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19 December was the 141th anniversary of the birth of Mileva Marić Einstein. But who remembers this brilliant scientist? While her husband, Albert Einstein is celebrated as perhaps the best physicist of the century, one question about his career remains: How much did his first wife contribute to his groundbreaking science? While nobody has been able to credit her with any specific part of his work, their letters and numerous testimonies presented in the books dedicated to her(1-5) provide substantial evidence on how they collaborated from the time they met in 1896 up to their separation in 1914. They depict a couple united by a shared passion for physics, music and for each other. So here is their story.

Mileva Marić was born in Titel in Serbia in 1875. Her parents, Marija Ruzić and Miloš Marić, a wealthy and respected member of his community, had two other children: Zorka and Miloš Jr. Mileva attended high school the last year girls were admitted in Serbia. In 1892, her father obtained the authorization of the Minister of Education to allow her to attend physics lectures reserved to boys. She completed her high school in Zurich in 1894 and her family then moved to Novi Sad. Mileva’s classmates described her as brilliant but not talkative. She liked to get to the bottom of things, was perseverant and worked towards her goals.

Albert Einstein was born in Ulm in Germany in 1879 and had one sister Maja. His father, Hermann, was an industrial. His mother, Pauline Koch came from a rich family. Albert was inquisitive, bohemian and rebel. Being undisciplined, he hated the rigor of German schools so he too finished his high school in Switzerland and his family relocated to Milan.


Mileva Marić in 1896 when she entered the Polytechnic Institute in Zurich

Albert and Mileva were admitted to the physics-mathematics section of the Polytechnic Institute in Zurich (now ETH) in 1896 with three other students: Marcel Grossmann, Louis Kollros and Jakob Ehrat. Albert and Mileva became inseparable, spending countless hours studying together. He attended only a few lectures, preferring to study at home. Mileva was methodical and organized. She helped him channel his energy and guided his studies as we learn from Albert’s letters, exchanged between 1899-1903 during school holidays: 43 letters from Albert to Mileva have been preserved but only 10 of hers remain(5). These letters provide a first-hand account on how they interacted at the time.

In August 1899, Albert wrote to Mileva: « When I read Helmholtz for the first time, it seemed so odd that you were not at my side and today, this is not getting better. I find the work we do together very good, healing and also easier.” Then on 2 October 1899, he wrote from Milan: “… the climate here does not suit me at all, and while I miss work, I find myself filled with dark thoughts – in other words, I miss having you nearby to kindly keep me in check and prevent me from meandering”.

Mileva boarded in a pension for women where she met her life-long friends Helene Kaufler-Savić and Milana Bota. Both spoke of Albert’s continuous presence at Mileva’s place, where he would come freely to borrow books in Mileva’s absence. Milan Popović, Helene’s grandson, published the letters Mileva exchanged with her throughout her life(4).

 By the end of their classes in 1900, Mileva and Albert had similar grades (4.7 and 4.6, respectively) except in applied physics where she got the top mark of 5 but he, only 1. She excelled at experimental work while he did not. But at the oral exam, Professor Minkowski gave 11 out of 12 to the four male students but only 5 to Mileva. Only Albert got his degree.

Meanwhile, Albert’s family strongly opposed their relationship. His mother was adamant. “By the time you’re 30, she’ll already be an old hag!” as Albert reported to Mileva in a letter dated 27 July 1900, as well as « She cannot enter a respectable family ”. Mileva was neither Jewish, nor German. She had a limp and was too intellectual in his mother’s opinion, not to mention prejudices against foreign people. Moreover, Albert’s father insisted his son found work before getting married.

In September 1900, Albert wrote to Mileva: “I look forward to resume our new common work. You must now continue with your research – how proud I will be to have a doctor for my spouse when I’ll only be an ordinary man.“ They both came back to Zurich in October 1900 to start their thesis work. The other three students all received assistant positions at the Institute, but Albert did not. He suspected that professor Weber was blocking him. Without a job, he refused to marry her. They made ends meet by giving private lessons and “continue[d] to live and work as before.“ as Mileva wrote to her friend Helene Savić.

On 13 December 1900, they submitted a first article on capillarity signed only under Albert’s name. Nevertheless, both referred to this article in letters as their common article. Mileva wrote to Helene Savić on 20 December 1900. We will send a private copy to Boltzmann to see what he thinks and I hope he will answer us.” Likewise, Albert wrote to Mileva on 4 April 1901, saying that his friend Michele Besso “visited his uncle on my behalf, Prof. Jung, one of the most influential physicists in Italy and gave him a copy of our article.”

The decision to publish only under his name seems to have been taken jointly. Why? Radmila Milentijević, a former history professor at City College in New York, published in 2015 Mileva’s most comprehensive biography(1). She suggests that Mileva probably wanted to help Albert make a name for himself, such that he could find a job and marry her. Dord Krstić, a former physics professor at Ljubljana University, spent 50 years researching Mileva’s life. In his well-documented book(2), he suggests that given the prevalent bias against women at the time, a publication co-signed with a woman might have carried less weight.

We will never know. But nobody made it clearer than Albert Einstein himself that they collaborated on special relativity when he wrote to Mileva on 27 March 1901: “How happy and proud I will be when the two of us together will have brought our work on relative motion to a victorious conclusion.”

 Then Mileva’s destiny changed abruptly. She became pregnant after a lovers’ escapade in Lake Como. Unemployed, Albert would still not marry her. With this uncertain future, Mileva took her second and last attempt at the oral exam in July 1901. This time, Prof. Weber, whom Albert suspected of blocking his career, failed her. Forced to abandon her studies, she went back to Serbia, but came back briefly to Zurich to try to persuade Albert to marry her. She gave birth to a girl named Liserl in January 1902. No one knows what happened to her. She was probably given to adoption. No birth or death certificates were ever found.

Earlier in December 1901, their classmate Marcel Grossman’s father intervened to get Albert a post at the Patent Office in Bern. He started work in June 1902. In October, before dying, his father granted him his permission to marry. Albert and Mileva married on 6 January 1903. Albert worked 8 hours a day, 6 days a week at the Patent Office while Mileva assumed the domestic tasks. In the evenings, they worked together, sometimes late in the night. Both mentioned this to friends, he to Hans Wohlwend, she to Helene Savić on 20 March 1903 where she expressed how sorry she was to see Albert working so hard at the office. On 14 May 1904, their son Hans-Albert was born.


Mileva and Albert’s wedding picture in 1903

Despite this, 1905 is now known as Albert’s “miracle year”: he published five articles: one on the photoelectric effect (which led to the 1921 Nobel Prize), two on Brownian motion, one on special relativity and the famous E = mc2. He also commented on 21 scientific papers for a fee and submitted his thesis on the dimensions of molecules. Much later, Albert told R. S. Shankland(6) that relativity had been his life for seven years and the photoelectric effect, for five years. Peter Michelmore, one of his biographers(7), wrote that after having spent five weeks to complete the article containing the basis of special relativity, Albert “went to bed for two weeks. Mileva checked the article again and again, and then mailed it”. Exhausted, the couple made the first of three visits to Serbia where they met numerous relatives and friends, whose testimonies provide a wealth of information on how Albert and Mileva collaborated.

Mileva’s brother, Miloš Jr, a person known for his integrity, stayed on several occasions with the Einstein family while studying medicine in Paris. Krstić(2) wrote: “[Miloš] described how during the evenings and at night, when silence fell upon the town, the young married couple would sit together at the table and at the light of a kerosene lantern, they would work together on physics problems. Miloš Jr. spoke of how they calculated, wrote, read and debated.” Krstić heard this directly from relatives of Mileva, Sidonija Gajin and Sofija Galić Golubović.

Zarko Marić, a cousin of Mileva’s father, lived in the countryside property where the Einsteins stayed during their visit. He told Krstić how Mileva calculated, wrote and worked with Albert. The couple often sat in the garden to discuss physics. Harmony and mutual respect prevailed. Gajin and Zarko Marić also reported hearing from Mileva’s father that during the Einstein’s visit to Novi Sad in 1905, Mileva confided to him: “Before our departure, we finished an important scientific work which will make my husband known around the world.” Krstić got this same information in 1961 from Mileva’s cousin, Sofija Galić Golubović, who was present when Mileva said it to her father.


Mileva, Albert and their son Hans-Albert in 1905

Desanka Trbuhović-Gjurić published Mileva’s first biography in Serbian in 1969(3). It later appeared in German and French. She described how Mileva’s brother often hosted gatherings of young intellectuals at his place. During one of these evenings, Albert would have declared: “I need my wife. She solves for me all my mathematical problems”, something Mileva is said to have confirmed.

In 1908, the couple constructed with Conrad Habicht an ultra-sensitive voltmeter. Trbuhović-Gjurić attributes this experimental work to Mileva and Conrad, and wrote: “When they were both satisfied, they left to Albert the task of describing the apparatus, since he was a patent expert.” It was registered under the Einstein-Habicht patent. When Habicht questioned Mileva’s choice not to include her name, she replied making a pun in German: “Warum? Wir beide sind nur ein Stein.“ (“Why? The two of us are but one stone”, meaning, we are one entity).

The first recognition came in 1908. Albert gave unpaid lectures in Bern, then was offered his first academic position in Zurich in 1909. Mileva was still assisting him. Eight pages of Albert’s first lecture notes are in her handwriting. So is a letter drafted in 1910 in reply to Max Planck who had sought Albert’s opinion. Both documents are kept in the Albert Einstein Archives (AEA) in Jerusalem. On 3 September 1909, Mileva confided to Helene Savić: “He is now regarded as the best of the German-speaking physicists, and they give him a lot of honours. I am very happy for his success, because he fully deserves it; I only hope and wish that fame does not have a harmful effect on his humanity.” Later, she added: “With all this fame, he has little time for his wife. […] What is there to say, with notoriety, one gets the pearl, the other the shell.”


Mileva and Albert in 1910.

Their second son, Eduard, was born on 28 July 1910. Up to 1911, Albert still sent affectionate postcards to Mileva. But in 1912, he started an affair with his cousin, Elsa Löwenthal while visiting his family who had moved to Berlin. They maintained a secret correspondence over two years. Elsa kept 21 of his letters, now in the Collected Papers of Albert Einstein. During this period, Albert held various faculty positions first in Prague, back in Zurich and finally in Berlin in 1914 to be closer to Elsa.

This caused their marriage’s collapse. Mileva moved back to Zurich with her two sons on 29 July 1914. In 1919, she agreed to divorce, with a clause stating that if Albert ever received the Nobel Prize, she would get the money. When she did, she bought two small apartment buildings and lived poorly from their income. Her son, Eduard stayed frequently in a sanatorium. He later developed schizophrenia and was eventually internalised. Due to these medical expenses, Mileva struggled financially all her life and eventually lost both buildings. She survived by giving private lessons and on the alimony Albert sent, albeit irregularly.

In 1925, Albert wrote in his will that the Nobel Prize money was his sons’ inheritance. Mileva strongly objected, stating the money was hers and considered revealing her contributions to his work. Radmila Milentijević quote from a letter Albert sent her on 24 October 1925 (AEA 75-364). ”You made me laugh when you started threatening me with your recollections. Have you ever considered, even just for a second, that nobody would ever pay attention to your says if the man you talked about had not accomplished something important. When someone is completely insignificant, there is nothing else to say to this person but to remain modest and silent. This is what I advise you to do.

