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In physics, Planck units are physical units of measurement defined exclusively in terms of five universal physical constants listed below, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units. Planck units have profound significance for theoretical physics since they elegantly simplify several recurring algebraic expressions of physical law by nondimensionalization. They are particularly relevant in research on unified theories such as quantum gravity. ==Overview== Originally proposed in 1899 by German physicist Max Planck, these units are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct. Planck units are only one system of several systems of natural units, but these units are not based on properties of any prototype object or particle (that would be arbitrarily chosen), but rather on only the properties of free space. The universal constants that Planck units, by definition, normalize to 1 are: *the gravitational constant, ''G'', *the reduced Planck constant, ''ħ'', *the speed of light in a vacuum, ''c'', *the Coulomb constant, (4π''ε''0)−1 (sometimes ''k''e or ''k''), and *the Boltzmann constant, ''k''B (sometimes ''k''). Each of these constants can be associated with at least one fundamental physical theory: ''c'' with special relativity and electromagnetism, ''G'' with general relativity and Newtonian gravity, ''ħ'' with quantum mechanics, ''ε''0 with electrostatics, and ''k''B with statistical mechanics and thermodynamics. Planck units are free of anthropocentric arbitrariness. Some physicists argue that communication with extraterrestrial intelligence would have to employ such a system of units in order to be understood.〔Michael W. Busch, Rachel M. Reddick (2010) "(Testing SETI Message Designs, )" (''Astrobiology Science Conference 2010'' ), April 26–29, 2010, League City, Texas.〕 Unlike the metre and second, which exist as base units in the SI system for historical reasons, the Planck length and Planck time are conceptually linked at a fundamental physical level. Natural units help physicists to reframe questions. Frank Wilczek puts it succinctly: While it is true that the electrostatic repulsive force between two protons (alone in free space) greatly exceeds the gravitational attractive force between the same two protons, this is not about the relative strengths of the two fundamental forces. From the point of view of Planck units, this is comparing apples to oranges, because mass and electric charge are incommensurable quantities. Rather, the disparity of magnitude of force is a manifestation of the fact that the charge on the protons is approximately the unit charge but the mass of the protons is far less than the unit mass. 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「Planck units」の詳細全文を読む スポンサード リンク
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