His scientific research never ceased, and he was publishing papers well into his nineties, making him one of the few scientists to have published at least one major paper in his field during every decade of his career, which in Bethe's case spanned nearly seventy years. PhysicistFreeman Dyson, once his doctoral student, called him "the supreme problem-solver of the 20th century",[16] and cosmologistEdward Kolb called him "the last of the old masters" of physics.[17]
His father accepted a position as professor and director of the Institute of Physiology at theUniversity of Kiel in 1912, and the family moved into the director's apartment at the institute. Initially, he was schooled privately by a professional teacher as part of a group of eight girls and boys.[23] The family moved again in 1915 when his father became the head of the new Institute of Physiology at theGoethe University Frankfurt.[20]
Bethe attended theGoethe-Gymnasium inFrankfurt, Germany. His education was interrupted in 1916, when he contractedtuberculosis, and he was sent toBad Kreuznach to recuperate. By 1917, he had recovered sufficiently to attend the localRealschule and the following year, he was sent to theOdenwaldschule, a private, coeducationalboarding school.[24] He attended theGoethe-Gymnasium again for his final three years of secondary schooling, from 1922 to 1924.[25]
Having passed hisAbitur, Bethe entered the University of Frankfurt in 1924. He decided to major inchemistry. The instruction in physics was poor, and while there were distinguished mathematicians in Frankfurt such asCarl Ludwig Siegel andOtto Szász, Bethe disliked their approaches, which presented mathematics without reference to the other sciences.[26] Bethe found that he was a poor experimentalist who destroyed his lab coat by spillingsulfuric acid on it, but he found the advanced physics taught by the associate professor,Walter Gerlach, more interesting.[26][27] Gerlach left in 1925 and was replaced byKarl Meissner, who advised Bethe that he should go to a university with a better school of theoretical physics, specifically theUniversity of Munich, where he could study underArnold Sommerfeld.[28][29]
Bethe entered the University of Munich in April 1926, where Sommerfeld took him on as a student on Meissner's recommendation.[30] Sommerfeld taught an advanced course on differential equations in physics, which Bethe enjoyed. Because he was such a renowned scholar, Sommerfeld frequently received advance copies of scientific papers, which he put up for discussion at weekly evening seminars. When Bethe arrived, Sommerfeld had just receivedErwin Schrödinger's papers onwave mechanics.[31]
For hisPhD thesis, Sommerfeld suggested that Bethe examineelectron diffraction incrystals. As a starting point, Sommerfeld suggestedPaul Ewald's 1914 paper onX-ray diffraction in crystals. Bethe later recalled that he became too ambitious, and, in pursuit of greater accuracy, his calculations became unnecessarily complicated.[32] When he metWolfgang Pauli for the first time, Pauli told him: "After Sommerfeld's tales about you, I had expected much better from you than your thesis."[33] "I guess from Pauli," Bethe later recalled, "that was a compliment."[33]
After Bethe received his doctorate,Erwin Madelung offered him an assistantship in Frankfurt, and in September 1928 Bethe moved in with his father, who had recently divorced his mother. His father had met Vera Congehl earlier that year and married her in 1929. They had two children, Doris, born in 1933, and Klaus, born in 1934.[34]
Bethe did not find the work in Frankfurt very stimulating, and in 1929 he accepted an offer from Ewald at theTechnische Hochschule inStuttgart. While there, he wrote what he considered to be his greatest paper,[35]Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie ("The Theory of the Passage of Fast Corpuscular Rays Through Matter").[36] Starting fromMax Born's interpretation of theSchrödinger equation, Bethe produced a simplified formula for collision problems using aFourier transform, which is known today as theBethe formula. He submitted this paper for hishabilitation in 1930.[35][37][38]
Bethe was known for his sense of humor, and withGuido Beck andWolfgang Riezler [de], two other postdoctoralresearch fellows, created ahoax paperOn the Quantum Theory of the Temperature of Absolute Zero where he calculated thefine structure constant from the absolute zero temperature in Celsius units.[41] The paper poked fun at a certain class of papers in theoretical physics of the day, which were purely speculative and based on spurious numerical arguments, such asArthur Eddington's attempts to explain the value of thefine structure constant from fundamental quantities in an earlier paper. They were forced to issue an apology.[42]