Mileva remained silent but her friend Milana Bota told a Serbian newspaper in 1929 that they should talk to Mileva to find out about the genesis of special relativity, since she was directly involved. On 13 June 1929, Mileva wrote to Helene Savić: ”Such publications in newspapers do not suit my nature at all, but I believe that all that was for Milana’s joy, and that she probably thought that this would also be a joy for me, as I can only suppose that she wanted to help me receive some public rights with regard to Einstein. She has written to me in that way, and I let it be accepted that way, for otherwise the whole thing would be nonsense.”


Mileva later on (unknown date)

According to Krstić(2), Mileva spoke of her contributions to her mother and sister. She also wrote to her godparents explaining how she had always collaborated with Albert and how he had ruined her life, but asked them to destroy the letter. Her son, Hans-Albert, told Krstić(2) how his parents’ “scientific collaboration continued into their marriage, and that he remembered seeing [them] work together in the evenings at the same table.” Hans-Albert’s first wife, Frieda, tried to publish the letters Mileva and Albert had sent to their sons but was blocked in court by the Einstein’s Estate Executors, Helen Dukas and Otto Nathan in an attempt to preserve the “Einstein’s myth”. They prevented other publications, including one from Krstić(2) on his early findings in 1974. Krstić mentions that Nathan even “visited” Mileva’s apartment after her death in 1948. On July 1947, Albert wrote to Dr Karl Zürcher, his divorce lawyer: “When Mileva will no longer be there, I’ll be able to die in peace.”

 Their letters and the numerous testimonies show that Mileva Marić and Albert Einstein collaborated closely from their school days up to 1914. Albert referred to it repeatedly in his letters, like when he wrote: « our work on relative motion”. Their union was based on love and mutual respect, which allowed them together to produce such uncommon work. She was the first person to recognize his talent. Without her, he would never have succeeded. She abandoned her own aspirations, happy to work with him and contribute to his success, feeling they were one unique entity. Once started, the process of signing their work under his unique name became impossible to reverse. She probably agreed to it since her own happiness depended on his success. Why did Mileva remain silent? Being reserved and self-effaced, she did not seek honors or public attention. And as is always the case in close collaborations, the individual contributions are nearly impossible to disentangle.

Pauline Gagnon

This article first appeared in Scientific American as an Opinion piece

To find out more about particle physics and dark matter, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN ».

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(1) Radmila Milentijević: Mileva Marić Einstein: Life with Albert Einstein, United World Press, 2015.

(2) Dord Krstić: Mileva & Albert Einstein: Their Love and Scientific Collaboration, Didakta, 2004.

(3) Desanka Trbuhović-Gjurić: Mileva Marić Einstein: In Albert Einstein’s shadow: in Serbian, 1969, German, 1982, and French, 1991.

(4) Milan Popović: In Albert’s Shadow, the Life and Letters of Mileva Marić, Einstein’s First Wife, The John Hopkins University Press, 2003.

(5) Renn and Schulmann, Albert Einstein / Mileva Marić, The Love Letters, Princeton University Press, 1992.

(6) Peter Michelmore, Einstein, Profile of the Man, Dodd, Mead & Company, 1962.

(7) R.S. Shankland, Conversation with Albert Einstein, Am. J. of Physics, 1962.


Le 19 décembre a marqué le 141ième anniversaire de naissance de Mileva Marić Einstein. Mais qui se souvient de cette brillante physicienne? Alors que son mari, Albert Einstein, est célébré comme étant peut-être le meilleur physicien du siècle, une ombre demeure sur sa carrière: quelles furent les contributions de sa première femme à son oeuvre scientifique? Même si personne n’a encore pu déterminer ses contributions exactes à son travail, leurs lettres et les nombreuses preuves présentées dans les livres consacrés à Mileva Marić(1-5) nous éclairent hors de tout doute sur la façon dont ils ont collaboré depuis leur rencontre en 1896 jusqu’à leur séparation en 1914. L’ensemble de ces documents dépeint le tableau d’un couple uni par une passion mutuelle pour la physique, la musique et l’un pour l’autre. Voici leur histoire.

Mileva Marić est née à Titel en Serbie en 1875. Ses parents, Marija Ruzić et Miloš Marić, un homme riche et respecté dans sa communauté, eurent deux autres enfants: Zorka et Miloš Jr. Mileva fréquenta l’école secondaire la dernière année où les filles y étaient encore admises. En 1892, son père obtint une autorisation du Ministre de l’Éducation pour qu’elle puisse assister aux cours de physique alors réservés qu’aux garçons. Elle compléta son secondaire à Zurich en 1894, date à laquelle sa famille déménagea à Novi Sad. Ses camarades de classe décrivirent Mileva comme étant brillante, mais peu bavarde. Elle aimait aller au fond de choses, était persévérante et marchait droit au but.

Albert Einstein est né à Ulm en Allemagne en 1879 et n’avait qu’une sœur, Maja. Hermann, son père, était un industriel et sa mère, Pauline Koch, était issue d’une famille riche. Albert était curieux, bohème et rebelle. Indiscipliné de nature, il détestait la rigueur des écoles allemandes et alla finir ses études secondaires en Suisse. Sa famille déménagea alors à Milan.

Mileva Marić en 1896 lorsqu’elle fut admise à l’Institut Polytechnique de Zurich

En 1896, Albert et Mileva furent admis dans la section de mathématiques et physique de l’Institut Polytechnique à Zurich (maintenant l’ETH) avec trois autres étudiants: Marcel Grossmann, Louis Kollros et Jakob Ehrat. Albert et Mileva devinrent vite inséparables, étudiant sans cesse ensemble. Il n’assista qu’à quelques cours, préférant étudier par lui-même. Mileva était méthodique et très organisée. Elle l’aidait à canaliser son énergie et guidait ses lectures comme nous le révèlent leurs lettres, échangées entre 1899 et 1903 durant les congés scolaires: 43 lettres d’Albert à Mileva ont été préservées mais seulement 10 lettres de Mileva subsistent(5). Ces lettres fournissent un témoignage direct sur la façon dont ils interagissaient à l’époque.

En août 1899, Albert écrit à Mileva : « Quand j’ai lu Helmholtz pour la première fois, il me semblait tout à fait inconcevable que tu ne sois pas à mes côtés et aujourd’hui, ça ne s’améliore pas. Je trouve le travail que nous faisons en commun très bon, curatif et aussi moins ardu.” Le 2 octobre 1899, il lui écrivit de Milan : “… le climat ici ne me convient pas du tout et, un certain travail me manquant, je me laisse aller à ruminer des idées noires – bref, je vois et sens que votre bienfaisante férule ne plane plus au-dessus de moi pour m’empêcher de divaguer “.

Mileva logeait dans une pension pour jeunes femmes où elle rencontra ses amies Helene Kaufler-Savić et Milana Bota. Toutes deux témoignèrent de la présence constante d’Albert chez Mileva, où il venait librement y emprunter des livres même en son absence. Milan Popović, le petit-fils d’Helene, a publié les lettres que Mileva écrivit à Helene tout au long de sa vie(4).

A la fin de leurs cours en 1900, Mileva et Albert avaient des résultats semblables (une moyenne de 4.7 et 4.6, respectivement) sauf en physique appliquée, où elle obtint la note maximale de 5, mais Albert, seulement 1. Elle excellait en travaux pratiques tandis qu’il n’y avait aucun talent. Cependant, lors de leur examen oral, le Professeur Minkowski accorda une note de 11 sur 12 aux quatre étudiants masculins, mais Mileva ne reçut que 5. Tous obtinrent leur diplôme sauf Mileva.

Entre temps, la famille d’Albert s’opposait fortement à leur relation. Sa mère était inflexible. « Quand tu auras 30 ans, elle sera déjà une vieille sorcière! », comme Albert le rapporta à Mileva dans une lettre datée du 27 juillet 1900, de même que “Elle ne peut pas entrer dans une famille convenable“. Mileva n’était ni juive, ni allemande. Elle boitait et était trop intellectuelle de l’avis de sa mère, sans compter les préjugés contre les étrangers. De son côté, le père d’Albert insistait pour que son fils trouve du travail avant de se marier.

En septembre 1900, Albert écrivit à Mileva : « Comme je me réjouis à l’avance de notre nouveau travail conjoint. Tu dois maintenant continuer avec ton investigation – comme je serai fier lorsque j’aurai un docteur comme compagne alors que je serai juste un homme ordinaire. » Les deux revinrent à Zurich en octobre 1900 commencer leur travail de thèse. Les trois autres étudiants se virent tous offrir des postes d’assistants à l’Institut, mais pas Albert. Il soupçonna le professeur Weber de malveillance. Pour joindre les deux bouts, ils donnèrent des leçons privées et « continuèrent à vivre et travailler comme avant », comme Mileva l’écrivit à son amie Helene Savić.

Le 13 décembre 1900, ils soumirent sous le seul nom d’Albert un premier article sur la capillarité. Néanmoins, tous deux référèrent à cet article dans leurs lettres comme leur article commun. Mileva écrivit à Helene Savić le 20 décembre 1900. « Nous enverrons une copie privée à Boltzmann pour voir ce qu’il pense et j’espère qu’il nous répondra. » De même, Albert écrivit à Mileva le 4 avril 1901, disant que son ami Michele Besso « a rendu visite à son oncle en mon nom, le Prof. Jung, un des physiciens les plus influents en Italie et lui a aussi donné une copie de notre article. »

La décision de publier sous le seul nom d’Albert semble avoir été prise en commun. Pourquoi ? Radmila Milentijević, ancienne professeure d’histoire au City College de New York, a publié en 2014 la biographie la plus complète à ce jour sur Mileva(1). Elle suggère que Mileva voulait probablement aider Albert à se faire un nom, pour qu’il puisse trouver un travail et l’épouser. Dord Krstić, ancien professeur de physique à l’Université de Ljubljana, passa près de 50 ans à enquêter sur la vie de Mileva. Dans son livre(2) fort bien documenté, il suggère qu’une publication co-signée avec une femme aurait pu en réduire l’impact étant donné les préjugés sexistes de l’époque.

Nous ne le saurons jamais. Mais personne ne peut être plus clair qu’Albert Einstein sur l’existence de leur collaboration sur la relativité spéciale lorsqu’il écrivit à Mileva le 27 mars 1901 : «Comme je serai heureux et fier quand nous aurons tous les deux ensemble mené notre travail sur le mouvement relatif à une conclusion victorieuse ! »

C’est à ce moment que le destin de Mileva bascula. Suite à une escapade amoureuse au Lac de Côme, elle tomba enceinte. Toujours sans emploi, Albert refuse toujours de l’épouser. C’est avec un avenir on ne peut plus incertain que Mileva tenta sa seconde et dernière chance à l’examen oral en juillet 1901. Cette fois, c’est le professeur Weber, celui qu’Albert soupçonnait de bloquer sa carrière, qui lui refuse la note de passage. Forcée d’abandonner ses études, elle retourna en Serbie, mais revint brièvement à Zurich pour essayer en vain de persuader Albert de l’épouser. Elle donna naissance à une petite fille nommée Liserl en janvier 1902. Personne ne sait ce qui lui est arrivé. Elle fut probablement donnée en adoption. Aucun acte de naissance ou de décès n’a été retrouvé.

