Silvan "Sam" Schweber (1928-2017)

I was quite saddened to hear about the passing of Sam Schweber, one of the foremost and most scholarly historians of physics of the last half century. Schweber occupied a rather unique place in the annals of twentieth century physics history. He was one of a select group of people - Abraham Pais, Jeremy Bernstein and Jagdish Mehra were others - who knew many of the pioneers of quantum mechanics and theoretical physics personally and who excelled as both scientists and historians. His work was outstanding as both historiography as well as history, and he wrote at least half a dozen authoritative books.

Schweber got his PhD with Arthur Wightman at Princeton University, but his real break came when he went to Cornell for a postdoc with the great Hans Bethe. Schweber became close friends with Bethe and his official biographer; it was a friendship that lasted until Bethe's demise in 2005. During this time Schweber authored a well received textbook on quantum theory, but he was just getting started with what was going to be life's work.

Schweber became known for two high achievements. Probably the most important one was his book "QED and the Men Who Made It" which stands as the definitive work on the history of quantum electrodynamics. The book focused on both the personal background and the technical contributions of the four main contributors to the early history of QED: Richard Feynman, Julian Schwinger, Sin Itiro Tomonaga and Freeman Dyson. It's one of those rare books that can be read with profit by both technically competent physicists as well as laymen, since the parts concerning the informal history of physics and personal background of the main participants are as fascinating to read about as the technical stuff. Other prime participants like Hans Bethe, Robert Oppenheimer and Paul Dirac also make major appearances. If Schweber had written just that book and nothing else, he would still have been remembered as a major historian. The book benefited immensely from Schweber's unique combination of talents: a solid understanding of the technical material, a sound immersion in the history of physics, a personal friendship with all the participants and a scholarly style that gently guided the reader along. 

But Schweber did not stop there. His other major work was "In the Shadow of the Bomb", a superb contrasting study of Hans Bethe and Robert Oppenheimer, their background, their personalities, their contributions to physics and nuclear weapons and their similarities and differences. It's a sensitive and nuanced portrait and again stands as the definitive work on the subject.

Two other Schweber contributions guaranteed his place as a major historian. One was another contrasting study, this time comparing Oppenheimer and Einstein. And finally, Schweber put the finishing touches on his study of Bethe's life by writing "Nuclear Forces: The Making of the Physicist Hans Bethe". This book again stands as the definitive and scholarly study of Bethe's early life until World War 2. It's a pity Schweber could not finish his study of the second half of Bethe's remarkably long and productive life.

Another valuable contribution that Schweber made was to record a series of in-depth interviews with both Freeman Dyson and Hans Bethe for the Web of Stories site. These interviews span several hours and are the most detailed interviews with both physicists that I have come across: they will always be a cherished treasure.

Schweber's style was scholarly and therefore his books were not as well known to the layman as they should be. But he did not weigh down his writing with unnecessary baggage or overly academic-sounding phrases. His books generally strike a good balance between academic and popular writing. They are always characterized by meticulous thoroughness, a personal familiarity with the topic and an intimate knowledge of the history and philosophy of science.

By all accounts Schweber was also a real mensch and a loyal friend. When Oppenheimer's student David Bohm became the target of a witch hunt during the hysterical McCarthy years, Schweber and Bohm were both at Princeton. Bohm was dismissed by the university which worried far more about its wealthy donors and reputation than about doing the right thing. Schweber went to the office of Princeton's president and pleaded with him to reinstate Bohm. The president essentially threw Schweber out of his office.

Schweber spent most of his career at Brandeis University near Boston. I was actually planning to see him sometime this year and was in the process of arranging a letter of introduction. While I now feel saddened that I will miss meeting him, I will continue to enjoy and be informed by the outstanding books he has penned and his unique contributions to the history of science.

Big things come in little packages: How Willis Lamb's tiny measurement revolutionized 20th century physics

It's the end of World War 2. Scientists and especially physicists have spent the last four years working on military hardware, culminating in radar and the atomic bomb. Many of these talented men and women are eager to go back to their university campuses and resume normal civilian life; some of them are distraught at their role in engineering such horrific weapons and want to return to the carefree life of fundamental physics research which they knew before the war.

