Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [8]
Planck was a schoolboy when he first encountered the law of the conservation of energy. It struck him 'like a revelation' he said later, because it possessed 'absolute, universal validity, independently from all human agency'.23 It was the moment he caught a glimpse of the eternal, and from then on he considered the search for absolute or fundamental laws of nature 'as the most sublime scientific pursuit in life'.24 Now Planck was just as spellbound reading Clausius' formulation of the second law of thermodynamics: 'Heat will not pass spontaneously from a colder to a hotter body.'25 The later invention of the refrigerator illustrated what Clausius meant by 'spontaneously'. A refrigerator needed to be plugged into an external supply of energy, in this case electrical, so that heat could be made to flow from a colder to a hotter body.
Planck understood that Clausius was not simply stating the obvious, but something of deep significance. Heat, the transfer of energy from A to B due to a temperature difference, explained such everyday occurrences as a hot cup of coffee getting cold and an ice cube in a glass of water melting. But left undisturbed, the reverse never happened. Why not? The law of conservation of energy did not forbid a cup of coffee from getting hotter and the surrounding air colder, or the glass of water becoming warmer and the ice cooler. It did not outlaw heat flowing from a cold to a hot body spontaneously. Yet something was preventing this from happening. Clausius discovered that something and called it entropy. It lay at the heart of why some processes occur in nature and others do not.
When a hot cup of coffee cools down, the surrounding air gets warmer as energy is dissipated and irretrievably lost, ensuring that the reverse cannot happen. If the conservation of energy was nature's way of balancing the books in any possible physical transaction, then nature also demanded a price for every transaction that actually occurred. According to Clausius, entropy was the price for whether something happened or not. In any isolated system only those processes, transactions, in which entropy either stayed the same or increased were allowed. Any that led to a decrease of entropy were strictly forbidden.
Clausius defined entropy as the amount of heat in or out of a body or a system divided by the temperature at which it takes place. If a hot body at 500 degrees loses 1000 units of energy to a colder body at 250 degrees, then its entropy has decreased by –1000/500 = –2. The colder body at 250 degrees has gained 1000 units of energy, +1000/250, and its entropy has increased by 4. The overall entropy of the system, the hot and cold bodies combined, has increased by 2 units of energy per degree. All real, actual processes are irreversible because they result in an increase in entropy. It is nature's way of stopping heat from passing spontaneously, of its own accord, from something cold to something hot. Only ideal processes in which entropy remains unchanged can be reversed. They, however, never occur in practice, only in the mind of the physicist. The entropy of the universe tends towards a maximum.
Alongside energy, Planck believed that entropy was 'the most important property of physical systems'.26 Returning to Munich University after his year-long sojourn in Berlin, he devoted his doctoral thesis to an explora-tion of the concept of irreversibility. It would be his calling card. To his dismay, he 'found no interest, let alone approval, even among the very physicists who were closely concerned with the topic'.27 Helmholtz did not read it; Kirchhoff did, but disagreed with it. Clausius, who had such a profound influence on him, did not even answer his letter. 'The effect of my dissertation on the physicists of those days was nil', Planck recalled with some bitterness even 70 years later. But driven by 'an inner compulsion', he was undeterred.28 Thermodynamics, particularly the second law, became the focus of Planck's research as he began his academic career.29