by John Gribbin
Although economic conditions in Berlin were terrible after the war and there was political chaos, the presence of colleagues such as Planck to discuss physics with encouraged Einstein to keep his base there, even though he was offered a special professorship back in Zurich, in the comfortable safe haven of Switzerland. But that did not stop him travelling abroad at almost every opportunity when invited – he even agreed to visit Zurich to give a regular series of lectures each year, not least since payment in the stable Swiss currency would go a long way to ease his living conditions in Berlin. In 1921 he made his first visit to the United Sates, a lecture tour as part of a fund-raising programme for the Hebrew University in Jerusalem. His reception by the public resembled that of a modern pop star, and his lectures on relativity theory drew packed houses, while he was also awarded civic receptions and honorary degrees.
Considering his global status by 1921, it might seem that the Nobel Committee were rather tardy in awarding Einstein their Physics Prize. In fact, he had been nominated every year since 1910 except for 1911 and 1915. Most of the people who did receive the award in those years thoroughly deserved it, although the curious exception is Niles Dalén, a Swedish inventor given the 1912 Prize for his automatic regulator to control gas-fuelled lighthouse lamps. The same year, Einstein was nominated unsuccessfully for the Special Theory of Relativity. And in 1916, when Einstein was nominated for his work in molecular physics, no award was made. In 1921, the award was deferred, and then awarded to Einstein the following year, while Niels Bohr received the 1922 Prize. The citation for Einstein’s award referred to ‘services to theoretical physics, and especially for his discovery of the law of the photoelectric effect’. But the award was presented ‘without taking into account the value that will be accorded your relativity and gravitation theories after these are confirmed in the future’. One way of interpreting this is that the Nobel Committee still weren’t convinced of the importance of the General Theory; a more generous interpretation, which I favour, is that they were carefully leaving the door open for the possible award of a second Prize once more tests had fully confirmed the validity of his theory of gravitation. Either way, no such second award was ever made, not least because over the next two decades there would be a wealth of deserving physicists receiving Nobel Prizes for their work on quantum physics. So, although Einstein did receive the Nobel Prize, it was specifically not for his masterwork! Not that this mattered to Mileva and the boys, who received the financial rewards while Albert had the glory.
At the time the award was announced, the Einsteins were in Japan. He was there on a lecture tour as a guest of the publishing house Kaizosha, following a successful tour by Bertrand Russell the previous year. Before Russell left Japan, his hosts asked him to name the three most important living people, so that they could be invited to follow in his footsteps; Russell gave them just two names, Lenin and Einstein. Since Lenin was busy at the time, they invited Einstein. The invitation was particularly welcome as it meant Einstein would be away from Germany for at least six months, from October 1922, at a time when the political situation was deteriorating following the assassination by right-wing extremists of Walther Rathenau, a Jew who was the Minister of Reconstruction. This was part of a pattern of growing anti-Semitism and violence.
By the time the Einsteins returned to Europe in 1923, his fame had reached even greater heights, thanks to the award of the Nobel Prize, but Germany was plunging deeper into the depths of economic collapse, runaway inflation and violence, which paved the way for the rise of Nazism. Amidst this turmoil it is doubly surprising that at the relatively grand old age (for a theoretical physicist) of 45 Einstein would still produce one last piece of really significant science. But he did not do it alone.
A last quantum hurrah
Einstein’s fame and status in the scientific community meant that he was sent streams of communications from aspiring scientists, as well as from his established colleagues, from all over the world. In June 1924 he received a letter from a young Indian physicist, Satyendra Bose, based in the city then known as Dacca.a The letter accompanied an unpublished scientific paper in which Bose developed Einstein’s ideas on light and radiation in new ways. Bose had found a new way to derive the equation for black-body radiation, without assuming that light behaved as a wave at all but simply as a quantum gas obeying a new kind of statistical law. Einstein had cut his scientific teeth on statistical laws, and immediately saw the importance of this discovery. He first translated Bose’s paper (which was written in English) into German and got it published in the Zeitschrift für Physik, then picked up the idea and developed it further himself, applying the new statistics not just to particles of light but, under appropriate circumstances, to molecules and atoms.
