Introduction To Quantum Mechanics
Quantum mechanics is the body of scientific principles that explains the behavior of matter and its interactions with energy on the scale of atoms and subatomic particles and how these phenomena could be related to everyday life (see: Schrodinger's cat).
Classical physics explains matter and energy at the macroscopic level of the scale familiar to human experience, including the behavior of astronomical bodies. It remains the key to measurement for much of modern science and technology; but at the end of the 19th Century observers discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. Coming to terms with these limitations led to the development of quantum mechanics, a major revolution in physics. This article describes how physicists discovered the limitations of classical physics and developed the main concepts of the quantum theory that replaced them in the early decades of the 20th century. These concepts are described in roughly the order they were first discovered; for a more complete history of the subject, see History of quantum mechanics.
Some aspects of quantum mechanics can seem counter-intuitive or even paradoxical, because they describe behavior quite different than that seen at larger length scales, where classical physics is an excellent approximation. In the words of Richard Feynman, quantum mechanics deals with "nature as She is — absurd."
Many types of energy, such as photons (discrete units of light), behave in some respects like particles and in other respects like waves. Radiators of photons (such as neon lights) have emission spectra that are discontinuous, in that only certain frequencies of light are present. Quantum mechanics predicts the energies, the colours, and the spectral intensities of all forms of electromagnetic radiation.
Quantum mechanics ordains that the more closely one pins down one measure (such as the position of a particle), the less precise another measurement pertaining to the same particle (such as its momentum) must become.
Put another way, measuring position first and then measuring momentum does not have the same outcome as measuring momentum first and then measuring position; the act of measuring the first property necessarily introduces additional energy into the micro-system being studied, thereby perturbing that system.
Even more disconcerting, pairs of particles can be created as "entangled twins." As is described in more detail in the article on Quantum entanglement, entangled particles seem to exhibit what Einstein called "spooky action at a distance," matches between states that classical physics would insist must be random even when distance and the speed of light ensure that no physical causation could account for these correlations.
Read more about Introduction To Quantum Mechanics: The First Quantum Theory: Max Planck and Black Body Radiation, Photons: The Quantisation of Light, The Quantisation of Matter: The Bohr Model of The Atom, Wave-particle Duality, Development of Modern Quantum Mechanics, Copenhagen Interpretation, Dirac Wave Equation, Quantum Entanglement, Quantum Field Theory, Quantum Electrodynamics, Interpretations, Applications
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