Steven Weinberg’s Life for Physics

Abstract

This is a personal review of Steven Weinberg’s scientific autobiography “A Life in Physics”. A reflection on both, personal aspects and scientific milestones of Professor Weinberg’s role-model life is conducted to honour his lasting accomplishments as a great physicist, academic teacher, and public activist in progressing high-energy particle theory and theoretical cosmology, and in raising public support for fundamental physics.

1. Introduction

Steven Weinberg was a giant among his fellow theoretical physicists and a versatile, deep thinker who influenced fundamental physics in a sustainable way. A non-exhaustive list of important contributions by Professor Weinberg to the development of modern physics is the following: nonperturbative, symmetry-based approaches to the strong interactions (Current Algebra [1], Spectral Sum Rules [2], broken, continuous symmetries [3], effective field theory of soft pions [4,5], nonlinear representations of global symmetries [1]), radiative corrections in gauge theory and gravitation [6], physics of critical phenomena in quantum field theories [7], renormalization group [8], Electroweak Sector of the Standard Model of particle physics [9], chiral symmetry breaking in quantum Chromodynamics (QCD) and anomalies in current conservations [10], cosmological constant [11], anthropic principle [12], cosmological perturbations and inflation [13], gravitational waves [14], nonlinear generalization of quantum mechanics [15], implications of the large-N expansion in QCD [16], conformal field theory [17], Effective-Field-Theory approach to quantum gravity [18], and supersymmetric quantum field theories [19]. Steven Weinberg has written textbooks on quantum field theory [20,21,22] and on Einstein’s theory of gravitation with applications in cosmology [23,24]. These are the textbooks I grew up with and came to appreciate highly because they not only teach the facts but also convey “a point of view” (Marvin Goldberger, [25]). They are carried by a personal, physically pragmatic, yet mathematically abstract attitude in approaching puzzles like the large mass of the would-be Goldstone boson ${\eta }^{\prime }$ or the large decay rate of ${\pi }^0$ into two photons [21]. I also liked how the metric tensor of cosmology was approached via the theory of maximally symmetric spaces and subspaces. When studying his textbooks on quantum field theory and his papers on Effective Field Theories and Spectral Sum Rules as a student and young researcher, I encountered multi-faceted, well-balanced arguments. To me, they were reason, guidance, and optimism within a subject that, apparently, had some loose ends.

I have never met Steven Weinberg. Yet, he has influenced me through his books, ground-breaking research, and as a public figure, and so I feel an urge to voice my sympathy and appreciation towards this great researcher, academic teacher, and influential proponent of popular discourse. Scientifically, the adoption of the electroweak mixing angle [26] in his famous article A Model of Leptons [9] is an example of Steven Weinberg’s important insights. Not only was this idea impressively confirmed experimentally, but, as I believe, the very foundations of quantum physics will require its explanatory power in ways that operate beyond the highly successful Standard Model of particle physics, which Steven Weinberg co-invented. This concerns in particular a deep understanding of particle–wave duality for elementary fermions and radiation, which field quantization after the fact is unable to provide, see below.

A few months ago, Steven Weinberg’s scientific autobiography “A Life in Physics” [25] appeared with Cambridge University Press, monitored by Professor Louise Weinberg, Steven’s wife. In the remainder of this review, I present and scientifically/personally reflect on some of the milestones and incidents described in this honest, warm-hearted, and intellectually stimulating account. In doing so, I focus on points in the book which resonate most with my own scientific viewpoints and also with my appreciation of his life-long, loving partnership and friendship with Louise. Topics I will not touch upon in this review include his services to the JASON Defense Advisory Group of several United States governments. In what follows, all quotes are from [25].

2. Drawn to Science

Steven describes his upbringing in a family tradition of civil servants as intellectually stimulating. His father, whose own father was an immigrant from Romania, made sure that there was sufficient reading for his boy in buying used books “by the bagful”. He describes his mother as “a beautiful, chic, quick-witted city girl from Berlin”. His interests in physics were sparked by tinkering with a chemistry set, thereby encountering the rules of atoms binding into molecules. Explaining those rules is physics, and so Steven started reading popular books by “serious physicists” like George Gamow and Sir James Jeans. He also acknowledges how Robert J. Oppenheimer’s public presence in post-war America favorably influenced his decision to become a theoretical physicist: “If to be a physicist was to be a priest celebrating some arcane mystery, still, it was possible to be a worldly one”.

Institutionally, becoming a physicist thus started early for Steven by enrolling in the famous Bronx High School of Science. Some courses offered there transpired to be outdated, however, and important subjects like calculus were not even covered. This prompted ambitious pupils, including his friends Gerald Feinberg and Sheldon Glashow, to acquire this knowledge privately, at times in cooperation, sometimes competitively, in isolation. So, in Steven’s opinion, Bronx High School of Science’s most important influence was to bring together aspiring and truly interested contemporaries with whom one could rub shoulders and compete.

Starting his university studies at Cornell, Steven points out the high quality of the mathematics education there and how Cornell’s curriculum strengthened his conviction that physics needs to be learned from scratch, including the edifices of classical mechanics, electromagnetism, and thermodynamics. At Cornell, he also met his future wife, Louise. Steven honestly speaks about his classic sophomore slump due to over-work and “dislike of the earnestness of the Telluride mystique”, Telluride House being the fraternity housing him initially. There were also certain emotional twists. But then “Recoiling from my sophomore slump, it became easy in my junior year at Cornell not to waste time”. This newly acquired stance stuck for a lifetime: “I am still a compulsive worker. Unless I am with Louise, or doing something for her, I am happiest sitting at my desk calculating. A few years ago, a journalist asked me how I managed to get so much done, and I told him that the trick was not going to church and not skiing”. During his years at Cornell, a summer job at Bell Labs in New York raised appreciation of this unique institution and its famous staff, including Philip Anderson, Claude Shannon, John Bardeen, Arno Penzias, and Robert Wilson. Steven clearly understood why this scientific institution was so attractive to top researchers: “Bell Labs was supported by a regulated monopoly, AT&T, which could include research costs in the justification for its rates. It was a violation of free-market orthodoxy, […] When a federal court in 1982 decreed the breakup of the AT&T monopoly, I feared that it would be the end of Bell Labs”. Before leaving Cornell, both Steven and Louise graduated with distinction. They were also married. Being on the acing team of the national mathematics contest, Steven won a “beautiful little gold medal, my first medal, that Louise sometimes wears on a necklace”. The next rung on the career ladder was research at the Bohr Institute in Copenhagen, a visit enabled by a National Science Foundation (NSF) grant to pursue doctoral studies abroad.

3. Years of Wandering

In Copenhagen, a first scientific engagement was alpha decay, a research topic suggested by another American visitor: David Frisch. Although acknowledging its use to efficiently acquire knowledge on an unknown subject, Steven soon gave up on this activity since he felt much more drawn to the newly established field of Quantum Electrodynamics. Feynman, Schwinger, and Tomonaga recently showed how to tame divergent loop integrals by redefinitions of the free parameters of the theory (renormalization). The ensuing finite theoretical results for observables like atomic level splittings and the electron’s magnetic moment were in perfect agreement with experiment. While old-guard physicists like Wolfgang Pauli, Paul Dirac, and some of their students (Gunnar Källén) dismissed the occurrence of infinities even in intermediate steps of a perturbation calculation as unphysical the new generation (fortunately) was more pragmatic: as long as the calculational method works in producing results that match experimental data there is nothing wrong with it. The need for renormalization in perturbative calculations within a four-dimensional Quantum Gauge Theory originates from the assumption of a trivial ground state: in perturbation theory, the a priori estimate of the vacuum is empty space. Yet, renormalization compensates for this in-built feature of the perturbative approach by prescribing the physical value of the calculated observable at an external energy–momentum transfer Q (a renormalization condition fixing the size of the finite expression after infinity is subtracted) which the full theory, subject to a more general gauge symmetry and a nonperturbative calculation—implying a non-trivial vacuum resolved at Q—actually should produce intrinsically. The glory of a renormalized field theory is expressed by the fact that, as energy–momentum transfer is changed away from this regime, the theoretical value of the observable remains finite and agrees with experiment. There is profound predictive power in this if only a finite number of renormalization conditions need to be imposed, that number arising from the structure of the defining Lagrangian (renormalizability). Importantly, for four-dimensional Quantum Gauge Theory, subject to Abelian or non-Abelian gauge groups of the Lie type with or without spontaneous gauge-symmetry breaking of a certain kind, renormalizability was proven in [27,28,29].

Being in Copenhagen, Steven describes the great Niels Bohr as a person with a tendency to ironic humor expressed by statements like “Einstein should stop telling the almighty how to behave” or, when asked whether he seriously believed in a horseshoe to be a lucky charm, “No, of course I don’t believe it. But they say it works even if you don’t believe in it”. When Bohr positioned Louise next to his own seat at a dinner party in his grand house, provided by the Carlsberg foundation, Steven suspected that this was “because she was so pretty”.

