Lise Meitner finally fled Nazi Germany in July 1938. There were two reasons she had been able to remain in Berlin long after many of her colleagues and contemporaries had departed. Meitner had been born in Vienna and, although she had Jewish ancestry, her Austrian citizenship had provided protection from Nazi racial laws. She had been working at the independent, privately-funded Kaiser Wilhelm Institute in Berlin-Dahlem since 1913. Meitner had been able to retain her position at a time when Jewish faculty members had been expelled from government-funded universities. This all changed in early 1938. In March, she lost her status as a foreign national following the annexation of Austria. Shortly after the Anschluss, Meitner was denounced by a colleague at the institute.1 It was time for her to leave Germany.
Meitner first traveled to the Netherlands. A courageous Dutch physicist, Dirk Coster, helped bring her over the border. Meitner had been forced to abandon her possessions in Berlin and arrived in the Netherlands carrying only a few small pieces of luggage. She also had in her possession a diamond ring that had been a gift from her colleague and collaborator, the radiochemist Otto Hahn. The ring had belonged to Hahn’s mother and he had instructed Meitner to use it to bribe the guards at the border if necessary.2 Meitner stayed only briefly in the Netherlands before leaving for Sweden.
A Walk in the Woods
Meitner spent her first Christmas in exile in Kungälv, a small town near Gothenburg.3 She was joined there by her nephew, Otto Frisch, a physicist who had taken refuge at the Niels Bohr Institute in Copenhagen. In his biography, Frisch described their famous walk in the woods and their discussion about the nature of nuclear fission.4 Before recounting this conversation, a few background details may prove useful.
In 1932, James Chadwick discovered the neutron. It was immediately clear that this new particle could penetrate and transform nuclei due to its electrical neutrality. Experimental studies were subsequently undertaken by Enrico Fermi and his group in Rome. They examined a number of elements before they turned their attention to uranium and began bombarding it with neutrons. But Fermi already knew what he was going to find before any experiments had taken place: neutron absorption would transform the uranium nuclei. As we now know, uranium becomes neptunium, which in turn becomes plutonium. Fermi was certain that his team had created these transuranic elements. Before she left Germany, Meitner, along with Hahn and a young collaborator named Fritz Strassmann, were engaged in repeating these experiments.5 After her departure, Hahn and Meitner had managed to maintain communication. It was Hahn and Strassmann’s latest experimental results that Meitner and Frisch were discussing when they took a walk in the woods.6 Frisch’s story begins with the arrival of the mail from Berlin:
When I came out of my hotel room after my first night in Kungälv I found Lise Meitner studying a letter from Hahn and obviously worried by it. … Its content was indeed so startling that I was at first inclined to be skeptical. Hahn and Strassmann had found that those three substances [formed by bombarding uranium with neutrons] were not radium, chemically speaking; indeed they had found it impossible to separate them from the barium which, routinely, they had added in order to facilitate the chemical separations. They had come to the conclusion, reluctantly and with hesitation, that they were isotopes of barium.7
Frisch asked whether Hahn and Strassmann might have made a mistake, but his aunt was adamant:
No, said Lise Meitner; Hahn was too good a chemist for that. But how could barium be formed from uranium? No larger particles than protons or helium nuclei (alpha particles) had ever been chipped away from nuclei, and to chip off a large number [this must be a reference to the number of protons] not nearly enough energy was available. Nor was it possible that the uranium nucleus could have been cleaved right across. A nucleus was not like a brittle solid than can be cleaved or broken.8
The pair then weighed up Niels Bohr’s liquid drop model for nuclear fission in light of these new developments:
George Gamow had suggested early on, and Bohr had given good arguments that a nucleus was much more like a liquid drop. Perhaps a drop could divide itself into two smaller drops in a more gradual manner, by first becoming elongated, then constricted, and finally being torn rather than broken in two? We knew that there were strong forces that would resist such a process, just as the surface tension of an ordinary liquid drop tends to resist its division into two smaller ones. But the nuclei differed from ordinary drops in one important way: they were electrically charged, and that was known to counteract the surface tension.9
Frisch continues:
At that point we both sat down on a tree trunk … and started to calculate on scraps of paper. The charge of a uranium nucleus, we found, was indeed large enough to overcome the effect of the surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly, unstable drop, ready to divide itself at the slightest provocation, such as the impact of a single neutron.10
The pair then proceeded to work out for the very first time the physical characteristics of nuclear fission:
But there was another problem. After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and a very large energy, about 200 MeV in all; where could that energy come from? Fortunately Lise Meitner remembered the empirical formula for computing the masses of nuclei and worked out that the two nuclei formed by the division of a uranium nucleus would be lighter than the original uranium nucleus by about one-fifth the mass of a proton. Now whenever mass disappears energy is created, according to Einstein’s formula E = mc2, and one-fifth of a proton mass was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!11
In her biography of Meitner, Ruth Lewin Sime writes of this episode: “Frisch’s story … conveys all the excitement and delight of a truly new idea, the first recognition that a nucleus can split, and the first understanding of how and why it does.”12
The story of Meitner and Frisch’s walk in the woods also brings to mind an Italian saying, “Se non è vero, è ben trovato” (Even if it is not true, it is well conceived). Many years ago, when I first read Frisch’s account, I found it charming, but not entirely believable. The part of the story concerning the conservation of energy certainly rings true. I find it harder to believe that the pair went for a walk carrying scraps of paper and writing implements, and sat on a tree trunk together calculating. The energy conversion calculations were sufficiently simple that pen and paper would not have been needed.
