Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [13]
The next day Rubens visited Planck to reassure him. 'He came to tell me that after the conclusion of the meeting he had that very night checked my formula against the results of his measurements,' Planck remembered, 'and found satisfactory concordance at every point.'46 Less than a week later, Rubens and Kurlbaum announced that they had compared their measurements with the predictions of five different formulae and found Planck's to be much more accurate than any of the others. Paschen too confirmed that Planck's equation matched his data. Yet despite this rapid corroboration by the experimentalists of the superiority of his formula, Planck was troubled.
He had his formula, but what did it mean? What was the underlying physics? Without an answer, Planck knew that it would, at best, be just an 'improvement' on Wien's law and have 'merely the standing of a law disclosed by a lucky intuition' that possessed no more 'than a formal significance'.47 'For this reason, on the very first day when I formulated this law,' Planck said later, 'I began to devote myself to the task of investing it with true physical meaning.'48 He could achieve this only by deriving his equation step by step using the principles of physics. Planck knew his destination, but he had to find a way of getting there. He possessed a priceless guide, the equation itself. But what price was he prepared to pay for such a journey?
The next six weeks were, Planck recalled, 'the most strenuous work of my life', after which 'the darkness lifted and an unexpected vista began to appear'.49 On 13 November he wrote to Wien: 'My new formula is well satisfied; I now have also obtained a theory for it, which I shall present in four weeks at the Physical Society here [in Berlin].'50 Planck said nothing to Wien either of the intense intellectual struggle that had led to his theory or the theory itself. He had strived long and hard during those weeks to reconcile his equation with the two grand theories of nineteenth-century physics: thermodynamics and electromagnetism. He failed.
'A theoretical interpretation therefore had to be found at any cost,' he accepted, 'no matter how high.'51 He 'was ready to sacrifice every one of my previous convictions about physical laws'.52 Planck no longer cared what it cost him, as long as he could 'bring about a positive result'.53 For such an emotionally restrained man, who only truly expressed himself freely at the piano, this was highly charged language. Pushed to the limit in the struggle to understand his new formula, Planck was forced into 'an act of desperation' that led to the discovery of the quantum.54
As the walls of a blackbody are heated they emit infrared, visible, and ultraviolet radiation into the heart of the cavity. In his search for a theoretically consistent derivation of his law, Planck had to come up with a physical model that reproduced the spectral energy distribution of blackbody radiation. He had already been toying with an idea. It did not matter if the model failed to capture what was really going on; all Planck needed was a way of getting the right mix of frequencies, and therefore wavelengths, of the radiation present inside the cavity. He used the fact that this distribution depends only on the temperature of the blackbody and not on the material from which it is made to conjure up the simplest model he could.
'Despite the great success that the atomic theory has so far enjoyed,' Planck wrote