Auparavant, en décembre 1901, le père de leur camarade de classe Marcel Grossman obtint pour Albert un poste à l’Office des Brevets à Berne, où il débuta en juin 1902. En octobre, juste avant sa mort, son père lui accorda la permission de se marier. Albert épousa Mileva le 6 janvier 1903. Albert travaillait 8 heures par jour, 6 jours semaine tandis que Mileva assumait les tâches ménagères. En soirée, ils travaillaient ensemble, parfois tard dans la nuit. Les deux le mentionnèrent à des amis, lui à Hans Wohlwend, elle à Helene Savić le 20 mars 1903, se désolant de le voir travailler si dur au bureau. Leur fils Hans-Albert naquit le 14 mai 1904.


Photo de marriage de Mileva et Albert en 1903

Malgré cette charge de travail, 1905 devint « l’année miraculeuse » d’Albert où il publia cinq articles: un sur l’effet photoélectrique (ce qui lui valut le Prix Nobel en 1921), deux sur le mouvement Brownien, un sur la relativité restreinte et un contenant la célèbre équation E = mc2. Il soumit des commentaires sur 21 articles scientifiques contre rémunération de même que sa thèse sur les dimensions des molécules

Bien plus tard, Albert confia à R. S. Shankland(6) que la relativité avait été sa vie pendant sept ans et l’effet photoélectrique, cinq ans. Peter Michelmore, un de ses biographes(7), écrivit qu’après avoir passé cinq semaines à compléter l’article sur la relativité restreinte, Albert « passa deux semaines au lit pendant que Mileva relisait inlassablement l’article avant de le poster ». Épuisé, le couple part en Serbie pour une première de trois visites où ils rencontrèrent de nombreux parents et amis. Les témoignages de ces derniers foisonnent d’information sur la façon dont Albert et Mileva collaboraient à l’époque.

Le frère de Mileva, Miloš Jr, une personne reconnue pour son intégrité, séjourna à plusieurs reprises chez les Einstein durant ses études de médecine à Paris. Krstić(2) écrivit: « [Miloš] décrivit comment en soirée et durant la nuit, quand le silence tombait sur la ville, le jeune couple s’assoyait à la table, et à la lumière d’une lampe au kérosène, travaillait à des problèmes de physique. Miloš Jr. mentionna comment ils calculaient, écrivaient, lisaient et débattaient. » Krstić recueillit ce témoignage directement de la marraine de Mileva, Sidonija Gajin et de sa cousine, Sofija Galić Golubović.

Zarko Marić, un cousin du père de Mileva, vivait dans la maison de campagne où les Einstein séjournèrent durant leurs visites. Il raconta à Krstić comment Mileva calculait, écrivait et travaillait avec Albert. Le couple s’assoyait souvent au jardin pour discuter de physique. L’harmonie et le respect mutuel prévalaient. Gajin et Zarko Marić rapportèrent aussi que le père de Mileva leur confia que lors de la visite des Einstein à Novi Sad en 1905, Mileva lui dit: « Nous venons de terminer un travail de recherche scientifique très important qui va rendre mon mari célèbre. » Krstić récolta les mêmes propos de la cousine de Mileva, Sofija Galić Golubović, qui était présente lorsque Mileva parla à son père.

Desanka Trbuhović-Gjurić a publié la première biographie de Mileva en serbe en 1969(3). Cet ouvrage paru plus tard en allemand puis en français. Elle y décrit comment le frère de Mileva accueillait souvent de jeunes intellectuels chez lui. Lors d’une de ces soirées, Albert aurait déclaré: « J’ai besoin de ma femme. Elle résout pour moi tous mes problèmes mathématiques », fait que Mileva aurait confirmé.

ae_mm_son_1905Mileva et Albert avec leur fils Hans-Albert en 1905

En 1908, le couple construisit avec Conrad Habicht un voltmètre ultrasensible. Trbuhović-Gjurić attribue ce travail expérimental à Mileva et Conrad. Elle écrit : “« Quand [Mileva et Conrad] furent tous les deux satisfaits, ils laissèrent à Albert le soin de décrire cet appareil, en expert des brevets». Ce fut enregistré sous le nom d’Einstein-Habicht. Quand Habicht interrogea Mileva sur son choix de ne pas y inclure son nom, elle répondit en faisant un jeu de mots en allemand : « Warum ? Wir beide sind nur ein Stein. » (Pourquoi ? Nous deux ne sommes qu’une seule pierre”, signifiant, nous ne faisons qu’un.)

La reconnaissance vint enfin en 1908. Albert fut invité à donner des cours non rémunérés à Berne, puis on lui offrit un premier poste académique à Zurich en 1909. Mileva l’aidait toujours. Huit pages des premières notes de cours d’Albert sont rédigées de sa main, de même qu’une lettre écrite en 1910 en réponse à Max Planck qui avait sollicité l’avis d’Albert. Ces deux documents se trouvent dans les Archives d’Albert Einstein (AEA) à Jérusalem. Le 3 septembre 1909, Mileva confia à Helene Savić : « Mon mari […] est maintenant perçu comme le meilleur physicien de langue allemande et on le couvre d’honneur. Je suis très heureuse pour son succès parce qu’il le mérite pleinement; je souhaite simplement et espère que la gloire n’aura pas d’effets adverses sur son humanité. » Plus tard, elle ajouta : « Avec toute cette gloire, il a peu de temps pour sa femme. […] Que peut-on faire, avec la notoriété, une personne reçoit la perle, l’autre la coquille. »


Mileva et Albert en 1910

Leur deuxième fils, Eduard, vint au monde le 28 juillet 1910. Jusqu’à 1911, Albert envoyait toujours des cartes postales affectueuses à Mileva. Mais en 1912, il commença une relation avec sa cousine, Elsa Löwenthal, lors d’une visite à sa famille qui avait déménagé à Berlin. Ils entretinrent une correspondance secrète pendant plus de deux ans. Elsa conserva 21 des lettres d’Albert, qu’on retrouve aujourd’hui dans Collected Papers of Albert Einstein. Durant cette période, Albert occupa différents postes de professeur d’abord à Prague, de retour à Zurich et finalement à Berlin en 1914 afin de se rapprocher d’Elsa.

Cela causa l’effondrement de leur mariage. Mileva retourna à Zurich avec ses deux fils le 29 juillet 1914. En 1919, elle consentit à divorcer, exigeant d’inclure une clause dans leur contrat de divorce stipulant que si Albert recevait le Prix Nobel, elle seule obtiendrait l’argent. Lorsqu’elle le reçut, elle acheta deux petits immeubles et vécut maigrement de leurs revenus. Son fils, Eduard séjourna à plusieurs reprises dans un sanatorium. Il souffrit plus tard de schizophrénie et dut finalement être interné. En raison de ces dépenses médicales, Mileva eut de graves soucis financiers toute sa vie et éventuellement perdit les deux immeubles. Elle survécut en donnant des cours particuliers et grâce à la pension alimentaire qu’Albert lui envoyait, bien qu’irrégulièrement.

En 1925, Albert voulut inclure dans son testament que l’argent du Prix Nobel constituait l’héritage de ses fils. Mileva s’y opposa fortement, lui rappelant que cet argent était le sien propre et envisagea de révéler ses contributions au travail d’Albert. Radmila Milentijević cite une lettre qu’Albert lui adressa le 24 octobre 1925 (AEA 75-364). « Mais tu m’as fait vraiment rire quand tu as commencé à me menacer de tes mémoires. T’est-il jamais venu à l’esprit, ne serait-ce qu’une seconde, que personne ne prêterait la moindre attention à tes salades si l’homme dont tu parles n’avait pas accompli quelque chose d’important? Quand une personne est quelqu’un de complètement insignifiant, il n’y a rien d’autre à dire à cette personne que de rester modeste et de se taire. C’est ce que je te conseille de faire. »

Mileva est resté silencieuse mais son amie Milana Bota déclara à un journal serbe en 1929 que Mileva pourrait les renseigner sur l’origine de la relativité restreinte, puisqu’elle y avait directement contribué. Le 13 juin 1929, Mileva écrivit à Helene Savić : « De telles publications dans les journaux ne correspondent pas du tout à ma nature mais je crois que cela a fait plaisir à Milana et qu’elle a probablement pensé que cela me ferait plaisir aussi et que, d’une certaine façon, cela m’aiderait à obtenir certains droits vis-à-vis d’Einstein aux yeux du public. Elle m’a écrit en ce sens, et je l’accepte ainsi, autrement tout cela n’aurait pas beaucoup de sens. »


Mileva Marić quelques années plus tard (date inconnue)

Selon Krstić(2), Mileva parla de ses contributions à sa mère et sa sœur. Elle écrivit aussi à ses parrain et marraine comment elle collabora avec Albert et comment il avait ruiné sa vie, mais leur demanda de détruire sa lettre. Son fils, Hans-Albert, confia à Krstić comment “la collaboration scientifique de ses parents continua après leur mariage et qu’il se rappelait les voir travailler ensemble en soirée à la même table.” La première femme d’Hans-Albert, Frieda, essaya de publier les lettres que Mileva et Albert avaient envoyé à leurs fils, mais fut bloquée en cour par les exécuteurs testamentaires d’Einstein, Helen Dukas et Otto Nathan afin de préserver le « mythe Einstein ». Ils empêchèrent aussi d’autres publications, y compris lorsque Krstić(2) voulu publier ses premières découvertes en 1974. Krstić mentionne que Nathan « visita » même l’appartement de Mileva après sa mort en 1948. En juillet 1947, Albert écrivit au Dr Karl Zürcher, l’avocat qui avait réglé son divorce : « Lorsque Mileva ne sera plus de ce monde, je pourrai mourir en paix. »

Leurs lettres et les nombreux témoignages attestent que Mileva Marić et Albert Einstein collaborèrent étroitement depuis leur rencontre jusqu’à 1914. Albert le mentionna à plusieurs reprises dans ses lettres, comme lorsqu’il écrivit : “notre travail sur mouvement relatif“. Leur union était faite d’amour et de respect mutuel. C’est ce qui leur a permis de produire ensemble un travail hors du commun. Elle fut la première à reconnaître son talent. Sans elle, il n’aurait jamais réussi. Elle abandonna ses propres aspirations, heureuse de travailler avec lui et de contribuer à son succès, sentant qu’ils ne faisaient qu’un. Une fois enclenché, il devint impossible de faire marche arrière sur le processus de signer leur travail sous le seul nom d’Albert. Elle l’avait probablement accepté puisque son propre bonheur dépendait de son succès. Pourquoi Mileva est-elle restée silencieuse? Étant de nature discrète, elle ne recherchait pas les honneurs ou l’attention publique. Et comme dans tous les cas de collaboration étroite, les contributions individuelles de chacun sont presque toujours impossibles à départager.

Pauline Gagnon

Cet article fut d’abord publié en anglais au Magazine Scientific American dans la section Opinions

Pour en savoir plus sur la physique des particules et la matière sombre, consultez mon livre “Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels“.

Pour être au courant des nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou inscrivez-vous sur cette liste de distribution

Références :
(1) Radmila Milentijević
: Mileva Marić Einstein : Vivre avec Albert Einstein, Éditions L’Age d’Homme, 2014.
(2) Dord Krstić: Mileva & Albert Einstein: Their Love and Scientific Collaboration, Didakta, 2004.

(3) Desanka Trbuhović-Gjurić Mileva Marić Einstein : Dans l’ombre d’Albert Einstein : en serbe, 1969, allemand, 1982 et français, 1991.
(4) Milan Popović: In Albert’s Shadow, the Life and Letters of Mileva Marić, Einstein’s First Wife, The John Hopkins University Press, 2003.