To reconnect the country's leading physicists with each other and with the great research problems which they left behind, the National Academy of Sciences decides to organize a series of conferences on the frontiers of physics. It's not hard to decide who should lead these conferences. Robert Oppenheimer has just led the wartime Los Alamos laboratory which produced the first nuclear weapons to high fame and glory. At Los Alamos and before at Berkeley, Oppenheimer has been widely acknowledged as the founder of the modern school of American theoretical physics and a man whose intellectual mastery of a wide array of disciplines is unmatched. It seems natural to have Oppenheimer be in charge of this post-war re-organization of physics in the country.

Oppenheimer and the National Academy of Sciences put together a list of the scientists they want to invite. Except for the famous Solvay Conferences organized in Europe during a more peaceful time, it's hard to think of another scientific gathering that attracted such an unprecedented constellation of talent. A dozen or more of the attendees have already won Nobel Prizes or would go on to win them; some for work which they would present during the conferences. The list of names is an all-star list in every respect: Hans Bethe, Enrico Fermi, Isidor Rabi, Robert Serber, Victor Weisskopf, Edward Teller, Abraham Pais, John Wheeler, Richard Feynman, Julian Schwinger and Hendrik Kramers. The meeting brings together both the new stars and the old guard (I mentioned Bohr and Dirac earlier, but as M Tucker points out in the comments section, they were present at a later conference: more on this equally interesting meeting in a future post).

Some of the participants at the Shelter Island conference:
Lamb (far left), Oppenheimer, (on arm rest) Feynman (seated
and writing) and Schwinger (second from right)

The first conference takes place in June 1947 at a tiny island called Shelter Island, situated in the jaws of the Long Island crocodile. The exclusive list of attendees gets escorted by a special police escort through major towns during their bus ride. Their selection as attendees, the cutting edge topics at the conference and Oppenheimer's leadership all make it clear that the center of physics has decidedly shifted from Europe to the United States. Shelter Island would go down in history as one of the most important conferences in the history of 20th century physics, but the participants don't quite know it yet.

One attendee in particular, a young protege of Oppenheimer's from Columbia University, is perhaps not as well known as the others: Willis Lamb. Lamb comes from a robust working class household and has obtained both his undergraduate and graduate degrees at Berkeley. Right before the war he got married to a German emigre, and as the story goes, for some time the authorities forbade him from walking on the beach and confiscated his shortwave radio for fear that he might be sympathetic with his wife's German compatriots and might try to communicate with German submarines. During the war he has worked on microwave radar with Rabi and others at Columbia. What is also perhaps not as well known is Lamb's versatility as a physicist. He is an experimental physicist now, but he got his PhD with Oppenheimer at Berkeley in the 1930s. This makes him one of the few scientists around to excel in both theoretical and experimental physics. Lamb's presence is already consequential since the participants at Shelter Island are pondering a discovery he made right after the war: a discovery important enough to be enshrined with his name - the Lamb Shift. The Lamb Shift will herald a new age in physics.

To understand the Lamb Shift, let's descend deep into the world of the atom with its electrons, protons and neutrons. Let's look at the simplest atom, hydrogen. As most of us have learnt in high school and college, electrons exist in energy levels defined by atomic orbitals. Each electron is defined by four so-called quantum numbers. In hydrogen, for the principal quantum number 2, the lone electron can exist in two orbitals defined by the secondary or angular quantum number: 2S and 2P. During the 1930s, in the heyday of quantum mechanics, the great English physicist Paul Dirac had worked out that the energy of the electron in these levels should be the same. Dirac's theory which also achieved the feat of marrying Einstein's special theory of relativity to the new quantum mechanics was the spectacular culmination of a decade of revolution in physics, a revolution led by men like Heisenberg, Bohr and Born, going back all the way to Einstein and Planck at the beginning of the century. The Dirac theory promises to be the icing on the cake of quantum mechanics, and its prediction of equivalent energies for the 2S and 2P orbitals of the hydrogen atom seems solid and indisputable.