After Copenhagen, Princeton University was the place chosen to continue PhD work. Steven describes Princeton as one of the rare places where the younger faculty and graduate students entertained casual relations: lunching together, sharing ideas about hot matters in physics, and students confidently speaking up in seminars. “Of course I was insufferable, but I enjoyed feeling like a grown-up”. Graduate students were expected to do research rather than coursework. His professors at Princeton were luminaries like Eugene Wigner, Arthur Wightman, and John Archibald Wheeler who all put their marks on Steven’s perception about the virtues of symmetry in quantum mechanics, an axiomatic approach to quantum field theory, and Feynman’s/Dirac’s path-integral formulation of quantum mechanical and quantum field theoretic transition amplitudes, respectively. Diving again into Quantum Electrodynamics as a research subject, he had difficulties accepting Feynman’s hand-waving arguments. Nor did he appreciate Schwinger’s and Tomonaga’s unnecessarily formalistic approaches. It was Freeman Dyson who came to Steven’s intellectual rescue, showing in his papers that Feynman’s rules were a straight consequence of the principles of electrodynamics and quantum mechanics without the use of path integrals. Talking to Dyson directly was a different matter, though: “[…] he seemed to me brusque and unhelpful”. Eventually, Sam Treiman and Marvin Goldberger, both being young faculty full of modern knowledge about the weak and the strong interactions, respectively, were the ones Steven would discuss physics with on a regular basis. Treiman agreed to supervise his PhD thesis on a topic that would examine weak decay and conversion processes by particle emissions and reabsorptions that are subject to the strong interactions. For this, the standard perturbative techniques known to work for Quantum Electrodynamics did not apply. To handle the strong-interaction physics, Steven initiated a research area whose general importance became clear only much later and today is known as Effective Field Theories. It was due to Sam Treiman and T. D. Lee that Steven was offered an instructorship at Columbia University for the spring of 1957: “I took my final oral exam, and left Princeton for New York and my girl that spring”.

To be at Columbia in early 1957 was to be at the epicenter of a fundamental physics quake on a worldwide scale. A year before, T. D. Lee and C. N. Yang had proposed that the weak interactions would not obey a symmetry known as space-inversion parity. Concretely, this meant that for one and the same particle species, judged by their direction of motion, those particles that spin left-handedly interact differently from those spinning right-handedly. Experimentally, evidence for such a disparity was generated at Columbia by Chien-Shung Wu in the beta-decay of the cobalt-60 nucleus and, at the same time, by Richard Garwin, Leon Lederman, and Marcel Weinrich in the chain of decays of pions to muons to electrons. Independently, Jerome Friedman and Valentine Telegdi found parity violation in pion decay at the University of Chicago. Although the Standard Model of particle physics very successfully describes parity violation of the weak interactions by the V–A structure of its weak currents, we have, to this day, no real theoretical explanation of this phenomenon. Possibly, it has to do with self-gravitating, ultralight, yet non-relativistic particles (axions, see below) whose high number densities and collective quantum correlations define dark matter and dark energy in our universe. So far, this idea is purely speculative, however. It is not backed up by concrete model calculations. I. I. Rabi was another great experimentalist at Columbia whom Steven admired for his accomplishments in discovering nuclear magnetic resonance and the measurement of the electron’s magnetic moment—an important test for Quantum Electrodynamics. Stimulated by such a strong environment, Steven Weinberg shifted his research interests to the theory of weak interactions, collaborating with his friend Gary Feinberg and Pasha Kabir on the subject. First, they addressed the experimental fact that a muon cannot decay into an electron and a photon. Today, this is a feature of the Standard Model, which in the mid-1950s was far from being established. Namely, as pointed out by Feinberg, the two neutrinos that are produced in such a decay, thought to be mediated by a hypothetical, charged, intermediate, and heavy particle, could in principle fuse again to produce an asymptotic photon. In the Standard Model, the fusing of two neutrinos that are of different flavors (from different generations) is impossible, explaining why there is no photon in the final state. But with the rudimentary understanding of weak interactions of the pre-Standard-Model epoch, Feinberg’s paper raised a serious problem.

Steven describes his first face-to-face encounter with the great Murray Gell-Mann at a conference on weak interactions at Gatlinburg in 1958, when he reminded Gell-Mann of a particular symmetry assumption that he tacitly made in modeling the weak interaction. Such a symmetry prohibits so-called second-class currents. “This episode began my long and rocky interaction with Murray”.

At the end of 1958, the only tenure-track assistant professorship available in theoretical physics at Columbia was offered to Gary Feinberg. As advised by Louise, Steven declined an extension of his postdoc position at Columbia and decided to move on to accept a non-academic long-term position in the Radiation Laboratory at Berkeley. This position was predicted by Marvin Golberger to turn into “a professorship on the Berkeley campus”.

The first few weeks at Berkeley, Steven found himself in bed to convalesce from a slipped disk. To distract him from pains and boredom, Louise had bought the 1939 textbook “An Introduction to Stellar Structure” by Subrahmanyan Chandrasekhar, who would go on to win the Nobel Prize in Physics for elucidating the physical processes involved in star formation and evolution. Reading this book opened a new world of astrophysics to Steven—a love at first sight—which matured into him becoming active in this new research field a few years later. I know exactly how Steven must have felt: for a nuclear/particle physicist that I was considering myself after being PhDed, it was a horizon-widening and fundamentally rewarding experience to connect to research questions in cosmology like inflation, the formation of cosmic structure, or the synthesis of the light elements in the very early universe. For Steven, similar feelings must have been triggered when he started immersing himself into Chandra’s exposition of the mathematics of gravitational stabilization of a burning fusion plasma inside a star and the ensuing energy and particle transport to the star’s surface.

The Rad Lab comprised of a decades-old tradition in experimental particle physics which culminated in the commissioning of the Bevatron in 1954, and Steven quickly caught on using the data it produced to check what is known as the Delta $I={\displaystyle \frac{1}{2}}$ rule for kaon decays and then to compute the absorption rate of muons into proton–proton–muon compounds called “mulecules”. He also met Abdus Salam at Rad Lab, who, jointly with Sheldon Glashow, would win a Nobel Prize in Physics with Steven 20 years later. They both had worked on the problem of renormalization in Quantum Electrodynamics, and they both liked each other’s work. Steven also came into contact with Geoffrey Chew, who was heading the theoretical physics group at Berkeley and who pursued the S-matrix approach to fundamental interactions. This program posits that the only things one should seriously try to target in particle scattering are the probability amplitudes, subject to a few constraints like unitarity, various symmetries, and maximum analyticity in the Lorentz-invariant (complex) variables associated with the transition. According to Chew’s approach, these are all ingredients required to pinpoint the scattering process. On the other hand, quantum electrodynamics and the weak interactions were successfully described by quantum field theory, and so Steven tended to believe that the S-matrix was a mere consequence of quantum field theory (also for the strong interactions) rather than an independent, fundamental structure. Yet, it was due to Geoffrey Chew that Steven received an invitation from Berkeley to become an assistant professor on a tenure-track position, an offer which he happily accepted.

4. Once Around the Globe

Partially funded by a Sloan Fellowship, Steven took an almost year-long leave of absence from Berkeley to work and travel around the world, starting in the fall of 1961. The first stop was Tokyo. What Steven and Louise were seeing and experiencing during their travels reminds me of the smaller tours (a few weeks each) that my wife Karin and I undertook when visiting institutions along the US east coast (New York University, University of Delaware, Stony Brook, University of Connecticut, MIT, Harvard University) and west coast (University of California at Los Angeles and Santa Barbara, Caltech) in the years 2004 and 2005 on a grant from Deutsche Forschungsgemeinschaft to report my results on Yang–Mills thermodynamics. Our host, Marc Wise at Caltech, said that we were acting like a rock band on tour. (Karin was five months pregnant with Cattleya when traveling in California).

Steven describes his respectful encounter with Sin-Itiro Tomonaga at the University of Tokyo. Tomonaga was one of the co-founders of Quantum Electrodynamics, thus had acquired a high position in administration, and would win a Nobel Prize in Physics. Steven’s own take on administrative responsibility as a reward for success in research was the following: “I realized that, whether or not I could make contributions of importance, the path of administration was a road I myself would leave untraveled”. I agree with this attitude. After all, what is a scientist’s value for a supportive society if he or she is constantly frustrated and hindered by the conflict of organizing rather than doing science, and not being able to do what he or she can do best, what, however strenuous and mind-wracking it may be, he or she derives genuine happiness from, and what earned him or her distinction in the first place? Thinking, learning, understanding, thinking, learning, …On the other hand, those scientists, who take an interest in administrative tasks and bring in their hands-on scientific experience to wisely allocate resources and to create opportunities responsibly, have my highest respect. Still, I believe that it is mutually beneficial to science and society if a limited number of mind workers are allowed to intensely immerse themselves in conducting science throughout their career. This was the source of the successes in physics and mathematics scored by German research universities during the early twentieth century and associated with great scientists like Max Planck, Max von Laue, Albert Einstein, Max Born, Felix Klein, Emmy Noether, David Hilbert, and Edmund Landau. Identifying and nurturing the talents of the next generation was an integral part of this phenomenon. Being advised well by his far-sighted wife Louise, Steven Weinberg’s enduring great success as a scientist is explained by the very fact that “the path of administration […] I myself would leave untraveled”.

After Tokyo, step visits to Hong Kong and Singapore, and a longer stay in India (Mumbai and New Delhi) followed. Because of delayed visas to Russia, a visit to Moscow could not take place, and further travel to London had to be diverted via Tel Aviv, Istanbul, and Athens. A chance encounter with Larry Wilets, a friend from Copenhagen, helped organize a visit to the Weizmann Institute of Science in Rehovot. Finally, arriving in London, Steven met a stimulating environment within the theory group at Imperial College, which was led by Abdus Salam (Steven’s co-recipient of the Nobel Prize in Physics on Electroweak Unification). Other important physicists resided at Imperial, including Tom Kibble, who developed a theory of topologically non-trivial fluctuations in cosmology, and Ray Streater, who, together with Arthur Wightman at Princeton, was developing a mathematically rigorous foundation of quantum field theory.

