Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [36]
In 1819 two French scientists, Pierre Dulong and Alexis Petit, measured the specific heat capacity, the amount of energy needed to raise the temperature of a kilogram of a substance by one degree, for various metals from copper to gold. For the next 50 years no one who believed in atoms doubted their conclusion that 'the atoms of all simple bodies have exactly the same heat capacity'.84 It therefore came as a great surprise when, in the 1870s, exceptions were discovered.
Imagining that the atoms of a substance oscillated when heated, Einstein adapted Planck's approach as he tackled the specific heat anomalies. Atoms could not oscillate with just any frequency, but were 'quantised' – able to oscillate only with those frequencies that were multiples of a certain 'fundamental' frequency. Einstein came up with a new theory of how solids absorb heat. Atoms are permitted to absorb energy only in discrete amounts, quanta. However, as the temperature drops, the amount of energy the substance has decreases, until there is not enough available to provide each atom with the correct-sized quantum of energy. This results in less energy being taken up by the solid and leads to a decrease in specific heat.
For three years there was hardly a murmur of interest in what Einstein had done, despite the fact that he had shown how the quantisation of energy – how at the atomic level energy comes wrapped up in bite-sized chunks – resolved a problem in a completely new area of physics. It was Walter Nernst, an eminent physicist from Berlin, who made others sit up and take note as they discovered that he had been to see Einstein in Zurich. Soon it was clear why. Nernst had succeeded in accurately measuring the specific heats of solids at low temperatures and found the results to be in total agreement with Einstein's predictions based on his quantum solution.
With each passing success his reputation soared ever higher, and Einstein was offered an ordinary professorship at the German University in Prague. It was an opportunity he could not refuse, even if it meant leaving Switzerland after fifteen years. Einstein, Mileva and their sons Hans Albert and Eduard, who was not yet one, moved to Prague in April 1911.
'I no longer ask whether these quanta really exist', Einstein wrote to his friend Michele Besso soon after taking up his new post. 'Nor do I try to construct them any longer, for I now know that my brain cannot get through in this way.' Instead, he told Besso, he would limit himself to trying to understand the consequences of the quantum.85 There were others who also wanted to try. Less than a month later, on 9 June, Einstein received a letter and an invitation from an unlikely correspondent. Ernst Solvay, a Belgian industrialist who had made a substantial fortune by revolutionising the manufacture of sodium carbonate, offered to pay 1,000 francs to cover his travel expenses if he agreed to attend a week-long 'Scientific Congress' to be held in Brussels later that year from 29 October to 4 November.86 He would be one of a select group of 22 physicists from across Europe brought together to discuss 'current questions concerning the molecular and kinetic theories'. Planck, Rubens, Wien and Nernst would be attending. It was a summit meeting on the quantum.
Planck and Einstein were among the eight asked to prepare reports on a particular topic. To be written in French, German or English, they were to be sent out to the participants before the meeting and serve as the starting point for discussion during the planned sessions. Planck would discuss