📚 The Michelson-Morley Experiment and Its Significance | 迈克尔逊-莫雷实验及其意义
The Michelson-Morley experiment, performed in 1887 by Albert A. Michelson and Edward W. Morley, is one of the most famous and important experiments in the history of physics. It was designed to detect the motion of Earth through the hypothetical ‘luminiferous aether’ — yet it produced a null result that fundamentally challenged existing ideas about light and motion.
迈克尔逊-莫雷实验由阿尔伯特·A·迈克尔逊和爱德华·W·莫雷于1887年完成，是物理学史上最著名、最重要的实验之一。该实验旨在检测地球在假想的”发光以太”中的运动——然而它给出的零结果从根本上挑战了当时关于光和运动的既有观念。
1. The Aether Hypothesis | 以太假说
In the 19th century, physicists believed that light waves, like all waves, required a medium through which to travel. Since light travels from the Sun to Earth across empty space, it was proposed that a mysterious invisible substance, called the ‘luminiferous aether’, filled all of space and served as this medium.
在19世纪，物理学家认为光波与其他波动一样，需要一种介质才能传播。由于光从太阳穿越虚空到达地球，人们便提出一种神秘且不可见的物质——”发光以太”——充满整个空间，充当这一传播介质。
According to this theory, the aether was assumed to be absolutely stationary in space. Earth, moving in its orbit around the Sun at about 30 km/s, should experience an ‘aether wind’ blowing past it — much like the wind felt when cycling on a still day.
根据这一理论，以太被认为在空间中是绝对静止的。地球以约30 km/s的速度绕太阳公转，应当感受到一股”以太风”迎面吹来——就像在无风的日子里骑车时感受到的气流一样。
The crucial question was: can this aether wind be detected? Michelson and Morley set out to measure it using an extremely sensitive optical instrument called the Michelson interferometer.
关键问题是：这种以太风能否被探测到？迈克尔逊和莫雷着手用一台名为迈克尔逊干涉仪的极其灵敏的光学仪器来测量它。
2. The Michelson Interferometer | 迈克尔逊干涉仪
The Michelson interferometer splits a single beam of monochromatic light into two perpendicular beams, reflects them from mirrors, and then recombines them to produce an interference pattern.
迈克尔逊干涉仪将一束单色光分成两束互相垂直的光束，使其经反射镜反射后重新汇合，从而产生干涉图样。
| Component | 组件 | Function | 功能 |
| Light source (monochromatic) | 光源（单色） | Emits a single wavelength of light | 发出单一波长的光 |
| Beam splitter | 分束器 | Splits light into two perpendicular paths | 将光分成两条垂直路径 |
| Mirror M₁ | 反射镜M₁ | Reflects light back along path 1 | 将光沿路径1反射回来 |
| Mirror M₂ | 反射镜M₂ | Reflects light back along path 2 | 将光沿路径2反射回来 |
| Detector / screen | 探测器/屏幕 | Displays interference fringes | 显示干涉条纹 |
The two light paths are made as equal in length as possible. If light travels at the same speed in both directions, the two beams will return in phase and constructively interfere. If one path takes longer, a phase difference appears and the interference pattern shifts.
两条光路的长度被尽可能调整得相等。如果光在两个方向上的速度相同，两束光将以相同相位返回并发生相长干涉。若某一路径耗时更长，则会出现相位差，干涉图样随之发生移动。
3. Predicted Effect of the Aether Wind | 以太风的预期效应
Suppose the apparatus is oriented so that one arm (length L) is parallel to Earth’s velocity v through the aether, and the other arm is perpendicular to it. Let c be the speed of light relative to the aether.
假设仪器的一条臂（长度为L）与地球在以太中的速度v平行，另一条臂与之垂直。设c为光相对于以太的速度。
For the parallel arm, the outbound and return journeys take different times. The total time is:
对于平行臂，光出发和返回的旅程用时不同。总时间为：
t∥ = L/(c – v) + L/(c + v) = 2Lc/(c² – v²)
For the perpendicular arm, the light must ‘angle’ slightly against the aether flow, analogous to a swimmer crossing a river. Its total time is:
对于垂直臂，光必须略微”倾斜”以对抗以太流，类似于游泳者横渡河流。其总时间为：
t⊥ = 2L/√(c² – v²)
The time difference between the two paths is therefore:
因此两条路径之间的时间差为：
Δt = t∥ – t⊥ ≈ Lv²/c³
Using L ≈ 11 m, v ≈ 3 × 10⁴ m/s (Earth’s orbital speed), and c ≈ 3 × 10⁸ m/s, the predicted fringe shift is only about 0.4 of a fringe — small but detectable with their apparatus.
取L ≈ 11 m，v ≈ 3 × 10⁴ m/s（地球公转速度），c ≈ 3 × 10⁸ m/s，预测的条纹移动仅为约0.4个条纹——虽然微小，但用他们的仪器仍可检测到。
4. The Experimental Design | 实验设计
Michelson and Morley mounted the interferometer on a massive stone slab floating in a pool of mercury. This allowed the entire apparatus to be rotated smoothly and continuously without vibrations.
