Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality

by Manjit Kumar

  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 blackbody radiation theory, while Einstein had been assigned his quantum theory of specific heat. Although Einstein was accorded the honour of giving the final talk, a discussion of his quantum theory of light was not on the agenda.

  ‘I find the whole undertaking extremely attractive,’ Einstein wrote to Walter Nernst, ‘and there is little doubt in my mind that you are its heart and soul.’87 By 1910 Nernst believed that the time was ripe to get to grips with the quantum that he regarded as nothing more than a ‘rule with most curious, indeed grotesque properties’.88 He convinced Solvay to finance the conference and the Belgian spared no expense booking the plush Hotel Metropole as the venue. In its luxurious surroundings, with all their needs catered for, Einstein and his colleagues spent five days talking about the quantum. Whatever slim hopes he harboured for progress at what he called ‘the Witches’ Sabbath’, Einstein returned to Prague disappointed and complained of learning nothing that he did not know before.89

  Nevertheless, he had enjoyed getting to know some of the other ‘witches’. Marie Curie, whom he found to be ‘unpretentious’, appreciated ‘the clearness of his mind, the shrewdness with which he marshalled his facts and the depth of his knowledge’.90 During the congress it was announced that she had been awarded the Nobel Prize for chemistry. She had become the first scientist to win two, having already won the physics prize in 1903. It was a tremendous achievement that was overshadowed by the scandal that broke around her during the congress. The French press had learned that she was having an affair with a married French physicist. Paul Langevin, a slender man with an elegant moustache, was a delegate at the conference and the papers were full of stories that the pair had eloped. Einstein, who had seen no signs of a special relationship between the two, dismissed the reports as rubbish. Despite her ‘sparkling intelligence’, he thought Curie was ‘not attractive enough to represent a danger to anyone’.91

  Even though at times he appeared to waver under the strain, Einstein had been the first to learn to live with the quantum, and by doing so revealed a hidden element of the true nature of light. Another young theorist also learned to live with the quantum after he used it to resurrect a flawed and neglected model of the atom.

  Chapter 3

  THE GOLDEN DANE

  Manchester, England, Wednesday, 19 June 1912. ‘Dear Harald, Perhaps I have found out a little about the structure of atoms,’ Niels Bohr wrote to his younger brother.1 ‘Don’t talk about it to anybody,’ he warned, ‘for otherwise I couldn’t write to you so soon.’ Silence was essential for Bohr, as he hoped to do what every scientist dreams of: unveiling ‘a little bit of reality’. There was still work to be done and he was ‘eager to finish it in a hurry, and to do that I have taken off a couple of days from the laboratory (this is also a secret)’. It would take the 26-year-old Dane much longer than he thought to turn his fledgling ideas into a trilogy of papers all entitled ‘On the Constitution of Atoms and Molecules’. The first, published in July 1913, was truly revolutionary, as Bohr introduced the quantum directly into the atom.

  It was his mother Ellen’s 25th birthday when Niels Henrik David Bohr was born on 7 October 1885 in Copenhagen. She had returned to the comfort of her parents’ home for the birth of her second child. Across the wide cobbled street from Christianborg Castle, the seat of the Danish parliament, Ved Stranden 14 was one of the most magnificent residences in the city. A banker and politician, her father was one of the wealthiest men in Denmark. Although the Bohrs did not stay there long, it was to be the first of the grand and elegant homes in which Niels lived throughout his life.

  Christian Bohr was the distinguished professor of physiology at Copenhagen University. He had discovered the role of carbon dioxide in the release of oxygen by haemoglobin, and together with his research on respiration it led to nominations for the Nobel Prize for physiology or medicine. From 1886 until his untimely death in 1911, at just 56, the family lived in a spacious apartment in the university’s Academy of Surgery.2 Situated in the city’s most fashionable street and a ten-minute walk from the local school, it was ideal for the Bohr children: Jenny, two years older than Niels, and Harald, eighteen months younger.3 With three maids and a nanny to look after them, they enjoyed a comfortable and privileged childhood far removed from the squalid and overcrowded conditions in which most of Copenhagen’s ever-increasing inhabitants lived.

