Theory of relativity

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The term "theory of relativity" was based on the expression "relative theory" (Template:Lang-de) used by Max Planck in 1906, who emphasized how the theory uativity]]. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity" (Template:Lang-de).^[1]

Scope

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 When first published, relativity superseded a 200-year-old theory of mechanics stated by Isaac Newton.^[2]^[3]^[4]

The theory of relativity overturned the concept of motion from Newton's day, by positing that all motion is relative. Time was no longer uniform and absolute. Physics could no longer be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved spacetime. Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds.^[2]^[3]^[4]

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 n''eutron stars, black holes, and gravitational waves.Cite error: A <ref> tag is missing the closing </ref> (see the help page).

The special theory of relativity and the general theory of relativity are connected. As stated below, special theory of relativity applies to all inertial physical phenomena except gravity. The general theory provides the law of gravitation, and its relation to other forces of nature.^[5]

Special relativity

Special relativity is a theory of the structure of spacetime. It was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" (for the contributions of many other physicists see History of special relativity). Special relativity is based on two postulates which are contradictory in classical mechanics:

The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

The resultant theory copes with experiment better than classical mechanics, e.g. in the Michelson-Morley experiment that supports postulate 2, but also has many surprising 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: Moving clocks 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.
Mass–energy equivalence: $E=mc^{2}$ , energy and mass are equivalent and transmutable.
Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in a vacuum.

The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).

General relativity

General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion; an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special 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 that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:

Clocks run more slowly in deeper gravitational wells.^[6] This is called gravitational time dilation.
Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
Rays of light bend in the presence of a gravitational field.
Rotating masses "drag along" the spacetime around them; a phenomenon termed "frame-dragging".
The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light.

Technically, general relativity is a theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.

Experimental evidence

Relativistic effects such as

two-way light-speed isotropy (Michelson–Morley experiment),
velocity and gravitational time dilation (Ives–Stilwell experiment, Pound–Rebka experiment),
relativistic energy and momentum increase at high velocities,

and many other effects have been tested and confirmed in various experiments.

History

The history of special relativity consists of many theoretical results and empirical findings obtained by Albert Michelson, Hendrik Lorentz, Henri Poincaré and others. It culminated in the theory of special relativity proposed by Albert Einstein, and subsequent work of Max Planck, Hermann Minkowski and others.

General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915, with contributions by many others after 1915.

Minority views

Einstein's contemporaries did not all accept his new theories at once. However, the theory of relativity is now considered as a cornerstone of modern physics, see Criticism of relativity theory.

Although it is widely acknowledged that Einstein was the creator of relativity in its modern understanding, some believe that others deserve credit for it, see Relativity priority dispute.

References

^ Planck, Max (1906), The Measurements of Kaufmann on the Deflectability of β-Rays in their Importance for the Dynamics of the Electrons , Reading: Addison–ley, ISBN 0-201-04679-2 {{citation}}: Text "jour1905–1911)" ignored (help)
^ ^a ^b Cite error: The named reference relativity was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference spacetime was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference fitz-loren was invoked but never defined (see the help page).
^ Cite error: The named reference londontimes was invoked but never defined (see the help page).
^ Feynman, Richard Phillips; Morínigo, Fernando B.; Wagner, William; Pines, David; Hatfield, Brian (2002). Feynman Lectures on Gravitation. West view Press. p. 68. ISBN 0-8133-4038-1., Lecture 5

External links

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[1] Planck, Max (1906), The Measurements of Kaufmann on the Deflectability of β-Rays in their Importance for the Dynamics of the Electrons , Reading: Addison–ley, ISBN 0-201-04679-2 {{citation}}: Text "jour1905–1911)" ignored (help)

[relativity-2] Cite error: The named reference relativity was invoked but never defined (see the help page).

[spacetime-3] Cite error: The named reference spacetime was invoked but never defined (see the help page).

[fitz-loren-4] Cite error: The named reference fitz-loren was invoked but never defined (see the help page).

[londontimes-5] Cite error: The named reference londontimes was invoked but never defined (see the help page).

[6] Feynman, Richard Phillips; Morínigo, Fernando B.; Wagner, William; Pines, David; Hatfield, Brian (2002). Feynman Lectures on Gravitation. West view Press. p. 68. ISBN 0-8133-4038-1., Lecture 5

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