But now, Willis Lamb has found that the two levels are different in energy by a tiny amount. It's an amount tiny enough to be undetectable except by the most sophisticated techniques and experimenters, but it causes shock waves in the world of physics and cries for an explanation. The Lamb Shift would be to quantum mechanics what the perihelion of Mercury was to astrophysics. Lamb with his background in both experimental and theoretical physics is in a unique position to measure this difference. He knows enough quantum mechanics to understand Dirac's theory of the electron. He knows enough atomic spectroscopy to understand the experimental underpinnings of the two energy levels. And, thanks to his work with microwave radar during the war, he knows enough microwave spectroscopy in particular to use microwaves to delicately probe the energies of the two levels. Microwave radiation may seem intense - it can sear your food to a crisp after all - but microwaves are actually pretty low in frequency compared to ultraviolet or visible light. By wielding them the way a surgeon wields a fine scalpel, Lamb and his graduate student Robert Retherford have probed the 2S and 2P levels of the hydrogen atom without injecting enough energetic radiation to cause other spectroscopic transitions and contaminate the experimental output. The number he gets is 1000 megahertz, a number which is a fraction of the kinds of frequencies emitted in spectroscopy and which could only have been determined by an experimenter of the first rank.

The Lamb Shift causes ripples in physics because it seems to point at physics beyond the Dirac equation. It's one of those rare, precious measurements in science which seem to inaugurate an entire field of study, a tiny, elusive number that points to great truths. In fact even during the 1930s some prescient physicists, Oppenheimer and Heisenberg among them, had suspected that the two energy levels might be different. But when they tried to calculate the actual number they started getting an absurd value for it: infinity. Nobody has bettered that result, partly because there was no experimental number to compare it with, but now at last, there is a solid reference number which the theoreticians can calibrate their calculations against. It's a rudder which they can finally use to guide the ship of their collective imagination.

The participants at the Shelter Island conference take the Lamb Shift to heart. The discussions continue into the twilight hours. Suggestions are thrown around without definite follow ups. One can sense the fomenting of a movement, but the destination is unclear. It's also clear from the conference that it's going to be the young breed of physicists who's going to crack the puzzle. First comes Julian Schwinger whose hours-long talk is like a prodigious performance by a violin virtuoso. His dazzling equations leave the attendees breathless. Then comes Richard Feynman, irreverent and colloquial with a wholly new way of looking at quantum mechanics, a language of wiggles and pictures which leaves the participants befuddled. It would take some time for his way of thinking to sink in. The proceedings of the conference are now legendary, with someone asking "What the hell should I calculate next?", Isidor Rabi asking "Who ordered that?" in response to the announcement of the muon, and Oppenheimer holding the gathering mesmerized with his splendid command over language, lightning fast mind and propensity to instantly summarize all agreements and disagreements into a concise package. And yet the Lamb Shift beckons.

It takes the resources of Hans Bethe with his unmatched ability to pound calculations into workable numbers to make the first great move; it's no wonder that years later after Bethe's death, his then promising protege Freeman Dyson called him "the supreme problem solver of the twentieth century". After the conference, Bethe astounds everyone by calculating the Lamb shift from scratch. One of his strokes of insight is to realize that even a non-relativistic calculation which ignores the effects of special relativity can give a number which is pretty damn close to the experimental value: 1040 megahertz. This requires a shift of a reference frame, so to speak, since everyone seems to have assumed that a non-relativistic calculation would be too inaccurate and unrealistic. And, as part of a Bethe story that has passed into lore, he does the calculation on the train ride home to upstate New York.

By his own account, Hans Bethe did the first calculation of
the Lamb Shift on a train ride to Schenectady in New York

Bethe's calculation energizes the physics community. It breathes life into a new technique called renormalization which gets rid of the ugly infinities plaguing pre-war calculations. It propels Feynman, Schwinger and Dyson along with Japanese physicist Sin-Itiro Tomonaga to put the finishing touches on their theory of quantum electrodynamics which is presented in the rest of the series of the conferences. Quantum electrodynamics reveals a magical world of so-called virtual particles such as photons that can flit in and out of existence in an eye-blink as the electron transitions between the 2S and 2P energy levels. These particles may seem to violate the conservation of energy because of their sudden appearance and disappearance, but Heisenberg's uncertainty principle as applied to energy and time ensures that one can have virtual particles existing for a definite amount of time as long as there is a finite uncertainty in the value of their energies. That uncertainty manifests itself as a difference in energy which is precisely equivalent in terms of frequency to the Lamb Shift.

