Einstein’s Witches’ Sabbath

Einstein’s Witches’ Sabbath

Interview with an Author
Headshot of Franklin Lambert

Franklin Lambert. Credit: Mr. Clovis Neves

The Solvay Councils were landmark events in the modern history of physics. In 2021, we posted an interview with Jeffrey Orens, author of The Soul of Genius, which discusses early Solvay Councils. We are pleased to offer another view on the topic.

The following is an essay by Franklin Lambert, one of the two authors of Einstein’s Witches’ Sabbath and the Early Solvay Councils: The Untold Story, which was translated into English just last year. The title was inspired by Einstein’s famous declaration of having to interrupt his work “to attend a Witches’ sabbath in Brussels,” which appears in a letter to his friend Michele Besso. This book is the result of more than ten years of archival research, conducted in Brussels, Leiden, Haarlem, and Paris. Below is a summary of the book written by Professor Lambert, followed by a Q&A about his research for the book.

Cover for Einstein’s Witches Sabbath and the Early Solvay Councils

Cover for Einstein’s Witches Sabbath and the Early Solvay Councils: The Untold Story.  Top L-R: Hendrik Lorentz, Walther Nernst. Bottom: Ernest Solvay. EDP Sciences, Les Ulis (France), November 2021, ISBN: 978-2-75982669-8 Cover Design Credit: EDP Sciences

A Short Description of Einstein’s Witches’ Sabbath

The book is about the First Solvay Council on Physics: its origin, its purpose, and its main consequences. Before World War II, there were no regular international physics conferences, with the notable exception of the seven Solvay Councils. The first one took place in Brussels from October 30 to November 3, 1911. It was the result of an extraordinary chain of events. Impressed by Einstein’s specific heat paper of 1907, the Berlin chemist Walther Nernst wanted its quantum basis to be discussed by international experts. By a series of coincidences, the planned “Konzil” was convened in June 1911 by Ernest Solvay, the Belgian industrialist and scientific philanthropist. Thanks to chairman Hendrik A. Lorentz, the meeting was so successful that it prompted Solvay to found two International Institutes, one for physics in 1912 and one for chemistry in 1913.

To this day, more than fifty Councils have been organized by the International Solvay Institutes for Physics and Chemistry, with the active support of the Solvay family.

First Solvay Conference

First Solvay Congress, Brussels, 1911. The theme was the theory of radiation and the quanta. L-R seated at table: Walther Nernst; Marcel Brillouin; Ernest Solvay; Hendrik Lorentz; Emil Warburg; Jean Perrin; Wilhelm Wien; Marie Curie; Henri Poincare. L-R Standing: Robert Goldschmidt; Max Planck; Heinrich Rubens; Arnold Sommerfeld; Frederick Lindemann; Maurice de Broglie; Martin Knudsen; Friedrich Hasenohrl; G. Hostelet; Edouard Herzen; James Hopwood Jeans; Ernest Rutherford; Heike Kamerlingh-Onnes; Albert Einstein; Paul Langevin. Photographie Benjamin Couprie, Institut International de Physique Solvay, courtesy AIP Emilio Segrè Visual Archives. Catalog ID: Einstein Albert E1

Lorentz chaired five Councils, which testified the transition from classical to modern physics with the advent of the quantum era. The first Council stimulated many physicists’ interest in what would come to be known as quantum mechanics. The fifth, in 1927, supposedly solved the problem by marking, according to Heisenberg, the “completion” of quantum mechanics.

Lambert and Berends’s book Einstein’s Witches’ Sabbath (2019) is a new edition in English translation of a previous work1. It focuses on two decisive events: Solvay’s convening of the 1911 Council and his subsequent founding of the International Solvay Institute for Physics (ISIP).

Several authors have written about the first Physics Council and its pioneering role. However, singular aspects of this “improbable meeting” remained largely undiscussed. Nernst’s initiative raises important questions which had not yet been addressed:

  • Why did a physical chemist, known for his distrust of Planck’s theory, take this sudden interest in the quantum problem?
  • Why did he need Solvay to convene a meeting of twenty physicists?
  • Why was the first international conference on quanta organized in a country where nobody cared about the problem?
  • Why did Solvay sign invitation letters that had been drawn up by someone else?

The book aims at answering these questions on the basis of archival documents, some of which had never been disclosed2. It also deals with several questions regarding Solvay and his founding of ISIP:

  • Why did a man who wanted to promote his own physical theories found a Physics Institute in which he would have “nothing to see nor to say?"
    • (Quote by Solvay, appearing in a letter to Lorentz of May 20, 1912 (doc.27a, Fonds des Instituts Solvay, Service des Bibliothèques et Archives de l’Université Libre de Bruxelles).)
  • Why did he grant priority to ISIP after having told Wilhelm Ostwald that he would support his plan to create an International Chemistry Institute? And why did he fail in his attempt to found two similar Institutes at the same time, one for physics (ISIP) and one for chemistry (ISIC)?

