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'''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]].

'''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]].

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.

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 [[David Hilbert]] and [[Albert Einstein]] as an extension of the postulates of Special Relativity.<ref>"[T]he German mathematician David Hilbert submitted an article containing the correct field equations for general relativity five days before Einstein."[http://nobelprize.org/educational_games/physics/relativity/history-1.html Nobel Prize historical account]</ref> 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 ==

== 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.  

===Time dilation===

==Time dilation==

[[Image:Light cone.png|right|thumb|Light-cone diagram]]

[[Image:Light cone.png|right|thumb|Light-cone diagram]]

One important consequence of SR's postulates is that an observer in one reference frame will observe a clock in another frame to be "ticking" more slowly than in the observer's own frame. This can be proven mathematically using basic geometry.

One important consequence of SR's postulates is that an observer in one reference frame will observe a clock in another frame to be "ticking" more slowly than in the observer's own frame. This can be proven mathematically using basic geometry.

:<math>c</math> is the speed of light (3x10⁸ ms^-1).

:<math>c</math> is the speed of light (3x10⁸ ms^-1).

===Mass increase===

==Mass increase==

We also see that as a body moves with increasing velocity its [[mass]] also increases.  

We also see that as a body moves with increasing velocity its [[mass]] also increases.  

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.