The Lamb Shift achieves a flowering of theoretical physics that has not been seen since the heyday of quantum mechanics in the 1930s. Quantum electrodynamics becomes the most accurate theory of physics. It calculates the magnetic moment of the electron correctly to sixteen decimal places; later Richard Feynman famously compared this to measuring the distance between New York and New Orleans to within the width of a human hair. It uncovers a universe that is alive with virtual particles and fields; these particles even permeate an absolute vacuum and give rise to so-called vacuum energy. It gives voice to a new generation of American physicists whose descendants are still housed in the country's leading physics departments. These men and women not only develop quantum electrodynamics, but the techniques they pioneer - Feynman diagrams, renormalization, scattering matrices - are used in the development of all of particle physics in the future, culminating first in the Standard Model and finally in the discovery of the Higgs Boson seven decades later. Feynman, Schwinger and Sin-Itiro Tomonaga deservedly win Nobel Prizes. The Lamb Shift and its implications of a vacuum energy even helps Stephen Hawking postulate the presence of energetic radiation from black holes.

But none of this would have been possible without Willis Lamb, the perfect incarnation of theorist and experimentalist who was present at the right place at the right time. Lamb received the Nobel Prize in physics in 1955, and spent the rest of his career at Oxford, Yale and Arizona (where he moved so that his wife could find a faculty position). He mentored other successful students and developed another highly productive career in laser physics; ironically, one of his papers in this field is cited even more extensively than the one on the Lamb Shift. He lived a long and productive life and died in 2008. But it's the Lamb Shift that will go down in history as the opening shot which inaugurated a golden age of physics. As Freeman Dyson who was one of the prime participants in that saga complimented Lamb on his 65th birthday, 

"Those years, when the Lamb shift was the central theme of physics, were golden years for all the physicists of my generation. You were the first to see that this tiny shift, so elusive and hard to measure, would clarify our thinking about particles and fields."

And that's all we are, really, particles and fields. Happy 103rd birthday, Willis Lamb.

"Understand what I love about America": Physicist Hans Bethe's moving letter to his teacher Arnold Sommerfeld

Hans Bethe, one of the true giants of twentieth century science, was also one of Adolf Hitler's greatest gifts to the United States. Fired from his position at the University of Tubingen in 1933 because of his Jewish ancestry, Bethe moved first to Britain and then, after an invitation from a former colleague, to Cornell University in 1935.

At Cornell he loyally stayed put for the next 70 years, establishing one of the world's great centers of physics. During this time he became one of the most prolific physicists in history, a true generalist contributing major insights to fields as diverse as nuclear physics, astrophysics and nuclear reactor analysis. He was a superb teacher, and counted both Richard Feynman and Freeman Dyson among his august students. He served as director of the theoretical physics division of the Manhattan Project at Los Alamos and then, after he midwifed the birth of one of humanity's most violent creations, spent the rest of his life tirelessly campaigning for its control and elimination. Bethe won the Nobel Prize in 1968 for figuring out one of science's greatest puzzles - the mechanism of energy generation in stars. He consulted for at least five presidents, served on many government committees and was actively working on supernovae when he died at age 99. He combined great scientific gifts with even greater human gifts; he was widely known as the conscience of the physics community.

Perhaps no other evidence exists of Bethe's indomitable leadership of science in the United States and his loyalty to this country than the letter he wrote to Arnold Sommerfeld. Sommerfeld was one of the greatest physics teachers of the century, and under his tutelage flourished such outstanding physicists as Bethe, Heisenberg, Pauli and Debye. Sommerfeld was very fond of Bethe, offering him a position in Munich after Bethe had been fired from Tubingen. He stayed in Germany during the war, trying to precariously hold on to German science under the onslaught of the Nazis so that it could be built up again after the war. One of the foremost ways in which he intended to do this was to try to bring back his former student Hans Bethe. To incentivize Bethe, Sommerfeld who by this time was close to retirement offered Bethe his own chair of physics at Munich. This was a very tempting and distinguished offer.

In response Bethe wrote what must be one of the most moving letters from one scientist to another, from a student to a teacher, and from one human being to another for that matter. It is worth copying out in its entirety because it showcases words like "loyalty", "affection" and yes, "patriotism" in their most genuine form. It also showcases American physics during one of its great ages and tells us what it was about the United States that made it so attractive to people from all around the world. The italics are mine.

