It was exactly one hundred years ago on 14 December 1900 that the great physicist Max Planck (1858–1947) presented to the German Physical Society his revolutionary concept of the quantum of action, using which he successfully explained the spectrum of radiation emitted by a black body. Planck was aided in this momentous discovery by, first, his insistence on empirical veracity and second, the laws of thermodynamics. This history is narrated in several places1, I shall just briefly recall the main features.
It was exactly one hundred years ago on 14 December 1900 that the great physicist Max Planck (1858–1947) presented to the German Physical Society his revolutionary concept of the quantum of action, using which he successfully explained the spectrum of radiation emitted by a black body. Planck was aided in this momentous discovery by, first, his insistence on empirical veracity and second, the laws of thermodynamics. This history is narrated in several places1, I shall just briefly recall the main features.
Science has developed through a variety of investigations more or less over the time scale of human existence. On this scale, quantum mechanics is an extremely young field, existing essentially for only just over a century. Even our understanding of classical mechanics has existed for a comparatively long period—roughly having been formalized with Newton's equations published in his Principia Mathematica , in April 1686 (1687). Up until the very end of the nineteenth century, mechanics in the form of Newton's laws were comfortable, being understood and accepted by all who dealt with the motion of reasonable bodies. With the turn of the new century, however, mechanics would be shaken to its roots. The change began already on December 14, 1900, when Planck (1900) presented his new theory of black-body radiation to the German Physical Society, and this change would affect mechanics in physically small structures (and lead to quantum mechanics). Mechanics of large bodies would similarly be changed in 1905 with the arrival of Albert Einstein's (1905a) relativity theory. In this book, we will deal with only the first of these two great breakthroughs.
One of the first inklings of a problem in physics at the end of the 19th century was the nature of the radiation emission spectrum from a black body. This could not be explained in terms of established and irrefutable methods of statistical mechanics. It was Planck’s idea to divide up the energy spectrum of a series of harmonic oscillators each with a different state of energy but whose energy steps depended upon the frequency of the oscillation that marked the beginning of quantum physics. As a result, Planck introduced a fundamental constant of nature, Planck’s constant.
As expected when I started out on this project, this has proved to be a complex and, at times, difficult story. After all, what was involved was tearing up the foundations of classical physics, which had been extraordinarily successful in explaining the macroscopic world about us, and replacing it by something radically different and non-intuitive in terms of our everyday experience. But the effort involved has been more than repaid by the very much deeper appreciation I have gained of the extraordinary works of the pioneers of quantum mechanics, both the theorists and the experimenters. If the brilliant theoretical researches of Planck, Einstein, Bohr, Heisenberg, Born, Jordan, Schrödinger, Pauli, Dirac and many others form the central core of this story, it should be remembered that their researches were inspired by the equally brilliant achievements of experimental physics. Another huge bonus has been a deepened understanding of quantum mechanics itself – if only I had these insights more than 50 years ago when I first encountered the subject. There is a great deal more that could be said. I must reiterate that I have presented a somewhat streamlined version of the story in order to ensure that there is some continuous pathway, however tortuous, to the way in which the new understandings came about. For a full appreciation of the complexity of the story and the numerous blind alleys and diversions which took place, there is no substitute for in-depth absorption in Mehra and Rechenberg's magisterial exposition of the history of quantum theory.
The beginning of the twentieth century brought surprising nonclas- sical phenomena. Max Planck’s explanation of black-body radiation [1], the work of Albert Einstein on the photoelectric effect [2], and Niels Bohr’s model to account for the electron orbits around the nuclei [3] established what is now known as the old quantum theory. To describe and explain these effects, phenomenological models and theories were first developed, without any rigorous and global justification. In order to provide a complete explanation for the underlying physics of such new nonclassical phenomena, physicists were forced to abandon classical mechanics to develop novel, abstract, and imaginative formalisms.
Physicists often complain that the phrase "quantum leap" is used erroneously, if harmlessly, by people to describe a large jump or change. Yet this month the physics community is celebrating the 100th anniversary of what must count as the beginning of the biggest quantum leap in the history of science, whichever way you define the phrase. On 14 December 1900 Max Planck presented a paper on the radiation emitted by a black body at a meeting of the German Physical Society in Berlin. This event is generally taken to mark the start of the quantum century, though few people realized it at the time – not even Planck himself.
Toward the end of the nineteenth century, it became clear that some experiments cannot be explained on the basis of known physics, now referred to as classical physics. In 1900, Max Planck was able to correctly describe the distribution of blackbody radiation energy as a function of wavelength λ or frequency ν = c/λ. He had to accept the existence of energy packets (quanta) with energy E = hν. A few years later, Albert Einstein found that this unexpected relation between energy and frequency is also necessary in order to understand photoemission from metallic surfaces. A decade later, Niels Bohr arrived at the correct equation for the electronic spectrum of the hydrogen atom by applying “quantum conditions” on the motion of the electron around the nucleus and the Planck’s relation E = hν on the energy difference between the energy levels (1916). A decade later, Heisenberg, Born, and Jordan introduced “quantum mechanics” and de Broglie and Schrödinger (1925-1926) introduced “wave mechanics.”