迈克尔逊和莫雷将干涉仪安装在一块漂浮于汞池中的巨大石板台上。这使整个装置可以平稳、连续地旋转而不会产生振动。
As the apparatus rotated, the arm previously parallel to the aether wind would become perpendicular, and vice versa. This would cause the interference fringes to shift back and forth periodically as the instrument rotated.
当装置旋转时，原本与以太风平行的臂会变为垂直，反之亦然。这将导致干涉条纹在仪器旋转过程中周期性地来回移动。
To increase the effective path length, the light was reflected back and forth multiple times using additional mirrors, achieving an effective path length of about 11 metres.
为了增加有效路径长度，光线通过额外的反射镜多次往返反射，达到约11米的有效光程。
5. The Null Result | 零结果
To the surprise of the entire physics community, the experiment showed no measurable fringe shift. The interference pattern remained identical regardless of the orientation of the apparatus.
令整个物理学界惊讶的是，实验显示没有可测量的条纹移动。无论装置取向如何，干涉图样始终保持不变。
The observed fringe shift was at most 0.01 fringes, which is within the margin of experimental error. The expected 0.4 fringe shift was definitively absent.
观测到的条纹移动至多为0.01个条纹，处于实验误差范围之内。预期中的0.4个条纹移动完全不存在。
This null result was repeated at different times of the year (to account for variations in Earth’s velocity as it orbits the Sun) and by numerous other experimenters. The conclusion was always the same: there is no detectable aether wind.
这一零结果在不同季节（以考虑地球绕太阳公转时速度的变化）被重复验证，并被众多其他实验者重复。结论始终一致：不存在可探测的以太风。
6. Interpretation Difficulties | 解释的困境
The null result posed a serious problem. If the aether existed and Earth moved through it, some effect must be observable. Yet none was found.
零结果带来了一个严重的问题。如果以太存在且地球在它之中运动，那么必然应当观察到某种效应。然而什么也没有发现。
Various explanations were proposed. The most notable was the ‘length contraction hypothesis’ by George FitzGerald and Hendrik Lorentz, which suggested that objects contract slightly in the direction of motion through the aether:
人们提出了各种解释。最著名的是乔治·菲茨杰拉德和亨德里克·洛伦兹提出的”长度收缩假说”，即物体在通过以太运动的方向上会发生微小收缩：
L = L₀√(1 – v²/c²)
This contraction would exactly compensate for the expected time difference, explaining the null result. However, this explanation seemed ad hoc — it was invented solely to explain the experiment’s outcome.
这种收缩恰好补偿了预期的时间差，从而解释了零结果。然而，这一解释显得牵强附会——它似乎只是为了解释实验结果而专门提出的。
7. Einstein’s Revolution | 爱因斯坦的变革
In 1905, Albert Einstein published his special theory of relativity, which dispensed with the aether entirely. Einstein’s theory rested on two postulates:
1905年，阿尔伯特·爱因斯坦发表了狭义相对论，彻底摒弃了以太概念。爱因斯坦的理论建立在两条假设之上：
- Principle of relativity | 相对性原理：
The laws of physics are the same in all inertial reference frames. | 物理定律在所有惯性参考系中都是相同的。
- Constancy of the speed of light | 光速不变原理：
The speed of light in a vacuum is the same for all observers, regardless of their relative motion. | 真空中的光速对所有观测者都是相同的，与它们的相对运动无关。
These postulates elegantly explain the Michelson-Morley null result: since the speed of light is the same in all directions for all inertial observers, no time difference can ever be measured — there is no aether wind to detect.
这些假设优雅地解释了迈克尔逊-莫雷实验的零结果：既然光速在所有惯性观测者眼中的所有方向上都是相同的，就不可能存在可测量的时间差——也就没有以太风可供探测。
8. Key Consequences of Special Relativity | 狭义相对论的关键推论
The special theory of relativity leads to several profound consequences that are now well established in modern physics:
狭义相对论引出了几个深远的结果，如今已在现代物理学中被充分确立：
Time dilation | 时间膨胀： A moving clock runs slower relative to a stationary observer:
时间膨胀：运动中的时钟相对于静止观测者走得更慢：
Δt = Δt₀/√(1 – v²/c²)
Length contraction | 长度收缩： A moving object contracts along its direction of motion:
长度收缩：运动中物体沿运动方向收缩：
L = L₀√(1 – v²/c²)
Mass-energy equivalence | 质能等价： Energy and mass are interchangeable:
质能等价：能量和质量可以相互转化：
E = mc²
These equations are among the most important in physics and have been confirmed by countless experiments, including particle accelerators, GPS satellites, and nuclear power generation.
这些方程是物理学中最重要的方程之一，已被无数实验证实，包括粒子加速器、GPS卫星和核能发电。
9. The Experiment’s Historical Significance | 实验的历史意义
The Michelson-Morley experiment is often described as the ‘most famous failed experiment’ in physics — it failed to detect what it was designed to find, yet it changed the course of physics forever.
