The Travelling Years - Stockholm, Cambridge and Cornell

Abstract

While Herwig was moving around from place to place in Germany, establishing institutes and moving swiftly upwards through the academic ranks, he also had the opportunity to travel. While it was in Germany that he honed the skills as a scientific administrator and diplomat that would come to define his later career, it was on his travels that he did some of his most interesting research. “This time characterises to some extent my whole scientific life,” he recalled, “I moved up in energy every 10 years or so by a large factor, and this meant that at each step I had to re-establish myself in a new scientific community.”

While Herwig was moving around from place to place in Germany, establishing institutes and moving swiftly upwards through the academic ranks, he also had the opportunity to travel. While it was in Germany that he honed the skills as a scientific administrator and diplomat that would come to define his later career, it was on his travels that he did some of his most interesting research. “This time characterises to some extent my whole scientific life,” he recalled, “I moved up in energy every 10 years or so by a large factor, and this meant that at each step I had to re-establish myself in a new scientific community.”

Herwig had begun his academic career working in optics, in part because nuclear physics was not allowed in Germany in the immediate aftermath of the war, and partly because, as a student at the time, his research options were tightly constrained by the interests of his supervisor. Particle physics did not yet exist as a discipline and nuclear physics was still in its infancy, but his groundings in optics would set him on the path to a research career devoted to the exploration of the fundamental constituents of the universe, and the laws governing their behaviour. As Herwig explained, “in physics, we explore nature, looking deeper and deeper into the microcosm. The smaller the things you want to look at, the shorter the wavelength you have to use, and shorter wavelengths mean higher energy.”

The Visible Spectrum and Beyond

The field of optics in which Herwig cut his research teeth essentially relies on photons of low energy, visible light. The objects that can be studied with visible light cannot be smaller than the shortest wavelength that our eyes can perceive, typically around 380 nm, which corresponds to violet light. That’s roughly the size of a large virus, so it’s pretty small, but by comparison with the subatomic, and subnuclear worlds to which Herwig would later travel, it’s huge. An atom is about 1000 times smaller than a typical virus and the nucleus of an atom is 10,000 times smaller still. The protons and neutrons, building blocks of atomic nuclei, are a further ten times smaller than nuclei, and their constituents, quarks, are so small as to be considered point-like, with no discernible size at all.

We can’t see objects such as viruses with the naked eye, but optical magnifying techniques allow us to do so. To see things that are smaller, we need to go beyond the violet to ultraviolet, followed by x-rays and gamma rays, while at the other end of the spectrum, beyond what we can see, lies the infrared. “Max Planck told us that the smaller the wavelength of a photon of light, the greater its energy,” explained Herwig. “In physics one of the units used to measure energy is the electron volt, eV, literally the energy required to move one electron through a potential difference of one volt. The energy carried by even the most energetic of visible photons is only about 3 eV, which is also the kind of energy a torch battery provides to accelerate an electron to this energy.”

The current experiments at CERN’s Large Hardon Collider (LHC) probe matter with an energy measured in TeV: tera, or trillion, electron volts. This allows structures deep inside the nuclei of atoms to be resolved, but when Herwig started out, such energies were unimaginable anywhere except in the cosmic rays pervading the universe, and constantly bombarding the Earth. The LHC effectively behaves like a microscope that can explore objects about 10¹⁵, that’s a one followed by 15 zeros, times smaller than anything visible with an optical instrument.

It is difficult to pinpoint the precise origin of modern physics, but a good candidate is 1834, when Michael Faraday, working at the Royal Institution in London, demonstrated that the atoms making up every chemical element carry with them an electric charge that is an integral multiple of that carried by the hydrogen atom, indicating some commonality between the elements. Then, in 1879, Russian physicist Dmitry Mendeleev published the periodic table of the elements. In it, he classified all the known elements in order of their mass, and organised according to their properties. This proved to be a milestone in humankind’s understanding of the microcosm, allowing scientists to make predictions based on the observations of patterns that emerged from the natural order. Gaps in the table pointed to yet-to-be-discovered elements, while the underlying patterns pointed to some deeper substructure within the atoms, which remained to be discovered. One could say that Mendeleev’s insight marked the transition from chemistry to modern physics, and it was not long before evidence to support the substructure theory was found.

In 1895 Wilhelm Conrad Röntgen, working at the University of Würzburg in Germany, discovered eine neue Art von Strahlen—a new kind of rays. These rays bear his name in the German language, and are known as x-rays in English. Just like visible light, they are a form of electromagnetic radiation, but with an extremely short wavelength. A little over a decade later, in 1906, the British physicist Joseph John Thomson conducted an experiment that provided evidence for the first subatomic particle, the electron. Thomson had discovered the particle responsible for the observations of Faraday and Röntgen, and at the same time, established the field of particle physics long before it had a name. Röntgen was awarded the inaugural Nobel Prize for Physics in 1901, while Thomson received the prize in 1906. Further discoveries were soon to follow, notably that of Ernest Rutherford and his graduate students Hans Geiger and Ernest Marsden in 1911.

In the final year of the nineteenth century, Rutherford, working in Manchester, discovered what he called alpha and beta rays in the radioactive disintegration of uranium nuclei. In 1911, he designed an experiment to put the alpha rays to good use. At the time, the atom was famously modelled as being rather like a spherical plum pudding, with a mass of positive electric charge dotted with negative charge to give the atom overall neutrality. Thomson’s experiment had demonstrated that the negative charge was composed of electrons, but Rutherford wanted to find out about the rest of the atom, and in particular whether it really was built like a plum pudding. “It became clear at the beginning of the twentieth century,” explained Herwig, “that one could not see the details of atoms with normal light, so what do you do? Let’s say you have a fog, and you want to explore what’s happening in the fog. You can’t see into the fog, so you shoot particles or bullets, whatever you want, across the fog. If the bullets go through undisturbed, you conclude the fog’s empty, but if you find a few are deflected, by observing these deflections you can conclude what the structure of the objects inside the fog is.” In Rutherford’s experiment, the fog was a foil of gold, and the bullets were alpha particles. Geiger and Marsden saw that most of the alphas went straight through, but a small number were deflected, sometimes through very large angles. By analysing these deflections, they were able to conclude that the atoms of gold were mostly empty, but that at their core was something much smaller than the atom—the nucleus. “The nucleus is surrounded by shells filled by electrons that circulate around the nucleus,” said Herwig, “but most of the atom is empty, most of its mass is concentrated in the tiny nucleus at the centre.”

It was not long before the constituents of the nucleus had been identified. Positively charged protons, which balance the charge of the electrons in atoms, were discovered by Rutherford in experiments carried out between 1917 and 1919, and the electrically-neutral neutron by James Chadwick in 1932. It had been a journey of almost a century since Faraday, but these discoveries meant that by the time Herwig was discovering the field of physics at the University of Hamburg in the late 1940s, the foundations of nuclear and particle physics were firmly in place, although tantalisingly out of reach to researchers in post-war Germany, where investigations of the nucleus were forbidden.

A Sojourn in Stockholm with Lise Meitner

“In order to learn nuclear physics, one had to go somewhere else,” explained Herwig. “Fortunately, Fleischmann, my Professor at Hamburg, had an opportunity to propose me for a one-year Fellowship in Stockholm, which I accepted with pleasure.” The year was 1951, not long after Herwig’s marriage to Ingeborg, and even though moving would mean leaving his new bride behind in Hamburg, the opportunity to work with one of the pioneers of experimental nuclear physics, Lise Meitner, was one that Herwig could not refuse.

Austrian by birth, Lise Meitner is one of the least well-known greats of physics from the first half of the twentieth century. Known mostly for the fact that she did not share the 1944 Nobel Prize with Otto Hahn, her contributions to physics are as remarkable as the story of her life, which Herwig was to learn first-hand in Stockholm. “She was able to leave Berlin just in time,” recalled Herwig.