Portrait of Ernest Solvay. Public Domain.

Portrait of Ernest Solvay. Public Domain.

The book also highlights several consequences of the Brussels Council, such as its impact on the academic careers of two major actors: Einstein and Lorentz. It also reports on the Dutch physicist Lorentz's decisive role in the creation of ISIP.

Special attention is paid to the Institute’s main activity during the period 1912-1014, including the granting of research subsidies to physicists from all over the world. Forty projects benefited from a Solvay grant before the First World War, and six beneficiaries saw their work crowned by a Nobel Prize.

Two chapters are devoted to Solvay’s relationship with Ostwald and to the role of the International Association of Chemical Societies in the founding of ISIC.

A further point of attention is the attitude of former Council members during and after the Great War. Many pages are devoted to the personal relationships among physicists who took an active part in the quantum revolution. These relationships came under great strain during and after the War, but the Solvay Councils survived thanks to Lorentz’s conciliatory attitude, the understanding of Solvay, and their unwavering faith in the universal character of science.

The book’s 310 pages and 621 notes are the result of a long-standing collaboration between Professors Lambert and Berends. The authors were inspired and encouraged in their work by historian of science John L. Heilbron, who noted that Solvay was the man who made the Quantenkongress and ISIP possible, while Lorentz was the man who made them successful.

A Belgian-Dutch partnership seemed therefore an appropriate way to collect and to analyze the relevant archival sources that are dispersed in various countries (mainly in Belgium, the Netherlands and France) as a consequence of ISIP’s modus operandi3. Working together for more than ten years, the authors visited seven archival centers situated in:

  • Brussels (Free University of Brussels ULB Fonds des Instituts Solvay)
  • Haarlem (NHA, Archief Lorentz, and Teyler Museum, Lorentz’s Collection)
  • Leiden (Rijksmuseum Boerhaave, Archief Kamerlingh Onnes)
  • Paris (École supérieure de physique et de chimie industrielles [ESPCI], Fonds Langevin; Collège de France, Fonds Brillouin; Institut de France, Archives de l’Académie des sciences)

The authors’ joint efforts enabled them to shed new light on the complex history of the early Solvay Councils.


Interview with Professor Franklin Lambert

1. How did you become a historian of science? What are your research interests (besides the subject of your book)?

As a professor of mathematical physics at Brussels’ Free University (VUB), I spent most of my research investigating nonlinear dynamics in soliton systems, with an emphasis on the structure of hierarchies of integrable nonlinear evolution equations (citations of my publications on the subject are currently reported on Research Gate).

As a former theoretical particle physicist, I was asked in 1992 to lecture on the history of modern physics, more specifically on the transition from classical to quantum physics during the period 1900-1930. At about the same time, I was invited by Ilya Prigogine to join the board of the International Solvay Institutes for Physics and Chemistry (ISIPC) as a VUB-representative. To my surprise I discovered that the International Physics Institute, founded by Solvay in 1912 (Fonds des Instituts Solvay, Service des Bibliothèques et Archives de l’Université Libre de Bruxelles), had never been subjected to an in-depth study.

Albert Einstein and Hendrik Lorentz

Albert Einstein and Hendrik Lorentz. AIP Emilio Segrè Visual Archives, Gift of W.J. Lugten. Catalog ID: Einstein Albert C31

One obvious reason was the absence in Brussels of essential ISIP documents, such as those that had been kept during 18 years in Copenhagen, before being transferred to Paris in 1929, at the request of Langevin, Lorentz’s successor at the head of ISIP’s Scientific Committee4. Other archival documents were located in Haarlem (NHA, Archief Lorentz), and in Leiden (Rijksmuseum Boerhaave, Archief Kamerlingh Onnes).

In 2003, I got involved as deputy-director of the ISIPC, in a systematic search of the Institutes’ archival documents that are scattered around the world. Three years later I got in contact with Frits Berends, a theoretical physicist from the University of Leiden, who shared my interest and granted me the benefit of his extensive documentation on Lorentz’s career (Frits and Dirk van Delft wrote a biography of Lorentz which was published in 2019 by Prometheus Amsterdam). Working in close collaboration, Frits and I started collecting available archival documents related to the convening by Solvay of the 1911 Council, and his subsequent founding of ISIP in partnership with Lorentz.

Thanks to this long-term search (we worked together for more than ten years), and to an unexpected stroke of luck5, we managed to grasp the meaning and the scope of the two singular developments that gave birth to the celebrated Solvay Councils on Physics and Chemistry: on the one hand the convening of the first international conference on quanta by a Belgian industrialist (Ernest Solvay), on the other his extraordinary decision to found a Physics Institute in which he would have “nothing to see, nor to say.”

