The radical years: general relativity and quantum mechanics

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 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 lensing and the distortion of time in gravitational fields.
Further information: History of general relativity

Niels Bohr (1885–1962)

The quantized theory of the atom gave way to a full-scale quantum mechanics in the 1920s. The quantum theory (which previously relied in the “correspondence” at large scales between the quantized world of the atom and the continuities of the “classical” world) was accepted when the Compton Effect established that light carries momentum and can scatter off particles, and when Louis de Broglie asserted that matter can be seen as behaving as a wave in much the same way as electromagnetic waves behave like particles (wave-particle duality). New principles of a “quantum” rather than a “classical” mechanics, formulated in matrix-form by Werner Heisenberg,Max Born, and Pascual Jordan in 1925, were based on the probabilistic relationship between discrete “states” and denied the possibility of causality. Erwin Schrödingerestablished an equivalent theory based on waves in 1926; but Heisenberg’s 1927 “uncertainty principle” (indicating the impossibility of precisely and simultaneously measuring position and momentum) and the “Copenhagen interpretation” of quantum mechanics (named after Bohr’s home city) continued to deny the possibility of fundamental causality, though opponents such as Einstein would metaphorically assert that “God does not play dice with the universe”.[37] Also in the 1920s, Satyendra Nath Bose's work on photons and quantum mechanics provided the foundation for Bose-Einstein statistics, the theory of the Bose-Einstein condensate, and the discovery of the boson.
Further information: history of quantum mechanics