(5) Renn and Schulmann, Albert Einstein / Mileva Marić, The Love Letters, Princeton University Press, 1992.

(6) Peter Michelmore, Einstein, Profile of the Man, Dodd, Mead & Company, 1962.

(7) R.S. Shankland, Conversation with Albert Einstein, Am. J. of Physics, 1962.


To celebrate the first five years of operation on board the International Space Station, Professor Sam Ting, the spokesperson for the Alpha Magnetic Spectrometer (AMS-02) Collaboration just presented their latest results at a recent seminar held at CERN. With a sample of 90 million events collected in cosmic rays, they now have the most precise data on a wide range of particles found in outer space.


source: ©NASA

Many physicists wonder if the AMS Collaboration will resolve the enigma on the origin of the excess of positrons found in cosmic rays. Positrons are the antimatter of electrons. Given that we live in a world made almost uniquely of matter, scientists have been wondering for more than a decade where these positrons come from. It is well known that some positrons are produced when cosmic rays interact with the interstellar material. What is puzzling is that more positrons are observed than what is expected from this source alone.

Various hypotheses have been formulated to explain the origin of these extra positrons. One particularly exciting possibility is that these positrons could emanate from the annihilation of dark matter particles. Dark matter is some form of invisible matter that is observed in the Universe mostly through its gravitational effects. Regular matter, everything we know on Earth but also everything found in stars and galaxies, emits light when heated up, just like a piece of heated metal glows.

Dark matter emits no light, hence its name. It is five times more prevalent than regular matter. Although no one knows, we suspect dark matter, just like regular matter, is made of particles but no one has yet been able to capture a particle of dark matter. However, if dark matter particles exist, they could annihilate with each other and produce an electron and a positron, or a proton and antiproton pair. This would at long last establish that dark matter particles exist and reveal some clues on their characteristics.

An alternative but less exotic explanation would be that the observed excess of positrons comes from pulsars. Pulsars are neutron stars with a strong magnetic field that emit pulsed light. But light is made of photons and photons can also decay into an electron and a positron. So both the pulsar and the dark matter annihilation provide a plausible explanation on the source of these positrons.

To tell the difference, one must measure the energy of all positrons found in cosmic rays and see how many are found at high energy. This is what AMS has done and their data are shown on the left plot below, where we see the flux of positrons (vertical axis) found at different energies (horizontal axis). The flux combines the number of positrons found with their energy cube. The green curve gives how many positrons are expected from cosmic rays hitting the interstellar material (ISM).

If the excess of positrons were to come from dark matter annihilation, no positron would be found with an energy exceeding the mass of the dark matter particle. They would have an energy distribution similar to the brown curve on the plot below as expected for dark matter particles having a mass of 1 TeV, a thousand times heavier than a proton. In that case, the positrons energy distribution curve would drop off sharply. The red dots represent the AMS data with their experimental errors shown by the vertical bars. If, on the other end, the positrons came from pulsars, the drop at high energy would be less pronounced.


source: AMS Collaboration

The name of the game is therefore to figure out precisely what is happening at high energy. But there are much fewer positrons there, making it very difficult to see what is happening as indicated by the large error bars attached to the data points at higher energy. These indicate the size of the experimental errors.

But by looking at the fraction of positrons found in all data collected for electrons and positrons (right plot above), some of the experimental errors cancel out. AMS has collected over a million positrons and 16 million electrons. The red dots on the right plot show the fraction of positrons found in their sample as a function of energy. Given the actual precision of these measurements, it is still not completely clear if this fraction is really falling off at higher energy or not.

The AMS Collaboration hopes however to have enough data to distinguish the two hypotheses by 2024 when the ISS will cease operation. These projections are shown on the next two plots both for the positrons flux (left) and the positron fraction (right). As it stands today, both hypotheses are still possible given the size of the experimental errors.


source: AMS Collaboration

There is another way to test the dark matter hypothesis. By interacting with the interstellar material, cosmic rays produce not only positrons, but also antiprotons. And so would dark matter annihilations but pulsars cannot produce antiprotons. If there were also an excess of antiprotons in outer space that could not be accounted for by cosmic rays, it would reinforce the dark matter hypothesis. But this entails knowing precisely how cosmic rays propagate and interact with the interstellar medium.

Using the AMS large sample of antiprotons, Prof. Sam Ting claimed that such excess already exists. He showed the following plot giving the fraction of antiprotons found in the total sample of protons and antiprotons as a function of their energy. The red dots represent the AMS measurements, the brown band, some theoretical calculation for cosmic rays, and the blue band, what could be coming from dark matter.


source: AMS Collaboration

This plot clearly suggests that more antiprotons are found than what is expected from cosmic rays interacting with the interstellar material (ISM). But both Dan Hooper and Ilias Cholis, two theorists and experts on this subject, strongly disagree, saying that the uncertainty on this calculation is much larger. They say that the following plot (from Cuoco et al.) is by far more realistic. The pink dots represent the AMS data for the antiproton fraction. The data seem in good agreement with the theoretical prediction given by the black line and grey bands. So there are no signs of a large excess of antiprotons here. We need to wait for a few more years before the AMS data and the theoretical estimates are precise enough to determine if there is an excess or not.


source: Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

The AMS Collaboration could have another huge surprise is stock: discovering the first antiatoms of helium in outer space. Given that anything more complex than an antiproton is much more difficult to produce, they will need to analyze huge amounts of data and further reduce all their experimental errors before such a discovery could be established.

Will AMS discover antihelium atoms in cosmic rays, establish the presence of an excess of antiprotons or even solve the positron enigma? AMS has lots of exciting work on its agenda. Well worth waiting for it!

Pauline Gagnon

To find out more about particle physics and dark matter, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN ».

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Pour célébrer les cinq premières années d’opération à bord de la Station spatiale internationale, le Professeur Sam Ting, porte-parole de la Collaboration Alpha Magnetic Spectrometer (AMS-02) vient de présenter leurs derniers résultats lors d’un récent séminaire tenu au CERN. Avec plus de 90 millions d’évènements recueillis dans les rayons cosmiques, ce groupe dispose des données les plus précises sur une vaste gamme de particules trouvées dans l’espace.


source: ©NASA

La question qui intrigue de nombreux scientifiques est de savoir s’ils pourront résoudre l’énigme de l’origine de l’excès de positrons trouvés dans les rayons cosmiques. Les positrons sont l’antimatière des électrons. Étant donné que nous vivons dans un monde fait presque uniquement de matière, les scientifiques se demandent depuis plus d’une décennie d’où émanent ces positrons. Il est bien connu que des positrons sont produits lorsque les rayons cosmiques interagissent avec la matière interstellaire mais on en observe bien plus que ce à quoi on s’attendait de cette seule source.

Des hypothèses diverses ont été formulées pour expliquer l’origine de ces positrons excédentaires. Une des plus fascinantes suggère que ces positrons pourraient venir de l’annihilation de particules de matière sombre. La matière sombre est une nouvelle forme de matière invisible qu’on détecte dans l’Univers par ses effets gravitationnels. La matière régulière, tout ce que nous voyons sur la Terre, mais aussi dans les étoiles et les galaxies, émet de la lumière lorsque chauffée, tout comme une pièce métallique irradie à haute température.

La matière sombre n’émet aucune lumière, d’où son nom. Elle est cinq fois plus répandue que la matière régulière. Personne ne le sait encore mais on soupçonne que cette matière, tout comme la matière ordinaire, soit faite de particules, mais on n’a toujours pas capturé de particules de matière sombre. Mais si de telles particules existaient, elles pourraient s’annihiler entre elles, produisant des électrons et des positrons, ou des paires de protons et d’antiprotons. Si un tel processus était établi, cela confirmerait enfin l’existence de particules de matière sombre et révèlerait quelques indices sur leurs caractéristiques.

Une explication alternative mais moins exotique serait que l’excès observé de positrons provienne de pulsars. Les pulsars sont des étoiles à neutrons ayant un fort champ magnétique et qui émettent de la lumière pulsée. Mais la lumière est faite de photons et les photons peuvent eux aussi produire des paires d’électrons et de positrons. Donc, les pulsars tout comme l’annihilation de matière sombre, fournissent une explication plausible quant à la source de ces positrons.

Pour les distinguer, il faut mesurer l’énergie des positrons captés dans les rayons cosmiques et voir combien on en trouve à haute énergie. C’est ce que AMS a fait et leurs résultats sont visibles dans le graphe de gauche ci-dessous où nous voyons le flux de positrons (axe vertical) trouvé à une énergie particulière (axe horizontal). Le flux combine le nombre de positrons trouvés avec leur énergie au cube. La courbe en vert donne le nombre de positrons produits lorsque des rayons cosmiques frappent de la matière interstellaire (ISM).

Si l’excès de positrons devait venir de l’annihilation de matière sombre, on ne trouverait aucun positron au-delà de l’énergie correspondant à la masse des particules de matière sombre. Ils auraient une distribution d’énergie semblable à la courbe en brun sur le graphe ci-dessous tel que prédit pour des particules de matière sombre ayant une masse de 1 TeV, soit mille fois plus lourd qu’un proton. Dans ce cas, la courbe de distribution d’énergie des positrons chuterait rapidement. Les points en rouge représentent les données d’AMS avec leurs erreurs expérimentales indiquées par les barres verticales. Par contre, si les positrons venaient de pulsars, la chûte à haute énergie serait moins prononcée.


source: Collaboration AMS

Toute la difficulté consiste à comprendre précisément leur comportement à haute énergie. Mais comme on y trouve moins de positrons, il est beaucoup plus difficile de voir ce qu’il en est comme l’indiquent les larges marges d’erreur associées aux mesures faites à plus haute énergie.

Mais si on mesure plutôt la fraction de positrons trouvés dans les données en combinant positrons et électrons, certaines des erreurs expérimentales s’annulent. AMS a rassemblé plus d’un million de positrons et 16 millions d’électrons. Les points en rouge sur le graphe de droite ci-dessus montrent la fraction de positrons trouvée dans leur échantillon en fonction de leur énergie. Malgré les pas de géants accomplis, la précision actuelle de ces mesures ne permet toujours pas d’établir clairement si cette fraction tombe abruptement à haute énergie ou pas.

La Collaboration AMS espère toutefois avoir assez de données pour distinguer les deux hypothèses d’ici à 2024, date à laquelle la Station Spatiale Internationale cessera ses opérations. On peut voir ces projections sur les deux graphes suivants tant pour le flux de positrons (à gauche) que pour la fraction de positrons (à droite). À ce jour, les deux hypothèses sont toujours valides étant donné la taille des erreurs expérimentales.


source: Collaboration AMS

L’hypothèse de la matière sombre peut aussi être testée d’une autre façon. En interagissant avec la matière interstellaire, les rayons cosmiques produisent non seulement des positrons mais aussi des antiprotons. Les annihilations de matière sombre pourraient aussi en produire mais pas les pulsars. Il faut donc déterminer s’il y a ou pas plus d’antiprotons dans l’espace que ce que les rayons cosmiques peuvent produire. Si c’était établi, ce serait un argument de plus contre l’hypothèse des pulsars. Mais pour ce faire, il faut savoir précisément comment les rayons cosmiques se propagent et interagissent avec la matière interstellaire.

