Editing History of physics (section)

====Special relativity====
{{further|History of special relativity}}
[[File:GPB circling earth.jpg|thumb|<!--Refining/rephrasing?:-->Einstein proposed that [[gravitation]] results from [[mass]]es (or their equivalent energies) [[Curvature of spacetime|curving ("bending")]] the [[spacetime]] in which they exist, altering the paths they follow within it.]]

Einstein argued that the speed of light was a constant in all [[Inertial frame of reference|inertial reference frames]] and that electromagnetic laws should remain valid independent of reference frame&nbsp;– assertions which rendered the ether "superfluous" to physical theory, and that held that observations of time and length varied relative to how the observer was moving with respect to the object being measured (what came to be called the "[[special relativity|special theory of relativity]]"). It also followed that mass and energy were interchangeable quantities according to the equation [[Mass–energy equivalence|''E''=''mc''<sup>2</sup>]]. In another paper published the same year, Einstein asserted that electromagnetic radiation was transmitted in discrete quantities ("[[Quantum|quanta]]"), according to a constant that the theoretical physicist [[Max Planck]] had posited in 1900 to arrive at an accurate theory for the distribution of [[blackbody radiation]]&nbsp;– an assumption that explained the strange properties of the photoelectric effect.

The special theory of relativity is a formulation of the relationship between physical observations and the concepts of space and time. The theory arose out of contradictions between electromagnetism and Newtonian mechanics and had great impact on both those areas. The original historical issue was whether it was meaningful to discuss the electromagnetic wave-carrying "ether" and motion relative to it and also whether one could detect such motion, as was unsuccessfully attempted in the Michelson–Morley experiment. Einstein demolished these questions and the ether concept in his special theory of relativity. However, his basic formulation does not involve detailed electromagnetic theory. It arises out of the question: "What is time?" Newton, in the ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'' (1686), had given an unambiguous answer: "Absolute, true, and mathematical time, of itself, and from its own nature, flows equably without relation to anything external, and by another name is called duration." This definition is basic to all classical physics.

Einstein had the genius to question it, and found that it was incomplete. Instead, each "observer" necessarily makes use of his or her own scale of time, and for two observers in relative motion, their time-scales will differ. This induces a related effect on position measurements. Space and time become intertwined concepts, fundamentally dependent on the observer. Each observer presides over his or her own space-time framework or coordinate system. There being no absolute frame of reference, all observers of given events make different but equally valid (and reconcilable) measurements. What remains absolute is stated in Einstein's relativity postulate: "The basic laws of physics are identical for two observers who have a constant relative velocity with respect to each other."

Special relativity had a profound effect on physics: started as a rethinking of the theory of electromagnetism, it found a new [[symmetry (physics)|symmetry law]] of nature, now called ''[[Poincaré symmetry]]'', that replaced [[Galilean symmetry]].

Special relativity exerted another long-lasting effect on [[dynamics (physics)|dynamics]]. Although initially it was credited with the "unification of mass and energy", it became evident that [[relativistic dynamics]] established a ''distinction'' between [[rest mass]], which is an invariant (observer independent) property of a [[particle]] or system of particles, and the [[energy]] and momentum of a system. The latter two are separately [[Conservation law (physics)|conserved]] in all situations but not invariant with respect to different observers. The term ''mass'' in [[particle physics]] underwent a [[semantic change]], and since the late 20th century it almost exclusively denotes the [[invariant mass|rest (or ''invariant'') mass]]. {{Further|mass in special relativity}}