Meitner had been invited to Sweden in June 1938 by Manne Siegbahn, a Nobel Prize winner and director of the physics department of the Nobel Institute of the Royal Swedish Academy of Sciences in Stockholm. It was literally a lifeline, although she had an agonising wait to find out whether Sweden would allow her to enter: with the Anschluss of Austria in March of that year, her passport was no longer valid. She escaped Germany by the narrowest of margins. While her colleagues at the Kaiser Wilhelm Institute in Berlin maintained the pretence that she had travelled to Vienna to visit relatives, she was smuggled out of Germany in early July into the Netherlands via a little-used railway crossing, and there she waited to find out whether she would be allowed to enter Sweden and take up Siegbahn’s offer. The good news arrived before the month was out, and she travelled on via Denmark, where she stayed with Niels and Margarethe Bohr, to Sweden, arriving at the beginning of August.

Already in her 60s, Meitner was close to retirement when she arrived in Sweden. “I think that Siegbahn thought that she’d settle down, have a quiet life, follow what’s going on in physics but not be an active researcher anymore,” said Herwig. “She was not expected to carry out any experimental activity in his institute, so after a short time, she decided to accept an invitation from the Kungliga Tekniska Högskolan (KTH), the Royal Institute of Technology, under Gudmund Borelius, where she could set up a modest experimental laboratory and continue her experimental work. I joined her there.”

Life in Stockholm was a revelation to Herwig, whose entire adult life had been lived under the influence of war or its aftermath. “Germany was still in ruins when I went to Stockholm,” he recalled, “and we just had enough to eat. The contrast was incredible. Sweden had not been involved in the war, and it was a paradise. There was enough food, no shortages at all. I enjoyed going to the opera and to concerts, and one thing that was incredible for me to see was that the street-sellers of varmkorv, hot sausages, were dressed in furs against the cold. In Germany, they were still in rags. Incredible.” One thing, however, was in short supply in Sweden: alcohol was rationed. This too contributed to the good life that Herwig was living. “The fixed ration for foreigners was relatively high, so I could buy spirits, and whenever I was invited to a party, I took a bottle along so I was always a welcome guest. I enjoyed life in Sweden.”

Life was not all fun and games, however, there was also serious work to be done, and Herwig hit it off well with his new supervisor. “With my Austrian roots, we had something in common,” said Herwig. “Apart from the physics, we immediately had a very close exchange of our personal histories, and she was happy to have someone to talk to about her early years. She told me her life story, and the difficulties she’d faced first as a woman and then as a Jew.” Before the First World War, it was very unusual for a woman to study physics, but Lise Meitner was fortunate in her teachers, and in 1906, she became one of the first women to earn a doctorate from the University of Vienna. “After her doctorate, she went to the Kaiser Wilhelm Institute, as it was called at the time, now it’s the Max Planck Institute, in Berlin. She went to the institute for radio chemistry where the director was Otto Hahn. They immediately became friends, but he could not do what he wanted and hire her in the normal way: women were formerly forbidden at that time to enter the institute, so she was accepted at the institute more-or-less secretly. She told me that in the beginning she was not allowed to enter through the main door and had to use a side door. Nor was she allowed to use the main laboratory in the institute, so Hahn had established a little laboratory in the basement in what had been a carpenter’s workshop, and it was there that she worked with Hahn.”

Lise Meitner had a good ear, which was put to good use in her relationship with Hahn. Both music lovers, they would sing together the famous lieder of Johannes Brahms as they worked, and Meitner was quick to criticise when Hahn was off-pitch. The two formed a strong working bond, he a nuclear chemist, she a physicist, at the time when the boundary between the fields was blurred and the talents of both were needed to interpret the results of experiments. Their partnership was cut short by the rise of Nazism, which forced Meitner’s escape. “She told me that she got all the help she needed from Hahn to get safely out of Germany. He accompanied her to the railway station, and gave her a diamond ring as a last resort in case she needed money.”

Hahn and Meitner’s greatest triumph came when she was in exile in Sweden. Back in Berlin, Hahn had been conducting experiments bombarding uranium nuclei, the heaviest to have been observed at the time, with neutrons in a bid to create heavier, so-called transuranic, elements. What they found was the opposite of what they expected, the lighter element barium appeared in the reaction products. “They found lighter elements and couldn’t explain it”, said Herwig. “So Hahn wrote a letter to Lise Meitner explaining his experimental results and asking for her help interpreting them. She got that letter just before the Christmas of 1938, which she was spending with her nephew, Otto Frisch, who was also a physicist. When the letter arrived, he was putting on his skis to go cross-country skiing, so with the fluttering letter in hand, she ran after him and called him back. He was reluctant, but she persuaded him. He took off his skis and when they sat down to discuss Hahn’s results, they had the idea that it must be nuclear fission, which nobody had expected.” In 1933, Frisch, also Austrian by birth but working in Hamburg at the time, had left Germany for London. When he and his aunt interpreted Hahn’s results, he immediately informed his colleagues there of the news. “Because of the political situation in Germany, Meitner and Frisch published separately from Hahn, with the latter’s paper describing the experiment, while Meitner and Frisch explained the physics behind it.”

Coming on the eve of the Second World War, the result caused a sensation. It showed that the atomic nucleus could be split, with all that implied. Frisch went on to work out, with Rudolf Peierls, the process for generating a nuclear explosion, and as a freshly minted British citizen, left to join the Manhattan Project in 1943. Hahn was awarded the Nobel Prize for Chemistry in 1944, as the war was drawing to a close. There was no award ceremony that year, and he was formally informed of the news while detained at His Majesty’s pleasure at Farm Hall, where many prominent German scientists were interrogated after the war.

“There were some rumours that Meitner and Frisch missed out on the Nobel Prize due to negative interventions from Hahn,” said Herwig. “She told me that’s just not true—Hahn had always recognised that as a nuclear chemist, he needed the expertise of physicists to interpret his results. I checked that later, many years later, when I met Hahn. I asked him if it was true. He said, ‘Of course, Lise was the only one who understood what we were doing. The Nobel files for the 1944 prizes are now open, and anyone can study the deliberations of the chemistry prize committee.’ The jury is still out as to why Meitner and Frisch were overlooked, but among the most likely explanations is that the chemists simply did not understand the importance of the physicists’ contribution.”

When Herwig arrived in Stockholm, all this was still recent history, but however she may have felt about it, the omission did not distract Meitner from her research, and Herwig was about to take his first big step up in energy. “Because the possibilities were relatively limited in Stockholm, she went back to some previous work that she had been doing,” recalled Herwig. “She had been quite essential in the discovery of the neutrino, much earlier than nuclear fission, when she was investigating nuclear beta decay.”

In 1930, Meitner had been in an unrivalled position to witness the birth of neutrino physics. She, along with Hans Geiger, was the recipient of a famous letter from Wolfgang Pauli, which opened: “Liebe Radioaktive Damen und Herren” (Dear radioactive Ladies and Gentlemen). Meitner and Geiger were involved with a physics gathering in Tübingen, and Pauli wanted to sound out the participants on a new idea he’d had.

One big dilemma in physics at the time was related to beta decay. We know now that beta decay happens when one of the neutrons in the nucleus becomes a proton. An electron is emitted in the process, ensuring that charge is conserved. In 1930, however, the neutron’s discovery was still two years into the future, and the dilemma was that the electrons emerged with a range of energies in a way that seemingly defied the sacrosanct principle of conservation of energy. “Meitner had already contributed much to the study of beta decay, demonstrating that the spectrum of emitted electrons was continuous,” explained Herwig, “and she had conducted a rather difficult experiment that showed that some of the energy was missing.”

