Chapter 1.Quantum Uncertainty
1900 was the year in which Lord Kelvin spoke of the triumphs of physics and the way Newton's theory of motion could be extended to embrace the phenomena of light and of heat. His address went on to mention "two clouds" that obscured the "beauty and clearness" of the theory. The first cloud involved the way light travels through space. The second cloud was the problem of how to distribute energy equally amongst vibrating molecules.
In his talk Kelvin proposed his own solution to these "two clouds." As it turned out, he was totally wide of the mark. Ironically, what Kelvin had taken to be clouds on the horizon were in fact two bombshells about to create a massive explosion in twentieth century physics. Their names were relativity and quantum theory. Both theories had something to say about light.
Light, physicists like Kelvin argued, is a vibration, and like every other vibration it should be treated by Newton's laws of motion. But a vibration, physicists argued, has to be vibrating in something. And so physicists proposed that space is not empty but filled with a curious jelly called "the luminiferous ether." But this meant that the speed of light measured in laboratories on earth - the speed with which vibrations appear to travel through the ether - should depend on how fast and in what direction the earth is going through the ether. And, since the earth rotates around the sun this direction is always varying, as is the speed should vary over a year. Scientists therefore expected to detect a variation in the speed of light measured at various times in the year. However, very accurate experiments showed that this was not the case. No matter how the earth moves with respect to the background of distant stars, the speed of light remains the same.
This mystery of the speed of light and the existence, or non-existence, of the ether was only solved with Einstein's Special Theory of Relativity, which showed that the speed of light is a constant, independent of how fast you or the light source is traveling.
The other cloud on Kelvin's horizon, the way energy is shared by vibrating molecules, was related to yet another difficult problem - the radiation emitted from a hot body. In this case the solution demanded a revolution in thinking that was just as radical as relativity theory - the quantum theory.