In London, Steven revisited Jeffrey Goldstone’s famous theorem on the emergence of one massless, spinless particle species for each of the spontaneously broken generators of a continuous, global symmetry represented by a Lie group. Discussions on this took place with Salam in London and Peter Higgs in Edinburgh, and earlier, with Goldstone himself during a summer workshop in Madison. This is why Goldstone was also included as an author on a subsequent paper. In the spring of 1962, the Weinbergs went to visit Italy, starting in Naples and continuing through Rome, Florence, Pisa, and Venice. In Rome, they made a new friend, Nicola Cabibbo, who later understood why an angle has to be introduced to rotate certain components of the first two generations of quarks (mass eigenstates) so as to render them eigenstates of the weak interactions. Gell-Mann called Cabibbo’s angle “that funny angle” to which Cabibbo remarked that it might be time for him to change his last name from “Cabibbo” to “Funny”. Steven and Louise also met Francis and Natalie Low in Rome. Of Francis, who was a professor at MIT, Steven said that “he was one of the wisest theorists anywhere” and how much he appreciated his paper with Gell-Mann on the beta function in Quantum Electrodynamics. The beta function determines how the electric charge changes with the resolution of the process it is probed at. Drawing on the beta function’s implications turned out to be crucial for pinpointing the non-Abelian gauge theory of the strong interactions, quantum chromodynamics, by a match with deep inelastic scattering of electrons by protons. These data suggested that the interactions between quarks and gluons inside a proton become weak with increasing energy–momentum transfer (Bjorken scaling). In contrast to Quantum Electrodynamics, the sign of beta is negative in quantum chromodynamics. Its computation, carried out by David Gross, David Politzer, and Frank Wilczek in 1973, was a generalization of Gell-Mann’s and Low’s work and, surprisingly, implied Bjorken scaling by what is now called “asymptotic freedom” (Nobel Prize in Physics in 2004).

Before crossing the Atlantic westwards, the last stop in Europe was CERN at Geneva since Steven was to attend the 1962 “Rochester” conference. With its new accelerator, the Proton Synchrotron, CERN could match the energies attainable at Rad Lab in Berkeley and Brookhaven National Laboratory near New York. While Steven’s talk on quasiparticles in strong-interaction physics did not resonate forcefully with the audience, he felt knighted by Oppenheimer stopping over at their table in a Geneva café to chat about some of Steven’s work he had read. During a coffee break, he also met Geoffrey Chew, who informed him that the Berkeley faculty had promoted him while he was traveling with Louise.

5. Young Professor and Electroweak Unification

When back at Berkeley, Sam Treiman and T. D. Lee were visiting for the summer. During a dinner at Steven’s and Louise’s new home, Lee wanted to know whether Steven would be willing to return to Columbia. Even though Steven declined this offer, it must have soothed some earlier emotions in association with a move to the west coast a few years earlier: “I felt a little more grown-up […]”. Giving courses on General Relativity at Berkeley and later at MIT, Steven decided to write a book on this subject: Gravitation and Cosmology. It is a wonderful book that, even 30 years after its publication in 1972, served me well in understanding the physical essence of Einstein’s field equations (the principle of equivalence), the construction of the metric for symmetric spaces, Killing vectors, the Friedmann–Lemaître–Robertson–Walker metric, redshift, spatial curvature, cosmography, and distance measures in an expanding spacetime. What goes for all of Steven Weinberg’s textbooks, this book is a well-balanced mix of mathematical abstraction and physical intuition, interspersed with quite personally and notationally distinct ways of approaching a given problem. I do recommend this book warmly to my students for an introduction to the subject.

In Berkeley, Steven started contemplating antimatter in relativistic quantum field theory. An old idea by Paul A. M. Dirac states that negative-energy solutions to the Dirac equation are, according to Pauli’s exclusion principle, neatly ordered occupants in a sea of spin-1/2 particles. The removal of one of these states through energy–momentum investment, in lifting that state to positive energy, creates a hole in the Dirac sea, thus describing pair creation of a particle and its antiparticle. Steven pointed out that the generation of antimatter does not rely on such a mechanism since integer-spin particles, such as the spin-zero ${\pi }^{\pm }$ particles, which do not obey Pauli’s exclusion principle, also come in particle–antiparticle pairs. Moreover, composite spin-1/2 particles like the neutron require a strong modification of Dirac’s theory to accommodate their anomalous magnetic moment, and it was not clear how the particle–antiparticle interpretation in Dirac’s theory extends to such neutral composites. Steven also pondered the question of what is more fundamental: the existence of massless particles of spin one and two or classical electrodynamics and gravity, which, when quantized, imply the existence of such particles. For spin-one particles, my personal answer to this question is the following: these particles and their quantum interactions emerge from SU(2) or SU(3) gauge principles in classical four-dimensional field theories solely involving mixed gauge fields (pure Yang–Mills) and defined on Euclidean spacetimes (temperature relating to the circumference of one compactified dimension). Such theories imply quantization by Planck’s Quantum of action, referring to the winding number (a topological invariant expressing that the group manifold of SU(2) coincides with the boundary manifold $S_3$ of four-dimensional Euclidean spacetime) of gauge–field configurations which constitute the thermal ground state. At sufficiently large energy–momentum transfer, the excitations of this ground state are thermalized spin-one, massless (and massive) (quasi-)particles. The concept of the thermal ground state appears to be applicable beyond thermalization. For example, the speed of propagation of gauge–field plane waves with low frequency and low wave-number is temperature independent and therefore does not refer to a particular, temperature-defining reference frame. On the other hand, for quantum gravity, Euclidean gauge–field theory may not be a good starting point to emerge classical waves (as in linearized Einstein gravity) on top of a background solution to Einstein’s equations and, at the same time, massless particles of spin two without ad hoc quantization. Rather, it could well be, as string theory proposes, that a quantum theory of gravity requires the consideration of extended objects to begin with. That Einstein’s equations turn out to be a consistency condition when quantizing such objects is an insightful result.

In 1964, the “Rochester” Conference took place at Dubna. It was there that Steven met Sidney Coleman, whom he came to admire for his deep understanding of modern physics, his brilliance in teaching, and his grounded humor: “If we make any progress in physics it is because we stand on the shoulders of dwarves”.

In the fall of 1966, Louise applied for law school and decided to enter the prestigious Harvard program. The Harvard physics department offered a distinguished position of Loeb Lecturer to Steven, supplemented by the remainder of the Sloan Fellowship, while Berkeley granted him a leave of absence. So, Steven and Louise could move to Cambridge together. Prior to this move, Steven started work on the implications of Current Algebra in association with spontaneously broken chiral symmetry for elastic pion–nucleon scattering. The incentive to do so was provided by Stephen Adler at Harvard and William Weisberger at Stanford, who demonstrated that the action of the axial current within nucleons produces results in agreement with beta-decay data. Steven’s results also agreed with the experiment and gave an alternative derivation of the Adler–Weisberger solution. Invoking, in addition, a mechanism that gives mass to the pion as a would-be Goldstone boson of spontaneous chiral symmetry breaking, the ensuing minute scattering rates among differently charged soft pions would also be in harmony with experiment. This was in contrast to the expectations of the S-matrix approach to the strong interactions. Steven’s results thus were discussed by the community and taken up by Roger Dashen in his report at the 1966 “Rochester” conference at Berkeley.

At Harvard, Steven had a revelation about the results of Current Algebra for the emission of several low-energy pions in high-energy reactions matching those obtained from calculating Feynman diagrams in a quantum field theory of pions with the proper symmetries: the sigma model. The simplest terms in expanding the amplitudes of pion scattering in the sigma model (tree-level) dominate those stemming from quantum loops because they are less suppressed due to lower powers of the exchanged energy–momentum. This was laying the foundations for the development of effective field theory a decade later. Using Current Algebra to compute the correlation function of two currents in the vacuum, Steven derived spectral sum rules which, with some further assumption, predicted that the mass of the chiral-partner meson to the $\rho$ meson, the $a_1$ meson, had to be larger by a factor $\sqrt{2}$. This is in agreement with the experiment. Developing an Effective Field Theory for pions was a spare-time activity for Steven, but eventually, following a suggestion by Julian Schwinger, led to a successful formulation (recovering previous results of the sigma model and Current Algebra) that only involved pion fields subject to chiral- and flavor-symmetry constraints.

Accepting an offer to become a professor at MIT, Steven arrived there in the fall of 1967. He describes the Center of Theoretical Physics, then directed by Victor Weisskopf, as a lively, active place in which he instantly felt comfortable. It was in his first fall at MIT when Steven, driven to explain the mass ratio of the $a_1$ and the $\rho$ mesons that his spectral sum rule was suggesting, turned to what is now known as SU(2) Yang–Mills theory, a non-Abelian gauge theory solely built from gauge fields, proposed by C. N. Yang and R. L. Mills in 1954. Pauli, who had encountered such a four-dimensional theory by zero-mode compactification of a higher-dimensional theory, was disenchanted with the Yang–Mills proposal on the grounds that this theory, perturbatively speaking, predicted three massless vector particles, only one of which (the photon) actually is seen in nature. In Yang–Mills theory, the associated gauge fields can acquire a mass by breaking the gauge symmetry. When this gauge-symmetry breaking is mediated by a gauge-invariant sector to be added to the pure Yang–Mills action, mass generation was first understood by Philip W. Anderson for the Abelian case (superconductivity) in 1963. One year later, this idea was extended by Brout and Englert, Higgs, as well as Guralnik, Hagen, and Kibble to non-Abelian gauge groups and relativistic field theory. In a theory with local rather than global continuous symmetry, the breaking thereof by non-trivial ground-state physics (modeled by a fundamental SU(2) charged scalar field in the Standard Model) also generates a Goldstone boson for each broken generator. Each Goldstone field is absorbed, however, by its respective gauge potential, such as to add a longitudinal polarization state to the two transverse polarization states.