Pauli’s solution to this dilemma was to propose that the decay involved an extra, neutral particle that escaped undetected, thereby accounting for the missing energy. He called this particle the neutron. With the discovery of what we now know as the neutron by James Chadwick in 1932, however, Pauli’s neutral particle was in need of a new name. Thanks to Enrico Fermi, it is today called the neutrino, or ‘little neutral one.’

Meitner’s indirect evidence for the neutrino had inspired Pauli’s missive, but direct evidence would have to wait until the 1950s. In a sign of how parsimonious Nobel committees can sometimes be, the physics prize for its discovery was only awarded to one of the experimenters, Frederick Reines, in 1995, by which time his co-researcher Clyde Cowan had passed away.

When Herwig arrived in Stockholm, the elusive neutrino was still evading detection, and Meitner was still investigating the finer details of beta-decay. “The task she gave me was to measure the beta spectrum by putting different absorbers in and measuring how many electrons with a certain energy passed through the absorbers,” said Herwig. “The first thing she taught me was to build a Geiger–Müller counter and then to set up an experiment with a radioactive source, an absorber and then the Geiger–Müller counter to measure the counting rate with different absorber thicknesses. It was an incredibly simple experiment compared to the hugely complicated magnetic spectrometer installations of today!”

Meitner was an exacting teacher. “She was critical in setting up the right geometry for the experiments,” Herwig recalled. “She told me she was famed for her good eyesight, and had been the scourge of dressmakers back in Vienna, where she could see at a glance whether the hem was straight.” And so it was with Herwig’s experiments. “She’d say: ‘Here it’s not my hem but your experiment, that’s not aligned properly!’ So I learned how to do an experiment properly and we published a paper.” It was not a great discovery, but it was a solid piece of nuclear physics, and it represented Herwig’s first big step up in energy from the few electronvolts of optics to the tens or hundreds of thousand electronvolts, keV, of beta decay and nuclear physics.

Fig. 5.1

Lise Meitner in her laboratory in Stockholm around 1950. During Herwig’s year in Stockholm, Meitner introduced him to experimental nuclear physics (©Department of Physics, KTH Royal Institute of Technology, Stockholm, All rights reserved)

Herwig’s stay in Stockholm was just a year, but he remembers it fondly. “I shared an office with a Swedish scientist, and I think we were the last scientists who worked with Lise Meitner, because after I left she very soon retired from the practical work,” he said. “So I am probably the last living physicist who still really joined her in her work.”

Without his wife, and despite his popularity at parties, Herwig spent most of his time in Stockholm working. “I lived relatively modestly and was able to save some money, so at the end of the academic year, I had saved enough to invite my wife to come and join me in Sweden. I could pay for her trip and support her stay for a few weeks.”

Ingeborg, who had stayed in Hamburg, still working for the British Military Government there, joined Herwig for his last month in Sweden in the summer of 1952. “I had been there through the whole winter, which was somewhat depressing – the long nights, no sun. I was not accustomed to that,” remembered Herwig. But with the summer came Ingeborg, and on 21 June, midsummer night, a big festival across the whole country. “I was invited with my wife to Lund University, and I was very much impressed that all students were in evening dress. In Germany no student would have appeared in evening dress in front of the rectorate. The rector gave a speech from the balcony and at the end of the speech they all threw their white caps, which are the identification of students in Sweden, into the air to mark the beginning of summer. It was a very nice and very impressive festival, and a fitting way to end my year in Sweden, where I got an introduction to nuclear physics, into another layer of the microcosm, but also an introduction to another culture. This was a very important stage in my life.”

On to Cambridge

It was not long after Herwig got back to Hamburg that he followed Rudolph Fleischmann to Erlangen, where he continued the work he’d begun with Lise Meitner in Stockholm. Before long, Fleischmann offered Herwig the chance of another sabbatical, this time in England. “Fleischmann was interested in nuclear physics, and he said: ‘if you go to England, to Cambridge, you can go to the famous institute where nuclear physics started with Rutherford at the beginning of the twentieth century and learn how to do nuclear physics with an accelerator.’ With Lise Meitner I had learned about radioactive decays, beta decay, but Fleischmann knew that the future lay with accelerators—at Heidelberg, he had worked with Walther Bothe, the Nobel Prize winner who had invented the coincidence method of detecting two particles emitted at the same time.”

In 1947, Lise Meitner’s nephew, Otto Frisch, had taken up a position as director of the nuclear physics department of Cambridge University’s Cavendish Laboratory. The department had acquired a modern version of a Van de Graaff accelerator operating at about 3 mega electronvolts (MeV), another step up in energy for Herwig. This time, conditions were sufficiently comfortable for the Schopper family that Ingeborg and Doris were able to join Herwig soon after he had arrived in Summer 1956 (see this chapter, In his own words: Learning about the English way).

“The scientist who was running the Van de Graaff was Denys Wilkinson. He was a young shooting star, quite well known already in Great Britain, and he was kind”, recalled Herwig. “I started an experiment at the Van de Graaff investigating the splitting of the deuteron, a nucleus consisting simply of a proton and a neutron, using energetic photons. I worked there for a few months, and in the end a publication came out, but what happened next was completely unexpected.”

Herwig had gone to Cambridge to learn about accelerators, and that’s how things started, but soon he became friends with Otto Frisch through the shared love of music that came with their Austrian heritage. Herwig was a regular visitor to the Frisch home, where he’d spend evenings listening to Otto play the piano. One day, Otto told him of an upcoming colloquium at the Harwell campus in Oxfordshire, and suggested that Herwig should attend. It was to be a life-changing experience. “The speaker was Abdus Salam, a Pakistani theorist who was already very well known,” recounted Herwig. “He was a professor at the University of London, and he received a Nobel Prize much later.”

Salam shared the 1979 Nobel Prize for Physics with the Americans Sheldon Glashow and Steven Weinberg for the unification of two of nature’s fundamental forces, electromagnetism and the weak nuclear force into a single electroweak theory. Back in the 1950s, however, his attention was firmly focused on the weak interaction itself, which is responsible for beta decay and plays a crucial role in energy production in the sun. Under different circumstances, Salam might have received science’s most coveted prize much sooner, as Herwig was to learn that day.

The chairperson of the colloquium was Wolfgang Pauli, who came to Harwell from his home in Switzerland just to chair this colloquium. Pauli was famed not only for his intellect, but also for his self-confidence: Pauli never apologised to anyone for being wrong, simply because he never was. At Harwell that day, however, a sense of humility was on display from this giant of physics.

“I learned something new that day,” said Herwig. “There was a concept of physics that I learned at this colloquium for the first time. Among the fundamentals of physics are some principles that were considered to be necessary for rational thought, ‘denknotwendig’ in German.” Among these is the one that Herwig learned from Abdus Salam: the concept of symmetry, which goes to the heart of modern physics, and has some very far-reaching consequences. “The concept of symmetry means that laws of nature do not change under certain operations,” explained Herwig.

The Principle of Symmetry Invariance

“The most fundamental symmetries are based on the idea that the results obtained by an experiment should not depend on the particular place where the experiment is carried out,” explained Herwig. “They should also be independent of the direction in which the experimental apparatus is oriented, and they should not be influenced by the moment at which the experimenter starts their watch. The full importance of these fundamental symmetries was first pointed out by Emmy Noether, who published a paper in 1918 showing that from each of these symmetries a conservation law of physics follows, namely the conservation of linear momentum, angular momentum and energy from the three I’ve just mentioned.”