Prior to her departure from Berlin, one of Meitner’s assistants had been a young physicist, Carl Friedrich von Weizsäcker.13 In 1935, Weizsäcker formulated the curve of binding energy. The curve, which represents the binding energy per nuclear particle, rises steeply for hydrogen, helium, lithium, and carbon. The curve starts to gradually level off after reaching the binding energy for oxygen, before flattening out and slowly falling away between the energies for iron and uranium.
This is, in fact, what makes fission energetically possible. A mass excess can be converted into the kinetic energy of the fission fragments. Using the liquid drop model, the curve can be parametrized, and from these parameters it is possible to find the binding energies of any nuclei that might be of interest. Meitner and Frisch knew that if barium was one of the fission products, the other must be krypton. The sum of the nuclear charges involved must be the same as that of uranium. Krypton is an inert gas and it simply floated out of Hahn’s apparatus. From the curve of binding energy, Meitner and Frisch knew the level of energy available for the fission fragments. This part of Frisch’s story seems credible. The idea that Meitner and Frisch were able to compute the parameters required for a spherical drop to become ellipsoidal and then distended until it finally splits seems less plausible. This is a nontrivial calculation that was first done by Bohr and John Wheeler.14
In any event, Frisch returned to Copenhagen, where he told Bohr of his walk with Meitner and their realization. Bohr understood immediately that they were right. At the time, Bohr was due to leave for the US with his assistant Léon Rosenfeld. He agreed not to tell anyone about the result until Frisch and Meitner had a chance to publish their findings. Alas, Bohr forgot to tell his assistant about this arrangement and as soon as Rosenfeld arrived in Princeton he told everyone the news.
The work of Meitner and Frisch was published in Nature as a brief letter entitled “Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction.”15 A few things about the letter are worth noting. This is the first and last time in the literature that the term “fission” appears in quotes. Frisch asked a biology colleague in Copenhagen what term they used for the cleaving of cells. It might also be noted that the names on the paper are not in alphabetical order. Meitner comes first. When it comes to claiming scientific priority family connections come second. Of more significance is what they left out—the neutrons. Along with the fission fragments neutrons are produced. If there are two or more, they can induce additional fissions and the process can grow exponentially. This is what transforms fission from a laboratory curiosity to serious technology. The neutrons were soon found and the race for the atomic bomb began. It is interesting to note that Frisch spent some of the war years at Los Alamos. Meitner was asked to go but declined.
A Life in Physics
Some biographers seem predestined to write about their subjects. Richard Ellmann’s masterful biography of James Joyce is a case in point.16 Richard Westfall’s Never at Rest, widely considered the authoritative biography of Isaac Newton, is another example.17 Ruth Lewin Sime’s A Life in Physics, a superbly researched account of the life and work of Meitner, should be considered among the very best scientific biographies. Sime holds a PhD in Chemistry from Harvard and has a deep understanding of Meitner’s work. She is not only the ideal biographer from a technical point of view, but she also understands from her own experience some of the obstacles Meitner faced in becoming a physicist. There were very few female physicists or chemists when Meitner embarked upon her career, and there are not many more today.18
When she was growing up, Meitner’s father was a successful lawyer. He was also a firm believer in the importance of women’s education, a belief that he put into practice with his five daughters. Despite showing an early aptitude for physics and mathematics, the young Lise first began training as a teacher. As was customary at the time, before students could enter university they had to pass a so-called Matura examination. In preparation for the exam, a small group, including Meitner, were tutored by a young Viennese physicist. When she took the exam, Meitner did so alongside fourteen other young women. Only four of them passed.
After enrolling at the University of Vienna in 1901, Meitner had the good fortune to have Ludwig Boltzmann as her teacher. Boltzmann was not only a superb theoretical physicist, but he was also a great lecturer.19 Boltzmann readily accepted female students in his lectures. Meitner’s thesis was in experimental physics and she took her degree in 1906, becoming only the second woman ever to earn a physics degree at the university.
After spending some time attempting to establish herself as independent researcher, it soon became clear to Meitner that there were few, if any, professional opportunities for her in Austria. And so she moved to Germany. In September 1907, Meitner arrived in Berlin hoping to study for a few more semesters. She ended up staying in the German capital for the next thirty years.