20 May 1947 

Dear Sommerfeld, 

I am very gratified and very honored that you have thought of me as your successor. If everything since 1933 could be undone, I would be very happy to accept this offer. It would be lovely to return to the place where I learned physics from you, and learned to solve problems carefully. And where subsequently as your Assistent and as Privatdozent I had perhaps the most fruitful period of my life as a scientist. It would be lovely to try to continue your work and to teach the Munich students in the same sense as you have always done: With you one was certain to always hear of the latest developments in physics, and simultaneously learn mathematical exactness, which so many theoretical physicists neglect today. 

Unfortunately it is not possible to extinguish the last 14 years. . . . For us who were expelled from our positions in Germany, it is not possible to forget. 

Perhaps still more important than my negative memories of Germany, is my positive attitude toward America. It occurs to me (already since many years ago) 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. Americans (nearly all of them) are friendly, not stiff or reserved, nor have a brusque attitude as most Germans do. It is natural here to approach all other people in a friendly way. Professors and students relate in a comradely way without any artificially erected barrier. Scientific research is mostly cooperative, and one does not see competitive envy between researchers anywhere. Politically most professors and students are liberal and reflect about the world outside—that was a revelation to me, because in Germany it was customary to be reactionary (long before the Nazis) and to parrot the slogans of the German National ["Deutschnationaler"] party. In brief, I find it far more congenial to live with Americans than with my German "Volksgenossen." 

On top of that America has treated me very well. I came here under circumstances which did not permit me to be very choosy. In a very short time I had a full professorship, probably more quickly than I would have gotten it in Germany if Hitler had not come. Although a fairly recent immigrant, I was allowed to work and have a prominent position in military laboratories. Now, after the war, Cornell has built a large new nuclear physics laboratory essentially "around me." And two or three of the best American universities have made me tempting offers. 

I hardly need mention the material side, insofar as my own salary is concerned and also the equipment for the Institute. And I hope, dear Sommerfeld, that you will understand: Understand what I love in America and that I owe America much gratitude (disregarding the fact that I like it here). Understand, what shadows lie between myself and Germany. And most of all understand, that in spite of my "no" I am very grateful to you for thinking of me. 

Yours, 

Hans

Given Bethe's warm feelings for the United States and his immense contributions to its scientific community, national security and humane ambitions, it shouldn't be surprising to read the letter which President Lyndon Johnson wrote to him on occasion of his 60th birthday. The letter said:

"You are not only an outstanding scientist, you are also a devoted public servant. The nation has asked for your help many times and you have responded selflessly. You have made profound contributions in the fields of atomic energy, arms control and military technology. And you have been an important source of the immense contribution which science and the university community are making to society as a whole. Our country is deeply indebted to you."

During a time when words like "patriotism" inspire cynicism and are often being used as fig leaves for hiding ulterior motives, Bethe's letter should be required reading for those who have forgotten what this country stood for, how its ethos of openness and excellence attracted talented men and women from around the world, and how the people it welcomed with open arms became true patriots. As we simmer in an election season in which we seem to be awash in slogans rousing the emotions of the lowest common denominator, Hans Bethe's moving letter to his former teacher should hopefully remind us of the best that this country can offer.

Letter source (which saved me the trouble of typing it out): The original can be found in both Silvan Schweber's biography of Bethe titled "Nuclear Forces" as well as Schweber's superb contrasting study of Oppenheimer and Bethe, "In the Shadow of the Bomb."

Robert Oppenheimer, Hans Bethe and the value of balance and compromise

Should you be involved in political causes and activism as a scientist? This was a question that squarely confronted many of the twentieth century's scientists, and it heralded a meld of politics and science that continues to challenge and haunt us today. No other scientific development of the 20th century pushed the problem to the fore as much as the advent of atomic energy, and in some sense, no two individuals showcased the dilemmas and promises inherent in the participation of scientists in political affairs more than Robert Oppenheimer and Hans Bethe. This difference in their perception seems to have played a very significant role in the divergent paths their lives took, and it is one that is well-explored for instance in Sam Schweber’s outstanding contrasting study of Oppenheimer and Bethe, “In the Shadow of the Bomb”.