For the second half of his scholarship, Bethe chose to go toEnrico Fermi's laboratory in Rome in February 1931. He was greatly impressed by Fermi and regretted that he had not gone to Rome first.[43] Bethe developed theBethe ansatz, a method for finding the exact solutions for theeigenvalues andeigenvectors of certain one-dimensional quantum many-body models.[44] He was influenced by Fermi's simplicity and Sommerfeld's rigor in approaching problems and these qualities influenced his own later research.[45]
The Rockefeller Foundation offered an extension of Bethe's fellowship, allowing him to return to Italy in 1932.[46] In the meantime, Bethe worked for Sommerfeld in Munich as aprivatdozent. Since Bethe was fluent in English, Sommerfeld had Bethe supervise all his English-speaking postdoctoral fellows, includingLloyd P. Smith fromCornell University.[47] Bethe accepted a request fromKarl Scheel to write an article for theHandbuch der Physik on thequantum mechanics of hydrogen and helium. Reviewing the article decades later,Robert Bacher andVictor Weisskopf noted that it was unusual in the depth and breadth of its treatment of the subject that required very little updating for the 1959 edition. Bethe was then asked by Sommerfeld to help him with thehandbuch article on electrons in metals. The article covered the basis of what is now calledsolid state physics. Bethe took a very new field and provided a clear, coherent, and complete coverage of it.[46] His work on thehandbuch articles occupied most of his time in Rome, but he also co-wrote a paper with Fermi on another new field,quantum electrodynamics, describing the relativistic interactions of charged particles.[48]
In 1932, Bethe accepted an appointment as an assistant professor at theUniversity of Tübingen, whereHans Geiger was the professor of experimental physics.[49][50] One of the first laws passed by the newNazi government was theLaw for the Restoration of the Professional Civil Service. Due to his Jewish background, Bethe was dismissed from his job at the university, which was a government post. Geiger refused to help, but Sommerfeld immediately gave Bethe back his fellowship at Munich. Sommerfeld spent much of the summer term of 1933 finding places for Jewish students and colleagues.[51]
Bethe left Germany in 1933, moving to England after receiving an offer for a position as lecturer at theUniversity of Manchester for a year through Sommerfeld's connection toWilliam Lawrence Bragg.[51] He moved in with his friendRudolf Peierls and Peierls' wife Genia. Peierls was a fellow German physicist who had also been barred from academic positions in Germany because he was Jewish. This meant that Bethe had someone to speak to in German and he did not have to eat English food.[52] Their relationship was professional as well as personal. Peierls aroused Bethe's interest innuclear physics.[53] AfterJames Chadwick andMaurice Goldhaber discovered thephotodisintegration ofdeuterium,[54] Chadwick challenged Bethe and Peierls to come up with a theoretical explanation of this phenomenon. This they did on the four-hour train ride from Cambridge back to Manchester.[55] Bethe would investigate further in the years ahead.[53]
In 1933, the physics department atCornell University,New York, was looking for a new theoretical physicist, and Lloyd Smith strongly recommended Bethe. This was supported by Bragg, who was visiting Cornell at the time. In August 1934, Cornell offered Bethe a position as an acting assistant professor. Bethe had already accepted a fellowship for a year to work withNevill Mott at theUniversity of Bristol for a semester, but Cornell agreed to let him start in the spring of 1935.[56] Before leaving for the United States, he visited theNiels Bohr Institute inCopenhagen in September 1934, where he proposed toHilde Levi, who accepted. The match was opposed by Bethe's mother, who despite having a Jewish background, did not want him to marry a Jewish woman.[57] A few days before their wedding date in December, Bethe broke off their engagement.[58]Niels Bohr andJames Franck were so shocked by this action by Bethe that he was not invited to the institute again until afterWorld War II.[57]
Together with Bacher and Livingston, Bethe published a series of three articles,[62][63][64] which summarized most of what was known on the subject of nuclear physics until that time, an account that became known informally as "Bethe's Bible". It remained the standard work on the subject for many years. In this account, he also continued where others left off, filling in gaps in the older literature.[65] Loomis offered Bethe a full professorship at the University of Illinois at Urbana–Champaign, but Cornell matched the position offered, and the salary of $6,000.[66] He wrote to his mother:
