At the beginning of the seventeenth century, knowledge about light was essentially the same as that which Ptolemy had summarized and then supplemented and developed by Alhazen,” says the Hungarian science historian Károly Simonyi in his brilliant “Cultural History of Physics”.
The fact that this was to change abruptly shortly afterwards is mainly due to the discovery of the telescope, but also to the development of increasingly precise clocks.
Serge Haroche tells the details of this story in his book.
The author is a professor at the Collège de France and the École Normale Supérieure (ENS) in Paris and, as an experimental physicist, has himself contributed significant insights into the nature of light.
For this he was awarded the Nobel Prize in Physics in 2012.
Of course, he is primarily concerned with light as a physical phenomenon, but for Haroche it is also important as a metaphor that is often used, such as in the French speech of the Siècle des Lumières for the epoch of the Enlightenment.
And it is also Haroche's job to provide enlightenment.
Before the author gets involved in the history of light, he provides insights into his own career.
He is now being asked more and more often how he got involved in researching light in the laboratory and in such a successful way.
We learn that he was very fond of astronomy from a young age and was fascinated by mathematical structures, such as the curious number pi, which appears in fields as diverse as geometry and statistics.
But as is so often the case, chance decides on the further lifelines.
Schrödinger's infamous cat
As a physics student at ENS in the 1960s, he was fortunate to attend lectures by excellent teachers and researchers who introduced him to the arcana of quantum physics.
Above all, Claude Cohen-Tannoudji should be mentioned, who was to receive the Nobel Prize in 1997 for his method of cooling atoms with laser light.
Haroche received his doctorate from him in 1971.
In fact, it was the laser, available since around 1960, that drove research in the ENS working group.
In the 1950s, two ENS scientists, Alfred Kastler and Jean Brossel, developed the optical pump that is important for the function of the laser, raising electrons to higher energy levels in the atom.
For this Kastler received the Nobel Prize in 1966, Brossel received nothing;
the history of the Nobel Prizes is always a history of those who have been ignored.
Unlike today, there was a great deal of freedom in the labs at the time, as Haroche wistfully notes in retrospect.
The heads of the working groups could mainly focus on research and support instead of bothering with third-party funding.
After this autobiographical introduction, the author turns to the history of light in four easy-to-read chapters.
Of course one encounters Galileo and Newton, Christiaan Huygens, Thomas Young, Augustin Fresnel, Michael Faraday and many others.
Newton envisioned light as consisting of small particles, while Huygens assumed the wave nature of light.
Not surprisingly, Newton's authority ensured the dominance of particle theory, which would only change with Young's interference experiments in the early nineteenth century.
An appeal for the freedom of basic research
Highlights of this story are Albert Einstein's work on the light quantum hypothesis and the special theory of relativity.
However, the light quanta of 1905 are not the later so-called photons.
As the quantum theory developed in the 1920s was supposed to teach, the latter do not behave like classical particles that are independent of one another, but, according to Einstein, “squat together relatively more frequently”.
According to the modern view of quantum theory, light (like matter) has both particle and wave properties, and it depends on the circumstances which aspect predominates.
The author dedicates a detailed presentation to the "oddities" of quantum theory, which of course also includes Schrödinger's notorious cat,
Light from the early days of the cosmos can be observed in the cosmic background radiation discovered in 1965.
The fact that there is anything other than light in the universe is due to the lack of symmetry between particles and antiparticles, which ensured a sufficient number of particles for our existence.
Otherwise, as Hans Blumenberg once put it, "an overwhelming situation from the point of view of absolute enlightenment: a lot of light, but no world that could ever have shone".
However, Haroche only touches on cosmology.
His focus is on laboratory physics and in particular the results of his own working group, which are presented in detail and carefully in the remaining chapters.
Without a solid knowledge of physics, you won't be able to enjoy this part throughout, but even then, the author's passion for his subject can be felt in every line.
Haroche and his group have succeeded in creating and controlling "Schrödinger cats" with single photons.
These individual photons can be studied using atoms and, in particular, make it possible to shed light on the transition from quantum properties to classical properties, but also to improve the accuracy of clocks.
It is this research that has been recognized by the Nobel Prize Committee.
Serge Haroche: "Light". A story.
Translated from the French by Ursula Held.
Klett-Cotta Verlag, Stuttgart 2022. 464 p., ill., hardcover, 35 euros.