Gauging the chiral and flavor symmetries of the strong interactions would render the $a_1$ meson massive and the $\rho$ meson massless since the flavor symmetry is unbroken. This contradicts the experiment, however. Something must have felt right about the gauge principle, even though it did not fit the main contemporary theoretical framework of dealing with the strong interactions. Then, in a leap of imagination, which likely originated with the former, intense analysis of the structure and role of weak currents, Steven realized that instead of applying the gauge principle to the strong interactions, it should serve well in explaining the occurrence of massless and massive spin-1 particles in electromagnetic and weak interactions. For some time already, massive vector particles were thought to mediate the weak interactions. Thus, if the underlying gauge group were only partially broken down to an Abelian subgroup, then the associated massless particle would be the photon $\gamma$, while three massive species would relate to the mediators of the weak interactions: $W^{\pm }$ and Z. Since the theory of the strong interactions, which would employ a different gauge group, was not yet available, Steven decided to consider only those matter particles, forming the electroweak currents, that solely were subject to electromagnetic and weak but not to the strong force. These leptons are organized into families, each family doublet composed of a charged and a neutral spin-1/2 particle (neutrino). Only the electron, belonging to the first family, is stable; charged leptons of the second and third families decay weakly, and neutrinos of all families mix among themselves due to tiny masses. The weak currents as constructed from the lepton doublets, which the $W^{\pm }$ and Z interact with, are left-handed and therefore do justice to experimentally secured parity violation in a maximum possible way. To break the original gauge symmetry U(1) × SU(2) down to U(1) a complex scalar doublet (the Higgs field which is fundamentally charged under SU(2)) was introduced, subject to a potential which enforces a non-trivial expectation value in the vacuum. Moreover, the gauge fields of U(1) and a U(1) subgroup of SU(2) are assumed to mix such that the physical, neutral fields $\gamma$ and Z are superpositions of them. The introduction of this mixing angle yields a relation between the weak and the electric charge. Due to the specification of the gauge-symmetry breaking through the fundamental Higgs field, the mixing angle is also determined by the ratio of the mass of $W^{\pm }$ to the mass of Z. If charged leptons emerge from thermal gauge fields in pure SU(2) Yang–Mills theory [30] with their energy density in the rest frame being localized within a spherical droplet thanks to thermodynamic stability, see also [31,32], then the mixing angle ${\theta }_W$ of two SU(2) factors with largely disparate Yang–Mills scales (in turn emergent due to the quantum physics of thermal ground states [33]) plays the same role as the one in Weinberg–Glashow–Salam Electroweak Theory [9,26,34]. Thermodynamically, the value of ${\theta }_W$ is determined from the zero of the mixed-theories plasma pressure. Since one of the SU(2) factors is in its deconfining phase (albeit subject to a sharp drop in temperature across the droplet boundary), the quantum oscillations of the plasma inside the droplet, whose frequency determines the droplet’s mass, also prevail in the environment. This matches the derivation of de Broglie’s wavelength by Lorentz boosting such an extended system.

Trust in the Model of Leptons [9] was rising slowly after its publication because it was not clear whether this theory was viable perturbatively in computing radiative corrections (renormalizability). Earlier, Sheldon Glashow [26] had proposed a model with the same structure of current–gauge–field interaction, but he had put in the masses of $W^{\pm }$ and Z by hand, thereby spoiling renormalizability. A little later, Abdus Salam independently constructed the Electroweak Theory with spontaneous gauge-symmetry breaking and expressed his belief that it was renormalizable. But this was not easy to see, and so work on Electroweak Unification lay dormant for a while. Today, Steven’s paper [9] is one of the most cited publications in high-energy physics. To a large part, this is because its renormalizability was shown to be linked to the gauge symmetry of the defining Lagrangian by Gerard ’t Hooft and Martin Veltman, as well as Ben W. Lee and Jean Zinn-Justin, more on this later. Steven also describes his struggles with so-called covariant gauge choices in the non-Abelian Electroweak Theory, required to exhibit Lorentz invariance but fixing the gauge incompletely by an innocent linear gauge condition. In such gauges, spurious gauge degrees of freedom arise whose effects must be canceled in the computation of physical transition amplitudes. For the time being, Steven handed this problem over to a graduate student.

During the attendance of the 1969 “Rochester” conference in Vienna, Steven was acting as a rapporteur on the subject of Current Algebra and used the opportunity to state his belief that the symmetries of the strong interactions should be used to construct Effective Field Theories which generalize the situation of quantum currents satisfying a certain algebra. Sitting in a Viennese beer cellar with Bruno Zumino and his wife Shirley, Steven and Louise were confronted with anti-Semitic remarks from an adjacent table. As is well known, Shoah’s sad, disastrous, and destructive character (most heartrendingly, the murder of more than six million humans by Germany’s nazis but also a mere extinction of Germany’s world prominence in science and art by a mindless, brutal expulsion of Jewish Europeans) was facilitated by extreme anti-Semitism all over Europe and culminated in a meticulously planned and executed mass extermination. In 1968, the memory of how common people were involved in this collective collapse of humanity was fresh in Germany and Austria. Being German, this paragraph in Steven’s book sends cold shivers of crunching disbelief, shame, and anger down my back. Steven reflects correctly when he says that “hostility to Jews in Europe may be […] overly concerned with Jewish successes in finance and the arts and sciences, successes seen as diminishing the opportunities of other Europeans” which means flaring-up envy implying frequently ignited, violent vileness that, very unfortunately, occurred in a stiffly organized, efficiently devastating form in Germany and most of the ambushed countries. Today’s Europeans are facing anti-Semitic currents again, which correlate with the rise of the extreme right in many of Europe’s parliaments. Our history calls for a confident and immediate decamouflage of their short-sighted and damaging attempts at twisting the truth and at demonizing minorities. What we do need is an inclusive, optimistic, constructive, science-, and education-based, and, foremostly, humane approach to solving the complex political and environmental problems of our times, and to opening new horizons of fertile scientific and technological exploration towards world peace, clean energy, space exploration, health, and societal wealth distribution.

At MIT, Steven became involved with political and committee activities, linked to missile defense strategies, the public reaction to the Vietnam war, the civil-rights movement, the Council of the American Physical Society, the Physics Advisory Committee of the Cambridge Electron Accelerator, and several committees of the American Academy of Arts and Sciences. It was Louise who “literally saved” Steven’s life when she advised him to refrain from any further service, e.g., in attending international meetings of various experts (in Louise’s words: “disheartened older men giving themselves something important looking to do”) “on the problems of the international order” if he “wanted to get anything done in physics”. Teaching graduate courses on General Relativity at MIT, Steven’s interests shifted more and more to cosmology and in particular to what quantum field theory implied for the value of the cosmological constant $\Lambda$, the quantity that associates with a negative pressure and was introduced by Albert Einstein to allow for a static solution of his field equations when simplified to describe homogeneous and isotropic space (Friedmann–Lemaître–Robertson–Walker metric). Naively speaking, the perturbative part of the Standard Model should contribute vacuum fluctuations to $\Lambda$’s associated energy density and pressure. The energy–momentum of these fluctuations is cut off at a scale M, which delineates the low-energy regime, where this quantum field theory is valid, from a high-energy regime where the Standard Model must be replaced by a more fundamental theory. In the absence of any high-energy completion of the Standard Model, it is not clear what the contribution of the high-energy vacuum fluctuations to $\Lambda$ is. Embarrassingly, in this way, the Standard Model produces a value which is by many orders of magnitude larger than the value extracted from cosmological observations of the Cosmic Microwave background or the apparent luminosity of objects of known intrinsic luminosity (standard candles) in dependence of their redshift. I have used the term “naive” because, to my mind, the above reasoning on how to compute $\Lambda$, necessarily subject to a fine-tuned cancelation of very large contributions, is flawed even conceptually. The Standard Model is a theory that efficiently describes the scattering and binding of well-prepared or observed asymptotic states, in principle subject to small coupling expansions above a trivial (empty) a priori estimate of the vacuum. These essentially free fluctuations of propagating matter and gauge fields give rise to the above-mentioned estimate of $\Lambda$. When the properties of a pure gauge-theory vacuum are addressed, from which these asymptotic states genuinely emerge, it turns out, however, that the entire phase structure of the Yang–Mills theory needs to be considered of which the so-called confining phase, relevant in describing the vacuum at large distance scales, can be argued to possess exactly vanishing energy density [33]. The re-introduction of the cosmologically observed value of $\Lambda$ but also that of cosmological dark-matter density could well be facilitated by pure Yang–Mills theoies, responsible for the emergence of the Standard Model’s elementary particles and their interactions, interacting with a certain would-be Goldstone field (an axion) created during the Big Bang. If such a framework indeed turns out to describe reality (a possibility subject to theoretical and observational validation or falsification in the years to come), then it would be clear why the problem of the smallness of $\Lambda$ cannot be settled within the Standard Model or any high-energy completion thereof. Steven describes his struggles in trying to reconcile the cosmological constant with particle physics during his time of teaching and research on the subject at MIT. His then-new book Gravitation and Cosmology advocates strongly the interpretation of the Cosmic Microwave background, discovered by Arno Penzias and Robert Wilson in 1964, as the late-time manifestation of an ever-cooling and expanding thermal state of the early universe. This contradicts a then-popular theory by Fred Hoyle in Cambridge (England) of an unchanging, steady-state universe in which matter is continually created to fill the voids that open up by expansion. Philosophically, the steady-state universe avoids the need to consider a beginning or an end of the universe, both of which are fertile grounds for religious narratives. However, the steady-state universe was ruled out by observation in favor of the Big Bang model (a ridiculing term coined by Hoyle himself).