The new statistical technique soon became known as ‘Bose-Einstein statistics’, and particles that obey Bose-Einstein statistics are called ‘bosons’. Einstein was able to use the new statistics to make predictions about the nature of thermodynamics, and in particular the behaviour of certain liquids at very low temperatures, where viscosity disappears and they become ‘superfluid’; the predicted superfluidity was observed experimentally in 1928, and the behaviour of bosons, or ‘bose condensates’, is still a matter of extreme interest to researchers today.
But the most far-reaching aspect of Einstein’s work in the mid-1920s came from the analogy between a quantum gas and a molecular gas. Einstein realised that it worked both ways. If the black-body radiation behaved in the same way as a molecular gas under some circumstances, and if light quanta were also known to have a wave-like nature, the implication was that molecules and other material objects must also have a wave-like nature.
Einstein published this conclusion, derived from Bose-Einstein statistics, in 1925; but there was no need for him to take it further and develop a complete wave theory of matter because someone else had already come up with the same idea from a different angle. The Frenchman Louis de Broglie had finished his PhD thesis in Paris in the spring of 1924 and submitted it to Paul Langevin, an old friend of Einstein. De Broglie was older than the average PhD student, having been born, at Dieppe, on 15 August 1892. He came from an aristocratic family and initially studied history at the Sorbonne, entering in 1909, being intended for a career in the diplomatic corps. But under the influence of his elder brother Maurice (seventeen years his senior), who became (very much against the wishes of his father) a pioneering researcher interested in X-ray spectroscopy, Louis began studying physics alongside history. Maurice had obtained his doctorate in 1908, and had been one of the scientific secretaries at the first Solvay Congress. Louis’ study of physics was interrupted in 1913 by what should have been a short spell of compulsory military service, but was extended when war broke out. During the First World War, he served in the radio communications branch of the army, operating for a time from the Eiffel Tower. It was because of army service and his switch from history that de Broglie’s scientific studies were delayed, so that his PhD thesis was only submitted (at the Sorbonne) in 1924, when he was in his thirties. But by then he had already published several important papers on the properties of electrons, atoms and X-rays, and he was one of the first physicists to accept fully the idea of light quanta, which he discussed in an article published in 1922.
The thesis developed from Albert Einstein’s earlier work – the work for which he had received the Nobel Prize, which showed that light (traditionally thought of as a wave) could also be explained in terms of particles (now known as photons) – to propose that ‘particles’ could also behave as waves, with the wave and particle aspects of the quantum entity linked by the equation wavelength × momentum = h, where h is Planck’s constant. De Broglie’s supervisor, Paul Langevin, was nonplussed by this and showed the thesis to Einstein. Einstein (who had just received Satyendra Bose’s paper on light quanta) wrote back to assure Langevin that de Broglie’s work was sound: ‘I believe,’ he said, ‘that it involves more than a mere analogy.’ De Broglie duly received
his PhD. In his second paper on the ‘Bose gas’, Einstein made a reference to de Broglie’s work which caught the attention of Erwin Schrödinger and started him down the road that led to wave mechanics.2
In plain English, in his thesis, de Broglie suggested that all material particles (electrons and the like) had a wavelength. Five years later, de Broglie received the Nobel Prize for his work.b By then, largely thanks to this breakthrough realisation that matter particles also have a wave-like nature, quantum theory was fully established, and Einstein had been in at both the beginning and end of the story. But in 1928 he had suffered another serious illness. In 1929, the year de Broglie received his Nobel Prize, Einstein was 50 years old, no longer a major player in the science game, and beginning to realise that the situation in Germany might soon become untenable.
