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Quantum mechanics is the science of the very small: the body of scientific principles that explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles. Classical physics explains matter and energy on a scale familiar to human experience, including the behaviour of astronomical bodies. It remains the key to measurement for much of modern science and technology. However, towards the end of the 19th century, scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain.〔(''Quantum Mechanics'' ) from National Public Radio〕 As Thomas Kuhn explains in his analysis of the philosophy of science, ''The Structure of Scientific Revolutions'', coming to terms with these limitations led to two major revolutions in physics which created a shift in the original scientific paradigm: the theory of relativity and the development of quantum mechanics.〔Kuhn, Thomas S. ''The Structure of Scientific Revolutions''. Fourth ed. Chicago; London: The University of Chicago Press, 2012. Print.〕 This article describes how physicists discovered the limitations of classical physics and developed the main concepts of the quantum theory that replaced it in the early decades of the 20th century. These concepts are described in roughly the order in which they were first discovered. For a more complete history of the subject, see ''History of quantum mechanics''. In this sense, the word ''quantum'' means the minimum amount of any physical entity involved in an interaction. Certain characteristics of matter can take only discrete values. Light behaves in some respects like particles and in other respects like waves. Matter—particles such as electrons and atoms—exhibits wavelike behaviour too. Some light sources, including neon lights, give off only certain discrete frequencies of light. Quantum mechanics shows that light, along with all other forms of electromagnetic radiation, comes in discrete units, called photons, and predicts its energies, colours, and spectral intensities. Some aspects of quantum mechanics can seem counterintuitive or even paradoxical, because they describe behaviour quite different from that seen at larger length scales. In the words of Richard Feynman, quantum mechanics deals with "nature as She is – absurd". For example, the uncertainty principle of quantum mechanics means that the more closely one pins down one measurement (such as the position of a particle), the less precise another measurement pertaining to the same particle (such as its momentum) must become. ==The first quantum theory: Max Planck and black-body radiation== Thermal radiation is electromagnetic radiation emitted from the surface of an object due to the object's internal energy. If an object is heated sufficiently, it starts to emit light at the red end of the spectrum, as it becomes red hot. Heating it further causes the colour to change from red to yellow, white, and blue, as light at shorter wavelengths (higher frequencies) begins to be emitted. A perfect emitter is also a perfect absorber: when it is cold, such an object looks perfectly black, because it absorbs all the light that falls on it and emits none. Consequently, an ideal thermal emitter is known as a black body, and the radiation it emits is called black-body radiation. In the late 19th century, thermal radiation had been fairly well characterized experimentally.〔A number of formulae had been created which were able to describe some of the experimental measurements of thermal radiation: how the wavelength at which the radiation is strongest changes with temperature is given by Wien's displacement law, the overall power emitted per unit area is given by the Stefan-Boltzmann law. The best theoretical explanation of the experimental results was the Rayleigh-Jeans law, which agrees with experimental results well at large wavelengths (or, equivalently, low frequencies), but strongly disagrees at short wavelengths (or high frequencies). In fact, at short wavelengths, classical physics predicted that energy will be emitted by a hot body at an infinite rate. This result, which is clearly wrong, is known as the ultraviolet catastrophe.〕 However, classical physics was unable to explain the relationship between temperatures and predominant frequencies of radiation. Physicists searched for a single theory that explained all the experimental results. thumb (red) and Wien approximation (blue). The first model that was able to explain the full spectrum of thermal radiation was put forward by Max Planck in 1900.〔This result was published (in German) as . English translation: "(On the Law of Distribution of Energy in the Normal Spectrum )".〕 He proposed a mathematical model in which the thermal radiation was in equilibrium with a set of harmonic oscillators. To reproduce the experimental results, he had to assume that each oscillator produced an integer number of units of energy at its single characteristic frequency, rather than being able to emit any arbitrary amount of energy. In other words, the energy of each oscillator was ''quantized''.〔The word ''quantum'' comes from the Latin word for "how much" (as does ''quantity''). Something which is ''quantized'', like the energy of Planck's harmonic oscillators, can only take specific values. For example, in most countries money is effectively quantized, with the ''quantum of money'' being the lowest-value coin in circulation. Mechanics is the branch of science that deals with the action of forces on objects. So, quantum mechanics is the part of mechanics that deals with objects for which particular properties are quantized.〕 The quantum of energy for each oscillator, according to Planck, was proportional to the frequency of the oscillator; the constant of proportionality is now known as the Planck constant. The Planck constant, usually written as , has the value of . So, the energy of an oscillator of frequency is given by : To change the colour of such a radiating body, it is necessary to change its temperature. Planck's law explains why: increasing the temperature of a body allows it to emit more energy overall, and means that a larger proportion of the energy is towards the violet end of the spectrum. Planck's law was the first quantum theory in physics, and Planck won the Nobel Prize in 1918 "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta".〔(【引用サイトリンク】 title=The Nobel Prize in Physics 1918 )〕 At the time, however, Planck's view was that quantization was purely a mathematical construct, rather than (as is now believed) a fundamental change in our understanding of the world. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Introduction to quantum mechanics」の詳細全文を読む スポンサード リンク
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