I am about the leading theoretician in America. That does not mean the best.Wigner is certainly better andOppenheimer andTeller probably just as good. But I do more and talk more and that counts too.[67]
Illustration of the proton–proton chain reaction sequence
Overview of the CNO-I cycle – the helium nucleus is released at the top-left step
But this did not account for the observation of elements heavier than helium. By the end of the conference, Bethe, working in collaboration withCharles Critchfield, had come up with a series of subsequent nuclear reactions that explained how the Sun shines:[70]
That this did not explain the processes in heavier stars was not overlooked. At the time there were doubts about whether the proton–proton cycle described the processes in the Sun, but more recent measurements of the Sun's core temperature and luminosity show that it does.[68] When he returned to Cornell, Bethe studied the relevantnuclear reactions and reactioncross sections, leading to his discovery of thecarbon-nitrogen-oxygen cycle (CNO cycle):[71][72]
The two papers, one on the proton–proton cycle, co-authored with Critchfield, and the other on the carbon-oxygen-nitrogen (CNO) cycle, were sent to thePhysical Review for publication.[73]
AfterKristallnacht, Bethe's mother had become afraid to remain in Germany. Taking advantage of her Strasbourg origin, she was able to emigrate to the United States in June 1939 on the French quota, rather than the German one, which was full.[74] Bethe's graduate studentRobert Marshak noted that theNew York Academy of Sciences was offering a $500 prize for the best unpublished paper on the topic of solar and stellar energy. So Bethe, in need of $250 to release his mother's furniture, withdrew the CNO cycle paper and sent it in to the New York Academy of Sciences. It won the prize, and Bethe gave Marshak $50 finder's fee and used $250 to release his mother's furniture. The paper was subsequently published in thePhysical Review in March. It was a breakthrough in the understanding of the stars, and would win Bethe theNobel Prize in Physics in 1967.[75][73] In 2002, at age 96, Bethe sent a handwritten note toJohn N. Bahcall congratulating him on the use of solar neutrino observations to show that the CNO cycle accounts for approximately 7% of the Sun's energy; the neutrino observations had started withRaymond Davis Jr., whose experiment was based on Bahcall's calculations and encouragement, and the note led to Davis's receiving a share of the 2002 Nobel Prize.[76]
Bethe married Rose Ewald, the daughter ofPaul Ewald, on September 13, 1939, in a simple civil ceremony.[77] She had emigrated to the United States and was a student at Duke University and they met while Bethe was lecturing there in 1937. They had two children, Henry and Monica.[78] (Henry was acontract bridge expert and former husband ofKitty Munson Cooper.)[79]
Bethe became anaturalized citizen of the United States in March 1941.[80] Writing to Sommerfeld in 1947, Bethe confided that "I am much more at home in America than I ever was in Germany. As if I was born in Germany only by mistake, and only came to my true homeland at 28."[81]
When the Second World War began, Bethe wanted to contribute to the war effort,[82] but was unable to work on classified projects until he became a citizen. Following the advice of theCaltechaerodynamicistTheodore von Kármán, Bethe collaborated with his friend Edward Teller on a theory of shock waves that are generated by the passage of a projectile through a gas. Bethe considered it one of their most influential papers. He also worked on a theory of armor penetration, which was immediately classified by the army, thus making it impossible for Bethe (who was not an American citizen at the time) to access further research on the theory.[83]
After receiving security clearance in December 1941, Bethe joined theMIT Radiation Laboratory, where he invented theBethe-hole directional coupler, which is used inmicrowavewaveguides such as those used inradar sets.[84] In Chicago in June 1942, and then in July at theUniversity of California, Berkeley, he participated in a series of meetings at the invitation of Robert Oppenheimer, which discussed the first designs for theatomic bomb. They went over the preliminary calculations byRobert Serber,Stan Frankel, and others, and discussed the possibilities of usinguranium-235 andplutonium. (Teller then raised the prospect of a thermonuclear device,Teller's "Super" bomb. At one point Teller asked if the nitrogen in the atmosphere could be set alight. It fell to Bethe andEmil Konopinski to perform the calculations demonstrating the virtual impossibility of such an occurrence.[85]) "The fission bomb had to be done," he later recalled, "because the Germans were presumably doing it."[86]
