The Day in History:

Einstein Introduces Special Relativity (1905)

Although Isaac Newton based his theory on absolute space and time, he also adhered to the Principle of relativity of Galileo Galilei. This stated all observers who move uniformly relative to each other are equal and no absolute state of motion can be attributed to any observer. During the 19th century the Aether Theory was widely accepted, mostly in the form given by James Clerk Maxwell. According to Maxwell all optical and electrical phenomena propagate in a medium. Thus it seemed possible to determine absolute motion relative to the aether and therefore to disprove Galileo"s Principle.

Those experiments and their failure lead to the development of the Maxwell-Lorentzian Electrodynamics by Hendrik Lorentz. Henri Poincaré formally completed this by stating the Relativity Principle as a general law of nature, including Electrodynamics and Gravitation. Albert Einstein eventually devised Special Relativity (SR) by completely re-interpreting Lorentzian Electrodynamics by changing the concepts of space and time and abolishing the aether. This paved the way to General Relativity. Subsequent work of Hermann Minkowski laid the foundations of Relativistic Field Theories.

Special relativity

Albert Einstein, 1921In September 1905 (received June 30), Albert Einstein published his annus mirabilis paper on what is now called Special Relativity. This paper contains — in the mathematical sense and with exception of the relativistic Doppler effect and aberration — no new results, but the derivation and the interpretation were radically new. Because of his axiomatic method, Einstein was able to derive all results on a few pages, while his predecessors needed many years of long, complicated work to arrive at the same mathematical formalism.

Einstein identified two fundamental principles, the Principle of Relativity and the Principle of the Constancy of Light, each founded on experience. Taken together (along with a few other tacit assumptions such as isotropy and homogeneity of space), these two postulates lead uniquely to the mathematics of Lorentz"s electrodynamics and special relativity. Lorentz and Poincaré had also adopted these same principles, as necessary to achieve their final results, but didn"t recognize that they were also sufficient, and hence that they obviated all the other assumptions underlying Lorentz"s initial derivations. Einstein"s paper also includes a fundamental new definition of space and time (all time and space coordinates in all reference frames are equal, so there is no "true" or "apparent" time) and the abolition of the aether.

It"s notable that Einstein"s paper contains no references to other papers. However, many historians of science like Holton or Miller have tried to find out possible influences on Einstein. Regarding the Relativity Principle, Einstein"s moving magnet and conductor problem (possibly after reading a book of August F?ppl) and the negative aether drift experiments (possibly the Michelson-Morley experiment) were important for him to accept that principle. Another possible source is Poincaré"s Science and Hypothesis, where he described the Principle of Relativity and which was read by him in 1904. Regarding the Principle of the Constancy of Light, Einstein himself stated that Lorentz"s theory (or the Maxwell-Lorentz electrodynamics) had considerable influence on his thinking. He said in 1909 and 1912 that he borrowed that principle from Lorentz"s stationary ether (which implies validity of Maxwell"s equations and the constancy of light in the ether frame), but he recognized that this principle together with the principle of relativity makes the ether useless. As he wrote in 1907 and in later papers, the apparent contradiction between those principles can be solved if it is realized that Lorentz"s local time is not an auxiliary quantity, but can simply be defined as time and is connected with signal velocity. Before Einstein, also Poincaré developed a similar physical interpretation of local time and noticed the connection to signal velocity, but contrary to Einstein he continued to argue that clocks in the aether show the true time, and moving clocks show the apparent time. Eventually, in 1953 Einstein described the advances of his theory (although Poincaré already stated in 1905 that Lorentz invariance is a general condition for any physical theory):

“ There is no doubt, that the special theory of relativity, if we regard its development in retrospect, was ripe for discovery in 1905. Lorentz had already recognized that the transformations named after him are essential for the analysis of Maxwell’s equations, and Poincaré deepened this insight still further. Concerning myself, I knew only Lorentz"s important work of 1895 [...] but not Lorentz"s later work, nor the consecutive investigations by Poincaré. In this sense my work of 1905 was independent. [..] The new feature of it was the realization of the fact that the bearing of the Lorentz transformation transcended its connection with Maxwell"s equations and was concerned with the nature of space and time in general. A further new result was that the "Lorentz invariance" is a general condition for any physical theory. This was for me of particular importance because I had already previously found that Maxwell"s theory did not account for the micro-structure of radiation and could therefore have no general validity. ”

Mass–energy equivalence

Already in §10 of his paper on electrodynamics, Einstein used the formula

for the kinetic energy of an electron (similar formulas were already used before Einstein by Wien, Poincaré, Abraham, Lorentz, and Hasen?hrl; see the description above). In elaboration of this, in November 1905 (received September 27) Einstein was the first to suggest that when a material body lost energy (either radiation or heat) of amount E, its mass decreased by the amount E/c2. So, he solved Poincaré"s radiation paradox from 1900. This led to the famous mass–energy equivalence formula: E = mc2. Einstein considered the equivalency equation to be of paramount importance because it showed that a massive particle possesses an energy, the "rest energy", distinct from its classical kinetic and potential energies.

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