Similar consequences follow from mirror reflections. If we observe nature through a mirror it looks exactly the same except that a left-handed glove becomes a right-handed glove and vice versa. “The laws of classical physics do not contain a definition of handedness,” said Herwig, “so there is no way to explain to an alien on a distant planet what a right-handed glove is. The only way would be to send one.”

Symmetries in physics seem very abstract, but they have very tangible consequences. For example, we know that as well as ordinary matter of the kind that we are made of, there also exists antimatter in the universe. Rather like the concept of handedness, what we call matter and what we call antimatter is arbitrary. “The only way to find out whether two pieces are of the same kind or not is to bring them together, since matter and antimatter annihilate on meeting,” explained Herwig. Indeed, observations indicate that most of the matter and antimatter that would have emerged from the Big Bang have annihilated, leaving just a small fraction of the matter behind: imperfect symmetries turn out to be important in physics as well.

Another symmetry is time reversal. This might seem surprising as we observe time always flowing in one direction—we get older, never younger—but all the laws of classical physics remain valid if we reverse the flux of time. “If one could make a film of the motion of the planets around the sun and show it to an astronomer,” said Herwig, “they would certainly be able to deduce Kepler’s laws, but they could not tell you whether the film was being shown forwards or backwards in time.” To explain why we get old, classical physics was obliged to introduce another concept, entropy, defined in such a way that it can only increase when moving to the future.

An Early Experiment on Mirror Reflection Invariance

The symmetry Herwig learned about at Harwell is mirror symmetry, along with its consequences for quantum theory. In physics, mirror symmetry goes by the name of parity. “We think that when you do experiments, nature can’t tell the difference between the observation and its mirror image—the laws of nature are the same for both,” said Herwig. Principles of symmetry have intrigued thinkers since time immemorial, you only have to look at nature to find them—in the human form, in the petals of flowers, or in rock crystals. For Herwig, the philosopher Immanuel Kant is a reference point. “Kant said that a necessary principle to investigate nature is to assume that there is a mirror invariance that nature cannot decide between, for example a right-handed glove is a mirror image of a left-handed glove”, he explained. “The same is true for nuts and bolts, there are right-handed bolts and left-handed bolts. A right-handed bolt will never fit into a left-handed nut, but this does not imply that left-handed nuts cannot exist, it just says the laws of nature must be invariant as far as mirror transitions are concerned.”

Surprising as it may seem, this principle translates directly to the world of quantum mechanics and the fundamental particles, which have a property called helicity, which can be right or left-handed. “In quantum mechanics, the equivalent principle is called helicity invariance,” Herwig continued, “and there is a law in quantum mechanics that if you have an invariance, it comes with a quantum number that does not change in reactions. This quantum number is called parity, and I taught myself all there was to know about it after this colloquium when I returned to Cambridge.”

Before 1956, physicists believed strongly that all laws of nature must be mirror invariant: that everything that could happen in physics would be identical in a mirror image. But Abdus Salam had come up with a two-component theory of the neutrino, which said that in beta decay, the emitted neutrinos must have a particular helicity, or handedness, whether in our universe or a mirror one, although his theory could not predict which helicity is preferred by nature in our universe.

Salam’s theory violated parity conservation, and at the beginning of the colloquium, Pauli, who never, never, admitted that something he had said was wrong, excused himself for having persuaded Abdus Salam not to publish his two-component theory, because some rumours were going around that an experiment had shown that parity conservation might be violated. “That’s what really caught my attention that day,” said Herwig. “Salam would probably have had the Nobel Prize if he’d published. As a consequence of that colloquium, my research took a new turn, back in the direction of beta decay.” What Herwig did not know when he set off for Harwell is that across the Atlantic there had been feverish activity, both theoretical and experimental, in elucidating the weak interaction.

As 1956 drew to a close, weak interaction physics was undergoing something of a revolution. A paper published in the summer by two Chinese-American theorists, Tsung-Dao Lee and Chen-Ning Yang, interpreted results from cosmic rays and from the Brookhaven National Laboratory in the US, as perhaps being an indication that parity was not conserved in weak interactions. Their paper did nothing less than challenge one of the basic tenets of physics, although in the summer, there was no watertight experimental evidence to show that they were right.

Lee’s home institute was Columbia University in New York City, where he knew the experimentalist Chien-Shiung Wu, who already enjoyed a powerful reputation. Wu had moved to the US from China in 1936, and worked on the Manhattan Project through the war, not to mention her experimental forays into beta decay, and she was the first woman to become a professor of physics at Columbia. Lee had discussed the paper he was about to publish with Yang with her before it was published, and by the time Herwig was listening to Salam’s words, she was already turning her formidable experience to providing the first definitive experimental proof that Lee and Yang were right.

“In their paper, Lee and Yang proposed four experiments to check whether mirror symmetry was true or not in weak interactions,” explained Herwig. “They proposed one experiment in beta decay which Mrs Wu immediately started to do, and in the autumn of ‘56, she had her first results. Her paper had not yet been published, but there were rumours that her results showed that parity invariance was wrong.” Those were the rumours that had brought Pauli to Harwell, where he said that if they were true, he owed Salam an apology. They were still just rumours, though, and Herwig remembers Pauli expressing scepticism, drawing on the Kantian ideal that reason must be based on principles of symmetry.

Herwig’s interest had nevertheless been aroused: here was an opportunity to make a major contribution to physics. When he got back to Cambridge, he read Lee and Yang’s paper, where he learned about the four experiments they proposed. “Two were experiments in nuclear physics, in beta decay, and one was the one that Wu was doing. The second predicted that if a beta decay is followed by a gamma emission, the emitted photons must have a definite helicity which means they would be circularly polarised.” Lee and Yang thought that this experiment could not be done, because they were not aware of any experimental method to measure the circular polarisation of the photons. Herwig, however, knew better. “When I read that, I jumped up because, before coming to Cambridge, I had done exactly that at Erlangen, in an experiment trying to measure the helicity of gamma rays.”

When Herwig reported back to Otto Frisch and asked whether he could do the experiment at the Cavendish, the answer he got was an enthusiastic yes. Herwig stopped his work on the Van der Graaf, and got back to work on beta decay. “After Christmas, I immediately started to do this experiment, and Frisch gave me all the support I needed.” Herwig’s work in Erlangen had shown that circular polarisation of gamma rays could be measured by scattering the gamma-ray photons from a magnetised iron cylinder. Although Herwig was given priority at the Cavendish lab’s workshops, he decided to build the apparatus himself to save time, winding the copper coil around the cylinder by hand, and feeling grateful for his early training in Hamburg that had taught him all the manual techniques that modern-day physicists don’t have to concern themselves with. The other essential ingredient for the experiment was a radioactive beta-emitting source. “Wu had done her experiment with a cobalt-60 source, and the beta decay in this nucleus is followed by a gamma emission,” said Herwig, “so I asked Frisch to get me such a source.”

As a fellow of the Royal Society, Frisch was very influential in the UK, and he was able to procure the source within a week. “With a cobalt-60 source, the same nucleus that Wu used, I started to do the measurements within a month, by the end of January, a record time for setting up a nuclear physics experiment,” said Herwig with a smile. “I had my first results in February, and they were published immediately.” The rapid publication time also owed much to Otto Frisch’s status in the UK nuclear physics community. “He was editor of The Philosophical Magazine, and he made sure that my paper was published within a week.” Herwig’s paper, ‘Circular polarization of γ-rays: Further proof for parity failure in β decay’, was received on 14 March 1957, and published the same day. C. S. Wu’s paper had appeared in the American journal Physical Review just one month earlier, on 15 February.

