Principle of relativity

The principle that the laws of physics have the same form in every reference frame within a specified class, such as inertial frames.

Reference frame: A coordinate system and clock arrangement used to describe positions and times of events. Relativity states which reference frames must share the same physical laws.
Galilean transformation: Equations relating space and time coordinates in inertial frames of classical mechanics. It preserves Newtonian mechanics when observers move at constant relative velocity.
Galileo Galilei: An Italian astronomer and physicist whose work helped establish mathematical and experimental approaches to nature. His ship thought experiment illustrated why uniform motion is not detectable by mechanical experiments alone.
Preferred frame: A reference frame singled out as physically distinguished from other frames. A preferred frame would undermine the equivalence asserted among the principle’s specified frames.
Inertial frame: A reference frame in which free bodies move at constant velocity unless acted on by a force. Classical and special relativity specify inertial frames as the class sharing their laws.
Lorentz transformation: Equations relating spacetime coordinates between inertial frames in special relativity. It preserves the laws of special relativity, including the invariant speed of light.
Isaac Newton: An English mathematician and physicist who formulated classical mechanics and the law of universal gravitation. His mechanics made inertial-frame equivalence a central principle of classical physics.
Lorentz ether theory: A theory retaining a stationary ether while explaining relativistic observations through contraction and local-time effects. It reproduces special-relativistic predictions while interpreting one frame as physically preferred.
Physical law: A rule or mathematical relation describing regularities in physical phenomena. The principle constrains how physical laws appear across the specified frames.
Galilean relativity: The classical principle that mechanics has the same form in all inertial frames. It is the Newtonian realization of the broader principle.

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