迈克尔逊-莫雷实验常被称为物理学中”最著名的失败实验”——它未能探测到原本设计要寻找的东西，却永远改变了物理学的进程。
Its significance can be summarised as follows:
其意义可总结如下：
| Aspect | 方面 | Significance | 意义 |
| Disproved the aether theory | 推翻了以太理论 | Showed that no aether wind exists, eliminating the need for a light medium | 证明以太风不存在，消除了光传播介质存在的必要性 |
| Supported the constancy of light speed | 支持了光速不变性 | Provided strong experimental evidence for Einstein’s second postulate | 为爱因斯坦的第二条假设提供了有力的实验证据 |
| Catalysed the development of special relativity | 促进了狭义相对论的发展 | Its null result was a key motivation for Einstein’s 1905 paper | 其零结果是爱因斯坦1905年论文的重要动因 |
| Advanced experimental technique | 推动了实验技术发展 | Michelson’s interferometer became a fundamental tool in modern physics | 迈克尔逊干涉仪成为现代物理学的基本工具 |
10. Modern Applications of the Interferometer | 干涉仪的现代应用
Although the original experiment failed in its stated goal, the interferometer design lives on in numerous modern applications:
尽管原初实验未能实现其既定目标，干涉仪的设计却延续至今，应用于众多现代科技中：
- LIGO gravitational wave detection | LIGO引力波探测： Michelson-type interferometers with arms 4 km long detected gravitational waves in 2015. | 臂长4公里的迈克尔逊型干涉仪于2015年探测到了引力波。
- Optical coherence tomography | 光学相干断层扫描： Used in medical imaging for retina and tissue examination. | 用于视网膜和组织检查的医学成像。
- Laser interferometry in precision measurement | 精密测量中的激光干涉测量： Used in semiconductor manufacturing and calibration. | 用于半导体制造和校准。
- Fourier-transform infrared spectroscopy | 傅里叶变换红外光谱： Used to identify chemical compounds. | 用于鉴别化合物。
These applications demonstrate that the experimental technique developed by Michelson and Morley has far outlived the aether hypothesis it was designed to test.
这些应用表明，迈克尔逊和莫雷所发展的实验技术远远超越了他们原本要验证的以太假说而被广泛沿用。
11. A-Level Exam Points | A-Level考试要点
For A-Level examinations, students should be able to:
对于A-Level考试，学生应能够：
- Explain the purpose of the Michelson-Morley experiment — to detect the aether wind | 解释迈克尔逊-莫雷实验的目的——探测以太风
- Describe the apparatus: beam splitter, two perpendicular mirrors, and interference pattern | 描述实验装置：分束器、两面垂直的反射镜和干涉图样
- State the principle of operation: interference of light split into two perpendicular paths | 阐述工作原理：光被分成两条垂直路径后发生干涉
- Explain why a fringe shift was expected (relative velocities along the two paths differ) | 解释为何预期出现条纹移动（两条路径上的相对速度不同）
- State the result: no measurable fringe shift was observed | 说明实验结果：未观察到可测量的条纹移动
- Explain that this null result supported Einstein’s postulate that the speed of light is constant for all inertial observers | 解释这一零结果如何支持爱因斯坦关于光速对一切惯性观测者恒定的假设
- Discuss the significance of the experiment in the development of special relativity | 讨论该实验在狭义相对论发展中的意义
Common exam pitfalls to avoid: do not say the experiment ‘proved that the aether does not exist’ as a direct statement — rather, it showed that the aether wind was undetectable and its null result is best explained by the constancy of light. Also, remember that Einstein’s derivation did not depend on the experiment, but the experiment provided important supporting evidence.
常见的考试误区：不要直接说实验”证明以太不存在”——更准确地说，它表明以太风无法被探测到，其零结果最好用光速不变来解释。同时要记住，爱因斯坦的推导并不依赖于这个实验，但实验为理论提供了重要的支持性证据。
12. Context: Einstein’s 1905 Paper | 背景：爱因斯坦1905年的论文
Interestingly, Einstein later stated that he was not explicitly aware of the Michelson-Morley result when he developed special relativity in 1905. He arrived at the theory primarily through theoretical reasoning based on Maxwell’s equations, which also imply a constant speed of light.
有趣的是，爱因斯坦后来表示，他在1905年发展狭义相对论时并不特别清楚迈克尔逊-莫雷的实验结果。他主要通过基于麦克斯韦方程组的理论推导得出该理论，而麦克斯韦方程组本身也蕴含了光速不变。
However, the Michelson-Morley experiment remains indispensable in physics pedagogy and in the historical narrative of relativity. It provides a clear, concrete example of how a carefully designed experiment can overturn an established theoretical assumption through a null result.
然而，迈克尔逊-莫雷实验在物理教学和相对论的历史叙事中仍然不可或缺。它提供了一个清晰、具体的例子，说明一个精心设计的实验如何通过零结果推翻一个既有的理论假设。
The experiment also earned Michelson the Nobel Prize in Physics in 1907, making him the first American to receive one — an honour for his optical precision instruments and the spectroscopic and metrological investigations conducted with them.
该实验还使迈克尔逊于1907年获得诺贝尔物理学奖，成为首位获此殊荣的美国人——表彰他在光学精密仪器以及借助这些仪器进行的光谱学和计量学研究方面的卓越贡献。
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