Happy Birthday, Quantum Mechanics!

Quantum mechanics was born on the 14th of December 1900 (by which time, it was generally agreed upon that most of the significant discoveries in Physics were made, and there remained only a little patchwork to be done in order to make it totally coherent and complete), when Max Planck explained the derivation of his radiation law at a meeting of the German Physical Society in Berlin. This presentation of Planck's described the key concepts of quantum mechanics and discussed some of the most recent and startling applications. Over the past century, this revolutionary new idea has profoundly altered our understanding of the universe, and the 20th century more or less belonged to Physics.

Interestingly, unlike for Newton, no apple fell on Planck's head. Nor even a light bulb! But they say that the glow of a light bulb left him confused. For, while the yellow glow of a light bulb suggested that most of its light/radiation was at visible frequency, the physics of the day would have predicted that a heated object should emit mostly shorter-wavelength, invisible radiation. Max Planck presented his explanation for this troubling observation, known as the black-body radiation problem, and the light-bulb started driving physicists crazy ever since! For, to solve his problem, Planck had had to invent the notion of the quantum, and as Feynmann puts in, 'We can safely say that nobody understnads Quantum Mechanics.… Do not keep asking yourself, if you can possibly avoid it, "But how can it be like that?"… Nobody knows how it can be like that.' Or to quote Murray Gell-Mann, who described quantum mechanics as 'that mysterious, confusing discipline which none of us really understands but which we know how to use'.

Nor was Planck's own view very different! Though he had to (reluctantly) announce that certain experimental results could only be understood if energy was emitted or absorbed in discrete packets (called 'quanta'), and not continuously as had been supposed to happen uptil then, and thus declare the arrival of Quantum Physics, the nature and significance of quanta made him very unhappy, as is evident from the following: 'He was really a revolutionary against his own will… He finally came to the conclusion, "It doesn't help. We have to live with quantum theory. And believe me it will expand. It will not only be optics. It will go in all fields. We have to live with it".' But the die was already cast, and there was not looking back. Nobody had the power to stop the onslaught of Quantum Mechanics, not even Planck, doubts and confusion of his later life notwithstanding!

[However, Plancks' situation was not as hapless as that of Paul Ehrenfest, whose problem of low self-esteem and constant doubts about his abilities was only compounded by the confusion created by the Bohr-Einstein debate about the interpretation of Quantum Mechanics, and who finally ended up killing himself in 1933.]