When Oppenheimer was put in charge of forming a secret weapons design laboratory,Los Alamos, he appointed Bethe director of the T (Theoretical) Division, the laboratory's smallest, but most prestigious division. This move irked the equally qualified, but more difficult to manage Teller andFelix Bloch, who had coveted the job.[87][88] A series of disagreements between Bethe and Teller between February and June 1944 over the relative priority of Super research led to Teller's group being removed from T Division and placed directly under Oppenheimer. In September it became part of Fermi's new F Division.[89]
Bethe's work at Los Alamos included calculating thecritical mass and efficiency ofuranium-235 and the multiplication ofnuclear fission in an exploding atomic bomb. Along withRichard Feynman, he developed a formula for calculating the bomb's explosive yield.[90] After August 1944, when the laboratory was reorganized and reoriented to solve the problem of theimplosion of theplutonium bomb, Bethe spent much of his time studying the hydrodynamic aspects of implosion, a job that he continued into 1944.[91] In 1945, he worked on theneutron initiator, and later, on radiation propagation from an exploding atomic bomb.[92] TheTrinity nuclear test validated the accuracy of T Division's results.[93] When it was detonated in the New Mexico desert on July 16, 1945, Bethe's immediate concern was for its efficient operation, and not its moral implications. He is reported to have commented: "I am not a philosopher."[94]
After the war, Bethe argued that a crash project for thehydrogen bomb should not be attempted,[95] although after PresidentHarry Truman announced the beginning of such a project and the outbreak of theKorean War, Bethe signed up and played a key role in the weapon's development. Although he saw the project through to its end, Bethe had hoped that it would be impossible to create the hydrogen bomb.[96] He later remarked in 1968 on the apparent contradiction in his stance, having first opposed the development of the weapon and later helping to create it:
Just a few months before, the Korean war had broken out, and for the first time I saw direct confrontation with thecommunists. It was too disturbing. Thecold war looked as if it were about to get hot. I knew then I had to reverse my earlier position. If I did not work on the bomb, somebody else would—and I had thought if I were around Los Alamos I might still be a force for disarmament. So I agreed to join in developing theH-bomb. It seemed quite logical. But sometimes I wish I were a more consistentidealist.[97]
After the H-bomb was made, reporters started to call Teller the father of the H-bomb. For the sake of history, I think it is more precise to say thatUlam is the father, because he provided the seed, and Teller is the mother, because he remained with the child. As for me, I guess I am themidwife.[97]
In 1954, Bethe testified on behalf ofJ. Robert Oppenheimer during theOppenheimer security hearing. Specifically, Bethe argued that Oppenheimer's stances against developing the hydrogen bomb in the late 1940s had not hindered its development, a topic which was seen as a key motivating factor behind the hearing. Bethe contended that the developments that led to the successfulTeller–Ulam design were a matter of serendipity and not a question of manpower or logical development of previously existing ideas. During the hearing, Bethe and his wife also tried hard to persuadeEdward Teller against testifying. However, Teller did not agree, and his testimony played a major role in the revocation of Oppenheimer'ssecurity clearance. While Bethe and Teller had been on very good terms during the prewar years, the conflict between them during theManhattan Project, and especially during the Oppenheimer episode, permanently marred their relationship.[98]
After the war ended, Bethe returned to Cornell. In June 1947, he participated in theShelter Island Conference. Sponsored by theNational Academy of Sciences and held at the Ram's Head Inn onShelter Island, New York, the conference on the "Foundations of Quantum Mechanics" was the first major physics conference held after the war. It was a chance for American physicists to come together, pick up where they had left off before the war, and establish the direction of post-war research.[99][100]
