28.06.2021 : Of a different era..

Many neutron star theorists spend their working lives delving into the intricacies of nuclear physics. While nuclear theories allow astrophysicists to interpret faint signals coming from exotic neutron stars; these celestial objects, in turn, act as unique laboratories (unlike any that can be built on Earth) for the nuclear physicists. Recently, my friend Sarmistha Banik organised a meeting to discuss such interconnections (between nuclear physics theory and neutron star observation) and to celebrate the sixtieth birthday of Prof. Debades Bandyopadhyaya (her academic guru) who has been working in this area for a very long time.

But neutron stars arrived later (discovered in 1967) on the scene. Nuclear Astrophysics has been walking hand-in-hand with nuclear physics since the beginning of the twentieth century. The last century decidedly belonged to physics. Even the world wars were fought on the back of physicists. Not surprisingly, many (or perhaps most) of the nuclear physicists (as well as nuclear astrophysicists) were involved in the process of Bomb making.

German-American Maria Geppert-Meyer (28.06.1906 - 20.02.1972) was one such nuclear physicist, whose nuclear shell model, explaining the properties of atomic nuclei, earned her the Nobel prize. [In the entire history of the Nobel Prize (1901 - 2021), only FOUR women physicists have been honoured - Marie Curie (1903), Maria Geppert-Mayer (1963), Donna Strickland (2018) and Andrea Ghez (2020). Though Geppert-Mayer had to wait for sixty long years after Curie, the last two have come in a quick succession and one hopes that the future would be reflective of the mores of our own times.]

Geppert-Mayer worked at Gottingen University for her doctoral thesis and moved to the US (after marrying Joseph Edward Mayer) in the 1930s. Then war came and she worked at Columbia University on the separation of uranium isotopes for the atomic bomb project. At around the same time Carl Friedrich von Weizsäcker (28.06.1912 – 28.04.2007), exactly six years younger than Geppert-Mayer and a former student of Gottingen, was also working on the atomic bomb project, albeit on the `wrong' side of the Atlantic.

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Weizsäcker, was a prominent member of the team (headed by Warner Heisenberg) that worked on the German Bomb-making project. In August 1939, Albert Einstein wrote to U.S. President Franklin D. Roosevelt to say - "the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated". Uranium-enrichment was one of the key steps of the Bomb project.

Heisenberg and Weizsacker, 1933

The opinions have been divided about the nature of Weizsacker's (and others') participation in the Bomb project. Robert Jungk's Brighter than a Thousand Suns, based on post-war interviews with Weizsacker, suggested that they intentionally held the program back. But letters and documents appearing later have hinted at a somewhat different version of the story. Even now, the story intrigues people urging modern playwrights to interpret the events in their own way. Farm Hall by David Cassidy and Operation Epsilon by Alan Brody are two such efforts in recent years. 

Who knows? Perhaps politics was in von Weizsäcker’s blood. His father was a career diplomat and his younger brother, Richard von Weizsäcker, was that West German president who presided over the reunification of Germany in 1990.

Nevertheless, after the war Weizsacker joined a group of prominent German physicists to protest against the nuclear armament of West Germany and in later life moved to philosophical and ethical issues - a behaviour not very different from many of the physicists who participated in the Manhattan Project. 

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Notwithstanding his war efforts (or lack thereof), von Weizsäcker would be remembered for his pre-war work on nuclear physics. Like Geppert-Mayer, he too worked on the atomic nuclei and developed (1937) a method, now known as the Bethe–Weizsäcker formula, to estimate the mass and other properties of atomic nuclei from the number of constituent neutrons and protons.

Weizsäcker also developed a theory of the Solar System formation with the important implication that other stars like our Sun are also likely to have planetary systems similar to our own. This theory was later corroborated by Russian-American physicist George Gamow (and others) after the War.   

However, I feel the most significant work of von Weizsäcker has been his discovery of the CNO cycle, in which carbon, nitrogen, and oxygen act as catalysts in a sequence of nuclear reactions that leads to the conversion of hydrogen into helium. Later (1939), Hans Bethe corroborated this through a more detailed work and the process came to be known as the Bethe–Weizsäcker cycle. Interestingly, Bethe's family had moved to Kiel in 1912, where Weizsacker was born that year. Bethe left Germany in 1933, but their intellectual endeavours converged again and again.

CNO cycle is one the two known processes through which the stars convert hydrogen into helium, and is expected to be dominant in stars that are somewhat heavier than our Sun. It is only in 2020 that the Borexino collaboration has finally detected the neutrinos from the Sun that bear distinct signatures of the CNO-cycle (Nature, 2020, 587, 577).

The Borexino neutrino detector lies deep under the Apennine Mountains in central Italy, patiently waiting for astrophysical neutrinos. Let us hope that such exciting detections from the "India-based Neutrino Observatory" (INO), planned to be constructed under the Bodi West Hills in Tamil Nadu, are not too far in the future.  

[Today also happens to be the birth-centenary of P. V. Narasimha Rao (28.06.1921 – 23.12.2004), the 9th Prime Minister (1991 - 1996), who opened up the Indian economy ushering in the vibrancy we have become so used to in the last few decades. But that is a completely different story requiring a completely different kind of storyteller. 😁]