In the fall of 1971, Gerard ’t Hooft, by appealing to an invention by Dirac and Feynman called the path-integral approach, showed in two long articles that Yang–Mills theories with spontaneous gauge-symmetry breaking are actually renormalizable. ’T Hooft also showed how this can be applied to the particular process of neutrino-electron scattering in Electroweak Theory. One or two years later, these results were confirmed by Ben Lee, Jean Zinn-Justin, and by the joint work of ’t Hooft and Martin Veltman, Gerard’s PhD thesis supervisor. The sanity of the Electroweak Theory was indicated nicely by electron–positron pair creation through neutrino–antineutrino annihilation, whose rate, in contrast to a model where the transition was mediated solely by $W^{\pm }$ vector bosons, does not blow up with increasing energy. This is because the inclusion of the neutral-current interaction facilitates a cancelation of strong separate increases due to $W^{\pm }$ and Z mediations. The provided theoretical backing of Electroweak Theory by its viability when including radiative corrections was a strong impetus to seek experimental confirmation of the theory’s predictions, in particular in particle reactions involving the neutral weak current. Steven therefore supported the Super Collider, which was proposed to be built in Texas. Interest in Electroweak Theory was growing among theorists, and great physicists like Freeman Dyson and Richard Feynman were publicly converting. The term “Standard Model” was proposed by Steven in the context of cosmology first, and, a few years later, was taken over for the gauge-theory-based modern theory of particle physics describing the electroweak and strong interactions. The idea behind this modest name was that in both cases, the model, despite being a highly successful, rich, and efficient framework to accommodate a wealth of data, was not to be taken as a dogma. Despite all its successes and theoretical depth, one has to keep in mind that the Standard Model of particle physics comprises 19 free parameters and a V–A structure of the weak currents, which is experimentally demanded but theoretically unexplained. In 1972, Steven was offered a professorship at Harvard University to replace Julian Schwinger, who was moving to the University of California at Los Angeles. He describes his internal struggles in accepting Harvard’s proposition since MIT had been very kind. But then Harvard remodeled its initial offer into an irrefutable one, including Schwinger’s wood-paneled office and a senior-scientist position at Harvard–Smithsonian Center for Astrophysics.

6. Harvard

Moving to Harvard in the fall of 1973, Steven, Sheldon Glashow, and Sidney Coleman were forming the senior faculty in theoretical high-energy physics, supplemented by a wealth of talented junior faculty, visitors, postdocs, and graduate students. This made for a stimulating and productive research environment. On the experimental side, it was Carlo Rubbia, having an outstanding understanding of theory and the technological aspects of detector design and particle accelerators and an “almost military commanding presence”, who was the colleague most likely to push a search for neutrino scattering by nuclei to test the Electroweak Theory. As one key process, this could prove the existence of weak neutral currents. However, a CERN group first reported the observation of a single event associated with a muon neutrino, originating from the decay of a high-energy pion, being scattered by an electron. The theoretical rate for this crucially depends on the electroweak mixing angle ${\theta }_W$. Unfortunately, the single event seen at CERN could not tightly determine the value of ${\theta }_W$. Later, in 1973, the Fermilab group, which Rubbia belonged to, did, however, constrain the ratio between charged and neutral current mediated scattering off atomic nuclei, which was in the range predicted by the Electroweak Theory when varying ${\theta }_W$ within reasonable bounds. By the end of 1973, the CERN group, in turn, measured the rates of neutrino and antineutrino scattering separately, giving, within errors, the same value of ${\theta }_W$. Also in 1973, the experimental fact that the strong interactions become weak at high energy–momentum transfers (Bjorken scaling) indicated the gauge principle obeyed by the theory of the strong interactions. Namely, David Gross, David Politzer, and Frank Wilczek showed that a theory of quarks and gluons (the gauge bosons of the strong interactions), which was based on the gauge group SU(3) and which did not invoke too many quark “flavors”, exhibits an energy–momentum dependence of its coupling strength (asymptotic freedom) that is in agreement with Bjorken scaling.

The idea of fundamental and electrically fractionally charged spin-1/2 particles of various flavors composing strongly interacting particles such as neutrons, protons, and pions had been put forward by Murray Gell-Mann and Georg Zweig in 1964. The new gauge theory of the strong interactions, quantum chromodynamics, requires quarks to possess a color charge in addition to electric charge. At short distance (or high energy–momentum transfer) quarks interact weakly, but, as distance increases, the force between them increases in such a way as to render them inseparable (confinement). While quark confinement can be shown by numerical simulations on spacetime lattices, there is, as of yet, no analytical proof for this phenomenon starting from the Lagrangian of Quantum Chromodynamics. Yet, as shown by Gerard ’t Hooft and others, quantum chromodynamics offers deep insights into the mechanism of spontaneous chiral symmetry breaking, mediated by certain gauge–field configurations that possess a winding number or topological charge (instantons). This was the symmetry-breaking assumed in Current Algebra and the effective low-energy theories that Steven was considering earlier to describe strong-interaction effects in weak processes. Renormalizability of quantum chromodynamics hinges on its gauge symmetry not being spontaneously broken, as is the case for the Electroweak Theory. So, the reason we cannot detect gluons and quarks directly must be an intrinsic one and cannot be introduced by external symmetry-breaking as induced by a Higgs field. All this explosive progress in high-energy physics was summarized in a talk given by Steven at an international conference in Aix-en-Provence in the summer of 1973.

Being associated with the Harvard–Smithsonian Center of Astrophysics, Steven started work on neutrino-mediated radiative transfer during star collapse, hoping to be able to describe the ejection of the outer layers due to a stream of neutrinos originating from proton-to-neutron conversion deep inside the star. This is a demanding calculation requiring powerful computing hardware and years of experience in writing code for the numerics. Therefore, it was not Steven but James Wilson, using the computing facilities at Livermore National Laboratory, who first demonstrated the effects of neutrino release on a star’s morphology.

I like how Steven expresses his sympathetic and supportive feelings about women physicists who, in increasing numbers, entered scientific careers, witnessed by him at Harvard. As a measure of today’s progress in this respect, he mentions how Louise was treated at the Palomar observatory in 1973 when, during a visit with her husband, she was not allowed in the observer’s cage according to a general rule about women. Much has irreversibly changed since then to the benefit of science as a human (and not only a male) enterprise.

In late 1974, a new particle, named $J/\psi$, was discovered by Sam Ting and Burt Richter’s groups at Brookhaven National Laboratory and Stanford Linear Accelerator, respectively. Even though $J/\psi$ weights more than three times as much as a proton, it is, by naive expectations of strong-interaction physics, remarkably stable. The resolution of the $J/\psi$ puzzle, put forward by David Politzer and Tom Appelquist, was a new, heavy quark flavor, called “charm”, which, in the quark–antiquark bound state that the $J/\psi$ represents, implies the strong interactions to become sufficiently weak for the $J/\psi$ decay to be dominated by the weak interactions, hence explaining the long lifetime. In extending the Electroweak Theory to quarks, it was shown by Glashow, Luciano Maiani, and John Iliopoulos, and later by Steven, that the charm quark was needed to explain the slow decay of the neutral K meson into a muon–antimuon pair. This decay would have been fast with only three light quark flavors. The four quarks “up”, “down”, “charm”, and “strange” form two doublets of which the lower components (“down” and “strange”) mix (Cabibbo angle). A third doublet of even heavier quarks, composed of the “bottom” quark and the “top” quark, was discovered at Fermi Lab by the group led by Leon M. Lederman in 1977 and by the CDF and D0 experiments in 1995. Taken together, the lower components of these three doublets mix through a unitary 3 × 3 (Kobayashi–Maskawa) matrix which generalizes Cabibbo’s proposal for quark mixing within the first two doublets and, by a complex phase, introduces a violation of space and charge inversion symmetry. The number of quark doublets matches the number of lepton doublets: there are three generations of fundamental fermions. This match can be understood as a consistency condition to not violate the gauge symmetry of the Standard Model theory by radiative effects, but it is not clear why there are three generations, what sets the values of the quark and charged lepton masses and those of the matrix elements of the Kobayashi–Maskawa matrix, that of ${\theta }_W$, and those of the couplings of the Standard Model at given momentum transfer. Moreover, there are undetermined parameters in the self-coupling of the Higgs field, and we have no theoretical means, which could be experimentally confirmed, to decide how the (feeble) values of the neutrino masses originate. The existence of neutrino masses is assured experimentally by neutrino mixing. Having said this, it is an undisputed fact that the Standard Model is a theory which encompasses all known phenomena and processes of particle physics up to the energy frontier of present-day colliders of the order of 10 Tera electron volts, and this is a remarkable success to which Steven contributed in an essential way. The approximate chiral symmetry of quantum chromodynamics is actually larger than the observed spectrum of light pseudoscalar mesons suggests. In 1976, Gerard ’t Hooft showed that the extra bit of symmetry is violated by quantum effects that are mediated by winding gauge–field configurations—instantons. Therefore, the associated would-be Goldstone bosons become massive. The value of the mass was estimated by Edward Witten and Gabriele Veneziano in 1979 in terms of a correlation function which measures the average effect of gauge–field winding within an ensemble of instantons representing the vacuum physics of quantum chromodynamics. Also, the mass of stable, strongly interacting particles like the proton, which are composed of light quarks (“up” and “down”), is almost entirely due to the gauge–field sector of quantum chromodynamics.

With the Standard Model of particle physics in place and hopefully awaiting more experimental confirmation, Steven took an interest in recent work by Ken Wilson, John Kogut, and Michael Fisher, who had set up and applied an impressive theoretical edifice to address critical phenomena in statistical physics. Their theory uses mathematical techniques which are similar to those employed in computing the running of coupling in quantum field theory, and thus Steven gave a mini-course on statistical physics for particle physicists at the Erice summer school in 1976.

In addition to neutrino scattering, where it clearly described the data, the Electroweak Theory was tested by three different groups in studying the rotation of the polarization state of light interacting with bismuth vapor. This rotation is induced by the mixed parity of electronic states due to Z-boson mediated interactions of electrons with the nucleus. While an experiment conducted at Novosibirsk confirmed the predictions of the Electroweak Theory, two alternative experiments, one at Seattle and one at Oxford, could not see the expected change in polarization state. Things still had to be sorted out.