  His father’s academic position and his mother’s social standing ensured that many of Denmark’s leading scientists and scholars, writers and artists were regular visitors to the Bohr home. Three such guests were, like Bohr senior, members of the Royal Danish Academy of Sciences and Letters: the physicist Christian Christiansen, the philosopher Harald Høffding and the linguist Vilhelm Thomsen. After the Academy’s weekly meeting, the discussion would continue at the home of one of the quartet. In their teens, whenever their father played host to his fellow Academicians, Niels and Harald were allowed to eavesdrop on the animated debates that took place. It was a rare opportunity to listen to the intellectual concerns of a group of such men as the mood of fin-de-siècle gripped Europe. They left on the boys, as Niels said later, ‘some of our earliest and deepest impressions’.4

  Bohr the schoolboy excelled at mathematics and science, but had little aptitude for languages. ‘In those days,’ recalled a friend, ‘he was definitely not afraid to use his strength when it came to blows during the break between classes.’5 By the time he enrolled at Copenhagen University, then Denmark’s only university, to study physics in 1903, Einstein had spent more than a year at the Patent Office in Bern.6 When he received his Master’s degree in 1909, Einstein was extraordinary professor of theoretical physics at the University of Zurich and had received his first nomination for the Nobel Prize. Bohr had also distinguished himself, albeit on a far smaller stage. In
1907, aged 21, he won the Gold Medal of the Royal Danish Academy with a paper on the surface tension of water. It was the reason why his father, who had won the silver medal in 1885, often proudly proclaimed, ‘I’m silver but Niels is gold’.7

  Bohr struck gold after his father persuaded him to abandon the laboratory for a place in the countryside to finish writing his award-winning paper. Although he submitted it just hours before the deadline, Bohr still found something to add, and handed in a postscript two days later. The need to rework any piece of writing until he was satisfied that it conveyed exactly what he wanted verged on an obsession. A year before he finished his doctoral thesis, Bohr admitted that he had already written ‘fourteen more or less divergent rough drafts’.8 Even the simple act of penning a letter became a protracted affair. One day Harald, seeing a letter lying on Niels’ desk, offered to post it, only to be told: ‘Oh no, that is just one of the first drafts for a rough copy.’9

  All their lives, the brothers remained the closest of friends. Apart from mathematics and physics they shared a passion for sport, particularly football. Harald, the better player, won a silver medal at the 1908 Olympics as a member of the Danish football team that lost to England in the final. Also regarded by many to be intellectually more gifted, he gained a doctorate in mathematics a year before Niels received his in physics in May 1911. Their father, however, always maintained that his eldest son was ‘the special one in the family’.10

  Dressed in white tie and tails as custom demanded, Bohr began the public defence of his doctoral thesis. It lasted just 90 minutes, the shortest on record. One of the two examiners was his father’s friend Christian Christiansen. He regretted that no Danish physicist ‘was well enough informed about the theory of metals to be able to judge a dissertation on the subject’.11 Nevertheless, Bohr was awarded his doctorate and sent copies of the thesis to men like Max Planck and Hendrik Lorentz. When no one replied he knew it had been a mistake to send it without first having it translated. Instead of German or French, which many leading physicists spoke fluently, Bohr decided on an English translation and managed to convince a friend to produce one.

  Whereas his father had chosen Leipzig and his brother Göttingen, German universities being the traditional place for high-flying Danes to complete their education, Bohr chose Cambridge University. The intellectual home of Newton and Maxwell was for him ‘the centre of physics’.12 The translated thesis would be his calling card. He hoped that it would lead to a dialogue with Sir Joseph John Thomson, the man he described later as ‘the genius who showed the way for everybody’.13

  After a lazy summer of sailing and hiking, Bohr arrived in England at the end of September 1911 on a one-year scholarship funded by Denmark’s famous Carlsberg brewery. ‘I found myself rejoicing this morning, when I stood outside a shop and by chance happened to read the address “Cambridge” over the door’, he wrote to his fiancée Margrethe Nørland.14 The letters of introduction and the Bohr name led to a warm welcome from the university’s physiologists who remembered his late father. They helped him find a small two-room flat on the edge of town and he was kept ‘very busy with arrangements, visits and dinner parties’.15 But for Bohr it was his meeting with Thomson, J.J. to his friends and students alike, which soon preyed on his mind.

  A bookseller’s son from Manchester, Thomson had been elected the third head of the Cavendish Laboratory in 1884 within a week of his 28th birthday. He was an unlikely choice, after James Clerk Maxwell and Lord Rayleigh, to lead the prestigious experimental research facility, and not just because of his youth. ‘J.J. was very awkward with his fingers,’ one of his assistants later admitted, ‘and I found it necessary not to encourage him to handle the instruments.’16 Yet if the man who won the Nobel Prize for discovering the electron lacked a delicate touch, others testified to Thomson’s ‘intuitive ability to comprehend the inner working of intricate apparatus without the trouble of handling it’.17

  The polite manner of the slightly dishevelled Thomson, the epitome of the absent-minded professor in his round-rimmed glasses, tweed jacket and winged collar, helped calm Bohr’s nerves when they first met. Eager to impress, he had walked into the professor’s office clutching his thesis and a book written by Thomson. Opening the book, Bohr pointed to an equation and said, ‘This is wrong.’18 Though not used to having his past mistakes paraded before him in such a forthright manner, J.J. promised to read Bohr’s thesis. Placing it on top of a stack of papers on his overcrowded desk, he invited the young Dane to dinner the following Sunday.