Both Oppenheimer and Bethe were precocious and were educated at the best universities in the world – Bethe at Munich and Oppenheimer at Göttingen. They met when Bethe fled the Nazis for the United States. Both of them became world-renowned for their accomplishments in research and teaching and for establishing world-class centers of physics; Oppenheimer at the University of California, Berkeley and Bethe at Cornell University. Early on Oppenheimer recognized Bethe as a truly outstanding theoretician and picked him to lead the important theoretical division of the Manhattan Project at Los Alamos. In turn Bethe enormously respected Oppenheimer's intellect, astonishingly quick mind and vast knowledge of diverse fields. After the war both Bethe and Oppenheimer served as top consultants to the government on atomic energy and defense. While Bethe spearheaded the development of physics in the country from Cornell, Oppenheimer served as director of the famed Institute for Advanced Study in Princeton, where he worked with individuals like Einstein, Dyson, Gödel and von Neumann. Both Bethe and Oppenheimer acted as wise men who others consulted for advice on important matters of science and policy. Both men remained very good friends till Oppenheimer's death in 1967; Bethe was one of three speakers at Oppenheimer’s memorial service.

On the other hand there were vast differences which partly owed their provenance to each man's personality and which were responsible for shaping their lives. Oppenheimer harbored a conflict of personality and self-doubt throughout his life. He was insecure in his Jewish identity whereas Bethe was largely indifferent to his Christian identity. While both men were prodigiously talented, Oppenheimer often searched for the center of his identity whereas Bethe was largely secure in his identity. Oppenheimer could be conceited, had a sharp tongue and made enemies, enemies who finally brought about his downfall in the government. Bethe on the other hand was one of the most balanced and strong-willed scientists of the century. He displayed remarkable equanimity and had rock solid self-confidence without a hint of arrogance. He could be calm under the most trying of circumstances and served as a sounding board on whom others could depend for sound advice. One of the reasons Oppenheimer picked Bethe rather than his volatile friend Edward Teller to lead the theoretical division of the project was because he knew that Bethe was far more likely to persevere, soothe egos and carry projects through to their end.

The differences in personality also led to each man's politics being quite different. While Bethe was avowedly liberal, his more balanced frame of mind and dedication to science kept him from actively pursuing radical political causes. Oppenheimer's soul-searching in the 30s led him to being associated with a variety of left-wing organizations on the West Coast. His brother, sister-in-law, wife, girlfriend and several students were active members of the Communist Party. While membership in the party was considered much more innocuous in the 30s than what it was later, I cannot imagine Bethe in a similar position. Such left-wing associations of course meant little of substance in Oppenheimer’s case and were common among intellectuals of that depressing decade; they mostly indicated nothing more than naive idealism, but nonetheless came to haunt Oppenheimer after the war. There is another interesting and unseemly difference between the two men’s personal relationships: Bethe was generally known to be very loyal to those he admired, whereas Oppenheimer was more political and attuned to what direction the wind was blowing in; this led to him denouncing a few of his left-wing colleagues as communists after the war.

A reading of Schweber’s book and other sources demonstrates that these great differences in personal traits were partly responsible for the path that each man's life took. With his plodding approach and enormous stamina, Bethe made contributions of astonishing breadth and depth to modern physics, won a Nobel Prize and continued to work until his death at the ripe age of 99. Oppenheimer's contributions were also outstanding but more limited. Perhaps his greatest contribution was the founding of modern theoretical physics in the United States, a school whose descendants continues to place the United States at the forefront of physics research. Scientifically his greatest feat was the discovery of black holes. Nonetheless, many people thought his contributions were not commensurate with his brilliance, partly because of his less focused approach and his being interested in several fields of study (among other things, he was as interested in French poetry and Sanskrit as he was in physics). Unlike Oppenheimer who in his friend Isidor Rabi’s words often saw the frontiers of physics as “surrounded by a fog”, Bethe was a practical man who stressed that he was not a philosopher, was self-assured in his science and always drove for hard agreement with experiment. He may not have been as quick as Oppenheimer, but he was more thorough in his approach to both physics and life.

Bethe served as a consultant to the government on important matters almost all his life because he could be more gentle and diplomatic than Oppenheimer, who with his sharp tongue and candid opinions quickly made powerful enemies. This led to him being hauled in front of a tribunal which revoked his security clearance. Wounded and depressed by this ungrateful action, Oppenheimer continued to write, teach and speak on science and society but could not influence government policy. On the other hand, Bethe continued to be more valuable as a government advisor throughout his life partly because he knew how to compromise and could be more diplomatic and modest than Oppenheimer. He could take courageous stands, but these stands often came in the form of well-argued and well-researched articles in magazines like Scientific American and the Bulletin of the Atomic Scientists. He was a key voice advising the government on the peaceful exploitation of atomic energy, and during the 80s he clashed with his old friend Edward Teller on the feasibility of Ronald Reagan’s Strategic Defense Initiative (‘Star Wars’). Even Republican administrations sought his advice.

