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'''Relativity''' refers to two theories in [[physics]], and to a principle which led to the first theory. The theories, both developed by [[Albert Einstein]], are '''special relativity''' (SR) and '''general relativity (GR). Special relativity is a theory which describes the laws of motion for non-accelerating bodies traveling at a significant fraction of the speed of light; at speeds approaching zero, special relativity is identical to Newton's Laws of Motion. General relativity is a theory which explains the laws of motion as viewed from accelerating reference frames and includes a geometric explanation for gravity.

'''Relativity''' refers to two theories in [[physics]], and to a principle which led to the first theory. Special relativity (SR) is a theory which describes the laws of motion for non-accelerating bodies traveling at a significant fraction of the speed of light.  At speeds approaching zero, special relativity is identical to Newton's Laws of Motion. Special relativity was developed by [[Hendrik Lorentz]], [[Henri Poincaré]] and [[Albert Einstein]].

Einstein derives special relativity from two postulates:

General Relativity (GR) is a theory which explains the laws of motion as viewed from accelerating reference frames and includes a geometric explanation for gravity.  This theory was developed by [[Albert Einstein]] as an extension of the postulates of Special Relativity.  A dramatic but later discredited claim by Sir [[Arthur Eddington]] of experimental proof of General Relativity in 1919 made Einstein a household name.

 

 

Special Relativity is a mathematical theory derived entirely from 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 identical in all inertial reference frames.''

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

Einstein's theory of Special Relativity was based on theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]], working on problems in electrodynamics and the [[Michelson-Morley experiment]], which had not found any sign of [[luminoferous ether]], which was believed to be the substance which carried electromagnetic waves. Special relativity alters [[Isaac Newton]]'s laws of motion by assuming that the speed of light will be the same for all observers, despite their relative velocities and the source of the light. (Therefore, if A sends a beam of light to B, and both measure the speed, it will be the same for both, no matter what the relative velocity of A and B. In Newtonian/Galiliean mechanics, If A sends a physical object at a particular velocity towards B, and nothing slows it, the velocity of the object relative to B depends on the velocities of the object and of B relative to A.)

In laymen's terms, these postulates are equivalent to the following:

 

# ''There is nothing special about [[earth]] or its [[solar system]] in terms of the laws of physics, as every location in the universe is indistinguishable from every other with respect to the laws of physics.

 

Special Relativity (SR) was initially developed by [[Henri Poincaré]] and [[Hendrik Lorentz]], working on problems in electrodynamics and the [[Michelson-Morley experiment]], which had not found any sign of [[luminoferous ether]], which was believed to be the substance which carried electromagnetic waves. Special relativity alters [[Isaac Newton]]'s laws of motion by assuming that the speed of light will be the same for all observers, despite their relative velocities and the source of the light. (Therefore, if A sends a beam of light to B, and both measure the speed, it will be the same for both, no matter what the relative velocity of A and B. In Newtonian/Galiliean mechanics, If A sends a physical object at a particular velocity towards B, and nothing slows it, the velocity of the object relative to B depends on the velocities of the object and of B relative to A.)

At low speeds (relative to light-speed), the Einstein-Lorentz relativity equations are equivalent to Newton's equations. The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

At low speeds (relative to light-speed), the Einstein-Lorentz relativity equations are equivalent to Newton's equations. The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

General relativity predicts that space-time is curved by massive bodies, so that near any massive body, the sum of the angles in a triangle is not exactly 180 degrees.  

Relativity is important for massive or fast-moving bodies: at low mass and low speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modeling the behavior of electrons requires that relativistic effects be taken into account (the chemically significant phenomenon of [[electron spin]] arises from relativity), and the course light passing through a region containing many massive bodies such as galaxies will be distorted ([[classical mechanics]], in which light travels with infinite speed in straight lines, does not predict this). These are both experimentally confirmed ([[electron spin]] was known before relativity arose, and telescopic observations confirm that galactic clusters distort the paths of the light passing through them).  

Relativity is important for massive or fast-moving bodies: at low mass and low speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modeling the behavior of electrons requires that relativistic effects be taken into account (the chemically significant phenomenon of [[electron spin]] arises from relativity), and the course light passing through a region containing many massive bodies such as galaxies will be distorted ([[classical mechanics]], in which light travels with infinite speed in straight lines, does not predict this). These are both experimentally confirmed ([[electron spin]] was known before relativity arose, and telescopic observations confirm that galactic clusters distort the paths of the light passing through them).  

 

General Relativity is a mathematical extension of Special Relativity. GR proposes that space-time is curved by massive bodies, so that near any massive body, the sum of the angles in a triangle is not exactly 180 degrees.  

General relativity successfully explains the seemingly anomalous precession of Mercury's perihelion.  While other explanations based in Newtonian gravity had been proposed, none were consistent with observation.

General relativity provides one explanation for the seemingly anomalous precession of Mercury's perihelion.  There are other explanations based in Newtonian gravity, such as factoring in the pull of the other planets on Mercury's orbit.  One Newtonian explanation requires a slight alternation to the precise inverse-square relation of Newtonian gravity to distance, which is disfavored by mathematicians due to its inelegance in integrating.

British Historian Paul Johnson declares the turning point in 20th century to have been when fellow Briton Sir [[Arthur Eddington]], an esteemed English astronomer, ventured out on a boat off Africa in 1919 with a local Army unit to observe the bending of starlight around the sun during a total eclipse.  Upon his return to England declared that his observations proven the theory of relativity.  In fact recent analysis of Eddington's work revealed that he was biased in selecting his data, and that overall his data were inconclusive about the theory of relativity. The prediction was later confirmed by more rigorous experiments, such as those performed by the [[Hubble Space Telescope]] {{fact}}.  Lorentz has this to say on the discrepancies between the empirical eclipse data and Einstein's predictions.

British Historian Paul Johnson declares the turning point in 20th century to have been when fellow Briton Sir [[Arthur Eddington]], an esteemed English astronomer, ventured out on a boat off Africa in 1919 with a local Army unit to observe the bending of starlight around the sun during a total eclipse.  Upon his return to England declared that his observations proven the theory of relativity.  In fact recent analysis of Eddington's work revealed that he was biased in selecting his data, and that overall his data were inconclusive about the theory of relativity. The prediction was later confirmed by more rigorous experiments, such as those performed by the [[Hubble Space Telescope]] {{fact}}.  Lorentz has this to say on the discrepancies between the empirical eclipse data and Einstein's predictions.

The prediction that light is bent by gravity is predicted both by Newtonian physics and relativity, but relativity predicts a larger deflection.

The prediction that light is bent by gravity is predicted both by Newtonian physics and relativity, but relativity predicts a larger deflection.

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.