Jacob Bekenstein 1947–2015

Jacob (Yacov) Bekenstein, a faculty member of the Racah Institute of Physics of the Hebrew University, Jerusalem, Israel, passed away on 16 August 2015 while visiting the Department of Physics in Helsinki.

Jacob made seminal contributions to the understanding of the properties of black holes and information storage in quantum field theory and general relativity. His powerful tool was Gedankenexperiments. This approach (in English, "thought experiments") has played a very important role in the developments of new ideas in physics: Galileo envisaged falling bodies, Maxwell had demons, Einstein chased light rays and studied elevators and Schrödinger mulled over the fate of cats. Jacob Bekenstein took these experiments to a whole new level.

He revolutionised the understanding of black holes after performing a Gedankenexperiment upon the suggestion of his adviser at Princeton University, John Wheeler. In the experiment, the content of a teacup was poured into a black hole. Jacob was able to transform the implied threat of the reduction of entropy in this very esoteric process (in apparent violation of the second law of thermodynamics) into the totally unexpected realisation that black holes have thermodynamical properties. It was a triumph of the human spirit. Not only did he ascribe to black holes properties such as entropy, which reflects the number of degrees of freedom, but he also brought about a revolution in the deep understanding of thermodynamics and the structure of space as seen at high-enough energies, where black holes and their like emerge and play a role.

Pioneering study

For well over a century, textbooks have been teaching us that thermodynamical properties such as entropy are extensive, namely that they are proportional to the volume of the system under study. However, for reasons of consistency, Jacob decreed that black holes are objects whose thermodynamical properties must be proportional only to the area of the system. The body of work in which these ideas were developed and argued dates from 1972. As the years pass, and even though the full implications of this deep insight are yet to be understood, appreciation of this pioneering study is only increasing.

Since the beginning of his work, Jacob pointed out the crucial role played by the properties of processing information in unlocking the secrets of black-hole and quantum gravity. His research led him to suggest bounds on the amount of information that can be stored in a given volume of space–time. This bound is called the "Bekenstein bound", and has direct implications on theories in which gravity seems to play no role. Jacob’s work, based again on Gedankenexperiments, raised a great deal of interest as well as controversy. He published it in 1981, and it has since passed several refinements and generalisations.

The very special nature of black holes uncovered by Jacob increased the realisation that information, in certain interesting circumstances at the very least, is stored on the boundary of those systems. This general feature was named "holography" – a concept borrowed from quantum optics. In particular, in one set of cases, that information is encoded in the form of a quantum-field theory where gravity is not even present. This goes under the name of AdS/CFT correspondence. Issues related to AdS/CFT correspondence are still the subject of active research in 2015, more than 40 years after Bekenstein’s seminal paper appeared.

Physics may be about uncovering the objective secrets of nature, but it is done by humans, and most humans appreciate due recognition and credit. Jacob’s ideas in the 1970s encountered both significant resistance and attempts to diminish his role. Not least from Stephen Hawking, who briefly mentions Bekenstein’s role in his bestselling book A Brief History of Time. He writes: "I was motivated partly by irritation with Bekenstein, who I felt misused my discovery." And then: "However, it turned out in the end that he [Bekenstein] was basically correct, though in a manner he had certainly not anticipated."

This reflects the enormous pressure that the 25 year-old Jacob had to withstand, essentially all alone. Jacob who, while being rather shy was very opinionated, answers Hawking’s remarks in his book Of Gravity, Black Holes and Information, where he writes: "I cannot, of course, do anything about another’s anger but…Nobody can be held to blame for giving in a published result a novel interpretation of his own." He also mentions that "I almost lost confidence then." One can only imagine the strength he needed to stick to his intuition. The main argument against him at the time was the conventional wisdom that black holes should have zero temperature and thus supposedly no entropy. So much for conventional wisdom. Out of this cocktail of emotions that Bekenstein and Hawking brewed, one following the other, breakthroughs came in the understanding of the physical properties of black holes.

A dedicated teacher

As mentioned previously, Jacob was highly opinionated. In his book, one can find critical remarks on aspects of string theory, as well as a line such as "aspiring students, beware" with regards to string theory and "beware of cheap interdisciplinary fads". I venture to say that in my opinion, some of this was self-critique, because both string theory and Jacob’s breakthroughs relied on the consistency of physical structures and seem painfully far from any near-future experimental verifications. In recent years, Jacob actually searched for some experimental evidence for quantum aspects of gravity itself. He was an excellent and dedicated teacher, and as such he also encouraged students to study advanced mathematics and improve their mastery of the English language.

