物理学 1887

论地球与光以太的相对运动

阿尔伯特·迈克耳孙与爱德华·莫雷

他们去寻找以太之风——却以极高的精度，发现：什么都没有。

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In depth · the introduction

一个世纪里，物理学家都笃信：光需要一片看不见的海洋才能传播。两位美国人造出了有史以来最精细的尺子，去感受那片海洋的水流——却什么也没感受到。

核心想法

在十九世纪，人人都同意光是一种波。可波，总要在某种东西里起伏：声波在空气里，海浪在水里。那么光也该在某种东西里起伏——一种填满整个空间的透明物质，名叫光以太。倘若它真的存在，那么以约每秒 30 公里绕太阳飞奔的地球，就该感到一阵「以太风」从身旁掠过——正如你把手伸出行驶的车窗，会感到空气呼呼地冲过来。

迈克耳孙与莫雷，正是要去感受这阵风。他们把一束光分成两半，让两半各自沿着两条成直角的臂行进，撞上镜子弹回，再重新合到一起。倘若以太风加快了一束、又减慢了另一束，那么合起来的光就会显出一种泄露天机的条纹图样——而旋转整台机器，会让条纹移动。他们造出的仪器，灵敏到能捕捉比预期弱二十倍的风。可条纹，从未移动。

它是如何诞生的

阿尔伯特·迈克耳孙痴迷于测量光。1881 年，他在德国造出干涉仪的第一个版本，得到一个令人困惑的、近乎零的结果——但他的计算里有个错误，而那次测试也还不够灵敏，难以确凿。回到克利夫兰，他与化学家爱德华·莫雷联手，把这场实验以巨大的规模重新搭建起来。

他们的杰作，与其说是物理，不如说同样是一项工程。为了扼杀颤动，他们把光学元件装在一块砂岩石板上，再让石板浮在一池液态水银上——于是这张沉重的工作台，能像唱机转盘上的唱片一样平稳地旋转，每六分钟缓缓转一圈。他们一小时又一小时地盯着条纹，一个季节又一个季节地谋划。一次又一次：什么都没有。物理学中最被期待的那阵风，根本就没在吹。

它为何重要

一个著名的「没有」，有时比一项发现更有力量。以太曾是物理学最根本的预设，而这场实验表明：根本探测不到什么以太风——这意味着，「光在一片固定而看不见的介质中行进」这幅令人安心的图景，有什么地方大错特错了。

人们花了将近二十年才消化它。这道谜题，帮着把一个激进的想法逼到了台前：光速对每个人都一样，无论他如何运动；而空间与时间本身，必须为了维持这一点而弯折。这个想法，就是爱因斯坦的狭义相对论；而迈克耳孙–莫雷的「零结果」，正是最常被援引为它扫清道路的那场实验。

一个可以想象的画面

想象河里有两名一模一样的游泳者，各自来回游同样的距离。一个笔直地横渡水流再游回来；另一个先逆流而上、再顺流而下。即便游速相同，水流耽搁他们的程度也不一样——那位逆流又顺流的，总要多花些时间。所以，倘若真有一股以太「水流」从地球身旁淌过，两束光返回时就该略微错开节拍，你会在条纹里看见它。迈克耳孙与莫雷把这场比赛一遍遍地重赛，把「泳池」朝各个方向旋转——可两名游泳者总是打平。根本没有水流。

它的位置

这场实验，正坐落在物理学史的一处转轴上。在它身后，是菲涅耳与麦克斯韦所构筑的伟大的光的波动说，那似乎要求一种以太；在它身前，是爱因斯坦 1905 年的相对论，把以太扔了出去、让光速成为绝对。迈克耳孙的干涉仪本身则从未退休：缩小到实验室里，它成了一件精密工具；放大到四公里长的臂，它成了 LIGO——那台在 2015 年捕捉到引力波的仪器，倾听着一种远比他们当年没能找到的以太风更微弱的、空间的涟漪。

The original document

Original source text

以太假说

A. A. Michelson & E. W. Morley · American Journal of Science · 1887 · §1

The discovery of the aberration of light was soon followed by an explanation according to the emission theory. The effect was attributed to a simple composition of the velocity of light with the velocity of the earth in its orbit.

On the undulatory theory, according to Fresnel, first, the ether is supposed to be at rest except in the interior of transparent media, in which secondly, it is supposed to move with a velocity less than the velocity of the medium in the ratio (n² − 1)/n², where n is the index of refraction.

[ … ]

If the earth were a transparent body, it might perhaps be conceded, in view of the experiments just cited, that the intermolecular ether was at rest in space, notwithstanding the motion of the earth in its orbit; but we have no right to extend the conclusion from these experiments to opaque bodies.

漂浮在水银上的仪器

§ The interferometer, floated to turn freely and continuously

The stone is about 1.5 meter square and 0.3 meter thick. It rests on an annular wooden float, 1.5 meter outside diameter, 0.7 meter inside diameter, and 0.25 meter thick. The float rests on mercury contained in the cast-iron trough.

The apparatus was revolved very slowly (one turn in six minutes) and observations of the fringes were made continually as the stone turned. By this means an accumulation of errors was avoided, and the displacement, if any, could be detected at once.

The distance D was about eleven meters, or 2×10⁷ wave-lengths of yellow light; hence the displacement to be expected was 0.4 fringe.

那个「零结果」

§ Comparing the observed shift with the shift predicted by a stationary ether

The actual displacement was certainly less than the twentieth part of this, and probably less than the fortieth part.

Considering the motion of the earth in its orbit only, this displacement should be 2D × v²/V² = 2D × 10⁻⁸. The distance D was about eleven meters; hence the displacement to be expected was 0.4 fringe — and yet it could not be found.

It appears, from all that precedes, reasonably certain that if there be any relative motion between the earth and the luminiferous ether, it must be small; quite small enough entirely to refute Fresnel's explanation of aberration.

The experiment will therefore be repeated at intervals of three months, and thus all uncertainty will be avoided.

Albert A. Michelson & Edward W. Morley · Cleveland, Ohio · 1887