物理学 1919

太阳引力场对光之偏折的测定

弗兰克·戴森、阿瑟·爱丁顿与查尔斯·戴维森

掠过太阳的星光偏折 1.75″——正如爱因斯坦所言，是牛顿所许的两倍。

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In depth · the introduction

1919 年，天文学家等着月亮遮住太阳——结果捕捉到星光偏折，偏折量恰是爱因斯坦此前大胆预言的那个数。

核心想法

爱因斯坦的广义相对论说：质量会弯曲其周围空间与时间的形状，而光走的是当下最直的路，便不得不顺着这弯曲而行。于是，一束掠过太阳的星光应当被略微弯折，一颗正贴着太阳的星，看上去应当被往外推开一点。

爱因斯坦给出了具体数字：紧贴太阳边缘的星，应向外移约 1.75 角秒——一个发丝般细的角度，却是用牛顿旧物理所能解释的两倍。整场检验，归根结底就是测出这极小的位移，再看它合的是爱因斯坦的数，还是较小的那个牛顿数。

事情的由来

麻烦在于，你根本看不见太阳旁边的星——它的强光把星都淹没了。唯一的例外是日全食：月球遮住太阳的那几分钟，群星会在变暗的白昼天空中冒出来。1919 年 5 月 29 日的这场日食恰到好处：被食的太阳将落在一团明亮密集的星群——毕星团——之中。

为防云遮，英国派出了两支观测队——一支去巴西的索布拉尔，一支去西非外海的普林西比岛，阿瑟·爱丁顿亲自前往。他们拍下被食太阳周围的群星，再与几个月后夜里拍的同一片星场比对。靠近太阳的星，果然向外移了。当年 11 月，弗兰克·戴森在伦敦公布这些数字，结果登上世界各地的头条，几乎在一夜之间把爱因斯坦变成在世最有名的科学家。诚实起见要加一句脚注：数据是带噪声的，一台成像模糊、给出相左结果的仪器被搁置一旁——这一取舍后来招致偏见之讥，不过其后的重新分析已确认了结论。

它为何重要

两个世纪以来，牛顿的引力一直是宇宙不容置疑的法则。1919 年的日食，是第一份硬证据，表明它并非定论——在足够重的天体附近，爱因斯坦那幅更奇异的弯曲时空图景，更贴近真实。它把广义相对论从优美的数学变成了受检验的物理，并开启了黑洞、膨胀的宇宙、引力波与 GPS 的一个世纪——而这一切，都倚靠着那几张日食底片所确认的理论。

一个日常画面

把空间的织物想象成一张绷紧的橡皮膜。在中央放一个重球——太阳——膜便绕它凹陷成一个坑。现在让一颗玻璃弹珠笔直地滚过膜面，离重球远远的：当它越过坑的斜坡时，会略微偏向，尽管没有任何东西从侧面推它。星光在太阳附近也是如此——不是被旧意义上的「力」拉扯，而是顺着太阳质量在空间本身凿出的山谷而行。弹珠越贴近重球，坡越陡、偏越急——这正是为什么掠过太阳边缘的光，偏折最大。

它的位置

这正是那场确认了本馆「爱因斯坦广义相对论（1916）」一篇所述理论的实验。它是一道枢纽：一端连着牛顿的《自然哲学的数学原理》（1687）——其引力正是在边缘处被它拉下王座——另一端连着随之而来的现代引力科学：LIGO 于 2016 年捕获的引力波、黑洞的物理，以及让天文学家得以称量暗物质的透镜星系。这一切，都建在那弯曲的时空之上——而几张照相底片，最先在那里抓住了太阳使光偏折的一幕。

The original document

Original source text

F. W. Dyson, A. S. Eddington & C. Davidson · Phil. Trans. R. Soc. Lond. A 220 (1920): 291–333

The question

The purpose of the expeditions was to determine what effect, if any, is produced by a gravitational field on the path of a ray of light traversing it.

Three alternatives

The report frames the test as a choice between three outcomes for a star seen close to the Sun's edge:

(1) The path is uninfluenced by gravitation. (2) The energy or mass of light is subject to gravitation in the same way as ordinary matter. If the law of gravitation is strictly the Newtonian law, this leads to an apparent displacement of a star close to the sun's limb amounting to 0″·87 outwards. (3) The course of a ray of light is in accordance with Einstein's generalised relativity theory. This leads to an apparent displacement of a star at the limb amounting to 1″·75 outwards.

The method

During the few minutes of totality on 29 May 1919, when the Moon hid the Sun's disc, each expedition photographed the field of stars surrounding it — a particularly rich part of the Hyades. Months later the same star field was photographed at night, with the Sun far away. Superposing the plates and measuring how far each star had shifted outward gives the deflection directly. Two sites were used against the risk of cloud: Sobral in northern Brazil, and the island of Príncipe in the Gulf of Guinea, where Eddington himself observed.

[ … ]

The measurements

Corrected to the Sun's limb, the well-exposed Sobral 4-inch plates gave 1.98″ ± 0.12″, and Eddington's Príncipe plates gave 1.61″ ± 0.30″ — both near Einstein's 1.75″ and far from the Newtonian 0.87″. A third set, the Sobral astrographic plates, gave a much smaller figure but had been blurred by the Sun's heat distorting the mirror, and was set aside. (Probable errors as quoted in the report.)

Conclusion

Thus the results of the expeditions to Sobral and Principe can leave little doubt that a deflection of light takes place in the neighbourhood of the sun and that it is of the amount demanded by Einstein's generalised theory of relativity, as attributable to the sun's gravitational field.

Royal Observatory, Greenwich · read to the Royal Society, 6 November 1919