The light from hot soot

This chapter describes a fascinating series of events in the history of science. They occurred at the beginning of the 20th century, and started quite innocuously with a disagreement between experimental evidence and theoretical prediction in the area of 'blackbody radiation'. The distribution of colours of light from hot surfaces, such as, burning coal or molten metal, did not make sense.

Classical thermodynamics, the well-established theory of heat and heat transfer, made excellent predictions of some of the general features of the spectrum, but when it came to specific models for the process, something was decidedly wrong

Wilhelm Wien produced a model which agreed quite well with experiment at short wavelengths, but in order to make it right at longer wavelengths, he had to 'fiddle' with the formula, or make a mathematical adjustment, for which there was no justification. Lord Rayleigh produced a more detailed model, which was right for long wavelengths, but the predicted energy of radiation shot off to infinity at the other end of the spectrum — the ultraviolet catastrophe.

In 1901, Max Planck made a revolutionary suggestion — an 'act of despair', as he called it. By restricting the allowed energy of light for a given wavelength, he was able to derive an exact formula, which fitted the experimental results everywhere. As a matter of fact it was just like Wien's 'fiddled' expression, but this time it was based on sound physical reasoning. Still, there was a heavy price to pay: Planck had to make the assumption that only certain levels of energy are possible for an electric oscillator, which are an integral number of quantum units hf.

Up to then, an inherent understanding in natural philosophy was that there are no restrictions on the possible values of physical entities. The suggestion that Nature is not continuous was so reactionary that Planck was afraid to publish his hypothesis, and it was in fact not generally accepted until the successful work of Niels Bohr in 1913. Physicists were reluctant to abandon long-established beliefs, with some notable exceptions, such as Albert Einstein, who in 1905 had developed the concept of quantisation even further in his theory of the photoelectric effect.

To follow this chapter in detail, some knowledge of basic mechanics and thermodynamics is helpful, but the reader should get a good feeling for the concepts by skimming through the more detailed parts of the discussion, particularly in Section 11.2. When you come to Sections 11.5.1-11.5.4, again skim through the mathematical discussion. If you are comfortable with the mathematics, however, you should enjoy following Planck's footsteps to one of the most fundamental discoveries in the history of physics

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