Astroparticle Physics

1.1 Discoveries in the 20th Century 5In parallel to these experimental observations, Einsteindeveloped his theories of special and general relativity (1905and 1916). The theory of special relativity is of paramountimportance for particle physics, while the prevailing domainof general relativity is cosmology. Einstein received the NobelPrize in 1921 not, however, for his fundamental theorieson relativity and gravitation, but for the correct quantummechanicalinterpretation of the photoelectric effect and theexplanation of Brownian motion. Obviously the Nobel committeein Stockholm was not aware of the outstanding importanceof the theories of relativity or possibly not evensure about the correctness of their predictions. This occurredeven though Schwarzschild had already drawn correct conclusionsfor the existence of black holes as early as 1916,and Eddington had verified the predicted gravitational bendingof light passing near the Sun during the solar eclipsein 1919. The experimental observation of the deflection oflight in gravitational fields also constituted the discovery ofgravitational lensing. This is when the image of a star appearsto be displaced due to the gravitational lensing of lightthat passes near a massive object. This effect can also lead todouble, multiple, or ring-shaped images of a distant star orgalaxy if there is a massive object in the line of sight betweenthe observer on Earth and the star (Fig. 1.7). It was only in1979 that multiple images of a quasar (double quasar) couldbe observed. This was followed in 1988 by an Einstein ringin a radio galaxy, as predicted by Einstein in 1936.theories of relativityblack holesgravitational lensingapparent positiondouble imagesEinstein ringobserversunstarFig. 1.7Gravitational lensing by a massiveobject:a) deflection of light,b) double images,c) Einstein ringIn the field of astronomy, stars are classified accordingto their brightness and colour of the spectrum (Hertzsprung–Russell diagram 1911). This scheme allowed a better understandingof the stellar evolution of main-sequence stars tostellar evolutionHertzsprung–Russelldiagram