实测宇宙线能谱为什么在约5×10^19 eV以上突然变陡,即所谓GZK截断?
Why does the measured cosmic-ray energy spectrum suddenly steepen above about 5×10^19 eV, the so-called GZK cutoff?
原因是超高能质子与宇宙微波背景光子之间的光致π产生反应。质子能量超过约5×10^19 eV后,便能经Δ共振与2.7 K背景光子作用并发射π介子,每次碰撞损失约两成能量,传播几十兆秒差距就跌回阈值以下;遥远天体发出的更高能质子根本无法抵达地球,能谱在高能端必然骤然压低。HiRes与皮埃尔·俄歇天文台的观测都看到了这一压低,证实了1966年提出的GZK理论预言。
The cause is photopion production: ultra-high-energy protons colliding with photons of the cosmic microwave background. Above roughly 5×10^19 eV a proton resonates with the 2.7 K background through the delta resonance and emits pions, shedding about one fifth of its energy per collision, so within a few tens of megaparsecs it falls back below threshold; higher-energy protons launched by remote sources simply never reach Earth, forcing the spectrum to plunge. HiRes and the Pierre Auger Observatory both observed this suppression, confirming the GZK prediction made in 1966.
难度 专业 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-11-04
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Greisen,《End to the Cosmic-Ray Spectrum?》,Physical Review Letters,1966 (DOI: 10.1103/physrevlett.16.748)；G. T. Zatsepin 与 V. A. Kuzmin,JETP Letters 第4卷78页,1966年
Greisen, 'End to the Cosmic-Ray Spectrum?', Physical Review Letters, 1966, DOI: 10.1103/physrevlett.16.748; G. T. Zatsepin and V. A. Kuzmin, JETP Letters 4, 78 (1966)
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1103/physrevlett.16.748
#GZK截断 #微波背景 #超高能宇宙线
静止寿命仅2.2微秒的宇宙线缪子,为什么大多能在衰变前穿过十几公里大气抵达海平面?
Why do most cosmic-ray muons, whose proper lifetime is a mere 2.2 microseconds, manage to cross a dozen kilometers of atmosphere and reach sea level before decaying?
因为狭义相对论的时间膨胀延长了它们的表观寿命。缪子生成于约15公里高空,若按静止寿命估算,即便以近光速飞行也几乎全数中途夭折;实测海平面缪子通量依然可观,且越快的缪子存活比例越高,对应地面系寿命按洛伦兹因子拉长。罗斯与霍尔1941年在山顶与海平面同时测量衰减速率,结果正合此预言,成为相对论最直接的日常证据之一。
Time dilation in special relativity stretches their apparent lifetimes. Born about fifteen kilometers overhead, muons would almost all die en route even at near-light speed were their rest-frame clock in charge; yet sizable numbers reach sea level, and the faster ones survive proportionally better because their Earth-frame lifetime is dilated by the Lorentz factor. Rossi and Hall compared decay rates at a mountain station and at sea level in 1941 and matched this prediction, turning atmospheric muons into one of the most direct everyday proofs of relativity.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-11-04
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Rossi & Hall,《Variation of the Rate of Decay of Mesotrons with Momentum》,Physical Review,1941 (DOI: 10.1103/physrev.59.223)
Rossi & Hall, 'Variation of the Rate of Decay of Mesotrons with Momentum', Physical Review, 1941, DOI: 10.1103/physrev.59.223
独立核验依据 · Independent verification: 核验依据 1 Crossref DOI 10.1103/physrev.59.223
#宇宙线缪子 #时间膨胀 #洛伦兹因子
1912年赫斯带静电计乘气球升至五千多米高空,他从放电速率随高度的变化中得出了什么结论?
What conclusion did Victor Hess draw in 1912 from how his balloon-borne electroscope discharged at altitudes above five kilometres?
他断定这种贯穿辐射来自地球之外,而非地面放射性。气球上升途中空气电离速率先随高度略有下降,越过约三千米后转而增大,到五千余米已达海平面的数倍;倘若辐射源于地表放射性物质,强度只会随高度单调衰减。因此必定存在一种穿透力极强的射线自宇宙空间射入大气,这就是后来命名的宇宙线。该实验开创了宇宙线物理,赫斯因此获得1936年诺贝尔物理学奖。
He concluded that this penetrating radiation came from beyond the Earth rather than from terrestrial radioactivity. As his balloons rose, the ionization rate first dipped slightly, then climbed past roughly three kilometres and reached several times the sea-level value above five; radiation from radioactive matter in the ground could only weaken with altitude. Some extremely penetrating rays had therefore to stream into the atmosphere from outer space, and these became known as cosmic rays. The flights opened cosmic-ray physics and brought Hess the 1936 Nobel Prize in Physics.
难度 入门 · 适合 大众
CC BY 4.0 · 来源待核验 · Source pending review
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出处备注：此条未指向带版次的可引用著作（通识性内容、机构文件或未展开的英文书名），本库不提供独立书目出处；如需精确引用，需由编辑补全具体篇目与版次。
V. F. Hess,Physikalische Zeitschrift 第13卷1084页,1912年
V. F. Hess, Physikalische Zeitschrift 13, 1084 (1912)
#宇宙线 #静电计 #大气电离