At the Racah Institute of Physics, the high-energy theorists sit on the higher floor; nevertheless, Jacob seemed to have a quite guarded respect for the field. He was very interested in the results coming (and not coming) out of the LHC.

Part of Jacob’s personality was that he practised religion in a private manner. He was named after Jacob in the bible, about whom it is said that he struggled with a formidable stranger (an angel, according to some), and did not lose. Very few faced the formidable challenges of presenting a theory of gravity with as much success as Jacob Bekenstein. One is also told about the biblical Yacov that he dreamed about: a ladder grounded on Earth and reaching heaven on which there was motion upwards and downwards. Jacob always made every research effort to be grounded on the Earth, but he did not shy away from climbing the ladder and bringing back his insights.

He will be dearly missed.

Jacob (Yacov) David Bekenstein was born in Mexico City on 1 May 1947. His father, Joseph Bekenstein, worked as a carpenter, and his mother, Esther (Nee) Vladaslavotsky, was a homemaker. They were Jewish immigrants from Poland to Mexico. He went on to graduate from Princeton University, obtaining a PhD in 1972 under John Wheeler. After time as a postdoctoral fellow at the University of Texas in Austin, he moved to the Ben-Gurion University of the Negev in Beersheba, where he eventually became chairman of the astrophysics department. In 1990, he joined the faculty of the Hebrew University of Jerusalem.

His awards include the Rothschild, Israel, Wolf and Einstein prizes in Physics. He was a member of the Israeli Academy of sciences and humanities.

Jacob is survived by his wife Bilha, his daughter Rivka (a PhD student in physics at the Technion) and his sons Yehonadav (a condensed-matter physicist, now a postdoctoral fellow at UC Berkeley) and Uriya (a PhD student in biology at the Hebrew University), as well as by his six grandchildren.

• Eliezer Rabinovici.

Bernard D’Espagnat 1921–2015

Bernard D’Espagnat, physicist and philosopher, passed away on 1 August 2015.

His father was a famous post-impressionist; one of his paintings can be seen in Musée d’Orsay. Bernard D’Espagnat entered the Ecole Polytechnique in Paris, but his studies were interrupted when he was sent to Vienna during the German occupation period for forced labour. After the war, he came back to France and finished his studies, starting his PhD in the Leprince Ringuet laboratory at the Ecole Polytechnique. After a year spent in Chicago he went to CERN, which was in its infancy in 1954. Indeed, D’Espagnat was the first member of the CERN Theory Group in Geneva (at that time there was another group of theorists affiliated to CERN but based in Copenhagen).

A year later, in 1955, he convinced Jacques Prentki, whom he had met at Ecole Polytechnique, to join him at CERN. As we know, Prentki played a very important role for the organization until his death in 2009, while D’Espagnat left CERN and became professor in Orsay (France) in 1961. The early work of D’Espagnat was mostly in collaboration with Prentki, mainly searching for symmetries of hadrons and their decays. In 20 publications, they had developed their own theoretical model, and later worked with Abdus Salam (1979 Nobel prizewinner) on the "global-symmetry" theory. Their exploratory work was very useful, despite the fact that, in 1961, Gell-Mann and Ne’eman independently (and, in parallel, Speiser and Thirring-Wess) developed the "good model" – the octet model in SU3. Indeed, looking for order in the jungle of hadrons is like playing the archeologist: you dig in different directions and different places and finally someone finds the truth.

In 1961, D’Espagnat published a paper, which I consider to be a turning point in his career. Two years earlier, the American physicists Day, Snow and Sucher had proposed a mechanism in which a negatively charged particle captured by a proton could cascade down to an S quantum state. In his paper, D’Espagnat pointed out that if a proton and an antiproton in an S state annihilate into a pair of neutral K0 mesons, it will unavoidably be a K01 and a K02 meson (this was before CP violation was discovered in 1964; today, these states are called K0s and K01). D’Espagnat’s theory was confirmed in an experiment carried out at CERN in 1962.