2. Which libraries and archives did you visit during the research process? How did that material figure into your book?

I already mentioned the Lorentz Collection at the NHA in Haarlem. These documents were a welcome complement to the letters written by Lorentz during the planning of the first Council on Physics, and during the founding of ISIP. These letters are part of the Brussels Collection “Fonds des Instituts Solvay,” kept at the “Service des Bibliothèques et Archives de l’Université Libre de Bruxelles”. Thanks to the help of Didier Devriese, a local archivist, we got access to the Lorentz-Solvay correspondence, as well as to Ernest Solvay's scientific diary, which is (curiously) part of the Archives of the Belgian Chemical Society (Archives de la Société chimique de Belgique, Service des Bibliothèques et Archives de l’Université Libre de Bruxelles). The Brussels documents and the NHA-sources of Haarlem allowed us to trace the steps leading to the 1911 Council (Ch. 1 and 2 of our book), and to the founding of ISIP (Ch. 4 and 5). These documents, and some further elements collected in Paris at the ESPCI6 (Fonds Langevin), enabled us to report on ISIP’s actions during its first years of existence (1912-1914) including the financial support of the Lebedew group in Moscow and the granting of subsidies to experimental physicists from all over the world (Ch. 4 and 7).

In order to present a faithful picture of the proceedings of the first Physics Council (Sect. 2.8), we consulted other Solvay documents located in Paris. These included the “Council Minutes” (Notes taken by M. de Broglie, Archives de l’Académie des sciences, Institut de France), and the original Solvay reports (Fonds Brillouin, Archives du College de France). Together with the Knudsen documents, located at the ESPCI (Fonds Langevin), they allowed us to describe in detail (Ch. 4 and 5) the founding of ISIP, the first meetings of its Scientific Committee, and the launching of the 1913 Council.

We felt it necessary to expand (Ch. 6) on Ernest Solvay’s other concerns during the months preceding and following the first Physics Council, in particular his reaction to Wilhelm Ostwald’s request to support his plan to found an International Chemistry Institute, in partnership with the IACS7. We therefore consulted a series of documents located in Mons (the Paul Otlet Collection at the Mundameum), London (the Frederick Donnan Collection at the UCL), Berlin (the Wilhelm Ostwald at the BBAW) and Paris (the Haller Collection at the ESPCI).

Last but not least, we wished to end our book (Ch. 9) with a brief account of the four post-war Councils which took place from 1921 to 1927 under Lorentz’s presidency. We therefore needed access to some relevant documents that are part of the Jagdish Mehra Collection in Houston. This could be done through the kind intervention of my late friend Mrs. Nadine Galland, who, as a former ISIPC Secretary, went to Houston and consulted the Mehra Collection with the valuable assistance of Prof. Linda Reichl.

3. What motivated you to write this book?

It was in the light of the results of our research that Frits Berends and I decided to write a book on the origin of the ISIPC and the famous Solvay Councils. This book, initially written in French8, appeared in September 2019, under the title9: Vous avez dit sabbat de sorcières? La singulière histoire des premiers Conseils Solvay. A translation of the book in English appeared in November 2021, under the more explicit title: Einstein’s Witches’ Sabbath and the Early Solvay Councils: The Untold Story.

4. Were you particularly surprised by anything you found in the course of your research?

I mentioned a “stroke of luck”: the discovery of Ernest Solvay’s scientific diary in a totally unexpected place (an Archive of the “Société Chimique de Belgique” which happens to be located at the Service des Bibliothèques & Archives de l’Université Libre de Bruxelles). This first-hand source enabled us to refute the customary claim that Solvay would have understood in 1911 that he couldn’t compete with his eminent guests, and that he would limit his future action to supporting the work of professional researchers. Solvay’s notes, written after the Council, are proof that this was not the case. The industrialist-investigator continued his research with new ardor, putting increased pressure on his assistants, Herzen and Hostelet. He went so far as to consider presenting his “gravito-materialitic” theory to the members of the French Physical Society, united for the circumstance with the members of the French Chemical Society, under the presidency of H. Poincaré (a project he abandoned after Poincaré’s unexpected death).

Another surprising point was the discovery in Haarlem (Lorentz’s Collection at the Teyler Museum) of a letter from Brillouin10 to Lorentz of January 29, 1912. In this long letter, Brillouin answers two questions that concerned Lorentz:

  • What was the global situation in France? Were there first rank researchers outside Paris who deserved to be considered as valuable representatives of French physics?
  • What was the situation in Paris relative to the Curie-Langevin affair? Was it possible to keep Marie Curie on the Committee without the scandal affecting Solvay?

Brillouin’s letter, of which long excerpts are quoted in our book, was unknown to Marie Curie’s biographer Françoise Giroud, who discussed the Curie-Langevin affair in her well-known book Une femme honorable. It was also unknown to the Langevin-Joliot family (as recognized in a letter by Hélène Langevin, Marie Curie’s granddaughter).