S’appuyant sur le vaste échantillon d’antiprotons recueillis par AMS, le Prof. Sam Ting a soutenu qu’un tel excès existe, présentant le graphe suivant à l’appui. On y voit la fraction d’antiprotons trouvés dans l’échantillon total de protons et des antiprotons en fonction de leur énergie. Les points en rouge représentent les mesures d’AMS, la bande brune, les calculs théoriques pour les rayons cosmiques et la bande bleue, ce qui pourrait venir de la matière sombre.


source: Collaboration AMS

Ce graphe suggère fortement un surplus d’antiprotons par rapport à ce que l’on s’attend des rayons cosmiques interagissant avec la matière interstellaire (ISM). Mais tant Dan Hooper qu’Ilias Cholis, deux théoriciens experts en la matière, s’objectent carrément, disant que l’incertitude sur les prédictions théoriques sont beaucoup plus grandes que ce que ce graphe suggère. Ils soutiennent que le graphe suivant (de Cuoco etal.) est de loin plus réaliste. Les points en rose représentent les données d’AMS pour la fraction d’antiprotons et le trait en noir, les prédictions théoriques avec leur marge d’erreur. Les deux concordent ou presque, suggérant l’absence de tout excès. Nous devrons patienter encore quelques années avant que les données d’AMS et les prédictions théoriques soient assez précises pour savoir s’il y a excès ou pas.


            source : Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

La Collaboration AMS pourrait nous réserver une autre belle surprise : la découverte d’antiatomes d’hélium dans l’espace. Étant donné l’extrême difficulté à produire une particule d’antimatière plus complexe qu’un antiproton, les scientifiques d’AMS devront trier d’énormes quantités de données et réduire toutes les erreurs expérimentales encore davantage avant qu’une telle découverte ne puisse être établie.

La découverte d’antihélium, ou celle d’un excès d’antiprotons ou encore la résolution de l’énigme des positrons, tout cela vaut bien la peine d’attendre encore quelques années. AMS a du beau pain sur la planche!
Pauline Gagnon

Pour en savoir plus sur la physique des particules et la matière sombre, consultez mon livre “Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels“.

Pour être au courant des nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou inscrivez-vous sur cette liste de distribution


Solving the Measurement Problem (Guest Post)

Wednesday, October 5th, 2016

The following is a guest posting from Ken Krechmer of the College of Engineering at Applied Science at the University of Colorado, at Boulder. 

Ken Krechmer

Ken Krechmer

The dichotomy between quantum measurement theory and classical measurement results has been termed: the measurement disturbance, measurement collapse and the measurement problem.   Experimentally it is observed that the measurement of the position of one particle changes the momentum of the same particle instantaneously.  This is described as the measurement disturbance.  Quantum measurement theory calculates the probability of a measurement result but does not calculate an actual measurement result.  What occurs that causes the quantum measurement probability to collapse into a classical measurement result?  Different approaches have been proposed to resolve one or both of these issues including hidden variables, non-local variables and decoherence, but none of these approaches appear to fully resolve both these aspects of the measurement problem.

Further complicating this measurement problem: 1. The quantum effect called entanglement is another measurement disturbance where the measurement of one particle instantaneously impacts a similar measurement of another, far remote, particle.  2. The quantum effect called uncertainty which defines the minimum variation between two measurement results and changes depending on the order of the two measurements.

Relational measurements and uncertainty,” also available at Measurement, resolves both aspects of the measurement problem by expanding the definition of a classical measurement to include sampling and calibration to a reference. Experimentally, it is well known that a measurement must be sampled and calibrated to a reference to establish a measurement result. This paper proves that the measurement collapse is due to the effect of sampling and calibration which is equal to the universal quantum measurement uncertainty.  The universal quantum measurement uncertainty has been verified in independent quantum experiments. Next, one quantum measurement is shown to instantaneously disturb another because one sampling and calibration process is applied to both measurement results.

 The paper resolves the dichotomy between quantum theory and classical measurement results, derives the quantum uncertainty relations using classical physics, unifies the measurement process across all scales and formally models calibration and sampling.


Ken Krechmer, University of Colorado (CU) Scholar in Residence, has taught a graduate level engineering course on standards and standardization at CU.  He authored prize winning papers on standards and standardization in 1995, 2000, 2006 and 2012. Krechmer co-founded the journal Communications Standards Review.  He was active in standardization committees in the ITU, ETSI, TIA, IEEE, and many consortia for over 20 years.  Krechmer is a Senior Member of the IEEE and a Member of Society of Engineering Standards.


The Delirium over Beryllium

Thursday, August 25th, 2016

This post is cross-posted from ParticleBites.

Article: Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays
Authors: J.L. Feng, B. Fornal, I. Galon, S. Gardner, J. Smolinsky, T. M. P. Tait, F. Tanedo
Reference: arXiv:1608.03591 (Submitted to Phys. Rev. D)
Also featuring the results from:
— Gulyás et al., “A pair spectrometer for measuring multipolarities of energetic nuclear transitions” (description of detector; 1504.00489NIM)
— Krasznahorkay et al., “Observation of Anomalous Internal Pair Creation in 8Be: A Possible Indication of a Light, Neutral Boson”  (experimental result; 1504.01527PRL version; note PRL version differs from arXiv)
— Feng et al., “Protophobic Fifth-Force Interpretation of the Observed Anomaly in 8Be Nuclear Transitions” (phenomenology; 1604.07411; PRL)

Editor’s note: the author is a co-author of the paper being highlighted. 

Recently there’s some press (see links below) regarding early hints of a new particle observed in a nuclear physics experiment. In this bite, we’ll summarize the result that has raised the eyebrows of some physicists, and the hackles of others.

A crash course on nuclear physics

Nuclei are bound states of protons and neutrons. They can have excited states analogous to the excited states of at lowoms, which are bound states of nuclei and electrons. The particular nucleus of interest is beryllium-8, which has four neutrons and four protons, which you may know from the triple alpha process. There are three nuclear states to be aware of: the ground state, the 18.15 MeV excited state, and the 17.64 MeV excited state.

Beryllium-8 excited nuclear states. The 18.15 MeV state (red) exhibits an anomaly. Both the 18.15 MeV and 17.64 states decay to the ground through a magnetic, p-wave transition. Image adapted from Savage et al. (1987).

Most of the time the excited states fall apart into a lithium-7 nucleus and a proton. But sometimes, these excited states decay into the beryllium-8 ground state by emitting a photon (γ-ray). Even more rarely, these states can decay to the ground state by emitting an electron–positron pair from a virtual photon: this is called internal pair creation and it is these events that exhibit an anomaly.

The beryllium-8 anomaly

Physicists at the Atomki nuclear physics institute in Hungary were studying the nuclear decays of excited beryllium-8 nuclei. The team, led by Attila J. Krasznahorkay, produced beryllium excited states by bombarding a lithium-7 nucleus with protons.

Preparation of beryllium-8 excited state

Beryllium-8 excited states are prepare by bombarding lithium-7 with protons.

The proton beam is tuned to very specific energies so that one can ‘tickle’ specific beryllium excited states. When the protons have around 1.03 MeV of kinetic energy, they excite lithium into the 18.15 MeV beryllium state. This has two important features:

  1. Picking the proton energy allows one to only produce a specific excited state so one doesn’t have to worry about contamination from decays of other excited states.
  2. Because the 18.15 MeV beryllium nucleus is produced at resonance, one has a very high yield of these excited states. This is very good when looking for very rare decay processes like internal pair creation.

What one expects is that most of the electron–positron pairs have small opening angle with a smoothly decreasing number as with larger opening angles.

Screen Shot 2016-08-22 at 9.18.11 AM

Expected distribution of opening angles for ordinary internal pair creation events. Each line corresponds to nuclear transition that is electric (E) or magenetic (M) with a given orbital quantum number, l. The beryllium transitionsthat we’re interested in are mostly M1. Adapted from Gulyás et al. (1504.00489).

Instead, the Atomki team found an excess of events with large electron–positron opening angle. In fact, even more intriguing: the excess occurs around a particular opening angle (140 degrees) and forms a bump.

Adapted from Krasznahorkay et al.

Number of events (dN/dθ) for different electron–positron opening angles and plotted for different excitation energies (Ep). For Ep=1.10 MeV, there is a pronounced bump at 140 degrees which does not appear to be explainable from the ordinary internal pair conversion. This may be suggestive of a new particle. Adapted from Krasznahorkay et al., PRL 116, 042501.

Here’s why a bump is particularly interesting:

  1. The distribution of ordinary internal pair creation events is smoothly decreasing and so this is very unlikely to produce a bump.
  2. Bumps can be signs of new particles: if there is a new, light particle that can facilitate the decay, one would expect a bump at an opening angle that depends on the new particle mass.

Schematically, the new particle interpretation looks like this:

Schematic of the Atomki experiment.

Schematic of the Atomki experiment and new particle (X) interpretation of the anomalous events. In summary: protons of a specific energy bombard stationary lithium-7 nuclei and excite them to the 18.15 MeV beryllium-8 state. These decay into the beryllium-8 ground state. Some of these decays are mediated by the new X particle, which then decays in to electron–positron pairs of a certain opening angle that are detected in the Atomki pair spectrometer detector. Image from 1608.03591.

As an exercise for those with a background in special relativity, one can use the relation (pe+ + pe)2 = mX2 to prove the result:


This relates the mass of the proposed new particle, X, to the opening angle θ and the energies E of the electron and positron. The opening angle bump would then be interpreted as a new particle with mass of roughly 17 MeV. To match the observed number of anomalous events, the rate at which the excited beryllium decays via the X boson must be 6×10-6 times the rate at which it goes into a γ-ray.

The anomaly has a significance of 6.8σ. This means that it’s highly unlikely to be a statistical fluctuation, as the 750 GeV diphoton bump appears to have been. Indeed, the conservative bet would be some not-understood systematic effect, akin to the 130 GeV Fermi γ-ray line.

The beryllium that cried wolf?

Some physicists are concerned that beryllium may be the ‘boy that cried wolf,’ and point to papers by the late Fokke de Boer as early as 1996 and all the way to 2001. de Boer made strong claims about evidence for a new 10 MeV particle in the internal pair creation decays of the 17.64 MeV beryllium-8 excited state. These claims didn’t pan out, and in fact the instrumentation paper by the Atomki experiment rules out that original anomaly.

The proposed evidence for “de Boeron” is shown below:


The de Boer claim for a 10 MeV new particle. Left: distribution of opening angles for internal pair creation events in an E1 transition of carbon-12. This transition has similar energy splitting to the beryllium-8 17.64 MeV transition and shows good agreement with the expectations; as shown by the flat “signal – background” on the bottom panel. Right: the same analysis for the M1 internal pair creation events from the 17.64 MeV beryllium-8 states. The “signal – background” now shows a broad excess across all opening angles. Adapted from de Boer et al. PLB 368, 235 (1996).

When the Atomki group studied the same 17.64 MeV transition, they found that a key background component—subdominant E1 decays from nearby excited states—dramatically improved the fit and were not included in the original de Boer analysis. This is the last nail in the coffin for the proposed 10 MeV “de Boeron.”

However, the Atomki group also highlight how their new anomaly in the 18.15 MeV state behaves differently. Unlike the broad excess in the de Boer result, the new excess is concentrated in a bump. There is no known way in which additional internal pair creation backgrounds can contribute to add a bump in the opening angle distribution; as noted above: all of these distributions are smoothly falling.

The Atomki group goes on to suggest that the new particle appears to fit the bill for a dark photon, a reasonably well-motivated copy of the ordinary photon that differs in its overall strength and having a non-zero (17 MeV?) mass.