Instant knowledge

The theory implies that, as seen in the annihilation of a proton–antiproton, if a K0s is detected, one knows instantly that the other decay product is a K01. The information seems to propagate at a velocity greater than the speed of light. John Bell later called this mechanism the "Bertlmann effect". The name comes from Reinhold Anton Bertlmann, a physicist living in Vienna who had the habit of wearing one blue sock and one red sock. If he put his socks in two envelopes and sent one to Moscow and the other to Geneva, the person who received the sock in Geneva would know instantly the colour of the sock received in Moscow. This is classically acceptable, but in quantum mechanics there is a subtle effect called entanglement, for instance seen in the decay of a π0 into two photons that show correlations in their polarizations. In fact, this is only an example of the many things that are difficult to swallow in the so-called "Copenhagen interpretation" of quantum mechanics. Even Einstein, who initiated quantum mechanics with his theory of the photoelectric effect, did not believe in it. He even said: "God does not play dice," criticizing the probabilistic aspect of the theory. However, John Bell proposed tests of quantum mechanics, which were experimentally checked by the French physicist Alain Aspect. In fact, in the ordinary life of physicists, quantum mechanics is tested every day. For instance, the energy levels of atoms are predicted with incredible accuracy by quantum mechanics.

In this complex situation, D’Espagnat tried to find a way out. His views, almost mystical, are certainly respectable. They imply a veiled reality. He has written several books in a marvellous style about this, in particular Conceptual Foundations of Quantum Mechanics. Others, Like Roland Omnès, have different points of view. D’Espagnat was rewarded by the prestigious Templeton prize in 2009, and was a member of the Académie française des Sciences Morales et Politiques. He was full of charm. We will sorely miss him

• André Martin.

Raymond Stora 1930–2015

A great figure of theoretical physics, Raymond Stora passed away on 20 July. He leaves behind an impressive body of work in the difficult domain of quantum field theory, as well as a vast empty space in the community of theoretical physicists.

Taking an equal interest in the use of quantum field theory in particle physics as in its mathematical structures, Raymond Stora significantly participated for more than half a century in important developments such as the relativistic invariance in quantum field theory or the instanton theory. In the 1970s, his work with C Becchi and A Rouet in the framework of the renormalisation of non-Abelian gauge theories – a basis for the unification of electromagnetic and weak interactions – led to the famous "BRS transformation", named after its authors, and which cannot be ignored by any student of theoretical physics.

Raymond Stora defended his thesis at MIT, then became a member of the Department of Theoretical Physics at the CEA in Saclay, from 1957 to 1970. He then joined the CNRS and became director of the Laboratory of Theoretical Physics in Marseilles. At the beginning of the 1980s, after several long-term visits to CERN, his career led him to Annecy, where he played a fundamental and vital role in the development of the group that was to become the LAPTh, Laboratory of Annecy-le-Vieux of Theoretical Physics. In the last few years, Raymond Stora was emeritus director of research at the CNRS, as well as emeritus researcher at the Theory Division of CERN.

International reach

Raymond Stora occupied several positions of responsibility at various national and international levels, in particular as president of the Theoretical Physics Commission of the CNRS and as director of the School of Physics at Les Houches. He was also awarded several national and international scientific prizes: the Joannidès Prize of the French Academy of Sciences, the Ricard Prize of the French Society of Physics, the Max-Planck Medal, and the Dannie Heineman Prize of Mathematical Physics. He was a corresponding member of the French Academy of Sciences as well as a Chevalier de la Légion d’Honneur.

A brilliant mathematical physicist and a particularly gifted particle physicist, Raymond Stora was a man of immense culture beyond the sciences. He loved arts and books, especially those of the 16th century, because they were the first printed texts to be published. But his main concern was in helping his fellow human beings to realise their full potential. He was a gentle, kind and fair man, exceptionally generous with the most precious thing: his time. With his eclectic knowledge, he spared no effort in guiding, correcting and advising people, encouraging without ever judging others, particularly young physicists. A man of conviction, he was driven by an incredible intellectual vivacity and extraordinary foresight. He was both demanding and tolerant, in his work as well as in his personal life: demanding towards himself, demanding for the truth, tolerant of different scientific ideas and tolerant of others.

Raymond was a master, a respected and beloved man. Some scientists deserve the term of humanist, and Raymond Stora was definitely one of them.

• His friends and colleagues.