  Initially delighted, as the weeks passed and the thesis remained unread, Bohr became increasingly anxious. ‘Thomson,’ he wrote to Harald, ‘has so far not been easy to deal with as I thought the first day.’19 Yet his admiration for the 55-year-old was undiminished: ‘He is an excellent man, incredibly clever and full of imagination (you should hear one of his elementary lectures) and extremely friendly; but he is so immensely busy with so many things, and he is so absorbed in his work that it is very difficult to get to talk to him.’20 Bohr knew that his poor English did not help. So with the aid of a dictionary he began reading The Pickwick Papers as he fought to overcome the language barrier.

  Early in November, Bohr went to see a former student of his father’s who was now the professor of physiology at Manchester University. During the visit, Lorrain Smith introduced him to Ernest Rutherford, who had just returned from a physics conference in Brussels.21 The charismatic New Zealander, he recalled years later, ‘spoke with characteristic enthusiasm about the many new prospects in physical science’.22 After being regaled with a ‘vivid account of the discussions at the Solvay meeting’, Bohr left Manchester charmed and impressed by Rutherford – both the man and the physicist.23

  On his first day, in May 1907, the new head of physics at Manchester University caused a stir as he searched for his new office. ‘Rutherford went up three stairs at a time, which was horrible to us, to see a Professor going up the stairs like that’, remembered a laboratory assistant.24 But within a few weeks the boundless energy and earthy no-nonsense approach of the 36-year-old had captivated his new colleagues. Rutherford was on his way to creating an exceptional research team whose success over the next decade or so would be unmatched. It was a group shaped as much by Rutherford’s personality as his inspired scientific judgement and ingenuity. He was not only its head, but also its heart.

  Born on 30 August 1871 in a small, single-storey wooden house in Spring Grove on New Zealand’s South Island, Rutherford was the fourth of twelve children. His mother was a schoolteacher and his father ended up working in a flax mill. Given the harshness of life in the scattered rural community, James and Martha Rutherford did what they could to ensure that their children had a chance to go as far as talent and luck would carry them. For Ernest it meant a series of scholarships that took him to the other side of the world and Cambridge University.

  When he arrived at the Cavendish to study under Thomson in October 1895, Rutherford was far from the exuberant and self-confident man he would become within a few years. The transformation began as he continued work started in New Zealand on the detection of ‘wireless’ waves, later called radio waves. In only a matter of months Rutherford developed a much-improved detector and toyed with the idea of making money from it. Just in time, he realised that exploiting research for financial gain in a scientific culture where patents were rare would harm the chances of a young man yet to make his reputation. As the Italian Guglielmo Marconi amassed a fortune that could have been his, Rutherford never regretted abandoning his detector to explore a discovery that had been front-page news around the world.

  On 8 November 1895, Wilhelm Röntgen found that every time he passed a high-voltage electric current through an evacuated glass tube, some unknown radiation was causing a small paper screen coated with barium platinocyanide to glow. When Röntgen, the 50-year-old professor of physics at the University of Würzburg, was later asked what he had thought on discovering his mysterious new rays, he
replied: ‘I did not think; I investigated.’25 For nearly six weeks, he did ‘the same experiment over and over again to make absolutely certain that the rays actually existed’.26 He confirmed that the tube was the source of the strange emanation causing the fluorescence.27

  Röntgen asked his wife Bertha to place her hand on a photographic plate while he exposed it to ‘X-rays’, as he called the unknown radiation. After fifteen minutes Röntgen developed the plate. Bertha was frightened when she saw the outlines of her bones, her two rings and the dark shadows of her flesh. On 1 January 1896, Röntgen mailed copies of his paper, ‘A New Kind of Rays’, together with photographs of weights in a box and the bones in Bertha’s hand, to leading physicists in Germany and abroad. Within days, news of Röntgen’s discovery and his amazing photographs spread like wildfire. The world’s press latched on to the ghostly photograph revealing the bones in his wife’s hand. Within a year, 49 books and over a thousand scientific and semi-popular articles on X-rays would be published.28

  Thomson had begun studying the sinister-sounding X-rays even before an English translation of Röntgen’s paper appeared in the weekly science journal Nature on 23 January. Engaged in investigating the conduction of electricity through gases, Thomson turned his attention to X-rays when he read that they turned a gas into a conductor. Quickly confirming the claim, he asked Rutherford to help measure the effects of passing X-rays through a gas. For Rutherford the work led to four published papers in the next two years that brought him international recognition. Thomson provided a brief note to the first, suggesting, correctly as it later proved, that X-rays, like light, were a form of electromagnetic radiation.