A major talking point at the conference was the discovery byWillis Lamb and his graduate student,Robert Retherford, shortly before the conference began that one of the two possible quantum states of hydrogen atoms had slightly more energy than that predicted by the theory ofPaul Dirac; this became known as theLamb shift. Oppenheimer and Weisskopf suggested that this was a result ofquantum fluctuations of the electromagnetic field, which gave the electron more energy. According to pre-warquantum electrodynamics (QED), the energy of the electron consisted of the bare energy it had when uncoupled from an electromagnetic field, and the self-energy resulting from the electromagnetic coupling, but both were unobservable, since the electromagnetic field cannot be switched off. QED gave infinite values for the self-energies; but the Lamb shift showed that they were both real and finite.Hans Kramers proposedrenormalization as a solution, but no one knew how to do the calculation.[99][10]
Bethe managed to perform the calculation on the train from New York toSchenectady, where he was working forGeneral Electric. He did so by realising that it was a non-relativistic process, which greatly simplified the calculation. The bare energy was easily removed as it was already included in the observed mass of the electron. The self energy term now increased logarithmically instead of linearly, making it mathematically convergent. Bethe arrived at a value for the Lamb shift of 1040 MHz, extremely close to that obtained experimentally by Lamb and Retherford. His paper, published in thePhysical Review in August 1947, was only three pages long and contained just twelve mathematical equations, but was enormously influential. It had been presumed that the infinities indicated that QED was fundamentally flawed, and that a new, radical theory was required; Bethe demonstrated that this was not necessary.[10][101]
His calculation of the Lamb shift has been called "the most important discovery in the history of the theory of quantum electrodynamics" by the physicistRichard Feynman, while physicist Paul Dirac called it "the most important calculation in physics for decades".[12]
One of Bethe's most famous papers is one he never wrote: the 1948Alpher–Bethe–Gamow paper.[102]George Gamow added Bethe's name (in absentia) without consulting him, knowing that Bethe would not mind, and againstRalph Alpher's wishes. This was apparently a reflection of Gamow's sense of humor, wanting to have a paper title that would sound like the first three letters of the Greek alphabet. As one of thePhysical Review's reviewers, Bethe saw the manuscript and struck out the words "in absentia".[103]
Bethe believed that theatomic nucleus was like aquantum liquid drop. He investigated thenuclear matter problem by considering the work conducted byKeith Brueckner onperturbation theory. Working withJeffrey Goldstone, he produced a solution for the case where there was an infinite hard-core potential. Then, working with Baird Brandow and Albert Petschek, he came up with an approximation that converted thescattering equation into an easily solveddifferential equation. This then led him to the Bethe-Faddeev equation, a generalisation ofLudvig Faddeev's approach to three-body scattering. He then used these techniques to examine theneutron stars, which have densities similar to those of nuclei.[104]
Bethe continued to do research onsupernovae, neutron stars,black holes, and other problems in theoretical astrophysics into his late nineties. In doing this, he collaborated withGerald E. Brown ofStony Brook University. In 1978, Brown proposed that they collaborate on supernovae. These were reasonably well understood by this time, but the calculations were still a problem. Using techniques honed from decades of working with nuclear physics, and some experience with calculations involving nuclear explosions, Bethe tackled the problems involved in stellargravitational collapse, and the way in which various factors affected a supernova explosion. Once again, he was able to reduce the problem to a set of differential equations, and to solve them.[105][106]
At age 85, Bethe wrote an important article about thesolar neutrino problem, in which he helped establish the conversion mechanism forelectron neutrinos intomuon neutrinos proposed byStanislav Mikheyev,Alexei Smirnov, andLincoln Wolfenstein to explain a vexing discrepancy between theory and experiment. Bethe argued that physics beyond theStandard Model was required to understand the solar neutrino problem, because it presumed that neutrinos have no mass, and therefore, cannot metamorphosize into each other; whereas the MSW effect required this to occur. Bethe hoped that corroborating evidence would be found by theSudbury Neutrino Observatory (SNO) inOntario by his 90th birthday, but he did not get the call from SNO until June 2001, when he was nearly 95.[107][108]
