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http://www.newworldencyclopedia.org/entry/Uncertainty_principle
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The uncertainty principle,' sometimes called the Heisenberg uncertainty principle, states that interaction and mechanical action come in quanta, that is, in discrete units. While one can detect an integral number of quanta, one cannot detect one half or two thirds of a quantum. The quantum of interaction joins the quanta of energy, time, and space in the current description of the world. The size of all these quanta is small—which is why they appear continuous—and determined by the size of Planck's Constant.
Thus, in quantum physics, the outcome of even an ideal measurement of a system is not deterministic, but instead is characterized by a probability distribution, and the larger the associated standard deviation is, the more "uncertain" one might say that characteristic is for the system. The Heisenberg uncertainty principle gives a lower bound on the product of the standard deviations of position and momentum for a system, implying that it is impossible to have a particle that has an arbitrarily well-defined position and momentum simultaneously. More precisely, the product of the standard deviations \Delta x \Delta p \geq \hbar/2, where \hbar (pronounced "h-bar") is Planck's constant, h, divided by 2π (the circumference of the unit circle).
The principle generalizes to many other pairs of quantities besides position and momentum. Examples of such complementary pairs are angular momentum about two different axes, and mass-energy in time, and can be derived directly from the axioms of quantum mechanics.
Note that the uncertainties in question are characteristic of the nature of the physical world. In any real-world measurement, there will also be additional uncertainties created by the non-ideal and imperfect measurement process. The uncertainty principle holds true regardless of whether the measurements are ideal (sometimes called von Neumann measurements) or non-ideal (Landau measurements). Note also that the product of the uncertainties, of order 10−35 Joule-seconds, is so small that the uncertainty principle has negligible effect on objects of macroscopic scale and things appear to be continuous rather than quantal, rather like the pixels of a photo on a computer screen that, from a distance, blend into a continuum.
The uncertainty principle was an important step in the development of quantum mechanics when it was discovered by Werner Heisenberg in 1927. It is connected to the observer effect—that observation requires interaction, and interaction involves a quantum of change, an unavoidable "disturbance" of the thing being observed. |
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楼主: terristwin
发表于 2009-06-02 03:08
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