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Hierarchy problem
In theoretical physics, the hierarchy problem is the large discrepancy between aspects of the weak force and gravity.〔http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-hierarchy-problem/〕 There is no scientific consensus on why, for example, the weak force is 1032 times stronger than gravity.
==Technical definition==
A hierarchy problem occurs when the fundamental value of some physical parameter, such as a coupling constant or a mass, in some Lagrangian is vastly different from its effective value, which is the value that gets measured in an experiment. This happens because the effective value is related to the fundamental value by a prescription known as renormalization, which applies quantum corrections to it. Typically the renormalized value of parameters are close to their fundamental values, but in some cases, it appears that there has been a delicate cancellation between the fundamental quantity and the quantum corrections. Hierarchy problems are related to fine-tuning problems and problems of naturalness.
Studying renormalization in hierarchy problems is difficult, because such quantum corrections are usually power-law divergent, which means that the shortest-distance physics are most important. Because we do not know the precise details of the shortest-distance theory of physics, we cannot even address how this delicate cancellation between two large terms occurs. Therefore, researchers are led to postulate new physical phenomena that resolve hierarchy problems without fine tuning.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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