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

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

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

In laymen's terms, these assumptions are equivalent to the following:{{fact}}

# ''It is impossible for any force or information ever to travel faster than the speed of light.''

# ''It is impossible for any force or information ever to travel faster than the speed of light.''

# ''There is nothing special about [[earth]], its [[solar system]] or even high-speed objects with respect to the laws of physics, which are assumed to be absolutely identical in every location and for every non-accelerating object in the universe.

# ''There is nothing special about [[earth]], its [[solar system]] or even high-speed objects with respect to the laws of physics, which are assumed to be absolutely identical in every location and for every non-accelerating object in the universe.

Both assumptions are subject to criticism, and are almost impossible to test. If either assumption is not completely true then Special Relativity fails as a mathematical model.  

Both assumptions are subject to criticism, and are almost impossible to test.{{fact}} (Why? It is very easy to accelerate a particle to 99% of the speed of light and to measure if a particle is exceeding the speed of light.) If either assumption is not completely true then Special Relativity fails as a mathematical model.{{fact}}

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.)

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 [[aether|luminiferous aether]], 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/Galilean 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.

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 essential for massive or fast-moving bodies; for electromagnetism; for light and other radiation; for quantum field theory; and for spin. Particles at low mass and low speed 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 ==

== General Relativity ==