgemeinerten Relativitätstheorie und einer Theorie der Gravitation" [Outline of a Generalized Theory of Relativity and of a Theory of Gravitation].Zeitschrift für Mathematik und Physik.62:225–261.
^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.Bibcode:1941JOSA...31..369I.doi:10.1364/JOSA.31.000369.
^Ashby, N. Relativity in the Global Positioning System.Living Rev. Relativ.6, 1 (2003).doi:10.12942/lrr-2003-1"Archived copy"(PDF). Archived fromthe original(PDF) on 5 November 2015. Retrieved9 December 2015.{{cite web}}: CS1 maint: archived copy as title (link)
^Francis, S.; B. Ramsey; S. Stein; Leitner, J.; Moreau, J.M.; Burns, R.; Nelson, R.A.; Bartholomew, T.R.; Gifford, A. (2002)."Timekeeping and Time Dissemination in a Distributed Space-Based Clock Ensemble"(PDF).Proceedings 34th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting:201–214. Archived fromthe original(PDF) on 17 February 2013. Retrieved14 April 2013.
^Hey, Tony; Hey, Anthony J. G.; Walters, Patrick (1997).Einstein's Mirror (illustrated ed.). Cambridge University Press. p. x (preface).ISBN978-0-521-43532-1.
Further reading
Einstein, Albert (2005).Relativity: The Special and General Theory. Translated by Robert W. Lawson (The masterpiece science ed.). New York: Pi Press.ISBN978-0-13-186261-6.
Einstein, Albert (2009).Einstein's Essays in Science. Translated by Alan Harris (Dover ed.). Mineola, New York: Dover Publications.ISBN978-0-486-47011-5.
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