1900年4月27日星期五，英国物理学家凯尔文勋爵发表了题为“十九世纪的云与热与光的动力学理论”的演讲，开头说：动态理论的美丽和清晰，它将热量和光线传递给是运动方式，目前被两片云遮住了。凯尔文接着解释说，“云”是两种无法解释的现象，他在完全理解宇宙的热力学和能量属性之前将其描述为需要填充的最后几个孔，用经典术语解释粒子的运动。这篇演讲以及归因于开尔文的其他评论（例如物理学家阿尔伯特迈克尔逊在1894年的演讲中）表明，他坚信当时物理学的主要作用是精确测量已知量，直到许多小数位的准确性。什么是“云”的意义开尔文所指的“云”是：无法检测到发光的以太，特别是迈克尔逊 – 莫利实验的失败。黑体辐射效应称为紫外线灾难。为什么这很重要。由于一个非常简单的原因，对这一演讲的引用变得有些受欢迎：开尔文勋爵尽可能地错了。而不是必须解决的细微细节，开尔文的两个“云”代替了理解宇宙的经典方法的基本限制。他们的决议引入了全新的（并且显然是未曾预料到的）物理领域，统称为“现代物理学”。事实上，马克斯普朗克在1900年解决了黑体辐射问题。（据推测，在凯尔文发表演讲之后。）在这样做时，他必须引用限制发射光能量的概念。这种“光量子”的概念在当时被视为一种简单的数学技巧，是解决问题所必需的，但它起作用。普朗克的方法精确地解释了黑体辐射问题中加热物体产生的实验证据。然而，在1905年，爱因斯坦进一步采用了这个概念，并用这个概念来解释光电效应。在这两种解决方案之间，很明显光似乎存在于能量的小包（或量子）（或光子，因为它们稍后将被称为）。一旦明确光线中存在光线，物理学家就会发现这些数据包中存在各种物质和能量，量子物理学的时代就此开始了。凯尔文提到的另一个“云”是迈克尔逊 – 莫利实验未能讨论发光的以太。这是当时物理学家认为渗透宇宙的理论物质，因此光可以像波浪一样移动。迈克尔逊 – 莫雷的实验是一个相当巧妙的实验，基于这样一种观点，即光会以不同的速度通过以太网移动，这取决于地球是如何穿过它的。他们构建了一种衡量这种差异的方法……但它没有奏效。似乎光的运动方向与速度没有关系，这与它穿过像以太这样的物质的想法不符。尽管如此，1905年爱因斯坦再次出现并在这一方面开球。他提出了狭义相对论的前提，并提出了一个假设，即光总是以恒定的速度移动。随着他发展相对论，很明显，发光醚的概念不再特别有用，因此科学家放弃了它。
On Friday, April 27, 1900, the British physicist Lord Kelvin gave a speech entitled “Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light,” which began: The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. Kelvin went on to explain that the “clouds” were two unexplained phenomena, which he portrayed as the final couple of holes that needed to be filled in before having a complete understanding of the thermodynamic and energy properties of the universe, explained in classical terms of the motion of particles. This speech, together with other comments attributed to Kelvin (such as by physicist Albert Michelson in a 1894 speech) indicate that he strongly believed the main role of physics in that day was to just measure known quantities to a great degree of precision, out to many decimal places of accuracy. What Is Meant by the “Clouds” The “clouds” to which Kelvin was referring were: The inability to detect the luminous ether, specifically the failure of the Michelson-Morley experiment. The black body radiation effect known as the ultraviolet catastrophe. Why This Matters. References to this speech have become somewhat popular for one very simple reason: Lord Kelvin was about as wrong as he could possibly have been. Instead of minor details that had to be worked out, Kelvin’s two “clouds” instead represented fundamental limits to a classical approach to understanding the universe. Their resolution introduced whole new (and clearly unanticipated) realms of physics, known collectively as “modern physics.” In fact, Max Planck solved the black body radiation problem in 1900. (Presumably, after Kelvin gave his speech.) In doing so, he had to invoke the concept of limitations on the allowed energy of emitted light. This concept of a “light quanta” was seen as a simple mathematical trick at the time, necessary to resolve the problem, but it worked. Planck’s approach precisely explained the experimental evidence resulting from heated objects in the black-body radiation problem. However, in 1905, Einstein took the idea further and used the concept to also explain the photoelectric effect. Between these two solutions, it became clear that light seemed to exist as little packets (or quanta) of energy (or photons, as they would later come to be called). Once it became clear that light existed in packets, physicists began to discover that all kinds of matter and energy existed in these packets, and the age of quantum physics began. The other “cloud” that Kelvin mentioned was the failure of the Michelson-Morley experiments to discuss the luminous ether. This was the theoretical substance that physicists of the day believed permeated the universe, so that light could move as a wave. The Michelson-Morley experiments had been a rather ingenious set of experiments, based on the idea that light would move at different speeds through the ether depending on how the Earth was moving through it. They constructed a method to measure this difference … but it hadn’t worked. It appeared that the direction of light’s motion had no bearing on the speed, which didn’t fit with the idea of it moving through a substance like the ether. Again, though, in 1905 Einstein came along and set the ball rolling on this one. He laid out the premise of special relativity, invoking a postulate that light always moved at a constant speed. As he developed the theory of relativity, it became clear that the concept of the luminous ether was no longer particularly helpful, so scientists discarded it.