This article is part of the “Building from Diversity” project. Written by Laura Paganini, science communicator at the INAF Osservatorio Astronomico di Brera

When you think about women that strongly impacted the history of science, Marie Curie will most likely come to your mind. Maria Salomea Skłodowska–Curie was the first woman to win the Nobel Prize, the only woman to do so twice and the only person to obtain it in two different scientific disciplines: physics and chemistry [1]

Marie and her husband, Pierre Curie, were two of the pioneers in the study of radioactivity. At the end of the 19th century, Antoine Henri Becquerel discovered that uranium emitted a type of radioactivity similar to X-rays, but its true nature was unknown. Fascinated by this new discovery, the Curies dedicated their life to studying radioactive elements and the origin of radioactivity, leading to a completely different comprehension of the atoms [2]. This is the part of the story we all know, and that made Marie Curie famous around the world. There is a lesser-known part of the story, documented in Marie’s laboratory notebooks and narrated by her daughter Eve in the book “Madame Curie. A biography” [3]. The story involves Cherenkov radiation – the same radiation used by the CTAO telescopes to study the high-energy gamma-ray Universe.

But let’s start with some scientific context. The radioactive materials, as those studied by Marie Curie, are formed by heavy atoms with nuclei composed of a large number of protons and neutrons that tend to “decay” spontaneously. In such elements, the ratio between the number of protons and neutrons is not energetically optimal, and the nucleus cannot be held together anymore. At that point, the nucleus needs to release energy to go back to a stable status. This is the so-called radioactive decay. Since energy can neither be destroyed nor created, but it can be converted, the atom reaches that stable energy by emitting either a photon or a particle. Through her examination of uranium, Marie proposed that this emission originated naturally and inherently from the atoms themselves, rather than being caused by external interactions. This hypothesis played a significant role in supporting the idea that atoms were divisible, a concept that had not yet been firmly established.

Today this is widely accepted, but, at the beginning of the 20th century, many of these physical mechanisms were not yet known, especially when involving atomic and subatomic phenomena and their interaction with light. When working with such a complex phenomenon, it is normal to be able to observe it without knowing how to explain it, especially if you are exploring the phenomenon for the first time ever, as in the case of the Curies!

Two radioactive elements that undergo this decay are polonium and radium, discovered by the Curies [4]. Marie once said: “Prodigious radium! Purified as a chloride, it appeared to be a dull white powder, which might easily be mistaken for common kitchen salt” [3]. So powerful, so complex, and yet, the solution of radium was simply the powder dissolved in water.

Today, we know that, in its decay, radium emits a helium nucleus (commonly known as “alpha particle”), giving rise to a chain of lighter nuclei and eventually electrons, which can move faster than light in the medium in which it is emitted (such as water). When this happens, a bluish light is emitted. Marie did not know it at the time, but she was looking at what years later would be known as the Cherenkov effect.

In 1910, Marie Curie, in fact, noticed the strange blue light and wrote in her notebook: “Nor was this the end of the wonders of radium. It also gave phosphorescence to a large number of bodies incapable of emitting light by their own means” [3, 5]. She wrongly attributed this blue emission to phosphorescence, which was the only phenomenon known at that time. However, the truth was that she was observing Cherenkov radiation, which would be officially discovered in 1934. Therefore, Marie Curie was one of the first people to observe Cherenkov radiation, albeit unknowingly. There were several others who came across this elusive radiation. In 1888, Oliver Heaviside wrote about it in a scientific paper (a) that was mostly disregarded: “If the speed of the motion exceeds that of light, the disturbances are wholly left behind the charge, and are confined within a cone.” And, in 1904, Arnold Sommerfeld theoretically predicted Cherenkov radiation, as well. But, once again, the scientific community missed the clue. It was not until 1922 that Marie’s French colleague, Leon Mallett, began studying the phenomenon, albeit still without much fortune. We had to wait another decade for the studies of Tamm, Frank, Vavilov and Cherenkov in 1934 to finally establish the true nature of this radiation [6].

And what was so overlooked in the 20^th century has now opened a new window of science: astroparticle physics, the newest field in astronomy and astrophysics. Cherenkov light can also be produced in the air when a gamma ray (photons, or light, with extremely high energy) arrives in the Earth’s atmosphere and produces a particle shower. These particles move faster than the speed of light in the air of the atmosphere giving rise to this bluish flash of light: Cherenkov light. It lasts barely a billionth of a second, so we cannot see it with our eyes, but with Cherenkov telescopes we can. And the CTAO will use these telescopes to unravel the high-energy Universe. From a dim light in Marie’s lab to opening a new field of study.

In Marie Curie’s own words (b): “I am among those who think that science has great beauty. A scientist in his laboratory is not only a technician: he is also a child placed before natural phenomena which impress him like a fairy tale. We should not allow it to be believed that all scientific progress can be reduced to mechanisms, machines, gearings, even though such machinery also has its beauty.”

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Article reviewed by Anna Wolter, researcher at the INAF Osservatorio Astronomico di Brera (Italy).

References

1. https://en.wikipedia.org/wiki/Marie_Curie
2. https://www.nobelprize.org/prizes/physics/1903/marie-curie/facts/
3. Madame Curie. A Biography, Eve Curie, 1947
4. https://www.nobelprize.org/prizes/themes/marie-and-pierre-curie-and-the-discovery-of-polonium-and-radium/
5. Handbook of Radioactivity Analysis (Third Edition), Michael F. L’Annunziata, 2012
6. APSNews (Volume 29, Number 11), 2020

(a) The paper “Electromagnetic waves, the propagation of potential, and the electromagnetic effects of a moving charge” was published in “The Electrician”, in 1888

(b) As quoted in Madame Curie: A Biography (1937) by Eve Curie Labouisse, as translated by Vincent Sheean, p. 341