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Definition: quantum mechanics from The Penguin Dictionary of Science

The study of systems that are on such a small scale that the ideas of ➤quantum state and ➤quantization must be applied. There are two fundamental differences between quantum mechanics and ➤classical physics (e.g. ➤Hamilton's equations). First, the parameters associated with a quantum system (e.g. the position and momentum of an electron) are directly affected by the act of measuring them. Second, the parameters are not precise values but are instead expressed in terms of probability distributions (e.g. the squared modulus of the ➤wavefunction is the probability distribution for position).

Summary Article: quantum mechanics
From Philip's Encyclopedia

Branch of physics that uses the quantum theory to explain the behaviour of elementary particles. According to quantum theory, all radiant energy emits and absorbs in multiples of tiny 'packets' or quanta. Atomic particles have wavelike properties and thereby exhibit a wave-particle duality. Sometimes the wave properties dominate, and other times the particle aspects dominate. The quantum theory uses four quantum numbers to classify electrons and their atomic states: energy level, angular momentum, energy in a magnetic field and spin. The exclusion principle says any two electrons in an atom cannot have the same energy and spin. A change in an electron, atom or molecule from one quantum state to another, called a quantum jump, is accompanied by the absorption or emission of a quantum. The quantum field theory seeks to explain this exchange. The interactions between quarks and between protons and neutrons are described by quantum chronodynamics. The idea that energy radiates and absorbs in packets was first proposed by German physicist Max Planck in 1900 to explain black body radiation. Using Planck's work, German-born US physicist Albert Einstein quantized light radiation, and in 1905 explained the photoelectric effect. He chose the name of photon for a quantum of light energy. In 1913, Danish physicist Niels Bohr used quantum theory to explain atomic structure and spectra, showing the relationship between the energy levels of an atom's electrons and the frequencies of radiation emitted or absorbed by the atom. In 1924, French physicist Louis de Broglie suggested that particles have wave properties, the converse having been postulated in 1905 by Einstein. In 1926, Austrian physicist Erwin Schrödinger used this hypothesis of wave mechanics to predict particle behaviour on the basis of wave properties, but a year earlier German physicist Werner Heisenberg had produced a mathematical equivalent to Schrödinger's theory without using wave concepts. In 1928, English physicist Paul Dirac unified these approaches while incorporating relativity into quantum mechanics. This predicted the existence of antimatter and helped develop the quantum electrodynamics theory of how charged subatomic particles interact within electric and magnetic fields. The superstring theory provides a possible answer to gravitational interaction. The complete, modern theory of quantum mechanics is the quantum field theory of quantum electrodynamics, also known as the quantum theory of light. It was derived by US physicist Richard Feynman in the 1940s. The theory predicts that a collision between an electron and a proton should result in the production of a photon of electromagnetic radiation, which is exchanged between the colliding particles. Quantum mechanics remains a difficult system because the uncertainty principle, formulated in 1927 by Heisenberg, states that nothing on the atomic scale can be measured or observed without disturbing it. This makes it impossible to know both the position and momentum of a particle.

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