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Relativity refers to two [[physics]] theories; general relativity (GR) and special relativity (SR), put foward by [[Albert Einstein]]. The Theory of Relativity is a geometrical theory of gravitation, while Special Relativity is a limiting case. Einstein formed two postulates around which the theory is based:

Relativity refers to two [[physics]] theories; general relativity (GR) and special relativity (SR), popularized by [[Albert Einstein]]. General relativity is a geometrical theory of gravitation, while special relativity is a limiting case. Einstein explains special relativity in terms of two postulates:

# ''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

# ''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

# ''The laws of physics are obeyed in all reference frames.''

# ''The laws of physics are obeyed in all reference frames.''

The theory of relativity was first proposed based on mathematical theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]]. This theory differs from [[Isaac Newton]]'s theory of gravitation by disposing with the idea of a universal, mutually agreeable scale of time (i.e. a universal clock that all times can refer to) and space (i.e. a universal sheet of "graph paper", which location refers to).  At low speeds (relative to light-speed), the Einstein-Lorentzian relativity equations are equivalent to Newton's formulas.

Relativity was first proposed based on mathematical theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]]. This theory differs from [[Isaac Newton]]'s theory of gravitation by disposing with the idea of a universal, mutually agreeable scale of time (i.e. a universal clock that all times can refer to) and space (i.e. a universal sheet of "graph paper", which location refers to).  At low speeds (relative to light-speed), the Einstein-Lorentzian relativity equations are equivalent to Newton's formulas.

The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

==Special relativity==

==Special relativity==

 

Special relativity is the limiting case of general relativity where all gravitational fields are weak.  

Special relativity is the limiting case of General relativity where all gravitational fields are weak. It is based on two postulates; one, that the laws of physics are identical to all [[inertial observers]], and two, that the [[speed of light]] ''in vacuo'' is a universal constant.

===Time dilation===

===Time dilation===

===Length contraction===

===Length contraction===

When two inertial reference frames move past each other in a straight line with constant relative velocity, an observer in one reference frame would observe a metre rule in the other frame to be shorter.

When two inertial reference frames move past each other in a straight line with constant relative velocity, an observer in one reference frame would observe a metre rule in the other frame to be shorter.

==General Relativity==

==General Relativity==

The GR field equations are

The Einstein field equations is

:<math> G_{uv} = 8\pi\, T_{uv} </math>

:<math> G_{uv} = 8\pi\, T_{uv} </math>

where ''G_uv'' is the [[Einstein curvature tensor]], and ''T_uv'' is the [[stress-energy tensor]], ''G_uv'' and ''T_uv'' are both rank 2 symmetric tensors.  The Einstein field equations is a system of [[partial differential equations]] that relates the curvature of space to the mass occupying the space.

where ''G_uv'' is the [[Einstein curvature tensor]], and ''T_uv'' is the [[stress-energy tensor]], ''G_uv'' and ''T_uv'' are both rank 2 symmetric tensors.  The GR field equations is a system of [[partial differential equations]] that relates the curvature of space to the mass occupying the space.

==Evidence for Relativity==

==Evidence for Relativity==

==Government Support for Relativistic research==

==Government Support for Relativistic research==

The Theory of Relativity enjoys a disproportionate share of federal funding of physics research today,<ref>The Democratic Congress insisted on the $250 million LIGO project despite substantial criticism by scientists that it was wasting scarce research dollars.  John Travis, "LIGO: a $ 250 million gamble; Laser Interferometer Gravitational-Wave Observatory; includes related article," ''Science'' p. 612 (Apr. 30, 1993).  "Adding to the acrimony is LIGO's $ 250 million price tag, which some hold responsible for NSF's recent funding woes."  ''Id.''</ref> much of it unsuccessful.  The $365 million dollar LIGO project, for example, has failed to detect the gravity waves predicted by relativity.<ref>http://www.npr.org/programs/atc/features/2002/sept/gravitywaves/index.html</ref>  However, more than twenty years of observing the [[pulsar]] pair PSR 1913+16 have shown its orbital period to be dropping at exactly the rate expected due to loss of orbital energy by gravitational radiation <ref>http://nobelprize.org/nobel_prizes/physics/laureates/1993/press.html</ref>, resulting in the 1993 Nobel Prize for physics being awarded to the discoverers of the pulsar pair, Joseph Taylor and Russell Hulse.

The Theory of Relativity enjoys a disproportionate share of federal funding of physics research today,<ref>The Democratic Congress insisted on the $250 million LIGO project despite substantial criticism by scientists that it was wasting scarce research dollars.  John Travis, "LIGO: a $ 250 million gamble; Laser Interferometer Gravitational-Wave Observatory; includes related article," ''Science'' p. 612 (Apr. 30, 1993).  "Adding to the acrimony is LIGO's $ 250 million price tag, which some hold responsible for NSF's recent funding woes."  ''Id.''</ref> much of it unsuccessful.  The $365 million dollar LIGO project, for example, has failed to detect the gravity waves predicted by relativity.<ref>http://www.npr.org/programs/atc/features/2002/sept/gravitywaves/index.html</ref>  However, more than twenty years of observing the [[pulsar]] pair PSR 1913+16 have shown its orbital period to be dropping at exactly the rate expected due to loss of orbital energy by gravitational radiation <ref>http://nobelprize.org/nobel_prizes/physics/laureates/1993/press.html</ref>, resulting in the 1993 Nobel Prize for physics being awarded to the discoverers of the pulsar pair, Joseph Taylor and Russell Hulse.