In 1996,Kip Thorne approached Bethe and Brown aboutLIGO, the Laser Interferometer Gravitational-Wave Observatory designed to detect the gravitational waves from merging neutron stars and black holes. Since Bethe and Brown were good at calculating things that could not be seen, could they look at the mergers? The 90-year-old Bethe quickly became enthused and soon began the required calculations. The result was a 1998 paper on the "Evolution of Binary Compact Objects Which Merge", which Brown regarded as the best that the two produced together.[109][110]
In 1968, Bethe, along with IBM physicistRichard Garwin, published an article criticising in detail the anti-ICBM defense system proposed by theDepartment of Defense. The two physicists described in the article that nearly any measure taken by the United States would be easily thwarted with the deployment of relatively simple decoys.[111] Bethe was one of the primary voices in the scientific community behind the signing of the 1963Partial Test Ban Treaty prohibiting further atmospheric testing of nuclear weapons.[112]
During the 1980s and 1990s, Bethe campaigned for the peaceful use ofnuclear energy. After theChernobyl disaster, Bethe was part of a committee of experts who analysed the incident. They concluded that the reactor suffered from a fundamentally faulty design and also that human error had contributed significantly to the accident. "My colleagues and I established," he explained "that the Chernobyl disaster tells us about the deficiencies of the Soviet political and administrative system rather than about problems with nuclear power."[113] Throughout his life Bethe remained a strong advocate for electricity from nuclear energy, which he described in 1977 as "a necessity, not merely an option."[114]
In the 1980s he and other physicists opposed theStrategic Defense Initiative missile system conceived by theRonald Reagan administration.[115] In 1995, at the age of 88, Bethe wrote an open letter calling on all scientists to "cease and desist" from working on any aspect of nuclear weapons development and manufacture.[116] In 2004, he joined 47 otherNobel laureates in signing a letter endorsingJohn Kerry for President of the United States as someone who would "restore science to its appropriate place in government".[117]
When Oppenheimer died, Oppie's long-time friend, Hans Bethe, assumed the mantle of the scientist of conscience in this country. Like Jefferson and Adams, Teller and Bethe would live on into the new century which they and their colleagues had done so much to shape.[118]
Bethe's hobbies included a passion for stamp-collecting.[119] He loved the outdoors and was an enthusiastic hiker all his life, exploring theAlps and theRockies.[120] He died in his home inIthaca, New York, on March 6, 2005, of congestive heart failure.[86] He was survived by his wife, Rose Ewald Bethe, and their two children.[121] At the time of his death, he was the John Wendell Anderson Professor of Physics, Emeritus, atCornell University.[122]
^James C. Keck Collected Works and Biography (Archived May 9, 2019, at theWayback Machine) has the class notes taken by one of Bethe's students at Cornell from the graduate courses on Nuclear Physics and on Applications of Quantum Mechanics he taught in the spring of 1947.
^"Interview with Hans Bethe by Charles Weiner at Cornell University". American Institute of Physics. November 17, 1967. Archived fromthe original on February 21, 2015. RetrievedApril 25, 2012.When asked by Charles Weiner if there was religion in his home, Bethe replied: "No. My father was, I think, slightly religious. I was taught to pray in the evening before going to bed, and I attended the Protestant religious instruction, which was given in the schools in Germany. I was also confirmed, and the instruction which I got in this connection got religion out of my system completely. It was never very strong before, and the confirmation had the consequence that I just didn't believe."
Brian, Denis (2001).The Voice Of Genius: Conversations With Nobel Scientists And Other Luminaries. Cambridge, Massachusetts: Perseus Pub.ISBN978-0-7382-0447-5.
Brown, Gerald E.; Lee, Sabine (2009).Hans Albrecht Bethe(PDF). Biographical Memoirs. Washington, D.C.: National Academy of Sciences. RetrievedJanuary 13, 2025.
Brown, Gerald E.; Lee, Chang-Hwan, eds. (2006).Hans Bethe and his Physics. New Jersey: World Scientific Publishing.ISBN981-256-609-0.
Schweber, Silvan S. (2012).Nuclear Forces: The Making of the Physicist Hans Bethe. Cambridge, Massachusetts: Harvard University Press.ISBN978-0-674-06587-1.
Szasz, Ferenc Morton (1992).British Scientists and the Manhattan Project: the Los Alamos Years. New York: St. Martin's Press.ISBN978-0-312-06167-8.OCLC23901666.
.png)
.jpg)