Theory part 1: Not a dark photon

With the Atomki result was published and peer reviewed in Physics Review Letters, the game was afoot for theorists to understand how it would fit into a theoretical framework like the dark photon. A group from UC Irvine, University of Kentucky, and UC Riverside found that actually, dark photons have a hard time fitting the anomaly simultaneously with other experimental constraints. In the visual language of this recent ParticleBite, the situation was this:


It turns out that the minimal model of a dark photon cannot simultaneously explain the Atomki beryllium-8 anomaly without running afoul of other experimental constraints. Image adapted from this ParticleBite.

The main reason for this is that a dark photon with mass and interaction strength to fit the beryllium anomaly would necessarily have been seen by the NA48/2 experiment. This experiment looks for dark photons in the decay of neutral pions (π0). These pions typically decay into two photons, but if there’s a 17 MeV dark photon around, some fraction of those decays would go into dark-photon — ordinary-photon pairs. The non-observation of these unique decays rules out the dark photon interpretation.

The theorists then decided to “break” the dark photon theory in order to try to make it fit. They generalized the types of interactions that a new photon-like particle, X, could have, allowing protons, for example, to have completely different charges than electrons rather than having exactly opposite charges. Doing this does gross violence to the theoretical consistency of a theory—but they goal was just to see what a new particle interpretation would have to look like. They found that if a new photon-like particle talked to neutrons but not protons—that is, the new force were protophobic—then a theory might hold together.

Schematic description of how model-builders “hacked” the dark photon theory to fit both the beryllium anomaly while being consistent with other experiments. This hack isn’t pretty—and indeed, comes at the cost of potentially invalidating the mathematical consistency of the theory—but the exercise demonstrates the target for how to a complete theory might have to behave. Image adapted from this ParticleBite.

Theory appendix: pion-phobia is protophobia

Editor’s note: what follows is for readers with some physics background interested in a technical detail; others may skip this section.

How does a new particle that is allergic to protons avoid the neutral pion decay bounds from NA48/2? Pions decay into pairs of photons through the well-known triangle-diagrams of the axial anomaly. The decay into photon–dark-photon pairs proceed through similar diagrams. The goal is then to make sure that these diagrams cancel.

A cute way to look at this is to assume that at low energies, the relevant particles running in the loop aren’t quarks, but rather nucleons (protons  and neutrons). In fact, since only the proton can talk to the photon, one only needs to consider proton loops. Thus if the new photon-like particle, X, doesn’t talk to protons, then there’s no diagram for the pion to decay into γX. This would be great if the story weren’t completely wrong.

Avoiding NA48

Avoiding NA48/2 bounds requires that the new particle, X, is pion-phobic. It turns out that this is equivalent to X being protophobic. The correct way to see this is on the left, making sure that the contribution of up-quark loops cancels the contribution from down-quark loops. A slick (but naively completely wrong) calculation is on the right, arguing that effectively only protons run in the loop.

The correct way of seeing this is to treat the pion as a quantum superposition of an up–anti-up and down–anti-down bound state, and then make sure that the X charges are such that the contributions of the two states cancel. The resulting charges turn out to be protophobic.

The fact that the “proton-in-the-loop” picture gives the correct charges, however, is no coincidence. Indeed, this was precisely how Jack Steinberger calculated the correct pion decay rate. The key here is whether one treats the quarks/nucleons linearly or non-linearly in chiral perturbation theory. The relation to the Wess-Zumino-Witten term—which is what really encodes the low-energy interaction—is carefully explained in chapter 6a.2 of Georgi’s revised Weak Interactions.

Theory part 2: Not a spin-0 particle

The above considerations focus on a new particle with the same spin and parity as a photon (spin-1, parity odd). Another result of the UCI study was a systematic exploration of other possibilities. They found that the beryllium anomaly could not be consistent with spin-0 particles. For a parity-odd, spin-0 particle, one cannot simultaneously conserve angular momentum and parity in the decay of the excited beryllium-8 state. (Parity violating effects are negligible at these energies.)


Parity and angular momentum conservation prohibit a “dark Higgs” (parity even scalar) from mediating the anomaly.

For a parity-odd pseudoscalar, the bounds on axion-like particles at 20 MeV suffocate any reasonable coupling. Measured in terms of the pseudoscalar–photon–photon coupling (which has dimensions of inverse GeV), this interaction is ruled out down to the inverse Planck scale.

Screen Shot 2016-08-24 at 4.01.07 PM

Bounds on axion-like particles exclude a 20 MeV pseudoscalar with couplings to photons stronger than the inverse Planck scale. Adapted from 1205.2671 and 1512.03069.

Additional possibilities include:

  • Dark Z bosons, cousins of the dark photon with spin-1 but indeterminate parity. This is very constrained by atomic parity violation.
  • Axial vectors, spin-1 bosons with positive parity. These remain a theoretical possibility, though their unknown nuclear matrix elements make it difficult to write a predictive model. (See section II.D of 1608.03591.)

Theory part 3: Nuclear input

The plot thickens when once also includes results from nuclear theory. Recent results from Saori Pastore, Bob Wiringa, and collaborators point out a very important fact: the 18.15 MeV beryllium-8 state that exhibits the anomaly and the 17.64 MeV state which does not are actually closely related.

Recall (e.g. from the first figure at the top) that both the 18.15 MeV and 17.64 MeV states are both spin-1 and parity-even. They differ in mass and in one other key aspect: the 17.64 MeV state carries isospin charge, while the 18.15 MeV state and ground state do not.

Isospin is the nuclear symmetry that relates protons to neutrons and is tied to electroweak symmetry in the full Standard Model. At nuclear energies, isospin charge is approximately conserved. This brings us to the following puzzle:

If the new particle has mass around 17 MeV, why do we see its effects in the 18.15 MeV state but not the 17.64 MeV state?

Naively, if the new particle emitted, X, carries no isospin charge, then isospin conservation prohibits the decay of the 17.64 MeV state through emission of an X boson. However, the Pastore et al. result tells us that actually, the isospin-neutral and isospin-charged states mix quantum mechanically so that the observed 18.15 and 17.64 MeV states are mixtures of iso-neutral and iso-charged states. In fact, this mixing is actually rather large, with mixing angle of around 10 degrees!

The result of this is that one cannot invoke isospin conservation to explain the non-observation of an anomaly in the 17.64 MeV state. In fact, the only way to avoid this is to assume that the mass of the X particle is on the heavier side of the experimentally allowed range. The rate for emission goes like the 3-momentum cubed (see section II.E of 1608.03591), so a small increase in the mass can suppresses the rate of emission by the lighter state by a lot.

The UCI collaboration of theorists went further and extended the Pastore et al. analysis to include a phenomenological parameterization of explicit isospin violation. Independent of the Atomki anomaly, they found that including isospin violation improved the fit for the 18.15 MeV and 17.64 MeV electromagnetic decay widths within the Pastore et al. formalism. The results of including all of the isospin effects end up changing the particle physics story of the Atomki anomaly significantly:

Parameter fits

The rate of X emission (colored contours) as a function of the X particle’s couplings to protons (horizontal axis) versus neutrons (vertical axis). The best fit for a 16.7 MeV new particle is the dashed line in the teal region. The vertical band is the region allowed by the NA48/2 experiment. Solid lines show the dark photon and protophobic limits. Left: the case for perfect (unrealistic) isospin. Right: the case when isospin mixing and explicit violation are included. Observe that incorporating realistic isospin happens to have only a modest effect in the protophobic region. Figure from 1608.03591.

The results of the nuclear analysis are thus that:

  1. An interpretation of the Atomki anomaly in terms of a new particle tends to push for a slightly heavier X mass than the reported best fit. (Remark: the Atomki paper does not do a combined fit for the mass and coupling nor does it report the difficult-to-quantify systematic errors  associated with the fit. This information is important for understanding the extent to which the X mass can be pushed to be heavier.)
  2. The effects of isospin mixing and violation are important to include; especially as one drifts away from the purely protophobic limit.

Theory part 4: towards a complete theory

The theoretical structure presented above gives a framework to do phenomenology: fitting the observed anomaly to a particle physics model and then comparing that model to other experiments. This, however, doesn’t guarantee that a nice—or even self-consistent—theory exists that can stretch over the scaffolding.

Indeed, a few challenges appear:

  • The isospin mixing discussed above means the X mass must be pushed to the heavier values allowed by the Atomki observation.
  • The “protophobic” limit is not obviously anomaly-free: simply asserting that known particles have arbitrary charges does not generically produce a mathematically self-consistent theory.
  • Atomic parity violation constraints require that the X couple in the same way to left-handed and right-handed matter. The left-handed coupling implies that X must also talk to neutrinos: these open up new experimental constraints.

The Irvine/Kentucky/Riverside collaboration first note the need for a careful experimental analysis of the actual mass ranges allowed by the Atomki observation, treating the new particle mass and coupling as simultaneously free parameters in the fit.

Next, they observe that protophobic couplings can be relatively natural. Indeed: the Standard Model Z boson is approximately protophobic at low energies—a fact well known to those hunting for dark matter with direct detection experiments. For exotic new physics, one can engineer protophobia through a phenomenon called kinetic mixing where two force particles mix into one another. A tuned admixture of electric charge and baryon number, (Q-B), is protophobic.

Baryon number, however, is an anomalous global symmetry—this means that one has to work hard to make a baryon-boson that mixes with the photon (see 1304.0576 and 1409.8165 for examples). Another alternative is if the photon kinetically mixes with not baryon number, but the anomaly-free combination of “baryon-minus-lepton number,” Q-(B-L). This then forces one to apply additional model-building modules to deal with the neutrino interactions that come along with this scenario.

In the language of the ‘model building blocks’ above, result of this process looks schematically like this:

Model building block

A complete theory is completely mathematically self-consistent and satisfies existing constraints. The additional bells and whistles required for consistency make additional predictions for experimental searches. Pieces of the theory can sometimes  be used to address other anomalies.

The theory collaboration presented examples of the two cases, and point out how the additional ‘bells and whistles’ required may tie to additional experimental handles to test these hypotheses. These are simple existence proofs for how complete models may be constructed.

What’s next?

We have delved rather deeply into the theoretical considerations of the Atomki anomaly. The analysis revealed some unexpected features with the types of new particles that could explain the anomaly (dark photon-like, but not exactly a dark photon), the role of nuclear effects (isospin mixing and breaking), and the kinds of features a complete theory needs to have to fit everything (be careful with anomalies and neutrinos). The single most important next step, however, is and has always been experimental verification of the result.

While the Atomki experiment continues to run with an upgraded detector, what’s really exciting is that a swath of experiments that are either ongoing or in construction will be able to probe the exact interactions required by the new particle interpretation of the anomaly. This means that the result can be independently verified or excluded within a few years. A selection of upcoming experiments is highlighted in section IX of 1608.03591:

Experimental searches

Other experiments that can probe the new particle interpretation of the Atomki anomaly. The horizontal axis is the new particle mass, the vertical axis is its coupling to electrons (normalized to the electric charge). The dark blue band is the target region for the Atomki anomaly. Figure from 1608.03591; assuming 100% branching ratio to electrons.

