Theory of relativity: Difference between revisions
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* Orbits [[precession|precess]] in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of [[Mercury (planet)|Mercury]] and in [[binary pulsar]]s). |
* Orbits [[precession|precess]] in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of [[Mercury (planet)|Mercury]] and in [[binary pulsar]]s). |
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* Even rays of [[light]] (which are weightless) [[General relativity#Bending of light|bend in the presence of a gravitational field]]. |
* Even rays of [[light]] (which are weightless) [[General relativity#Bending of light|bend in the presence of a gravitational field]]. |
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* [[Expansion of the universe|The Universe is expanding]], and the far parts of it are moving away from us faster than the [[speed of light]]. |
* [[Expansion of the universe|The Universe is expanding]], and the far parts of it are moving away from us faster than the [[speed of light]]. Observe that this does not contradict the theory of special relativity, since it is space itself that is expanding. |
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* [[Frame-dragging]], in which a rotating mass "drags along" the space time around it. |
* [[Frame-dragging]], in which a rotating mass "drags along" the space time around it. |
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Revision as of 20:11, 22 April 2007
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Spacetime_curvature.png/300px-Spacetime_curvature.png)
The theory of relativity, or simply relativity, refers specifically to two theories: Albert Einstein's special relativity and general relativity.
The term "relativity" was coined by Max Planck in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity.
Special relativity
Special relativity is a theory of the structure of spacetime. It was introduced in Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". 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 (Galileo's 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 has many surprising consequences. Some of these are:
- Time dilation: Moving clocks tick slower than an observer's "stationary" clock.
- Length contraction: Objects appear shorter along the direction in which they are moving to an outside observer.
- Relativity of simultaneity: two events that appear simultaneous to an observer A will not be simultaneous to an observer B if B is moving with respect to A.
- Mass-energy equivalence: per the relationship E = mc², energy and mass are equivalent and transmutable.
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 of the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion: In other words 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 intertially 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:
- Time goes slower at lower gravitational potentials. 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).
- Even rays of light (which are weightless) bend in the presence of a gravitational field.
- The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light. Observe that this does not contradict the theory of special relativity, since it is space itself that is expanding.
- Frame-dragging, in which a rotating mass "drags along" the space time around it.
Technically, general relativity is a metric 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 intertially.
References and links
See the special relativity references and the general relativity references. For information on the silent film produced on this subject, see The Einstein Theory of Relativity.
External links
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/38px-Wikisource-logo.svg.png)
- Living Reviews in Relativity — An open access, peer-referred, solely online physics journal publishing invited reviews covering all areas of relativity research.
- Relativity - How to explain Einstein's theory?
- Reflections on Relativity — A complete online course on Relativity.
- Relativity explained in words of four letters or less
- Briefing on Einstein's Theory of Relativity — A terse dose of insight on the subject.
- On the Electrodynamics of Moving Bodies
- Special Relativity Simulator
- A Relativity Tutorial at Caltech — A basic introduction to concepts of Special and General Relativity, as well as astrophysics.
- Relativity Gravity and Cosmology — A short course offered at MIT.
- Relativity in film clips and animations from the University of New South Wales.