In Berlin, Meitner was fortunate to find a second great mentor, Max Planck. She later described their first meeting:
He received me very kindly and soon afterwards invited me to his home. The first time I visited him there he said to me, “But, you are a Doctor already! What more do you want?” When I replied that I would like to gain some real understanding of physics, he just said a few friendly words and did not pursue the matter any further. Naturally I concluded that he could have no very high opinion of woman students, and possibly that was true enough at the time.20
Planck relented and Meitner was allowed to attend his lectures. She was also offered workspace in a laboratory and it was there that she met Hahn. Meitner was not allowed to work in the main laboratory, but was provided with a basement room. In due course, she and Hahn decided to begin working together.21 Their collaboration lasted thirty years. In 1913, Meitner joined Hahn at the newly created Kaiser Wilhelm Institute. It was her first real job and she remained at the Institute until she was forced to leave Germany.22
History Rewritten
After the invasion of Italy in late 1943, the US dispatched a team of investigators to Italy, France, and Germany in an effort to unearth details of the scientific work undertaken by the Nazis during the war and to prevent this information falling into the hands of the Soviets. Chief among their concerns was finding out whether German scientists had made any progress toward developing nuclear weapons. The operation had been ordered by General Leslie Groves, the director of the Manhattan Project. In a none-too-subtle nod to its leader, the code name chosen for the mission was “Alsos,” the Greek word for grove.23
In addition to collecting files, records, and other evidence, the Alsos personnel also took the most important German scientists into custody. The group of scientists they captured—including Hahn, Werner Heisenberg, and Weizsäcker, among others—were flown to England and interned at Farm Hall, a manor house near Cambridge.24 The group were unaware that microphones had been placed throughout the house. All their conversations were recorded and transcribed, including their reactions to the first atomic bomb being dropped at Hiroshima on August 6, 1945. The group appeared genuinely shocked at the news. Hahn remarked, “I don’t believe it… They are 50 years further advanced than we.”25
Upon further reflection, the German scientists felt compelled to produce their own account—a Lesart—of the research they had undertaken during the war. All in all, it is a deplorable document. Dated two days after the Hiroshima attack, the Lesart begins by noting the appearance in the media of “partly incorrect statements regarding the alleged work carried out in Germany on the atomic bomb,” and expresses a desire to “set out briefly the development of the work on the uranium problem.”26 It is an account with many shortcomings.
The Hahn discovery was checked by many laboratories, particularly in the United States, shortly after publication. Various research workers—Meitner and Frisch were probably the first—pointed out the enormous energies which were released by the fission of uranium. On the other hand, Meitner had left Berlin six months before the discovery and was not concerned herself in the discovery.27I wrote about this particular passage in Hitler’s Uranium Club:
This document is remarkable for what it does not say. Meitner is given lukewarm credit—“probably the first”—and her departure from Berlin is made to seem like some sort of natural event. No mention is made as to why she was forced to leave. There is also no mention of the fact that Hahn wrote her frequently asking for and obtaining her advice and even proposing collaboration, and that she and Frisch interpreted Hahn’s data to mean that fission had occurred, thus contributing in an essential way to the discovery.28
As it turned out, Meitner died before these documents and the transcripts from Farm Hall were declassified. Nonetheless, she had other reasons to be displeased with Hahn.
Some weeks after the Hiroshima attack, the 1944 Nobel Prize in Chemistry was awarded to Hahn, and Hahn alone, for “his discovery of the fission of heavy atomic nuclei.”29 Meitner’s contribution was not recognized. Despite appearing as a coauthor with Hahn on a landmark 1939 paper, Strassmann’s name is also conspicuous by its absence.30 In Cambridge, Heisenberg heard the announcement on the BBC and passed on the news.31 Much has been written over the years about Hahn’s Nobel Prize. Sime devotes numerous pages of her book to the topic.32 Meitner later wrote:
Surely Hahn fully deserved the Nobel Prize in Chemistry. There is really no doubt about it. But I believe that Frisch and I contributed something not insignificant to the clarification of the process of uranium fission—how it originates and that it produces so much energy, and that was something very remote from Hahn.33
After the war, Meitner continued working in physics but, in truth, her most important contributions were already behind her. In 1960, at age 82, Meitner moved to Cambridge to be near Frisch and his family. She never had children of her own and it is unclear whether she ever had a romantic relationship.
In 1966, Hahn, Meitner, and Strassmann were announced as joint winners of the Enrico Fermi Award, a lifetime achievement award administered by the US Department of Energy. As Sime notes in her book, the citations differed for each of the three recipients. Meitner’s citation was for “pioneering research in the naturally occurring radioactivities and extensive experimental studies leading to the discovery of fission.”34
Lise Meitner died in her sleep on October 27, 1968, just a few days before her ninetieth birthday.35