During the 1976–1977 academic year, Louise and Steven were following an invitation from Stanford Law School and the Stanford Physics Department. Steven did not feel overly welcome at Stanford, which in hindsight he attributed to an influence by the Caltech physics faculty regarding him as “that brassy, over-confident tyro”. So, in addition to pleasantly collaborating with newly arrived Helen Quinn, whom Steven knew from Harvard, he finished his popular book “The first three minutes” by working at home. Eventually, this book was translated into 23 languages, won several prizes, and brought together particle physicists and cosmologists. Disturbing news from Fermilab, where experimenters were seeing events in which a neutrino turned into three muons instead of one, incited Steven to ask Ben Lee to work with him at Stanford on an extension of Electroweak Theory. The experiments turned out to be wrong, a case to which Arthur Eddington’s remark “One should never believe any experimental result until it is confirmed by theory” applies. Ben Lee and Steven also worked on estimating the cosmological survival rate of massive neutrinos in dependence of their mass. This did not yet take into account the dominance of so-called dark matter, whose presence can be detected by the motion of galaxies in clusters, the motion of stars within a galaxy, and by theoretically describing the anisotropies of the Cosmic Microwave background. To Steven, working with Ben Lee had been extremely pleasant, and so he was shocked to learn that Ben had been killed in a car crash in the fall of 1977. In Steven’s view, Ben Lee was a role model: someone that “had tried to break through the barriers of specialization” and a person with “a remarkable sweet personality, […], so that knowing him had been a great pleasure”.

Back at Harvard, Steven learned of an important idea that Helen Quinn and Roberto Peccei had been pondering. This originated from the puzzle that an observable, which violates charge and space reversal symmetries—the electric dipole moment of the neutron—is measured to be extremely small, while in quantum chromodynamics there is no rationale for a term in the Lagrangian, giving rise to this symmetry violation, to be small. According to Peccei and Quinn, this term can be explained away dynamically if the parameter, which determines the strength of symmetry violation, is made a field—representing Goldstone bosons of a spontaneously broken chiral U(1) symmetry due to dynamics which takes place independently of quantum chromodynamics. This idea could provide a deep link between particles physics and the dark sector (dark matter and dark energy) of the universe if the scale (temperature or typical energy–momentum transfer) at which the Peccei–Quinn symmetry is broken (Peccei–Quinn scale) is close to the Planck scale $\sim {10}^{19}$ Giga electron volts. At the Planck scale, gravitation—an otherwise feeble force—becomes as strong as the other forces of the Standard Model. Frank Wilczek and Steven simultaneously published analyses assuming that the Peccei–Quinn symmetry was broken within quantum chromodynamics, coining the name axions for the associated Goldstone bosons. This proposal was ruled out by collider experiments, but a shift of the Peccei–Quinn scale to higher and higher values (ultimately, the Planck scale) guarantees that the axion interacts with ordinary matter more and more weakly and therefore could qualify as a dark particle. Depending on the spatial extent that collective quantum mechanical states (condensates) provide when axion particles interact gravitationally, they may associate with fuzzy dark matter or dark energy. Here, the term fuzzy refers to quantum mechanical interference effects which introduce fringes to the axion mass density along the edges of localized condensate lumps. The spatial extent of such an axion condensate, in turn, depends crucially on how large the axion mass is.

In the winter of 1977, Steven received the Dannie Heineman Prize for Mathematical Physics. During a visit to Washington, D.C., to testify before a congressional committee on science, technology, and space, he was also invited to the White House by the President’s science advisor, Frank Press, to meet Jimmy Carter.

A 1978 experiment at the Stanford Linear Accelerator with polarized electron beams scattered by deuterons revealed that the measured left–right asymmetry was just at the level predicted by Electroweak Theory with a mixing angle of ${\theta }_W\sim {26}^{\circ }$. This is the value already indicated by neutrino scattering. The year 1978 also saw the first invitations for Steven to receive honorary doctoral degrees, one from Knox College and one from the University of Chicago. After indicating to T.D. Lee at Columbia University that he would like to visit China, Steven promptly received an invitation from the Chinese Academy of Science. The trip to China could nicely be combined with the attendance of an international high-energy physics conference in Tokyo. So, the whole family went to Tokyo first and then on to China. In Beijing, Steven met Abdus Salam again and Édouard Brézin from the University of Paris, who is an expert on the theory of critical phenomena and had visited Harvard before. Back in the United States, Steven spoke at festivities commemorating the 100th anniversary of Einstein’s birth. Stephen Hawking and Werner Israel had asked Steven to contribute to a book they planned to publish for the Einstein centennial. In a viable quantum theory of gravitation, the renormalization program, which works in the case of the Standard Model, cannot be applied to remove infinities since the theory is based on the principle of equivalence of gravitation and inertia. Potentially, as Steven argued in his contribution to the Einstein centennial volume, this could be resolved by showing quantum gravitation to be running into an asymptotically safe theory. Namely, the theory would run into a fixed point at high energy–momentum, which is associated with a finite set of finite couplings. In 2+$ϵ$ dimensions, couplings can be expanded into power series in $ϵ$. Consequently, gravitation is shown to be asymptotically safe with a fixed point exhibiting a single, finite coupling. Trying to show that an analogy of this takes place close to the physical four dimensions of spacetime is an ongoing activity.

Towards the end of 1978, Louise received an invitation to spend the summer semester of 1979 teaching at the University of Texas Law School in Austin. It seems that Steven started to like Austin on the spot for its country music writing and performance, for its then quaint city center with its “truly beautiful” and “western in spirit” state Capitol building, and its college town character. After her summer term was over, Louise was offered a full professorship by the University of Texas Law School, “one that came to appreciate Louise’s genius”. The University of Texas Law School was and still is one of the highest-regarded jurisprudence faculties throughout the United States. As pleasant as the offer was, it introduced the problem of having to live separately, Louise in Austin, Steven in Cambridge. The decision was to make the Cambridge house the family center for Christmas and summer vacations and to meet there after each 13-week teaching stint.

Preparing a contribution to the festschrift for Julian Schwinger’s 60th birthday, Steven understood how in an effective Lagrangian for pion interaction, whose terms are separately subject to (spontaneously broken) chiral symmetry and which are ordered in ascending powers of derivatives of the pion field, an infinity arising from a loop diagram due to an interaction involving a lower order in derivatives can always be absorbed into a constant in front of an interaction term involving a higher number of derivatives. In this way, allowing loops to be mediated by interactions subject to a finite, maximum number of derivatives invokes only a finite number of constants to be fixed by experiment. After this fix, the theory has remarkable predictive power. It can also be systematically made more precise by including a higher number of maximum derivatives contributing to loop diagrams. This idea of effective field theory, which Steven pioneered, was taken up and developed for the strong interactions by various researchers, but it also can be applied to gravitation itself in positing that Einstein’s classical theory is just the leading term of an effective theory that considers quantum interactions for an energy–momentum domain beyond the Planck scale. This introduces a natural deviation from classical gravitation for high energies and momenta, and the naive non-renormalizability of the classical Einstein–Hilbert action no longer is a conceptual problem. Doing away with the constraints of renormalizability in the Standard Model by introducing higher-dimensional operators in its action, which respect its gauge invariance but may introduce violations of certain conservations laws like lepton or baryon number, Steven and, independently, Frank Wilczek and Tony Zee derived estimates of the proton (a baryon) lifetime in dependence of a scale W (perhaps $W\sim {10}^{15}$ Giga electron volts) which decides on the strength of these additional interactions. Also, there is a higher-dimensional operator that produces small neutrino masses. By terrestrial conversion into electrons, the detected solar electron neutrinos, whose initial flux density is thought to be known by calculations of their production rates in the solar core, exhibited only about a third of that flux density (modulo geometric flux–density thinning). This experimental fact can be explained by neutrino flavor oscillation, an idea proposed by Bruno Pontecorvo, which requires neutrino masses. Yet, experiments detecting neutrino oscillations cannot, as a matter of principle, fix the absolute neutrino masses but only mass differences. Based on a proposal that neutrinos are so-called center–vortex loops in confining phases of pure SU(2) Yang–Mills theories, my perception is that neutrino masses react extremely sensitively to how neutrinos are probed by their environments so that the conceptual assignment of a non-evolving mass to any given neutrino flavor could be misleading: Pontecorvo’s idea about quantum mechanical neutrino mixing due to a fixed mass matrix is technically simple, but it may have to be developed.

In October 1979, Steven learned that the Nobel Prize in Physics was awarded jointly to him, Abdus Salam, and Sheldon Glashow. This was, of course, happy news. Immediately though, Louise soberly gave Steven one of her far-sighted advice: “Now you have to write some unimportant papers”. This was to remind of Nobel laureates being prone to going for the next Big Thing and to stop doing the ordinary hard work of science. Following Louise’s advice, Steven kept on doing ordinary and hard scientific work, thus escaping “panjandrum-itis”. Guided by the physicist–historian Gerald Holton, Steven submitted to the notion of a Nobel Lecture having to be very well prepared because, in the future, it would be closely examined by historians of science. Therefore, Steven and Louise drove to a country hotel in the Berkshire Hills of Western Massachusetts to be in a quiet environment while preparing the talk. Steven describes how pleasant and welcoming the Swedish authorities were treating the Nobel laureates and their families upon arrival in Stockholm. The content of Steven’s Nobel Lecture, delivered to the Royal Swedish Academy of Science, was a summary of the Standard Model and why it must be regarded as a simple, unifying framework which, by a few global symmetry principles, the gauge principle, and renormalizability, is uniquely determined. He then went on to explain the idea of effective theories and how the Standard Model could be extended by non-renormalizable terms, which would violate baryon and lepton numbers and thus could lead to proton decay. Steven describes all the amenities and joyful events during Nobel Week, including the Prize ceremony, the Town Hall banquet, the ball at the Royal Palace, and the happy ending in the morning of “Sankta Lucia” day when the laureates and their families were woken up by a dozen blonde girls with candles in their headdresses, singing while serving coffee and saffron buns. Steven came back to Stockholm to attend Nobel Week as an invitee three more times. The 1999 Prize to Gerard ’t Hooft and Martin Veltman was one of those occasions because Steven, strongly supporting Hooft’s nomination for demonstrating the renormalizability of Electroweak Theory, was on the advisory panel and therefore invited by the Nobel Committee.