Exile
Although Einstein continued to carry out research and write scientific papers after his 50th birthday, hardly surprisingly he did nothing else to rank with his earlier achievements. Most of his later scientific life was devoted to an unsuccessful effort to find a single mathematical package (a unified theory) that would describe both the material world and the world of electromagnetic radiation, echoing the way Maxwell had unified the description of electricity and magnetism into one package. It was a noble effort, but doomed to failure given the limited understanding physicists had of the particle world, in particular, at the time. We shall not describe any of this work, but sketch the outlines of Einstein’s later years and mention just one influential idea that he had – although it did not work out as he had expected.
Clinging to the hope that the political situation in Germany might yet improve, Einstein avoided resigning from his post in Berlin as long as he could, but spent as much time as possible out of the country over the next few years. After spending the summer of 1930 in Berlin, he visited Brussels, London and Zurich (where he received an honorary doctorate from the ETH) in the autumn, returning briefly to Berlin before sailing for California from Antwerp on 2 December. The long voyage via New York and the Panama Canal ended in San Diego on 30 December, and Einstein stayed as a visiting professor at Caltech, in Pasadena, until mid-March 1931.
Once again, the return to Berlin was brief. In May, Einstein took up an invitation to spend a month giving a series of lectures in Oxford. The visit proved so successful that it led to an invitation for him to become a visiting fellow at one of the Oxford colleges, Christ Church, for five years, with an annual stipend of £400. He could come and go as he pleased, provided he spent some time each year in Oxford. As everyone involved realised, apart from the intrinsic attraction of such an offer, it provided an escape route if things got worse in Germany.
At the same time, Einstein was being courted by Caltech, with an offer of $7,000 for another visit the following winter, and an understanding that the arrangement could be made permanent if he wished. Once again, the Einsteins sailed on 2 December, and once again they arrived in California at the end of the year. It was during this visit that Einstein made the contact that would soon result in him finding a settled base.
The contact was with Abraham Flexner, a highly regarded American scientific administrator, who had obtained funding for a new research institute in Princeton and was on a headhunting expedition to the West Coast looking for eminent scientists to staff it. Who better than the most eminent scientist of the 20th century? Flexner realised that if he could lure Einstein to his new institute, his presence would act as a magnet for other scientists and ensure the success of the project. But he adopted a softly, softly approach to snaring his prize.
Einstein returned to Europe in March 1932, committed to spending the next winter at Caltech but with no plans for a permanent move. In the summer, while he was in Oxford, he met up with Flexner (still headhunting!) again. This time, the American offered Einstein a post at what would become the Institute for Advanced Study, on whatever terms Einstein wished. In June, a deal was agreed. Einstein would visit Princeton for six months each year, starting in the autumn of 1933, for a salary of $10,000 and all travelling expenses to be paid by the Institute. The arrangement came just in time. Following elections in July 1932 the Nazis became the largest party in the German government, and the writing was on the wall. Officially, when Albert and Elsa left for their third and last visit to Caltech in December 1932 Einstein was expected back in Berlin to take up his academic duties in April 1933, but as he locked the door of their home, Einstein turned to his wife and said: ‘Take a very good look at it. You will never see it again.’3
Adolf Hitler was appointed Reich Chancellor on 30 January 1933, and in the ensuing wave of anti-Semitism Einstein’s bank account was frozen, his house was ransacked and copies of a popular book he had written on relativity were publicly burned. Although the Einsteins returned to Europe at the end of March, they clearly could not return to Germany, but largely divided their time between Oxford and Belgium before leaving for Princeton in October. This was still not seen, at the time, as a permanent move, and it was expected that Einstein would return to Oxford the following summer. There were even moves to grant him British citizenship. In fact, after 1933 Einstein only left the United States once, travelling to Bermuda in May 1935 in order to make a formal application to re-enter the country as a permanent resident.
