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The theory of relativity was first proposed based on mathematical theory (derived from a series of thought experiments by Einstein) rather than observation.  The theory rests on two postulates that are themselves difficult to test, and then derives mathematically what the physical consequences should be (and hence experimentally testable predictions). Einstein's thought experiments pondered what would happen if the speed of light was constant in all frames of reference, and all laws of physics were the same in every (inertial) frame of reference no matter where it is or how fast it is traveling.  This theory improves on [[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). In general terms, relativity predicts that space-time can be curved by massive bodies, so that (for example) near a [[black hole]] the sum of the angles in a triangle is not exactly 180 degrees, time passes more rapidly away from a black hole than near it (for a distant observer) and other apparent violations of [[geometry]] and common sense.

The theory of relativity was first proposed based on mathematical theory (derived from a series of thought experiments by Einstein) rather than observation.  The theory rests on two postulates that are themselves difficult to test, and then derives mathematically what the physical consequences should be (and hence experimentally testable predictions). Einstein's thought experiments pondered what would happen if the speed of light was constant in all frames of reference, and all laws of physics were the same in every (inertial) frame of reference no matter where it is or how fast it is traveling.  This theory improves on [[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). In general terms, relativity predicts that space-time can be curved by massive bodies, so that (for example) near a [[black hole]] the sum of the angles in a triangle is not exactly 180 degrees, time passes more rapidly away from a black hole than near it (for a distant observer) and other apparent violations of [[geometry]] and common sense.

Relativity is important for massive or fast-moving bodies: at low mass or high speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modelling the behaviour 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, modelling the behaviour 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).  

One of the mathematical consequences of the theory - the well known equivalance between energy and matter, E=mc^2 - predicted the release of energy in nuclear reactions, explaining the source of the sun's fusion energy and spurring the development of fissile weapons (the [[atomic bomb]]). Another was the effect of high-relative-speed ("relativistic") travel on the passage of time: from this relativity was able to explain, and accurately predict, the anomalous orbit of the planet Mercury (travelling at high speed very close to the sun).  

One of the mathematical consequences of the theory - the well known equivalance between energy and matter, E=mc^2 - predicted the release of energy in nuclear reactions, explaining the source of the sun's fusion energy and spurring the development of fissile weapons (the [[atomic bomb]]). Another was the effect of high-relative-speed ("relativistic") travel on the passage of time: from this relativity was able to explain, and accurately predict, the anomalous orbit of the planet Mercury (travelling at high speed very close to the sun).