In the spring of 1980, Steven began to collaborate with Edward Witten, whom he describes as a remarkable physicist who is blessed with the “capability for incisive abstract mathematical thought” which hardly any physicist, probably not even the great Einstein and Bohr, possesses. Witten’s mathematical vision and penetrating analytical skills are generally accepted among professional mathematicians and, in particular, his profound contributions to topological quantum field theory. Sir Michael Atiyah’s Fields Medal laudatio for Witten, whom he characterizes as an outstanding bridge builder between theoretical physics and pure mathematics, contains the following quote: “First, and more conventionally, mathematicians have been spurred into learning some of the relevant physics and collaborating with colleagues in theoretical physics. Second, and more surprisingly, many of the ideas emanating from physics have led to significant new insights in purely mathematical problems, and remarkable discoveries have been made in consequence. […] In all this large and exciting field, which involves many of the leading physicists and mathematicians in the world, Edward Witten stands out clearly as the most influential and dominating figure”. Steven interacted with Edward Witten when he was a postdoc at Harvard in 1980. The recommendation letter for him by Marvin Goldberger contained a single sentence: “If you do not hire him you are crazy”. The work itself was concerned with the question of whether massless particles of spin higher than one can be obtained as composites in quantum field theories, which are formulated in terms of fields that associate with massless particles of spin one or less. The answer is no, and so the graviton, which carries spin two, needs to be regarded as a fundamental particle subject to the Einstein–Hilbert action or a generalization thereof.

After renting a house for Louise in Austin and after their daughter Elizabeth returned to college in Andover, the start of the academic year 1980–1981 saw a lonely Steven in Cambridge, and so serious doubts arose as to whether the family separation was a sustainable situation. During his visits to Austin, Louise and Steven enjoyed the warm social embeddings, the friendly, down-to-earth way of life in Texas, and the many cultural attractions of its capital city. Steven was falling more and more in love with the place and missing Louise so intensely, he began visiting Austin almost every weekend. This was temporarily ended when the physics department of the University of Texas at Austin offered a visiting semester to Steven. Being freed from frequent travel, Steven gave an undergraduate course at Austin that discussed various experiments that led to the discoveries of subatomic particles. The notes for this course were the basis for a later book first published with Scientific American. Steven and Louise, recalling a joke from a theater play, titled this book “The Discovery of Subatomic Particles”.

Back at Harvard in the fall of 1981, Steven received an invitation to a “Study Week on Cosmology and Fundamental Physics” at the Vatican, and he describes his stay as enjoyable. As it seemed, the Vatican astronomers were more comfortable with modern astrophysics than the community of particle physicists, who still had to grow used to a Big Bang marking the universe’s beginning. Shortly after the Vatican study week, Steven gave a talk at Berkeley in honor of Geoffrey Chew in which he stated that “Quantum Field Theory is riding very high […] But perhaps we will see another swing […] back into the direction of S-matrix theory”. He also said that Effective Field Theories could be seen as “the fulfillment of the S-matrix theory program […] ”. Who knows in what ways S-matrix theory will re-surface in the hot spots of theoretical physics research.

Comparing the physics departments at the University of Texas and Harvard University, Steven noticed a sense of community between faculty members at Austin in being “ambitious for each other” while there was an intense competitiveness with “an almost electric tension” at Cambridge. Both faculties had and were visited by very good physicists. At Austin, the intermingling with non-academic life came more naturally than in Cambridge. But most importantly, it was the misery of being separated from Louise which made him consider seriously giving up his very comfortably arrangements with Harvard and the many friends in Cambridge: “And none of it seemed to matter”. The chairman of the Austin physics department, Tom Griffy, and the president of the University of Texas, Peter Flawn, went out on a limb to establish a position that Steven could accept, invoking the resources of the whole community. The eventual, happy Austin arrangement was achieved by the Welsh Foundation endowing a chair for Steven, the Josey Regental Chair in Science, which came with research funds to support a theory group. In addition, Steven would be a member of the astronomy department of the University of Texas. He started his tenure in Austin in 1982 while officially still being on leave from Harvard. Steven urged the Harvard physics department to hire Frank Wilczek as his replacement. Unfortunately, Wilczek declined Harvard’s offer to go to the University of California at Santa Barbara.

7. University of Texas at Austin

Shortly after Christmas of 1983, Steven, with the help of Yuval Ne’eman, organized the first Jerusalem Winter School at the Hebrew University in Jerusalem. An urge to found this institution had arisen from wishing to support the Jewish cause in the little democracy of Israel, the only existing one in the Middle East. How valued scientific research is in Israel can be readily appreciated by the fact that this small nation’s present expenditure on civilian research and development is 5.5 % of its gross domestic product. This is far more than any other nation’s science spending. Steven finds strong words to point out Israel’s right to self-defense, also in facing the danger of a future nuclear attack by Iran. Israel should efficiently defend itself, yes. Yet, in my opinion, Israel’s and its neighbors’ future would be secured in a much more satisfactory, sustainable, and humane way if a good and well-ordered understanding among Jews and Arabs could be established, based on a peaceful co-existence and mutual appreciation of cultural heritages, religions, and historical accomplishments. In such a process, the present technological and societal advances of Israel, clearly expressed by a living democracy, prosperity, and high standards in developing science, the arts, and technology, should be an inviting and not a destructive one. To interest young physicists and physics students from neighboring Arab states was one goal that Steven had pursued in setting up the Jerusalem Winter School. Unfortunately, so far this has not happened. Let us hope that the future will be different in this respect. A wonderfully reflected Israeli attitude is expressed by D’vorah Ne’eman’s answer to Louise’s personal dislike of Orthodox Jewish outfits: “But of course those are the very ones who need safe harbor”. The Jerusalem Winter School is a lasting success and is presently directed by David J. Gross.

In the early 1980s, Steven worked on more speculative ideas such as supersymmetry and Kaluza–Klein theories. On the former part, the fact that the superpartners of the known particles described by the Standard Model were not found in accelerator experiments required a theoretical mechanism of breaking supersymmetry without destroying a salient feature which has to do with certain radiative corrections growing weakly with the energy–momentum of the process envisaged. The definite form of interactions, dictated by this highly constraining symmetry, is an interesting feature. Kaluza–Klein theories attempt to unify gravitation with other forces, subject to a given gauge group, in terms of higher-dimensional, pure gravitation subject to the Einstein–Hilbert action. For this to reduce to the Standard Model plus gravitation in four dimensions, the extra dimensions need to be compactified over the group manifold of its gauge group. In this context, one can relate some couplings of the Standard Model to the extradimensional extent of the manifold, say, to the $S_1\times S_3$ of Electroweak Theory. This is what Steven did. Extra dimensions have never been seen experimentally. Yet, Steven’s take on speculative research in theoretical physics, which happens far away from experimentally possible falsification, is quite positive: “[…] it is an essential part of the craft of theoretical physics to be willing to risk wasting time”. Yes, but only up to reasonable bounds.

While staying at the University of Oxford to give the annual Cherwell–Simon Lecture, a verbal attack on Steven and Louise being Jewish Americans was placed by a musicologist before dining at All Souls College. He greeted the couple by talking to another member of the College, “Get them out of here. Show them the garden”. Later, over dinner, he said to Louise, “You see, we don’t like you”. She responded slowly, “I am very sorry to hear that. Up to now, everybody has been so kind.”, and left the table while informing the offender that a neighbor in Cambridge, the composer Leon Kirchner, would be very interested to know. This polite and intelligent but unmistakable act of civil courage by Louise exposed the offender, who stutteringly tried to detain the couple, and there is a positive lesson to be learned from this.

In 1983, Steven, together with Edward Witten, Roman Jackiw, and Nicola Khuri, helped organize and attended a commemorative follow-up edition of the famous 1947 Shelter Island conference. This historic meeting marked the birth of Quantum Electrodynamics, triggered by Willis Lamb’s experimental result on a relative shift of two energy levels in the hydrogen atom, which, by Dirac’s equation, should have been degenerate. The 1983 Shelter Island conference was an occasion to bring the old-time heroes together with a new generation of outstanding theoreticians. The interesting phenomenon about this meeting on Shelter Island was that, in contrast to the many important experimental and theoretical contributions and ideas of 1947, no breakthrough could be reported and no challenge to the Standard Model was announced.

After arriving in Austin, Steven started recruiting theoretical physicists to the new theory group. The first hire was Willy Fischler, who made important contributions to axions and quantum gravity. Later, in 1984, Joe Polchinski was recruited. He turned into a very successful and versatile theoretician, e.g., in inventing D-branes in String Theory, the Polchinski–Strominger effective string theory, and in applying the renormalization group of Ken Wilson to certain effective theories, including the Abelian–Higgs model of superconductivity. Joe left Austin in 1992 to work at the Kavli Institute in Santa Barbara. Even though my work on nonperturbative Yang–Mills thermodynamics did not exactly fit into his circle of activities, he once invited me to spend one week at Santa Barbara in 2005, during which I came to know and appreciate him as a deep-thinking and kind contemporary. We had some contact after he became ill, and it was with great sadness that I learned of his untimely death in 2018. Over the years, the theory group at Austin was staffed further with theoreticians who all made important contributions, including Vadim Kaplunovsky, Can Kilic, Sonia Paban, Jacques Distler, and Katie Freese.