Today, the letter should be regarded as a complement to Marcel Brillouin’s Notes on the Curie-Langevin affair that are part of Léon Brillouin’s Collection at the Niels Bohr Library & Archives. These Notes are extensively discussed in Jeffrey Orens' book The Soul of Genius.

Einstein's mother, Pauline Einstein, née Koch.

Einstein's mother, Pauline Einstein, née Koch. Images of her brother, Caesar Koch (Einstein’s “favorite uncle”) are scarce. Hebrew University of Jerusalem Albert Einstein Archives, courtesy AIP Emilio Segrè Visual Archives. Catalog ID: Einstein Albert G37

5. Was there anything interesting that you found in the course of your research that didn’t make it into the book?

Additionally, do you have any projects that you would like to tell us about?

Thanks to my contacts with Einstein’s “Belgian family”, and more particularly with Nathalie and Jean Ferrard, I got access to some letters of Einstein written to his “favorite uncle” Caesar Koch. These letters, and the testimony of Koch’s granddaughters, Mrs. Ferrard and Mrs. Tréfois, made me realize that uncle Caesar had a decisive impact on Einstein’s early years. Indeed, it is known that young Einstein visited his uncle and his cousin Suzanne Koch-Gottschalk on several occasions. It is also known that Einstein sent his first scientific manuscript, written before his departure from Germany, to his uncle Caesar.

Einstein’s “Belgian” family members lived first in Antwerp, later in Brussels and in Liege. Caesar Koch died in 1941 in a small Belgian town, Court-St. Etienne, after having retained his Swiss citizenship for nearly fifty years. We have good reasons to believe that Uncle Caesar influenced Einstein in his choice to continue his studies at the ETH in Zurich, and that he was at the origin of the importance attached by Einstein to the preservation of his precious Swiss passport. Several other elements, notably the letters written by Einstein, and by his wife Elsa, to their common uncle Caesar, are further proof of the latter’s decisive role during Albert’s childhood.

On the other hand, it is clear that Einstein’s attendance of four Solvay Councils (1911, 1913, 1927 and 1930) and his election in 1926 as a member of the Solvay Scientific Committee for Physics, reinforced his early contacts with Belgium.

In particular, it is known that his friendship with Queen Elisabeth (of Belgium) resulted from an encounter during a dinner at the Royal Palace during the fifth Physics Council (October 1927). It is also known that Einstein came to Brussels in 1932 to prepare the seventh Physics Council, and that he resided at the Belgian coast from April to September 1933. Some episodes of Einstein’s career were closely connected with his attendance of the 1911 Council. They are discussed in our book (Ch. 3). Other important facts linking Einstein to Belgium will be reported, if all goes well, in a future book.


1 Franklin Lambert & Frits Berends, « Vous avez dit sabbat de sorcières? La singulière histoire des premiers Conseils Solvay », EDP Sciences, Les Ulis, 2019.

2 In particular the Notes from Solvay’s Daily Register for the years 1910-1914.

3 ISIP’s Administrative Commission operated in Brussels. Its Scientific Committee was chaired by Lorentz, who resided in Haarlem. Knudsen, the Scientific Secretary, lived in Copenhagen. His archives were sent to Paris in 1930 at the request of Langevin, Lorentz’s successor.

4 After the death of Lorentz in February 1928, the Secretary of ISIP’s Scientific Committee (Martin Knudsen) decided to transfer the Institute’s archives from Copenhagen to Paris.

5 The accidental discovery of Ernest Solvay’s scientific diary, a first-hand source erroneously filed in an unrelated archive of the Belgian Chemical Society.

6 We thank Mrs. Catherine Kounelis for having provided access to these crucial Solvay documents.

7 The International Association of Chemical Societies, a forerunner of IUPAC.

8 Most of the essential archival documents are written in French.

9 Einstein’s famous declaration of having to interrupt his work “to attend a Witches’ sabbath in Brussels” appears in a letter to his friend Michele Besso.

10 Marcel Brillouin was one of the two French members of the “provisional scientific committee” chaired by Lorentz and constituted during the Council at Solvay’s request. Marie Curie was the other French member. 


About the authors:

Frits Berends is emeritus professor of theoretical physics at Leiden University. He wrote, together with Dirk van Deft, a biography of Lorentz (in Dutch) which appeared in 2019 (Prometheus, Amsterdam). Franklin Lambert is emeritus professor of mathematical physics at the Vrije Universiteit Brussel. As a former deputy director of the International Solvay Institutes, he is currently involved in the recovery of the Institutes’ archival documents. Together with F. Berends and M. Eckert, he edited in 2015 “The Early Solvay Councils and the Advent of the Quantum Era”, EPJST, vol. 224.


Article organized by Corinne Mona.

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