Fig. 5.2

Circular Polarization of γ-rays: Further Proof for Parity failure in β Decay: Herwig’s paper demonstrating Lee and Yang’s theory of parity violation. Published in Philosophical Magazine in May 1957 [1], the paper is remarkable in many ways, not least that it is a single-author paper, which would be unheard of in modern-day particle physics. It remains the only paper showing that neutrinos and antineutrinos have opposite helicities using a single experimental set-up as shown (H. Schopper (1957) A Journal of Theoretical Experimental and Applied Physics, 2:17, 710–713 ©Taylor & Francis license, All rights reserved)

“My paper appeared in The Philosophical Magazine only a few weeks later than the paper of Wu, but of course, American physicists didn’t read The Philosophical Magazine, so it took several months until people became aware of my paper,” said Herwig, in a clear illustration that the centre of gravity of fundamental physics had moved yet further to the west. Nevertheless, the paper did eventually get him noticed. “Within half a year, it really put me on the landscape of international nuclear physics,” he recalled, “That‘s how I became known in the international area of nuclear physics, thanks to Frisch and thanks to this experiment.”

Fig. 5.3

Herwig discussing helicity with Chien-Shiung Wu. This photograph was probably taken at a conference in Geneva in 1958 (Herwig Schopper’s personal collection. ©Herwig Schopper, All rights reserved)

Herwig went on to repeat the experiment with a source that emitted positrons, the antimatter equivalent of electrons, followed by a gamma, and the result still stood. “I was able to show that the circular polarisation of the gamma rays following electron and positron beta decay have opposite helicities,” he said. “Still today, I think my experiment is the only one that has been able to show this with the same experimental equipment, just by changing a radioactive source. I’m very proud of that! And remarkably, my paper had only one author.”

The revelation that parity was not conserved once again showed the vagaries of the Nobel Prize. The 1957 Nobel Prize for Physics was awarded to Lee and Yang. Chien-Shiung Wu had to wait until 1978, when she was the inaugural recipient of the Wolf Prize, for recognition of her experiment. As time progressed, she was not silent on this omission, pondering, for example, in a 1964 MIT Symposium: “whether the tiny atoms and nuclei, or the mathematical symbols, or the DNA molecules have any preference for either masculine or feminine treatment.” But Herwig thinks there might have been another reason for this omission. “In order to carry out the Wu experiment the nuclear spins of the radioactive nuclei had to be oriented in one direction, which required extremely low temperatures not available at Columbia University,” he explained. “Hence she had to do the experiment with a cryogenic group in Washington and the paper was signed by more than three authors. The Nobel can be given only to three scientists and not to a group—a too restrictive rule for modern sciences.”

As time progressed and Herwig’s renown grew, he was invited to give talks at the American Physical Society and many other conferences and colloquia. “I got to know Lee and Yang—Lee became a friend,” he recalled. “I became good friends with Wu, and we met very often at conferences. In fact, in 1957, something very special happened to me, I was invited to an international conference in Israel together with Lee and Wu to talk about our results, and I think I was probably the first German to be invited to an Israeli conference after the war, so that was a big moment.”

Fig. 5.4

In 1988 Herwig was presented with the Weizmann Institute’s Gold Medal by former French Minister and President of the European Parliament, Simone Veil, Honorary President of the Pasteur-Weizmann Council, in recognition of his work to strengthen relations between CERN and Israel. On the right is J.-J. Brunschwig, Chair of the Institute’s Suisse-Romande delegation (©CERN, All rights reserved)

Looking back on his time in Cambridge, Herwig is led to some philosophical musing on the nature of knowledge, and our interpretation of results. “It turned out these experiments not only show that parity, the mirror symmetry, is violated, but also that the symmetry between matter and antimatter is violated,” he began. “I could show, at least with the same accuracy, and I think with even better accuracy than the Wu experiment, that this symmetry is also violated to the maximum possible degree theoretically allowed. When it became known that parity and matter–antimatter symmetry are both violated, theorists took that for granted automatically. But if you consider the now famous standard model of particle physics, this parity and charge symmetry violation is put in by hand. There’s no fundamental theoretical reason for the sign of helicity of the neutrino, it’s only the experimental measurements that justify it. Today, nobody talks anymore about parity violation, it’s taken as given, but people are still talking about matter–antimatter asymmetry, although both were put into the standard model by hand at the same time. But then people were surprised that matter–antimatter symmetry seems to be violated in the cosmos, as well as at the microscopic scale. In the cosmos, it seems that all galaxies consist of matter and not of antimatter. So people are getting excited about the matter–antimatter violation, but they are not excited anymore about the mirror equivalence violation. To me, this shows a strange behaviour of scientists concerning ‘revolutions’ in physics. Sometimes in physics, in theoretical physics, some fundamental concepts are taken as given, whereas others are not, although there are no clear theoretical arguments in favour of one and against the other. Many discussions between physicists and philosophers could be saved if one would accept such arbitrariness and agree that the experiments show what is realised in nature: logical thinking alone is not always enough. But let me emphasise that I still believe that a close co-operation between theory and experiment is necessary for any progress in science. The theories sometimes present a question and leave the experiments to decide how to answer it, but sometimes by pure chance, a new phenomenon is discovered without having been predicted. Both approaches are necessary to gain basic new knowledge.”

His year at Cambridge done, in autumn 1957, Herwig packed his bags for Erlangen, and took his new research interest with him.

A Year at Cornell

Back in Erlangen, Herwig introduced his students to beta decay, but the following year, 1958, he was off to Mainz to take up a new post. The university there was relatively young, and its physics department was mainly occupied with teaching. It was headed by Rudolf Kollath, who Herwig knew from Hamburg. In order to introduce more research, there were plans to create an institute for experimental nuclear physics. Herwig’s job, as well as becoming a professor, was to set up and direct this new institute. Although moving towards science administration, Herwig managed to keep his hand in with research. “We continued the nuclear beta decay experiments with some collaborators who had come with me,” he recalled. “We produced some interesting and detailed results that helped understand the weak interaction” (see Chap. 4, Moving to Mainz and the foundation of MAMI).

Fig. 5.5

Mainz’s young Professor Schopper made an appearance in the Fuldaer Zeitung newspaper in September 1958. He’d been invited to present the results of the experiment he conducted at Cambridge on parity violation at the second Atoms for Peace conference in Geneva in 1958 (©Fuldaer Zeitung, All rights reserved)

It wasn’t long, however, before an opportunity arose to plunge right back into new research. Fleischmann’s successor in Hamburg, Willibald Jentschke, needed someone to learn the art of doing physics at an electron machine in order to establish an outside user group for the new Deutsches Elektronen-Synchrotron, DESY, laboratory that Jentschke was setting up in Hamburg. He asked Herwig to go to the States for a year, and Herwig was keen to accept. He deferred the post he’d been offered at Karlsruhe, and in August 1960, the Schopper family, now four in number with the arrival of Andreas in October 1959, was once again on the move. Jentschke had arranged for Herwig to receive a Fellowship to work with Robert Rathbun Wilson, founder and director of Cornell University’s Laboratory for Nuclear Studies, which was home to a 1.4 billion electronvolt, or GeV, electron synchrotron at Ithaca, in upstate New York. At the time, it was the largest electron machine in the world.

“So I quit my job in Mainz, accepted the chair at Karlsruhe but started with a year’s leave of absence without pay,” explained Herwig. “My income would come from the Fellowship, so that’s what I did, and Bob Wilson happily accepted me. He had arranged the fellowship from the National Science Foundation. It was a lousy fellowship, I think it was $500 a month for the academic year, but I thought we could live there modestly on that.” The next hurdle was to obtain an immigration visa that would allow him to work in America. “It was very difficult for a German to get a visa for the USA,” said Herwig. Immigration visas to the US were rationed, and there was a quota for each European country. At the time he applied, the German quota was exhausted, but there were practically no Czechoslovaks who could immigrate to the United States. “I was lucky in this sense because I was born in Czechoslovakia, and the rules in the United States were, and still are, that to get a visa, your birthplace is decisive and not your actual citizenship.” So Herwig was eligible for an immigration visa on the Czechoslovak quota, but there were still more hoops to be jumped through. “I had to fill in a form and appear before a diplomatic representative at the American embassy, and I had to solemnly swear on all kinds of things.”