The year 1983 saw the discovery of the mediators of the weak force, the W and Z vector bosons, at CERN. This was honored by the Nobel Prize in Physics being awarded to Carlo Rubbia, leading the experimental team, and to Simon van der Meer for his work on focusing particle beams that was instrumental in this discovery. In 1985, Steven was awarded an honorary doctorate in literature from Washington College, a small liberal arts college in Maryland. In his address to the commencement session, he expressed his conviction that “no better way of teaching science to undergraduates than through its history”. Steven also describes his involvement with string theory and how he was impressed by this theory, naturally bringing in gravitation. Also, string theory does not have to be renormalized. Yet, he explains why he did not work on this theory for a long time: “[…] string theory has not led to any new predictions or explanations of quantitative details of the Standard Model […] No progress is made with it”. But he concedes that string theory will be part of the final answer. Whatever the final answer will turn out to be (a feat largely dependent on what definition of final answer us physicists can agree upon), certain deep ideas about quantum field theory and its connection to topology (e.g., BPS states), the holographic principle, and discrete symmetries that were first formulated in a stringy framework are destined to shape future directions of theoretical physics research.

In the spring of 1986, for the first time, Steven gave the summary talk at a “Rochester” Conference, which was held in Berkeley that year. He summarized the state of the field and voiced a plea for government funding of the next generation of particle accelerators. During the academic year 1986–1987 Steven gave a lecture at the University of Cambridge in honor of Paul Dirac, who had died in 1984. He spoke about nothing less than the ingredients of a final theory of physics: quantum mechanics, principles of symmetry, and the idea that, if string theory is truly fundamental then the fact that it does not require renormalization and therefore that it is not constrained by such a requirement (unlike the Standard Model) points to Effective Field Theories being similarly unconstrained. To me, such seeking of the final theory at extremely high energies—though understandable given the great successes of the Standard Model—falls short of the idea of emergence [35]. What if quantum mechanics is itself an emergent phenomenon and, as such, has implications beyond the Copenhagen Interpretation? What if the elementary matter particles of the Standard Model do not turn out to be vibration states of Planck-length strings but relate to experimentally more accessible phenomena, revealing themselves by their collective behavior at high temperature and density? What if the foundational ideas of quantum mechanics, e.g., Louis de Broglie’s proposal of the electron mass representing an electron’s intrinsic vibration need to be revisited and extended to make progress not in collider physics but in cosmology, astrophysics, high-density plasma physics, and in understanding strong electronic correlations in two-dimensional condensed-matter systems? What if parity violation of the weak interactions is a phenomenon linked to what the dark sector of the universe is made of? And what if the dark sector is a manifestation of a dynamically broken global symmetry whose quantum anomaly is invoked by the non-trivial topology of gauge fields in pure Yang–Mills theories, which also define visible matter, radiation, and their interactions?

A year later, the 300th anniversary of the publication of Newton’s great Principia led to another invitation for Steven to speak in Cambridge, England. Steven’s talk was a reductionist’s manifesto in the sense that theoretical particle physics is part of a long tradition of successes in decomposing a system into its parts and, in understanding the parts, to infer back onto the properties of the system. Starting with Galileo, through Kepler, Newton, Faraday, Maxwell, Planck, and Einstein, this process has brought us closer to uncovering the ultimate, simple, and surprisingly few laws of nature. In general, theories that are based on deeper principles clarify, explain, and embed the successful theories of the past, a prime example being Newton’s laws of motion, which, when applied to his universal law of gravitation and in using calculus, contain Kepler’s laws of planetary motion. Therefore, Newton’s set of theoretical principles plus a new calculational method explains Kepler’s laws, but they obviously describe much more, and this in a highly quantitative way. What about the quantum mechanics and statistics of point particles? Here, a deeper principle, gauge symmetry and conformal invariance of a four-dimensional action composed solely of gauge fields, all subjected to thermodynamics, could explain certain implications of quantum mechanical wave equations and, possibly, Pauli’s exclusion principle. This could also elucidate other open problems—for example, why the cosmological densities of dark energy and dark matter are so small on the scales of particle physics. According to Steven, the evolution of theoretical physics provides explanations of past principles. It generalizes those principles in unforeseen ways and thus suggests arrows of scientific explanation. I agree with him when he says that these “arrows seem to converge to a common source”. He calls this source “the final theory”. While Steven’s epoch was characterized by the search for a final theory in experimentally exploring shorter and shorter distances by processes taking place at higher and higher energies, I would expect that essential clues of the final theory will be found elsewhere in the future. We already obtain important hints from astrophysical and cosmological observations, where complementary messengers like gravitational waves, the entire electromagnetic spectrum, and neutrinos can be combined, but also from plasma and condensed-matter physics. For example, with the advent of ESA’s space-based, long-wavelength gravitational-wave interferometer LISA and by continuing the operation of the James Webb Space Telescope, a long-standing paradigm about the cosmic evolution of gravitating structure is likely to undergo essential modification: cold dark matter.

Colliders that probe the energy frontier are extremely expensive. The prime motivation for Steven, but also for many other high-energy physicists, to convince the US government to build a new proton collider (called the superconducting super collider (SSC)) was to clarify the precise mechanism for electroweak symmetry breaking. There were other motivations, like the discovery of superpartners and the production of dark-matter particles. Later, the theoretical possibility for the Large Hadron Collider (LHC) to create tiny black holes, due to extradimensional gravitational physics being accessed through the high energies invested into the collisions, has worried the public. Fortunately, there are compelling statistical arguments from cosmic-ray physics that LHC’s operation is safe. Steven describes his contributions to the campaign for the SSC, testifying before House and Senate Committees. The Reagan administration approved the project in 1987, and the Department of Energy (DOE) then began the site selection process. After a 19-month search period, the DOE announced that the SSC would be located in Texas at Waxahachie. This made a big splash in Texas, culminating in a joint session of the Texas Senate and House to celebrate. From 1989 to 1993, Steven served on the Science Policy Committee of the SSC Laboratory. In January 1989, the experimental physicist Roy Schwitters was made director of SSC. Schwitters had served in the team at Stanford Linear Accelerator, headed by Burt Richter, which discovered the $J/\psi$ particle. Later, he became the head of the CDF team at Fermi Lab, working at Harvard University. For Schwitters to leave Harvard, the University of Texas offered him a professorship when his time as the SSC director was over. During lobbying campaigns in Washington, Steven experienced some reactions from Congressmen; however, there were difficulties in funding the SSC. The fact that Texas was the site for administering the International Space Station by the Johnson Manned Spacecraft Center in Houston did not help either. Also, testimonies by influential physicists who did not work in high-energy physics were opposing the SSC on the grounds that the money could have been spent better on solving long-standing “inner” physical problems such as high-temperature superconductivity, turbulence, and important astrophysical questions. Influential physicists like Phil Anderson did not support the case made by the elementary particle-physics community that the only way to learn about fundamental physics is to collide particles at an ever-increasing energy. A similar opinion was voiced by James Krumhansl, whose testimony proved particularly harmful for the SSC because he was then the president of the American Physical Society. Another unhelpful fact was that the original cost of $ 4.4 billion was exceeded due to Congress’ slow release of funding in tranches, leading to a nominal cost increase as the dollar depreciated over time. In 1992, the House voted to delete spending for the SSC, a decision vetoed by the Senate. The process repeated itself in 1993, but this time the House voted to reject the House–Senate conference committee report created to support the favorable Senate vote. The committee agreed. This was the death of the SSC after a billion dollars had already been spent on digging the SSC tunnel and on preparations for manufacturing the superconducting magnets. Fortunately, CERN kept pushing the LHC, which discovered the Higgs particle in 2012 and kept confirming the Standard Model.

Towards the end of his memoirs Steven describes his struggle with the cosmological constant problem, and how he derived an “Anthropic Bound on the Cosmological Constant” in 1987 and again in 1997 which was in the right ballpark considering the value that was extracted in 1998 from cosmological observations of supernovae independently by the groups led by Adam Riess and Saul Perlmutter. This discovery also established today’s dominance of dark energy (the cosmological constant) over all other sources of cosmological expansion. As a consequence, we now know that the universe is not only expanding but that its expansion is accelerating. I agree with Steven when he says that “[…] quantum field theory might have a useful role to play in examining the early universe […]” and would add that some aspects of the present universe, like the CMB at large angles and the way how the presently dominating dark energy emerges and will evolve in the future, are subject to deeper understanding within the framework of quantum field theory, and in particular, of gauge–field theory.

In 1990, Steven and Louise served as Forum Fellows at the World Economic Forum at Davos, where, unfortunately, they experienced some mild form of anti-Semitism. In 1991, Steven was awarded the National Medal of Science by President George H. W. Bush. In 1992, Steven received an honorary PhD degree from the University of Padua, which was celebrating the inaugural lecture of Galileo Galilei in 1592. The university was also commemorating the 350th anniversary of Galilei’s death. In 1995 and 1996, Steven published his wonderful first two volumes of the textbook on the The Quantum Theory of Fields: Foundations and Modern Applications. A third volume on Supersymmetry was published in 2000. In the mid-1990s, the Hobby–Eberly Telescope, which is run by the University of Texas, was launched, and Steven gave a dedicatory address at its opening. The Hobby–Eberly Telescope is one of the world’s largest optical telescopes and a prototype of the James Webb Space Telescope, which arrived at Lagrange point L2 in January 2022 and started operation in July 2022.

8. Conclusions

I did enjoy reading Steven’s scientific memoirs very much. They testify to a rich, role-model life for fundamental physics, including his ground-breaking research in quantum field theory and cosmology, but also his profound teaching activities, public advocacy for high-energy physics, and his engagement in scientifically advising the US government. I am also enchanted by how Steven and Louise planned and lived their lives together, constantly supporting, enjoying, appreciating, and counseling each other.