The moral dilemmas generated by physics in the form of Oppenheimer and Bethe in the twentieth century have been picked up by other fields, most notably biology and climatology, in the twenty-first. As science grapples with more and more politically inflammatory topics – climate change, the teaching of evolution, stem cell research, recombinant DNA technology – scientists would do well to heed the lessons offered by Oppenheimer and Bethe. The question before many of them would be the following: should they take a more radical stance like Oppenheimer and try to change policy in the short-term, or should they be more plodding like Bethe and make modest but more solid contributions to policy in the long-term?

Bethe's Dictum: "Always work on problems for which you possess an unfair advantage"

Hans Bethe in his young days

Hans Bethe has long been a big hero of mine, not only because he was one of the greatest scientists of the twentieth century but also because he was one of its most conscientious. The sheer body of work he produced beggars belief, but so does his rocklike, steadfast determination on which others could rely in the most trying of times – and there was no dearth of such times during Bethe’s lifetime (1906-2005).

Bethe’s diversity of contributions to virtually every branch of physics was probably rivaled only by Enrico Fermi in the 20^th century. The seminal body of work that he produced encompasses every decade of his unusually long life, beginning with his twenties as a student of Arnold Sommerfeld in Munich and ending only a few months before his death at age 99. It ranges across almost every imaginable field of theoretical and applied physics: quantum mechanics, nuclear physics, quantum electrodynamics, astrophysics, solid state physics, nuclear weapons and nuclear reactor design, missile engineering. In addition there is the vast trove of documents featuring his key contributions to government policy over six decades. The sum total of this oeuvre is so large that it led one of Bethe's distinguished colleagues to joke that it must have been the result of a conspiracy crafted by many people who all decided to publish under the name "Hans Bethe".

What made Bethe so successful? Intelligence, certainly, but the twentieth century had no dearth of off-scale intelligent scientists, especially in physics. Coupled with very high intelligence were some other qualities that his fellow scientists noted: supreme powers of concentration and an indefatigable stamina (he could churn out hundreds of pages filled with equations sitting at one place from dawn to dusk with almost no mistakes), a facility with almost every mathematical tool and trick used in physics, and a remarkable versatility of talent that could combine mathematical rigor (which he learnt from Sommerfeld) with simplicity and physical intuition (which he learnt from a postdoctoral stint with Enrico Fermi).

To some extent many of these qualities are intrinsic and cannot be acquired, but others definitely can. Among the latter is a quality that’s best encapsulated in my favorite Bethe quote: “Always work on problems for which you possess an unfair advantage”. Since so many of modern physics’ ansatzs, rules and equations are named after Bethe, I will call this piece of advice ‘Bethe’s Dictum’.

I believe that Bethe’s Dictum was largely what allowed Bethe to achieve everything that he did, and I think it’s a profoundly useful dictum for the rest of us. How did Bethe himself apply this dictum? Here’s what I wrote in a review of Bethe’s recent biography written by his longtime friend and biographer Silvan Schweber:

It is not possible for us to mirror the extraordinary mental faculties of minds like Bethe and Einstein. But we can very much try to emulate their personal qualities which are more accessible if we persevere. In case of Bethe, one of his most important traits was an uncanny ability to sense his own strengths and limitations, to work on problems for which he "possessed an unfair advantage". Bethe knew he was not a genius like Dirac or Heisenberg. He could not sit in a chair and divine the deep secrets of the universe by pure thought. Rather, his particular strength was in applying a dazzling array of mathematical techniques and physical insight to concrete problems for which results could be compared with hard numbers from experiment. He could write down the problem and then go straight for the solution; this earned him the nickname "the battleship". 

Another important thing to learn from Bethe was that just like Fermi, he was willing to do whatever it took to get the solution. If it meant tedious calculations filling reams of paper, he would do it. If it meant borrowing mathematical tricks from another field he would do it. Of course, all this was possible because of his great intellect, formidable memory and extraordinary powers of concentration, but there is certainly much to learn from this attitude toward problem solving. The same approach helped him in other aspects of his life. He became extremely successful as a government consultant and scientific statesman partly because he knew when to compromise and when to push ahead.