Herwig duly presented himself at the nearest US diplomatic mission in Frankfurt and was presented with a long form to complete. “There were about two dozen questions that I had to answer, and after each question I had to swear. Would I respect the Constitution of the United States was one of the first questions. Then I was asked to swear that I would not murder the President, or organise prostitution in the United States, and so on. There were many, many questions, and I had to swear individually on all of them.”

Herwig survived the ordeal, and was awarded a visa, but it was a close thing. “I couldn’t help thinking about another physicist, Friedrich Houtermans, who’d told me about his experience of immigration to the United States. When he went to the American Embassy in Switzerland, they asked him the same questions, and after about twelve, he said, “Well, if you have more stupid questions like that, then I renounce my request for an immigration visa.” And he did. He didn’t get his visa. So when I was going through all these questions, swearing, I was thinking of Houtermans, and I was afraid that I would start to laugh. If I did, they might misinterpret my behaviour and believe I was not respecting the laws of the United States. But I controlled myself and got my visa.”

As air transport was still a luxury in the 1960s, the Schoppers boarded a ship in Hamburg, and were in for a choppy crossing. Doris, now five years old enjoyed the adventure, but baby Andreas did not appreciate the trip. Herwig spent much of the time in the ship’s pool, employing the kind of thinking that only a physicist could. “It was the first time that I crossed the Atlantic. I got seasick, but when I went into the swimming pool it immediately disappeared since the water did not follow the slow motions of the ship.” They arrived in New York after a week at sea, and were greeted, like so many before them, by the sight of lady liberty, lifting her light to guide the tired, the poor, and the huddled masses of the old world into the new. From New York, they travelled to Ithaca by plane, and were very soon installed in a house they’d rented at a very good price from a professor who was away on sabbatical. “It was a beautiful house with a little garden, indeed the first house we could have just for us. This was a great experience for my family.”

By the time the Schoppers arrived in Ithaca, Herwig’s research interest had moved on to the question of why different forces exist in nature, and what the properties of the fundamental building blocks of matter might be. At the time these were the protons and neutrons of atomic nuclei, since their constituents, the quarks, were not yet known. This required another jump in energy, from the MeV of nuclear physics to the GeV available at the Cornell electron synchrotron. It was a significant jump: 1 GeV is a thousand MeV.

By the 1960s, particle accelerators were firmly established as the research tools of choice for exploring the subatomic, and subnuclear worlds of particles and the forces that govern their interactions. Far from being a purely esoteric field of science, such studies of the quantum world can offer understanding of how the universe works at larger scales, and have gone on to deliver technologies that underpin much of modern society.

Back then, however, the field of particle physics was in its infancy, and Herwig was asking the question of whether the protons and neutrons of atomic nuclei had substructure, just as Rutherford, Geiger and Marsden had shown to be the case for atoms. “Since it had turned out that an atomic nucleus is not a single body but has a complicated structure, why couldn’t that happen for the proton and the neutron? Such investigations need smaller wavelengths, higher energies, and that became possible with the development of high-energy accelerators.”

Although much of the groundwork for high-energy particle accelerators had been done in Europe, and CERN had started its first big machine, the proton synchrotron, the year before, in 1960, Europe was still playing catch up. The US was the place to be for a physicist in pursuit of the high-energy frontier. Berkeley’s cosmotron had been running at its full energy of 3.3 GeV since 1953, and a massive linear accelerator was being planned at Stanford. Two miles in length, the Stanford linear accelerator remains to this day the world’s longest linear accelerator. The cosmotron was a proton accelerator, whereas the Stanford machine would be an electron machine, making them complementary in terms of the research they could support.

Herwig, however, was interested in a different kind of machine. The facility that Jentschke was planning for Hamburg was to be a circular electron machine, like the one at Cornell, and Herwig was there to learn how to do physics at such a facility. It was a choice guided by several factors, some political and some to do with the science at hand. On the political side, Jentschke was building a facility that would be complementary to the big proton machine at CERN, according to the principle of the ECFA pyramid. That guided the choice of electrons. “Jentschke’s idea was not to imitate the linear electron accelerator at Stanford, but to build a circular accelerator for electrons, even if it was more difficult to get to high energies.”

On the physics side, there are many parameters to consider when designing a machine for research. Protons are easier to accelerate to high energy in a circular machine than electrons because they lose much less energy when being forced to follow a circular path. Electrons, on the other hand, produce cleaner collisions. That makes protons the particles of choice when the highest energies possible are required, whereas electrons are better suited for precision studies. “If you want a high-energy electron machine, then a linear accelerator is the facility of choice,” explained Herwig, “although they are more expensive than a circular electron synchrotron.”

It was a Saturday afternoon when the Schoppers arrived in Ithaca, but Herwig was keen to explore his new workplace, so he left his family at home, and drove to the laboratory. “I’d never seen a large accelerator, so I immediately went there. Everything was open, there were no access controls, and I went directly to the accelerator hall. I got in with no problem at all, it was empty, nobody was there—it was Saturday afternoon. I walked around the accelerator, and came across a man sweeping the floor and so I started to ask him questions about life in the institute and some details about the accelerator. He gave me surprisingly informed answers, so after a while I said, “Look, you can’t be a janitor here, who are you?” He said, “I’m Bob Wilson.” I said, “What? You’re Bob Wilson, the director of the institute sweeping the floor on a Saturday afternoon?” “Oh,” he said, “it’s nothing special, when I was a young researcher, the first time I went to Berkeley, where the director was the famous Luis Alvarez, I found someone sweeping the floor there. I asked him, “Who are you?” “I’m Luis Alvarez,” he said, “the director of the institute.” This exchange was an eye-opener for Herwig, who was used to the European way of doing things, and an introduction to the more laid-back American approach of the time. “That was first time I met Bob Wilson. He was a fantastic person, and we became good friends. I tried much later to introduce the tradition of floor sweeping by directors at CERN but failed completely.”

Fig. 5.6

Robert Rathbun—Bob—Wilson, director of Cornell University’s synchrotron laboratory, introduced Herwig to experimental high-energy physics. Wilson went on to become the founding director of the National Accelerator Laboratory, today known as Fermilab. As well as being an excellent scientist, Wilson also brought his artistic sensibility to the Fermilab site, insisting that the buildings and urban architecture be designed according to a uniform aesthetic style (©Fermilab, All rights reserved)

Bob Wilson founded not just one leading laboratory for particle physics, but two. After his time at Cornell, he went on to establish the National Accelerator Laboratory, today’s Fermilab, near Chicago, and he was also a pioneer of hadron beam therapy for treating cancer, pointing out that protons deposit all of their energy at the end of their trajectories, unlike electrons or photons, which deposit energy smoothly along their paths. That makes protons good for pinpointing tumours and avoiding collateral damage to healthy tissue nearby. The GSI laboratory, founded by Christian Schmelzer in Darmstadt, later went on to become a pioneer of this technique, and Wilson’s idea is acknowledged in a stained glass window—one of several celebrating the science of the nearby laboratory—in a nearby church dating from the twelfth century in Wixhausen.

