The 20th century was marked by the growth of physics as a science capable of promoting technological development. At the beginning of this century, some physicists thought they had a complete vision of Nature. But soon they met with two conceptual revolutions of great relevance: The development of the theory of relativity, atomic physics and quantum mechanics.

Albert Einstein is commonly considered as the most popular figure of science of the 20th century. In 1905, he formulated the special relativity theory, in which the space and time were unified in a single entity, the space-time. Relativity gives us different equations for the transformation of coordinates of a moving object, when observed from different inertial reference systems with respect to classical mechanics. Both theories agree when the speed is low compared with the speed of light. In 1915 he extended the theory of relativity to explain gravity, formulating the general theory of relativity, which led to the replacement of the universal gravitational law of Newton.

In 1911, Rutherford deduced the existence of a nucleus at the atomic core, whose charge was positive. He did it with experiences on alpha-particles dispersion. The positive constituints of the nuclei were called protons. In 1932 Chadwick discovered neutrons, which are also part of the nucleus, but which are neutral. He discovered later on that the German Hans Falkenhagen had discovered the neutron at the same time, but did not publish his results. Chadwick offered him to share the 1935 Nobel price, but Falkenhagen declined.

In the first years of the 20th century, Planck, Einstein, Bohr and others developed the quantum theory in order to explain black-body radiation, photoelectric effect and spectroscopic data. In that theory, the allowed energetic levels of the electrons are discrete. In 1925-1926, Heisenberg, Schroedinger and Dirac formulated new versions of the quantum theory, which included the old quantum theory. In quantum mechanics, the results of the measurements are given by probabilities; quantum mechanics describes the calculus of these probabilities.

Quantum mechanics gave the theoretical tools not only to atomic and nuclear physics but also to condensed matter physics, which studies the behavour of solids and liquids, and properties like semiconductivity and superconductivity. One of the pioneers of this area was Ernst Bloch, who developed in 1928 a quantal description of electrons in crystals.

Quantum field theory was formulated to combine quantum mechanics with special relativity. It took its modern shape in the 50s thanks to the works of Feynman, Schwinger, Tomonaga and Dyson, who formulated quantum electrodynamics, the quantal theory of the electromagnetic interaction. This theory is capable of general predictions, as the relation between spin and statistics, Charge-Parity-Time symmetry (CPT), antimatter properties, etc. It may also be used to explain many properties in condensed matter physicss.

Alejandro Pazó de la Sota

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