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In physics, an infrared divergence or infrared catastrophe is a situation in which an integral, for example a Feynman diagram, diverges because of contributions of objects with very small energy approaching zero, or, equivalently, because of physical phenomena at very long distances. The infrared (IR) divergence only appears in theories with massless particles (such as photons). They represent a legitimate effect that a complete theory often implies. One way to deal with it is to impose an infrared cutoff and take the limit as the cutoff approaches zero and/or refine the question. Another way is to assign the massless particle a fictitious mass, and then take the limit as the fictitious mass vanishes. The divergence is usually in terms of particle number and not empirically troubling, in that all measurable quantities remain finite.〔〔 (Unlike in the case of the UV catastrophe where the energies involved diverge.) ==Bremsstrahlung example== When an electric charge is accelerated (or de-accelerated) it emits Bremsstrahlung radiation. Semiclassical electromagnetic theory, or the full quantum electrodynamic analysis, shows that an infinite number of soft photons are created. But only a finite number are detectable, the remainder, due to their low energy, falling below any finite energy detection threshold, which must necessarily exist. 〔, pages 177-184 and appendix A6〕 However even though most of the photons are not detectable they can't be ignored in the theory; Quantum electrodynamic calculations show that the transition amplitude between ''any'' states with a finite number of photons vanishes. Finite transition amplitudes are obtained only by summing over states with an infinite number of soft photons.〔 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「Infrared divergence」の詳細全文を読む スポンサード リンク
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