The ability to pick problems for which you possess an unfair advantage, to selectively apply your strengths and minimize your weaknesses, is important in all walks of life. And yet it is easy to overlook this match between abilities and problems because too often we choose to study what’s fashionable, what’s “cool” or the "in thing", or what seems to attract the most funding rather than what our intellect and personality is best suited for. I got a minor taste of Bethe’s Dictum myself when I was in college. I was intensely interested in physics then and had almost made up my mind to major in it. And yet my father who clearly knew Bethe’s Dictum without knowing anything about Bethe wisely counseled me to seriously consider chemistry, since he thought that my abilities would be more suited to that field. In retrospect I think he was absolutely right. I am sure I would have enjoyed studying physics and might have even become a passable physicist, but I have little doubt that my tendency to think more broadly than deeply is better suited to chemistry and biology, where one cannot derive most facts from first principles and where memory and connections between various sub-fields can play a more important role than raw mathematical ability and intelligence. I suspect many fields of experimental physics are similar.

Bethe’s Dictum is especially important in a world which suffers from an extravaganza of choices for professional and interdisciplinary study. The dictum is also important when deciding whether to learn something new or maximize the use of something old; it hints at achieving a balance of these activities. And it’s especially important advice for young people who are just starting out in your career. It’s perfectly fine to try to study something which you are passionate about, but passion can only take you so far. The hard fact is that talents and interests may not always overlap, and down the road on which lies interest without talent also lies frustration. In the long term it might be far better to study something which may not be your absolute top interest but for which you possess an unfair advantage in terms of your temperament and skill set. It’s probably the most important lesson we can learn from Hans Bethe’s extraordinary, long and satisfyingly lived life. 

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Hans Bethe; his life, work and times

Hans Bethe was one of the most important and extraordinary scientists of the twentieth century. The sheer depth and breadth of his work is hard to comprehend. He made important contributions to nuclear physics, quantum electrodynamics, particle physics, solid state physics and astrophysics. He was a great teacher who founded a world-class center of physics at Cornell University. He won a Nobel Prize for explaining one of the oldest problems in science, the problem of the source of solar energy. He counted the greatest physicists of the century among close friends and colleagues. He played a key role in the development of the atomic and hydrogen bombs, served as a top consultant to government, valuably contributed to arms control and worked ceaseless till the ripe age of 99. He was famous not just for his science but for his wisdom and humanism, rock solid self confidence and equanimity of mind.

All these contributions and qualities would be almost impossible to capture in any one volume. Yet "Hans Bethe And His Physics" does this admirably. Several chapters span Bethe's personal traits, his work in science and public policy. Many chapters are written by close friends, students and colleagues. Accounts range from semi-technical descriptions of Bethe's science to fond personal reminiscences. The chapters provide a detailed picture of a great scientist and human being.

Probably the most valuable chapter is one by Chris Adami of Caltech who as a student spent a summer with Bethe and a close collaborator and friend, Gerald Brown of Stony Brook. The chapter is essentially a distillation of Adami's daily diary. His ruminations and first hand accounts provide a rare personal glimpse into Bethe's mind and life. Adami narrates several talks with Bethe that ranged from discussions of Bethe's childhood and education to the most current research in physics. The description of everyday life during that summer is endearing and provides wonderful insight into Bethe's personal side, including his fondness for chocolate ice cream, roast beef and history. The book would honestly be worth reading for this lengthy chapter alone. As Adami nostalgically notes, on the last day of that summer, Bethe who hated sentimental goodbyes shook Adami's hand and simply said, "Carry on". Adami says we should all remember and follow those simple words.

The rest of the chapters in the book focus on Bethe's work in various branches of physics and arms control. The chapters also include some accounts by Bethe himself on his work in solar nucleosynthesis and his recent contributions. He continued to be remarkably productive even into his 90s and during his later years worked on supernovas and on the great mystery of solar neutrinos.

Hans Bethe's life was a kaleidoscope of twentieth century physics and he was one of the most important particpants in this journey. While a book covering every aspect of his vast contributions in detail would be too big, this book is an excellent compendium that provides essential insight into this great man's science, life and work. Highly readable and recommended.