Soon after taking this step, the Einsteins were able to buy a house in Princeton, 112 Mercer Street – they had previously been living in a rented apartment. But Elsa did not enjoy the new property for long. She became ill during the summer and never fully recovered. She died in the house on Mercer Street on 20 December 1936. Einstein, always independently-minded, seems to have quickly got over the loss. By this time his divorced step-daughter Margot was acting as his housekeeper and looking after him, while he had an efficient secretary, Helen Dukas, who protected him from outside intrusions. As he wrote to his friend Max Born not long after Elsa died:
I have settled down splendidly here. I hibernate like a bear in its cave, and really feel more at home than ever before in all my varied existence. This bearishness has been accentuated further by the death of my mate, who was more attached to human beings than I.4
Spooky action at a distance
The year before Elsa died, Einstein made his last important contribution to science, although he would have been astonished at the way an idea he presented in 1935 was developed, and at its importance in the applied physics of the 21st century.
The idea, known from the initials of the authors of the paper in which it was presented as ‘the EPR experiment’ (although Einstein was the brains behind it) was originally put forward in 1935 by Einstein and two of his colleagues in Princeton, Boris Podolsky and Nathan Rosen. It was a ‘thought experiment’ (like the freely falling lift) intended to demonstrate (as they thought) the logical impossibility of quantum mechanics. Einstein had no expectation that such an experiment could, or would, ever be carried out. But the basic idea was adapted by David Bohm in the 1950s and refined by John Bell in the 1960s to become a practicable experiment that was actually carried out in the 1980s, most notably by Alain Aspect and his team in Paris, establishing that nature really does behave in a noncommonsensical way.
In the original version of the thought experiment – sometimes referred to as the ‘EPR paradox’, although it is not really a paradox – the EPR team imagined a pair of particles that interact with one another and then separate, flying far apart and not interacting with anything else at all until the experimenter decides to look at one of them. At the time the particles interact, it is possible to measure the total momentum of the system, and this cannot change if they do not interact with anything else. So if the experimenter chooses, much later, to measure the momentum of one particle, it is possible to calculate the momentum of the other particle, far away, by subtracting the measured momentum from the total. We know that quantum physics requires that by measuring the momentum of the first particle we destroy information about its position, because of Heisenberg’s uncertainty principle.
But the EPR team suggested that quantum uncertainty could be circumvented by measuring the momentum of the first particle and the position of the second particle, while calculating the momentum of the second particle in the way we have outlined. The only alternative would be that by measuring the momentum of the first particle we destroy information about the position of the second particle (or prevent such information ever existing), instantaneously, no matter how far apart they are.
Einstein referred to this as a ‘spooky action at a distance’, arguing that it was both logically absurd and impossible for any communication to travel faster than light, so quantum theory must be flawed. But the experiments show that this kind of ‘non locality’ is indeed a feature of the quantum world, and that measurements made on one particle of such a quantum pair really do affect its counterpart, instantaneously, no matter how far away that counterpart may be. This is the opposite of what Einstein (and, indeed, John Bell) expected. Even more profoundly, the experiments show that non-locality is not solely a feature of quantum theory, but a feature of the physical world. Whatever theory is the best description of reality, it must include non-locality, just as any satisfactory theory of gravity must include the inverse square law. And this is more than abstract theorising. This ‘entanglement’ of quantum entities is now being put to practical use in the infant technology of quantum computing, in developing uncrackable quantum codes, and (soon) a totally secure quantum internet.5 It has become one of the most enduring and important parts of Einstein’s legacy, albeit in the opposite way from what he had in mind. But it really was his scientific swansong.
The final years
Over the next few years, most of what was left of Einstein’s family also came to America. Hans Albert, who had completed a PhD in engineering at the ETH in 1936, arrived in 1937 with his wife and son and settled in the United States; he died in 1973. But Eduard, who had developed signs of serious mental illness in the early 1930s, ended his days (in 1965) in a psychiatric hospital in Switzerland. In 1939, Maja and her husband Paul Winteler had to leave Italy, where the Fascists were in power. Paul was refused entry to the United States and stayed in Geneva; Maja joined the household in Princeton.