In 1911 Rutherford describes in the paper "The scattering of alpha and beta particles by matter and the structure of the atom" experiments which led him to formulation of the `planetary theory' of atoms, as opposed to the `plum pudding theory' which was commonly accepted at that time.
Kamerlingh-Onnes found (in 1911) that when cooling the pure mercury tubes to the temperature of 4.2 °K their electrical resistance suddenly dropped to zero. He called this phenomenon ‘superconductivity’. Kamerlingh-Onnes obtained similar results with some other metals, for instance tin and lead and in 1914 he established a permanent current, or what he called a ‘persistent supercurrent’, in a superconductive coil of lead.
Cosmic rays are high-energy particles that travel throughout the Milky Way Galaxy. Some of them originate from the Sun, but the majority comes from sources outside the solar system. Some, very energetic, come from the outskirts of the Universe. Cosmic-ray particles entering the Earth atmosphere collide with atoms and molecules and create secondary showers of particle and gamma rays.
Before the 1950s, cosmic rays were the only source of high-energy particles. This is why they played a crucial role in scientific studies of the atomic nuclei and their components; they also brought first experimental evidence of time dilation in agreement with the relativistic theory. With the advent of powerful particle accelerators in the 1950s, the range of studies of cosmic rays has been limited but research work continues because cosmic rays contain particles with energies far beyond those attainable under laboratory conditions.
Niels Bohr began with the assumption that electrons were orbiting the nucleus, much like the Earth orbits the Sun. From the classical electrodynamics point of view, a charge travelling in a circular path should lose energy by emitting electromagnetic radiation. If the ‘orbiting’ electron loses energy, it should end up spiralling into the nucleus – no atom would be stable.
Bohr borrowed the idea of energy quanta from Planck. He proposed that only orbits of certain radii, corresponding to defined energies, are "permitted", and therefore stable.
Einstein showed that Newton's law of gravitation was only approximately correct, breaking down in the presence of very strong gravitational fields.
The general theory of relativity predicts that the orientation of Mercury's orbit is found to undergo a slow precession in space over time.
Einstein's theory predicts that the direction of light propagation should bend in a gravitational field, in the manner which is double compared to the predictions from Newton’s gravitation theory. A striking consequence is gravitational
lensing.
The general theory of relativity predicts that light coming from a strong gravitational field should have its wavelength shifted to larger values (what astronomers call a ‘red shift’), again contrary to Newton's theory. A detailed observation (by R. V. Pound and G.A. Rebka in 1960) indicated such a red shift, for gamma photons, using Mössbauer effect, and that its magnitude is correctly given by Einstein's theory.
During the solar eclipse in Brazil of May 29, 1919, A. Eddington took pictures of the stars in the region around the Sun. According to the theory of general relativity, stars near the Sun would appear to have been slightly shifted because their light had been curved by its gravitational field. Eddington's observations confirmed Einstein's theory, and were hailed both as German-English post-war “de-freezing” and as a proof of general relativity over the Newtonian model.
In 1919 E. Rutherford discovered that the nuclei of certain light elements, such as nitrogen, could be "disintegrated" by the impact of energetic alpha particles coming from some radioactive sources (like polonium), and that during this process fast protons were emitted. P. Blackett later proved, with the cloud chamber, that the nitrogen in this process was actually transformed into an oxygen isotope, so that Rutherford was the first to deliberately transmute one chemical element into another.