We highlight one particularly interesting search: recently a joint team of theorists and experimentalists at MIT proposed a way for the LHCb experiment to search for dark photon-like particles with masses and interaction strengths that were previously unexplored. The proposal makes use of the LHCb’s ability to pinpoint the production position of charged particle pairs and the copious amounts of D mesons produced at Run 3 of the LHC. As seen in the figure above, the LHCb reach with this search thoroughly covers the Atomki anomaly region.


So where we stand is this:

  • There is an unexpected result in a nuclear experiment that may be interpreted as a sign for new physics.
  • The next steps in this story are independent experimental cross-checks; the threshold for a ‘discovery’ is if another experiment can verify these results.
  • Meanwhile, a theoretical framework for understanding the results in terms of a new particle has been built and is ready-and-waiting. Some of the results of this analysis are important for faithful interpretation of the experimental results.

What if it’s nothing?

This is the conservative take—and indeed, we may well find that in a few years, the possibility that Atomki was observing a new particle will be completely dead. Or perhaps a source of systematic error will be identified and the bump will go away. That’s part of doing science.

Meanwhile, there are some important take-aways in this scenario. First is the reminder that the search for light, weakly coupled particles is an important frontier in particle physics. Second, for this particular anomaly, there are some neat take aways such as a demonstration of how effective field theory can be applied to nuclear physics (see e.g. chapter 3.1.2 of the new book by Petrov and Blechman) and how tweaking our models of new particles can avoid troublesome experimental bounds. Finally, it’s a nice example of how particle physics and nuclear physics are not-too-distant cousins and how progress can be made in particle–nuclear collaborations—one of the Irvine group authors (Susan Gardner) is a bona fide nuclear theorist who was on sabbatical from the University of Kentucky.

What if it’s real?

This is a big “what if.” On the other hand, a 6.8σ effect is not a statistical fluctuation and there is no known nuclear physics to produce a new-particle-like bump given the analysis presented by the Atomki experimentalists.

The threshold for “real” is independent verification. If other experiments can confirm the anomaly, then this could be a huge step in our quest to go beyond the Standard Model. While this type of particle is unlikely to help with the Hierarchy problem of the Higgs mass, it could be a sign for other kinds of new physics. One example is the grand unification of the electroweak and strong forces; some of the ways in which these forces unify imply the existence of an additional force particle that may be light and may even have the types of couplings suggested by the anomaly.

Could it be related to other anomalies?

The Atomki anomaly isn’t the first particle physics curiosity to show up at the MeV scale. While none of these other anomalies are necessarily related to the type of particle required for the Atomki result (they may not even be compatible!), it is helpful to remember that the MeV scale may still have surprises in store for us.

  • The KTeV anomaly: The rate at which neutral pions decay into electron–positron pairs appears to be off from the expectations based on chiral perturbation theory. In 0712.0007, a group of theorists found that this discrepancy could be fit to a new particle with axial couplings. If one fixes the mass of the proposed particle to be 20 MeV, the resulting couplings happen to be in the same ballpark as those required for the Atomki anomaly. The important caveat here is that parameters for an axial vector to fit the Atomki anomaly are unknown, and mixed vector–axial states are severely constrained by atomic parity violation.
KTeV anomaly

The KTeV anomaly interpreted as a new particle, U. From 0712.0007.

  • The anomalous magnetic moment of the muon and the cosmic lithium problem: much of the progress in the field of light, weakly coupled forces comes from Maxim Pospelov. The anomalous magnetic moment of the muon, (g-2)μ, has a long-standing discrepancy from the Standard Model (see e.g. this blog post). While this may come from an error in the very, very intricate calculation and the subtle ways in which experimental data feed into it, Pospelov (and also Fayet) noted that the shift may come from a light (in the 10s of MeV range!), weakly coupled new particle like a dark photon. Similarly, Pospelov and collaborators showed that a new light particle in the 1-20 MeV range may help explain another longstanding mystery: the surprising lack of lithium in the universe (APS Physics synopsis).

Could it be related to dark matter?

A lot of recent progress in dark matter has revolved around the possibility that in addition to dark matter, there may be additional light particles that mediate interactions between dark matter and the Standard Model. If these particles are light enough, they can change the way that we expect to find dark matter in sometimes surprising ways. One interesting avenue is called self-interacting dark matter and is based on the observation that these light force carriers can deform the dark matter distribution in galaxies in ways that seem to fit astronomical observations. A 20 MeV dark photon-like particle even fits the profile of what’s required by the self-interacting dark matter paradigm, though it is very difficult to make such a particle consistent with both the Atomki anomaly and the constraints from direct detection.

Should I be excited?

Given all of the caveats listed above, some feel that it is too early to be in “drop everything, this is new physics” mode. Others may take this as a hint that’s worth exploring further—as has been done for many anomalies in the recent past. For researchers, it is prudent to be cautious, and it is paramount to be careful; but so long as one does both, then being excited about a new possibility is part what makes our job fun.

For the general public, the tentative hopes of new physics that pop up—whether it’s the Atomki anomaly, or the 750 GeV diphoton bumpa GeV bump from the galactic center, γ-ray lines at 3.5 keV and 130 GeV, or penguins at LHCb—these are the signs that we’re making use of all of the data available to search for new physics. Sometimes these hopes fizzle away, often they leave behind useful lessons about physics and directions forward. Maybe one of these days an anomaly will stick and show us the way forward.

Further Reading

Here are some of the popular-level press on the Atomki result. See the references at the top of this ParticleBite for references to the primary literature.

UC Riverside Press Release
UC Irvine Press Release
Nature News
Quanta Magazine
Quanta Magazine: Abstractions
Symmetry Magazine
Los Angeles Times


What is “Model Building”?

Thursday, August 18th, 2016

Hi everyone! It’s been a while since I’ve posted on Quantum Diaries. This post is cross-posted from ParticleBites.

One thing that makes physics, and especially particle physics, is unique in the sciences is the split between theory and experiment. The role of experimentalists is clear: they build and conduct experiments, take data and analyze it using mathematical, statistical, and numerical techniques to separate signal from background. In short, they seem to do all of the real science!

So what is it that theorists do, besides sipping espresso and scribbling on chalk boards? In this post we describe one type of theoretical work called model building. This usually falls under the umbrella of phenomenology, which in physics refers to making connections between mathematically defined theories (or models) of nature and actual experimental observations of nature.

One common scenario is that one experiment observes something unusual: an anomaly. Two things immediately happen:

  1. Other experiments find ways to cross-check to see if they can confirm the anomaly.
  2. Theorists start figure out the broader implications if the anomaly is real.

#1 is the key step in the scientific method, but in this post we’ll illuminate what #2 actually entails. The scenario looks a little like this:

An unusual experimental result (anomaly) is observed. One thing we would like to know is whether it is consistent with other experimental observations, but these other observations may not be simply related to the anomaly.

An unusual experimental result (anomaly) is observed. One thing we would like to know is whether it is consistent with other experimental observations, but these other observations may not be simply related to the anomaly.

Theorists, who have spent plenty of time mulling over the open questions in physics, are ready to apply their favorite models of new physics to see if they fit. These are the models that they know lead to elegant mathematical results, like grand unification or a solution to the Hierarchy problem. Sometimes theorists are more utilitarian, and start with “do it all” Swiss army knife theories called effective theories (or simplified models) and see if they can explain the anomaly in the context of existing constraints.

Here’s what usually happens:

Usually the nicest models of new physics don't fit! In the explicit example, the minimal supersymmetric Standard Model doesn't include a good candidate to explain the 750 GeV diphoton bump.

Usually the nicest models of new physics don’t fit! In the explicit example, the minimal supersymmetric Standard Model doesn’t include a good candidate to explain the 750 GeV diphoton bump.

Indeed, usually one needs to get creative and modify the nice-and-elegant theory to make sure it can explain the anomaly while avoiding other experimental constraints. This makes the theory a little less elegant, but sometimes nature isn’t elegant.

Candidate theory extended with a module (in this case, an additional particle). This additional model is "bolted on" to the theory to make it fit the experimental observations.

Candidate theory extended with a module (in this case, an additional particle). This additional model is “bolted on” to the theory to make it fit the experimental observations.

Now we’re feeling pretty good about ourselves. It can take quite a bit of work to hack the well-motivated original theory in a way that both explains the anomaly and avoids all other known experimental observations. A good theory can do a couple of other things:

  1. It points the way to future experiments that can test it.
  2. It can use the additional structure to explain other anomalies.

The picture for #2 is as follows:

A good hack to a theory can explain multiple anomalies. Sometimes that makes the hack a little more cumbersome. Physicists often develop their own sense of 'taste' for when a module is elegant enough.

A good hack to a theory can explain multiple anomalies. Sometimes that makes the hack a little more cumbersome. Physicists often develop their own sense of ‘taste’ for when a module is elegant enough.

Even at this stage, there can be a lot of really neat physics to be learned. Model-builders can develop a reputation for particularly clever, minimal, or inspired modules. If a module is really successful, then people will start to think about it as part of a pre-packaged deal:

A really successful hack may eventually be thought of as it's own variant of the original theory.

A really successful hack may eventually be thought of as it’s own variant of the original theory.

Model-smithing is a craft that blends together a lot of the fun of understanding how physics works—which bits of common wisdom can be bent or broken to accommodate an unexpected experimental result? Is it possible to find a simpler theory that can explain more observations? Are the observations pointing to an even deeper guiding principle?

Of course—we should also say that sometimes, while theorists are having fun developing their favorite models, other experimentalists have gone on to refute the original anomaly.


Sometimes anomalies go away and the models built to explain them don’t hold together.


But here’s the mark of a really, really good model: even if the anomaly goes away and the particular model falls out of favor, a good model will have taught other physicists something really neat about what can be done within the a given theoretical framework. Physicists get a feel for the kinds of modules that are out in the market (like an app store) and they develop a library of tricks to attack future anomalies. And if one is really fortunate, these insights can point the way to even bigger connections between physical principles.

I cannot help but end this post without one of my favorite physics jokes, courtesy of T. Tait:

 A theorist and an experimentalist are having coffee. The theorist is really excited, she tells the experimentalist, “I’ve got it—it’s a model that’s elegant, explains everything, and it’s completely predictive.”The experimentalist listens to her colleague’s idea and realizes how to test those predictions. She writes several grant applications, hires a team of postdocs and graduate students, trains them,  and builds the new experiment. After years of design, labor, and testing, the machine is ready to take data. They run for several months, and the experimentalist pores over the results.

The experimentalist knocks on the theorist’s door the next day and says, “I’m sorry—the experiment doesn’t find what you were predicting. The theory is dead.”

The theorist frowns a bit: “What a shame. Did you know I spent three whole weeks of my life writing that paper?”


Les grandes percées sont rares en physique. La recherche est plutôt jalonnée d’innombrables petites avancées et c’est ce qui ressortira de la Conférence Internationale de la Physique des Hautes Énergies (ICHEP) qui s’est ouverte hier à Chicago. On y espérait un pas de géant mais aujourd’hui les expériences CMS et ATLAS ont toutes deux rapporté que l’effet prometteur observé à 750 GeV dans les données de 2015 avait disparu. Il est vrai que ce genre de choses n’est pas rare en physique des particules étant donné la nature statistique de tous les phénomènes que nous observons.