“Bob Wilson was a great personality, he was not only an excellent physicist, but he was also proud to be a painter and sculptor,” said Herwig. “Some of his pieces of art were even exhibited at Harvard University, and even if real artists were indulgent of his artistic efforts he was very proud of them. Bob also cared about the environment when it was not yet a very common concern. He designed the new lab building—the high rise—an extraordinary architectural landmark for the whole region. He also designed the electricity pylons of the laboratory, which have a certain touch of Japanese beauty. In addition, he introduced a breeding herd of bison to the laboratory’s prairie land. As the herd thrived, on special occasions, for example large international conferences, delegates would be treated to bison grilled on a large open fire. Bob’s wife was also fantastic and very good at art. I was very happy that we became friends, not only from the professional point of view, but also privately.”

Finding the director sweeping the floor on a Saturday afternoon was not the only culture shock in store for Herwig. “I was very much impressed by the friendliness and helpfulness of our neighbours,” he explained. “When we moved into the house, the neighbours immediately came and asked my wife how they could help us to settle down, so that really impressed us very much.” When Herwig started to work at the laboratory and regular night shifts became a way of life, he’d take a nap through the morning and Ingeborg would prepare lunch for when he awoke. After lunch they’d pay a visit to one of the many beautiful state parks that Ithaca had to offer. To Herwig, it seemed idyllic, but to Ingeborg’s friends, it was an affront. On one such day, Ingeborg was invited for lunch, but declined because of the arrangement she had with Herwig. “I almost started a revolution when I said I couldn’t come to the wives’ lunch because I was preparing lunch for my husband,” she explained to Herwig. “I met with a complete lack of understanding: they said, ‘it’s not acceptable that a wife cannot go to a ladies’ lunch because she has to prepare lunch for her husband, can’t he prepare something for himself?’” It was the beginning of the 1960s, and European and American ways of doing things were worlds apart. “They gave up on me,” said Herwig, “but I was still impressed with them, and with the fact that when my daughter went to school, she didn’t only have to remember the names and achievements of all the Presidents, but also those of their wives. I discovered that the influence of women in the United States was already much stronger than it was in Europe at the time.”

At Cornell, Herwig learned all about the merits of linear versus circular machines, and protons versus electrons as tools for research. “The advantage of electrons compared to heavier particles is that it’s easier with them to investigate the structure of a small thing like a proton,” he explained. “If you use protons as the probe, you get all kinds of complications because of the complicated inner structure of the proton itself. These complications are very interesting in their own right, but if you want to know the structure of a proton, it’s better to use electrons since they have no inner structure.” For Herwig’s new research interests, that dictated an electron machine, but the advantages did not stop there. “A circular electron machine is also better to do such measurements because you can use an internal target that is installed inside the magnet ring of the accelerator itself,” he continued. “If you want to investigate the structure of the proton, you can’t bombard a single proton, you use a liquid hydrogen target. Protons are the nuclei of hydrogen atoms, so they are packed very densely in liquid hydrogen. And if you want to study all the details, you need many, many electrons to sound out the structure of the proton. Because the proton is so small, most of the electrons you fire at the target pass straight by. In a linear accelerator, you have one shot for each bunch of electrons you accelerate, but in a circular machine with an internal target, if the electrons miss the protons the first time round, they come back again and again and again. You can get more precise results more easily.”

One experiment in particular attracted Herwig’s attention. Robert Hofstadter had pioneered the technique for investigating the structure of nuclei and individual nucleons (protons and neutrons) by bombarding them with high-energy electrons. It was to win him a half share of the 1961 Nobel Prize in Physics, the citation for which reads: “The experimental method used by Hofstadter is connected with the principles of the ordinary electron microscope. Here the possibilities to observe details are increased by raising the voltage which accelerates the electrons.” The machine Hofstadter had used for his pioneering work was at Stanford—the two-mile long machine that was on the cards and had a strong pedigree based on smaller linear accelerators going back to the 1940s.

“By measuring the deflection of the electrons from the proton,” explained Herwig, “he determined the radius of the protons and also of heavier atomic nuclei for the first time, and by scattering electrons from deuterons, which consist of one proton and one neutron, he could also measure the radius of neutrons.” Herwig wanted to take this research a step further, and he recognised that with the Cornell synchrotron and its internal target, he was in exactly the right place to do so.

“The proton carries a positive electric charge that is not concentrated in a point but has a local distribution characterised by an average radius,” said Herwig. “But although the overall charge of the proton is positive it also contains some negative charges.” The same is true for the neutron whose total charge is zero because the positive and negative charges compensate each other. If electric charges are not at rest but move around they produce magnetic fields characterised by magnetic moments that are again not concentrated in a point but have a local distribution. What Herwig wanted to do was to scatter electrons from protons or neutrons, a process through which they would be deflected by the electric charges and also by the magnetic fields, in order to measure the sizes of the particles and the distribution of electric charge and magnetic moment inside them—the so-called proton and neutron form factors.

“The problem is that protons can be used directly as a target but there exist no free stable neutrons,” explained Herwig, “so the best one can do is to use deuterium which consists of nuclei containing one proton and one neutron. If one has measured first the properties of the proton one can then extract the properties of the neutron from the measurements on deuterium, but evaluating the experimental data is somewhat complicated.”

“In order to get sufficient data, one has to observe scattered electrons at many different angles,” continued Herwig. “This is where the advantages of a circular accelerator with an internal target come in.” Cornell’s electron synchrotron would make it easy for him to collect data from millions of interactions between the electron beam and the protons and deuterons in the target. His next challenge was to build a spectrometer to measure the energy of the scattered electrons. “There was a difficulty,” he realised. “If you have an internal target, access is limited because the electrons are kept in orbit by magnets, and they limit the space around an internal target, so we had to invent a new kind of spectrometer that could be installed close to the internal target in a circular machine.”

Bob Wilson gave Herwig the task of constructing such a spectrometer, and he hit on the idea of using a quadrupole magnet. The kind of magnets we’re most familiar with have two poles, labelled north and south, but electromagnets can be made with as many north and south poles as you can fit into the space available. Quadrupoles have two of each, and were already in use in accelerators for focusing the beams. “So I designed such a quadrupole spectrometer,” he said, “It was built at the main mechanical workshop of the institute, and it worked very well for studying electron scattering from an internal target.”

With the large amount of data they collected, Herwig and the Cornell team were able to make good measurements of the proton and neutron radii and form factors. “I did something that I am still a little bit proud of: I corrected a mistake that the people had done at Stanford,” smiled Herwig. “I showed that although the measurements at SLAC were right, the determination of the electric and the magnetic form factors at SLAC were wrong, so we determined the electric and magnetic form factors of the proton and the neutron in a proper way, and we presented our results at meetings of the American Physical Society. But I must say Cornell was not very good in public relations, unlike Stanford, so I think Cornell never got the credit they deserved for this work.”

Another skill that Herwig picked up at Cornell was running both the experiment and the accelerator at the same time. Unlike today’s big research facilities where accelerator physics and particle physics are distinct disciplines, and the control rooms for accelerators and experiments are far apart, Cornell’s accelerator had been built by the physicists who used it, and it was natural for the control rooms to be adjacent to each other. “We had to operate two things at the same time: the accelerator and the experiment. The control rooms for the two were very close together, only a few metres apart, so we were running back and forth from the control desk of the accelerator to that of the experiment, and operating both at the same time. It was beautiful because you had full control. It gave me the chance to learn how to operate a complicated machine like a synchrotron.”

Bob Wilson also provided Herwig with a role model for a hands-on laboratory director. “I remember one remarkable event,” he recounted. “One night we realised that we needed a measurement at a large angle of more than 90°, but we could not get the quadrupole spectrometer to cover such an angle because one of the accelerator magnets was in the way, so at three o’clock in the morning, Bob took a hacksaw and cut a corner off the magnet.” Herwig took note, but was wary of putting what he’d seen into practice. “Even as Director-General of CERN I would never have dared to touch a machine element without the permission of the expert! Times have changed.”

