Max Planck and the birth of quantum theory, part 1

The greatest crisis physics has ever known came to a head over afternoon tea on Sunday, October the 7th 1900 at the home of Max Planck in Berlin. The son of a professor of jurisprudence, Planck had held the chair in theoretical physics at the University of Berlin since 1889 specializing in thermodynamics, the science of heat change and energy conservation. He could easily have been sidetracked into a different career. At the age of 16 having just entered the University of Munich he was told by Philipp von Joly, a professor there, that the task of physics was more or less complete. The main theories were in place, all the great theories had been made and only a few minor details needed filling in here and there by generations to come. It was a view disastrously wrong but widely held at the time, fueled by technological triumphs and the seemingly all pervasive power of Newton's mechanics and Maxwell's electromagnetic theory. Planck later recalled why he persisted with physics. The outside world is something independent from man, something absolute and the quest for the laws which apply to this absolute appeared to me as the most sublime scientific pursuit in life. The first instance of an absolute in nature that struck Planck deeply even as a high school student was the law of conservation of energy, the first law of thermodynamics. This says that you can't make or destroy energy, only change it from one form to another or to put it another way the total energy of a closed system, one that energy can't either enter or leave, always stays the same. Later Planck became equally convinced, but mistakenly it turned out, that the second law of thermodynamics is an absolute. The second law, which includes the statement that you can't turn heat into mechanical work with a hundred percent efficiency, crops up today in science in all sorts of different guises. It forms an important bridge between physics and information. When the second law was first introduced in the 1860s by Rudolf Clausius in Germany and William Thompson later Lord Kelvin in Britain, however, it was in a form that came to be known as the entropy law. Physicists and engineers of this period were obsessed with steam engines and for good reason. Steam engines literally powered the industrial revolution, so making them work better and more efficiently was a vital economic concern. An important early theoretical study of heat engines had been done in the 1820s by the French physicist Sadi Carnot, who showed that what drives a steam engine is the fall or flow of heat from higher to lower temperatures, like the fall of a stream of water that turns a mill wheel. Clausius and Thompson took this concept and generalized it. Their key insight was that the world is inherently active and that whenever an energy distribution is out of kilter, a potential or thermodynamic force is set up that nature immediately moves to quell or minimize. All changes in the real world can then be seen as consequences of this tendency. In the case of a steam engine, pistons go up and down, a crank turns, one kind of work is turned into another, but this is always at the cost of a certain amount of waste heat. Some coherent work, the atoms of the piston all moving in the same order, turns into incoherent heat, hot atoms bouncing around at random. You can't throw the process into reverse any more than you can make a broken glass jump off the floor and reassemble itself on a table again. You can't make an engine that will run forever. The reason the engine runs in the first place is because the process is fundamentally unbalanced. Would-be inventors of perpetual motion machines take note. Whereas the first law of thermodynamics deals with things that stay the same or in which there's no distinction between past, present and future, the second law gives a motivation for change in the world and a reason why time seems to have a definite preferred direction. Time moves relentlessly along the path toward cosmic dullness. Kelvin spoke in doom-mongering terms of the eventual heat death of the universe, when in the far future there will be no energy potentials left and therefore no possibility of further meaningful change. Clausius coined the term entropy in 1865 to refer to the potential that's dissipated or lost whenever a natural process takes place. The second law in its original form states that the world acts spontaneously to minimize potentials or equivalently to maximize entropy. Time's arrow points in the direction the second law operates toward the inevitable rise of entropy and the loss of useful thermodynamic energy. For Max Planck the second law and the concept of entropy held an irresistible attraction, the prospect of an ultimate truth from which all other aspects of the external world could be understood. These ideas formed the subject of his doctoral dissertation at Munich and lay at the core of almost all his work until about 1905. It was a fascination that impelled him toward the discovery for which he became famous, yet ironically this discovery and the revolution it sparked eventually called into question the very separation between humankind and the world, between subject and object, for which Planck held physics in such high esteem. Planck wasn't a radical or a subversive in any way, he didn't swim instinctively against the tide of orthodoxy. On the contrary, having come from a long line of distinguished and very respectable clergymen, statesmen and lawyers, he tended to be quite staid in his thinking. At the same time he also had a kind of aristocratic attitude to physics that led him to focus only on big basic issues and to be rather dismissive of ideas that were more mundane and applied. His unswerving belief in the absoluteness of the entropy version of the law, which he shared with few others, left him in a small minority in the scientific community. It also curiously led him to doubt the existence of atoms, and that was another irony given how events turned out. Like other scientists of his day, Planck was intrigued by why the universe seemed to run in only one direction, why time had an arrow, why nature was apparently irreversible and always running down. He was convinced that this cosmic one-way street could be understood on the basis of the absolute validity of the entropy law, but here he was out of step with most of his contemporaries. The last decade of the 19th century saw most physicists falling into line behind an interpretation of the second law that was the brainchild of the Austrian physicist Ludwig Boltzmann. It was while Boltzmann was coming of age and completing his studies at the University of Vienna that Clausius and Thomson were hatching the second law, and Clausius was defining entropy and showing how the properties of gases could be explained in terms of large numbers of tiny particles dashing around and bumping into one another on the walls of their container, the so-called kinetic theory of gases. To these bold new ideas in the 1870s, Boltzmann added a statistical flavor. Entropy, for example, he saw as a collective result of molecular motions. Given a huge number of molecules flying here and there, it's overwhelmingly likely that any organized starting arrangement will become more and more disorganized with time. Entropy will rise with almost but not total certainty. So although the second law remains valid according to this view, it's only in a probabilistic sense. Some people were upset by Boltzmann's theory because it just assumed from the outset without any attempt to prove that atoms and molecules exist. One of the biggest critics was Wilhelm Ostwald, the father of physical chemistry and Nobel Prize winner in 1909, who wanted to rid physics of the notion of atoms and base it purely on energy, a quantity that could be observed. Like other logical positivists, people who accept only what can be observed directly and who discount speculation, Ostwald stubbornly refused to believe in anything he couldn't see or measure. Boltzmann eventually killed himself because of depression brought on by such persistent attacks on his views. Planck wasn't a logical positivist. Far from it. Like Boltzmann, he was a realist who time and again attacked Ostwald and the other positivists for their insistence on pure experience. Yet he also rejected Boltzmann's statistical version of thermodynamics because it cast doubt on the absolute truth of his cherished second law. It was this rejection, based more on a physical rather than a philosophical argument, that led him to question the reality of atoms. In fact, as early as 1882, Planck decided that the atomic model of matter didn't jibe with the law of entropy. There will be a fight between these two hypotheses that take the life of one of them, he predicted. And he was pretty sure he knew which was going to lose out. In spite of the great successes of the atomistic theory in the past, we will finally have to give it up and to decide in favor of the assumption of continuous matter. By the 1890s, Planck had mellowed a bit in his stance against atomism. He'd come to realize the power of the hypothesis, even if he didn't like it. Yet he remained adamantly opposed to Boltzmann's statistical theory. He was determined to prove that time's arrow and the irreversibility of the world stemmed not from the whim of molecular games of chance, but from what he saw as the bedrock of the entropy law. And so as the century drew to a close, Planck turned to a phenomenon that led him, really by accident, to change the face of physics. I hope you'll join me in part two for the continuation of the story of how quantum theory began.