Sur chaque figure, l’axe vertical indique le nombre d’évènements trouvés contenant une paire de photons dont la masse combinée apparaît sur l’axe horizontal en unités de GeV. (À gauche) Les points en noir représentent les données expérimentales recueillies et analysées jusqu’à présent par la Collaboration CMS, soit 12.9 fb-1, à comparer aux 2.7 fb-1 disponibles en 2015. Le trait vertical associé à chaque point représente la marge d’erreur expérimentale. En tenant compte de ces erreurs, les données sont compatibles avec ce à quoi on s’attend pour le bruit de fond, tel qu’indiqué par la courbe en vert. (À droite) Une nouvelle particule se serait manifestée sous forme d’un pic tel que celui en rouge si elle avait eu les mêmes propriétés que celles pressenties dans les données de 2015 à 750 GeV. Visiblement, les données expérimentales (points noirs) reproduisent simplement le bruit de fond. Il faut donc conclure que ce qui avait été aperçu dans les données de 2015 n’était que le fruit d’une variation statistique.

Mais dans ce cas, c’était particulièrement convainquant car le même effet avait été observé indépendamment par deux équipes qui travaillent sans se consulter et utilisent des méthodes d’analyse et des détecteurs différents. Cela avait déclenché beaucoup d’activités et d’optimisme : à ce jour, 540 articles scientifiques ont été écrits sur cette particule hypothétique qui n’a jamais existé, tant l’implication de son existence serait profonde.

Mais les théoriciens et théoriciennes ne furent pas les seuls à nourrir autant d’espoir. Beaucoup d’expérimentalistes y ont cru et ont parié sur son existence, un de mes collègues allant jusqu’à mettre en jeu une caisse d’excellent vin.

Si beaucoup de physiciens et physiciennes avaient bon espoir ou étaient même convaincus de la présence d’une nouvelle particule, les deux expériences ont néanmoins affiché la plus grande prudence. En l’absence de preuves irréfutables de sa présence, aucune des deux collaborations, ATLAS et CMS, n’a revendiqué quoi que ce soit. Ceci est caractéristique des scientifiques : on parle de découvertes seulement lorsqu’il ne subsiste plus aucun doute.

Mais beaucoup de physiciens et physiciennes, moi y compris, ont délaissé un peu leurs réserves, non seulement parce que les chances que cet effet disparaisse étaient très minces, mais aussi parce que cela aurait été une découverte beaucoup plus grande que celle du boson de Higgs, générant du coup beaucoup d’enthousiasme. Tout le monde soupçonne qu’il doit exister d’autres particules au-delà de celles déjà connues et décrites par le Modèle standard de la physique des particules. Mais malgré des années passées à leur recherche, nous n’avons toujours rien à nous mettre sous la dent.

Depuis que le Grand collisionneur de hadrons (LHC) du CERN opère à plus haute énergie, ayant passé de 8 TeV à 13 TeV en 2015, les chances d’une découverte majeure sont plus fortes que jamais. Disposer de plus d’énergie donne accès à des territoires jamais explorés auparavant.

Jusqu’ici, les données de 2015 n’ont pas révélé la présence de particules ou phénomènes nouveaux mais la quantité de données recueillies était vraiment limitée. Au contraire, cette année le LHC se surpasse, ayant déjà produit cinq fois plus de données que l’année dernière. On espère y découvrir éventuellement les premiers signes d’un effet révolutionnaire. Des dizaines de nouvelles analyses basées sur ces données récentes seront présentées à la conférence ICHEP jusqu’au 10 août et j’en reparlerai sous peu.

Il a fallu 48 ans pour découvrir le boson de Higgs après qu’il fut postulé théoriquement alors qu’on savait ce que l’on voulait trouver. Mais aujourd’hui, nous ne savons même pas ce que nous cherchons. Cela pourrait donc prendre encore un peu de temps. Il y a autre chose, tout le monde le sait. Mais quand le trouverons nous, ça, c’est une autre histoire.

Pauline Gagnon

Pour en savoir plus sur la physique des particules et les enjeux du LHC, consultez mon livre : « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels».

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Giant leaps are rare in physics. Scientific research is rather a long process made of countless small steps and this is what will be presented throughout the week at the International Conference on High Energy Physics (ICHEP) in Chicago. While many hoped for a major breakthrough, today, both the CMS and ATLAS experiments reported that the promising effect observed at 750 GeV in last year’s data has vanished. True, this is not uncommon in particle physics given the statistical nature of all phenomena we observe.


On both plots, the vertical axis gives the number of events found containing a pair of photons with a combined mass given in units of GeV (horizontal axis) (Left plot) The black dots represent all data collected in 2016 and analysed so far by the CMS Collaboration, namely 12.9 fb-1, compared to the 2.7 fb-1 available in 2015. The vertical line associated with each data point represents the experimental error margin. Taking these errors into account, the data are compatible with what is expected from various backgrounds, as indicated by the green curve. (Right) A new particle would have manifested itself as a peak as big as the red one shown here if it had the same features as what had been seen in the 2015 data around 750 GeV. Clearly, the black data points pretty much reproduce the background. Hence, we must conclude that what was seen in the 2015 data was simply due to a statistical fluctuation.

What was particularly compelling in this case was that the very same effect had been observed by two independent teams, who worked without consulting each other and used different detectors and analysis methods. This triggered frantic activity and much expectation: to date, 540 scientific theory papers have been written on a hypothetical particle that never was, so profound the implications of the existence of such a new particle would be.

But theorists were not the only ones to be so hopeful. Many experimentalists had taken strong bets, one of my colleagues going as far as putting a case of very expensive wine on it.

If many physicists were hopeful or even convinced of the presence of a new particle, both experiments nevertheless had been very cautious. Without unambiguous signs of its presence, neither the ATLAS nor the CMS Collaborations had made claims. This is very typical of scientists: one should not claim anything until it has been established beyond any conceivable doubt.

But many theorists and experimentalists, including myself, threw some of our caution to the air, not only because the chances it would vanish were so small but also because it would have been a much bigger discovery than that of the Higgs boson, generating much enthusiasm. As it stands, we all suspect that there are other particles out there, beyond the known ones, those described by the Standard Model of particle physics. But despite years spent looking for them, we still have nothing to chew on. In 2015, the Large Hadron Collider at CERN raised its operating energy, going from 8 TeV to the current 13 TeV, making the odds for a discovery stronger than ever since higher energy means access to territories never explored before.

So far, the 2015 data has not revealed any new particle or phenomena but the amount of data collected was really small. On the contrary, this year, the LHC is outperforming itself, having already delivered five times more data than last year. The hope is that these data will eventually reveal the first signs of something revolutionary. Dozens of new analyses based on the recent data will be presented until August 10 at the ICHEP conference and I’ll present some of them later on.

It took 48 years to discover the Higgs boson after it was first theoretically predicted when we knew what to expect. This time, we don’t even know what we are looking for. So it could still take a little longer. There is more to be found, we all know it. But when will we find it, is another story.

Pauline Gagnon

To find out more about particle physics, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN ».

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Earlier last month, Romania became the 22nd Member State of the European Organisation for Nuclear Research, or CERN, home to the world’s most powerful atom-smasher. But the hundred Romanian scientists working on experiments there have already operated under a co-operation agreement with CERN for the last 25 years. So why have Romania decided to commit the money and resources needed to become a full member? Is this just bureaucratic reshuffling or the road to a more fruitful collaboration between scientists?

Image: CERN

On 18th July, Romania became a full member state of CERN. In doing so, it joined twenty one other countries, which over the years have created one of the largest scientific collaborations in the world. Last year, the two largest experimental groups at CERN, ATLAS and CMS, broke the world record for the total number of authors on a research article (detailing the mass of the Higgs Boson).

To meet its requirements for becoming a member, Romania has committed $11mil USD towards the CERN budget this year, three times as much as neighbouring member Bulgaria and more than seven times as much as Serbia, which holds Associate Membership, aiming to follow in Romania’s footsteps. In return, Romania now holds a place on CERN’s council, having a say in all the major research decisions of the ground-breaking organization where the forces of nature are probed, antimatter is created and Higgs Bosons discovered.

Romania’s accession to the CERN convention marks another milestone in the organisation’s history of international participation over the last sixty years. In that time it has built bridges between the members of nations where diplomacy and international relations were less than favourable, uniting researchers from across the globe towards the goal of understanding the universe on its most fundamental level.

CERN was founded in 1954 with the acceptance of its convention by twelve European nations in a joint effort for nuclear research, the year where “nuclear research” included the largest ever thermonuclear detonation by the US in its history and the USSR deliberately testing the effects of nuclear radiation from a bomb on 45,000 of its own soldiers. Despite the Cold War climate and the widespread use of nuclear physics as a means of creating apocalyptic weapons, CERN’s founding convention alongside UNESCO, which member states adhere to today, states:

“The Organization shall provide for collaboration among European States in nuclear research of a pure scientific and fundamental character…The Organization shall have no concern with work for military requirements,”

The provisional Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research) was dissolved and its legacy was carried by the labs built and operated under the convention it had laid and the name it bore: CERN. Several years later in 1959, the British director of the Proton Synchrotron division at CERN, John Adams, received a gift of vodka from Soviet scientist Vladimir Nikitin of the Dubna accelerator, just north of Moscow, and at the time the most powerful accelerator in the world. 

The vodka was to be opened in the event the Proton Synchrotron accelerator at CERN was successfully operated at an energy greater than Dubna’s maximum capacity: 10 GeV. It more than doubled the feat, reaching 24 GeV, and with the vodka dutifully polished off, the bottle was stuffed with a photo of the proton beam readout and sent back to Moscow.

John Adams, holding the empty vodka bottle in celebration of the Proton Synchroton’s successful start (Image: CERN-HI-5901881-1 CERN Document Server)

Soviet scientists contributed more than vodka to the international effort in particle physics. Nikitin would later go on to work alongside other soviet and US scientists in a joint effort at Fermilab in 1972. Over the next few decades, ten more member states would join CERN permanently, including Israel, its first non-European member. On top of this, researchers at CERN now join from four associate member nations, four observer states (India, Japan, USA and Russia) and holds a score of cooperation agreements with other non-member states.

While certainly the largest collaboration of this kind, CERN is certainly no longer unique in being a collaborative effort in particle physics. Quantum Diaries is host to the blogs of many experiments all of whom comprise of a highly diverse and internationally sourced research cohort. The synchrotron lab for the Middle East, SESAME, expected to begin operation next year, will involve both the Palestinian and Israeli authorities with hopes it “will foster dialogue and better understanding between scientists of all ages with diverse cultural, political and religious backgrounds,”. It was co-ordinated in part, by CERN.

I have avoided speaking personally so far, but one needs to address the elephant in the room. As a British scientist, I speak from a nation where the dust is only just settling on the decision to cut ties with the European Union, against the wishes of the vast majority of researchers. Although our membership to CERN will remain secure, other projects and our relationship with european collaborators face uncertainty.

While I certainly won’t deign to give my view on the matter of a democratic vote, it is encouraging to take a look back at a fruitful history of unity between nations and celebrate Romania’s new Member State status as a sign that that particle physics community is still, largely an integrated and international one. In the short year that I have been at University College London, I have not yet attended any international conferences, yet have had the pleasure to meet and learn from visiting researchers from all over the globe. As this year’s International Conference on High Energy Physics kicks off this week, (chock-full of 5-σ BSM discovery announcements, no doubt*), there is something comforting in knowing I will be sharing my excitement, frustration and surprise with like-minded graduate students from the world over.

Kind regards to Ashwin Chopra and Daniel Quill of University College London for their corrections and contributions, all mistakes are unreservedly my own.
*this is, obviously, playful satire, except for the case of an announcement in which case it is prophetic foresight.