Editing History of physics (section)

===Albert Einstein's theory of relativity===
[[File:Einstein patentoffice.jpg|thumb|upright|[[Albert Einstein]] (1879–1955), ca. 1905]]
In 1905, a 26-year-old German physicist named [[Albert Einstein]] (then a [[patent clerk]] in [[Bern]], Switzerland) showed how measurements of time and space are affected by motion between an observer and what is being observed. Einstein's radical [[theory of relativity]] revolutionized science. Although Einstein made many other important contributions to science, the theory of relativity alone is one of the greatest intellectual achievements of all time. Although the concept of relativity was not introduced by Einstein, he recognised that the [[speed of light]] in vacuum is constant, i.e., the same for all observers, and an absolute upper limit to speed. This does not impact a person's day-to-day life since most objects travel at speeds much slower than light speed. For objects travelling near light speed, however, the theory of relativity shows that clocks associated with those objects will run more slowly and that the objects shorten in length according to measurements of an observer on Earth. Einstein also derived the equation, {{nowrap|1=''E'' = ''mc''<sup>2</sup>}}, which expresses the [[Mass–energy equivalence|equivalence of mass and energy]].

====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}}

====General relativity====
{{further|History of general relativity}}
By 1916, Einstein was able to generalize this further, to deal with all states of motion including non-uniform acceleration, which became the general theory of relativity. In this theory, Einstein also specified a new concept, the curvature of space-time, which described the gravitational effect at every point in space. The curvature of space-time replaced Newton's universal law of gravitation. According to Einstein, gravitational force in the normal sense is an illusion caused by the geometry of space. The presence of a mass causes a curvature of space-time in the vicinity of the mass, and this curvature dictates the space-time path that all freely-moving objects follow. It was also predicted from this theory that light should be subject to gravity&nbsp;– all of which was verified experimentally. This aspect of relativity explained the phenomena of light bending around the sun, predicted black holes as well as properties of the [[Cosmic microwave background radiation]]&nbsp;– a discovery rendering fundamental anomalies in the classic Steady-State hypothesis. For his work on relativity, the photoelectric effect and blackbody radiation, Einstein received the Nobel Prize in 1921.

The gradual acceptance of Einstein's theories of relativity and the quantized nature of light transmission, and of [[Niels Bohr's model of the atom]] created as many problems as they solved, leading to a full-scale effort to reestablish physics on new fundamental principles. Expanding relativity to cases of accelerating reference frames (the "[[general relativity|general theory of relativity]]") in the 1910s, Einstein posited an equivalence between the inertial force of acceleration and the force of gravity, leading to the conclusion that space is curved and finite in size, and the prediction of such phenomena as [[gravitational lens]]ing and the distortion of time in gravitational fields.