The Schoppers’ stay in Ithaca came to an end at the beginning of the summer of 1961, but the family’s American adventure wasn’t over. “In America, professors and also fellows were paid for nine months of the academic year, and for three months they could do what they wanted,” said Herwig. “My fellowship was for the nine months, but we had lived modestly and saved enough money to spend another three months in the United States. I had been invited to give some talks in other universities and laboratories, but for the last six weeks we decided to take a road trip across the United States. I had bought an old Chevrolet, and we went from Ithaca across the great plains and the Yellowstone Park before arriving in San Francisco. I combined this private trip with invitations to laboratories like Los Alamos, where the atomic bomb was developed during the war, and to Stanford. In this way we got to see the beautiful United States, for example driving along the Pacific coast from San Francisco to L.A. We appreciated how easy it was during such a long trip to take care of our baby of less than a year old: in all the motels and restaurants people were extremely accommodating in helping us with Andreas, so we all really enjoyed this trip through the USA, though I don’t think Andreas remembers much.”

While the Schoppers were enjoying their holiday, political developments in Germany were taking a sinister turn, and would force the family to take a difficult decision: on 13 August, the Berlin wall started to go up. “Bob Wilson was very kind and offered me a job,” said Herwig. “Nobody knew what would happen in Germany, even a civil war was not excluded. Since I had an immigration visa, there would have been no problem staying, and in time I would have got United States citizenship, but after long discussions with my wife, we decided to go back to Europe. On the one hand we were fascinated by the American way of life, which had many, many advantages. It’s a big country, with apparently no limits, it was fantastic. In the States we could travel for weeks using the same language, the same currency, and staying in the same type of hotels. Beautiful. Europe appeared small, and in a way provincial, with all the diversity of different countries and different languages, but we realised the advantages and the beauty of these diversities, and European culture and way of life, so we decided to go back. I went to Karlsruhe and took up my professorship there.”

Through the course of three postings, to Stockholm, Cambridge and Cornell, Herwig had completed the transition from optics to particle physics. A total jump in energy of nine orders of magnitude in two steps.

In His Own Words: Learning About the English Way

“My entry to England gave me an interesting lesson in how things worked there in the 1950s. At that time, of course, aeroplanes practically didn’t exist, so surface travel was the only way. I took the overnight ferry from Hamburg to Harwich and arrived, I think, at about five o’clock in the morning. The first thing was I had to go to the immigration office, and they asked me what I wanted to do in the UK. I said, “I have an invitation to Cambridge University for a year.” The officer replied, “well, if you want to stay for a year, do you have a visa?” I explained that my fellowship was arranged in a hurry, and I didn’t have time to apply for a visa, which would have taken several weeks or months, but pointed out that I was allowed to enter as a German for three months as a tourist. Within those three months, I planned to apply for a visa. He said, “you can’t do that, if you want to stay for a year, you should have applied for a visa beforehand. I’m sorry, my hands are bound. You have to leave the United Kingdom on the same boat you came on.” I asked him whether he would have allowed me to enter if I had told him that I wanted to stay for three months. He said, “yes, of course, but you told me you wanted to stay for a year.” I’d have been better off not telling the truth, but now I was stuck. The boat was scheduled to stay in Harwich the whole day, and sail back to Hamburg overnight. The Immigration officer called a policeman to escort me back to the boat and make sure I didn’t try to escape.

So I had breakfast with a policeman, and I really felt like a criminal. But then, at nine o’clock, I learned that there was a change of immigration officer, so I asked the policeman if he’d let me try again, and he took me back to the immigration office, but I got the same answer: “no, our hands are bound. You have to leave the United Kingdom on the same boat you arrived.” I asked him why, and he explained it to me. I really only understood the full implication of what he said much later, but it really was an interesting lesson. He told me that since England was never occupied by Napoleon, the Napoleonic Code was never introduced there. The Napoleonic Code meant that law was written down, law was executed according to the written laws. In England, not having been occupied by Napoleon, the legal courts had to refer to previous cases and not to written law. So the immigration officer was afraid that by letting me enter the United Kingdom, he would create a precedent and all German physicists could turn up, refer to the Schopper case, and they would have to be let in. He didn’t want to take that responsibility, which I can understand, but I had one last try anyway. I said, “Professor Frisch at Cambridge is waiting for me, and he will be surprised if I don’t arrive. Can I at least make a telephone call and inform him about my situation?” He said, “yes, of course, that you can do.” So the policeman took me to the nearest telephone booth and I tried to call Frisch. I got his secretary, who told me that he was giving a lecture and couldn’t talk to me, but she promised to inform him as soon as possible.

Fig. 5.7

(Courtesy of Cavendish Laboratory, University of Cambridge, © University of Cambridge, All rights reserved)

Otto Frisch pictured at Cambridge around 1970 with Sweepnik, a device he designed to analyse bubble chamber images

I went back to the boat, still accompanied by the policeman, to wait. We had lunch together, and then, at three o’clock, the immigration officer came in and said, “you are allowed to enter the United Kingdom.” I was baffled, and I asked what had happened to make him change his mind. He explained. “Well, Professor Frisch has called us, and we learned that he is a fellow of the Royal Society and that changed the situation.” That would have been completely impossible in a European continental country: no scientific society would have had sufficient reputation to change such a decision!

So I was allowed to enter the United Kingdom, and within three months I applied for a visa. It was still a rather bureaucratic process, but I got a visa for the whole year. That immigration officer had given me a very interesting lecture concerning the different way that laws in Europe were applied, and it wasn’t the last lesson I learned about English society that year.

At Cambridge University, I learned that the courtyards of the colleges are, in principle, private, but for many centuries the practice has been that the public can walk through them. In order to keep that up, however, it is necessary that, at least once a year, two members of the public have to walk through the courtyards and give neutral testimony to each other. These old institutions are full of stories like that, which seem quaint, but I liked them. Another that amused me concerned a penalty given to a student for a rather unusual misdemeanour. At that time, all the students and scientists had to wear gowns. I also had to wear a black gown. One day they caught a student swimming naked in the River Cam, he was called before a provost and he was told that it’s forbidden to swim naked in the Cam and they wanted him to pay a few pounds as a penalty, but they couldn’t find a precedent case where a student was punished for swimming in the Cam. In the end, he was punished for not wearing his gown while swimming in the Cam, and they managed to fine him, so that was another nice lesson in English law.

My learning curve was not confined to the university. When my wife and daughter arrived, we had difficulties finding an apartment. We didn’t have much money, so I couldn’t afford a house, and we ended up renting an apartment in an old house. The landlady was very nice, and she lived on the ground floor. We were on the first floor. The house didn’t have central heating and winter was coming. There was a gas fire, but to keep it going, you had to feed it with a shilling every two hours, I think it was: a shilling to keep the room warm. So we had to learn to get up during the night to put in a shilling into the heater, so as not to freeze to death. There was no hot running water, and once a week, on Saturday, the water was heated and the whole family could have a hot bath.

Our landlady had children of her own, and she included my daughter in their games. We were happy there, but again I learned something about the English mentality. One day, she asked me what I did. I said, “I’m with Professor Frisch at the university. I thought she would be impressed that he was a professor there, and a member of the Royal Society. But instead, she observed “Oh, he’s a German immigrant.” When I explained to her that Frisch was the nephew of Lise Meitner, that he’d had to leave Europe as a refugee, just as Meitner had gone to Sweden, and he had been in England for more than 20 years, fully recognised in academic circles, she only said, “no, no, a pear is a pear, and an apple is an apple. He’s not British, he will always remain an apple and never become a pear.” She didn’t mean any harm, but that was a lesson in English mentality: not in academic circles, but among ordinary people. Later, we found a more modern apartment, with central heating and hot water.”