by Pierre Grivet
January 1, 1967
On the left bank of the Seine, not far from the Latin Quarter, is the Institut de France. It is there that M. de Broglie, Nobel Prize laureate in 1929 and perpetual secretary of the Académie des Sciences, works. Prof. Pierre Grivet is asking him some questions.
Grivlet: Mr. De Broglie, you received the Nobel Prize in Physics in 1929 for your discovery of electron waves, which are now named after you. Could you please explain the significance of these brand new waves and how they deeply changed our conception of mechanics, and in fact, created a new science: wave mechanics?
de Broglie: Yes, it was in 1929 that I received the Nobel Prize in Physics for the discovery of the wave nature of electrons. It was the result of a very long reflection. In 1905, when I was 13, Mr. Einstein discovered that in light, there are not only waves, which were already well-known, but also particles, which enabled him to explain the photoelectric effect, which was still unexplainable at the time. I had also studied a lot about the theory of relativity. It was after all of these studies that I came up with the idea that the idea of a wave should be extended to all concrete particles, and in particular to electrons. This deeply changed particle mechanics because up until then, we could use well-known laws to calculate the trajectory of particles, but after the introduction of wave mechanics, the particle trajectory was determined based on the trajectory of a wave. As a result, some wave-like behavior would appear even in the case of particles like electrons. For example, if there are many electrons associated with the same wave, it can happen in some circumstances that these electrons spread in a way that corresponds to diffraction phenomena which are well known for light. The experimental confirmation of the existence of these diffraction phenomena in electrons provided verification of my ideas in 1927.
Grivlet: These studies were made in 1923-1924. Could you please tell us if your work on radioelectricity, when you were enrolled in the military at the Eiffel Tower during the 1st world war, contributed to the development of these ideas.
de Broglie: What I can tell you is that it was in 1923 when I wrote my first notes on wave mechanics. I then revisited the topic in a more elaborate way for my PhD thesis in 1924. However, during my military conscription in 1913, I had already begun to think about this topic in a very elementary way, admittedly because I was still very young. I may have been more efficient if serious circumstances had not changed the course of my life for a few years. During the First World War, I was mobilized for five years at the Eiffel Tower where I dedicated myself to radioelectricity. This may have delayed my work, but it was not totally useless for the development of my ideas. During those five years, I had to study a lot about electromagnetic waves and electrons and became familiar with them at a time when radio, which was still at an embryonic stage, was developing at a very fast pace. It was during this time that three-electrodes tubes were introduced for the reception and emission of electromagnetic waves. I learned a lot about practical things, thought a lot about waves and electrons, and this may have prepared me for the work I accomplished in later years.
Grivlet: Is it true that you are what we call today a "pure theoretician" and that you made all your discoveries using only a pen and a sheet of paper? Or was the fact that your older brother was an experimental physicist and the head of a prestigious research laboratory important for the evolution of your ideas?
de Broglie: It is certainly true that I am, in a way, a pure theoretician. I have never done any experimental work myself, and it is always at my desk, with a pen in hand, that I work on theoretical questions. However, it is also true that during my youth, I had contact with experimental groups. Not only at the Eiffel Tower, as I just mentioned, where I was working in a laboratory, but also at my brother's lab, where we worked on X-rays. I did quite a bit of work with him and he taught me a lot. I also did some theoretical work on the experimental results obtained in my brother's lab. So, at that time, I certainly acquired knowledge that became very useful later for the development of my theoretical work.
Grivlet: Your discovery was so revolutionary that it was not immediately accepted. Is it true that you based it on the theory of relativity, and that Einstein was the first to acknowledge its extreme importance? He immediately asked one of his students, Mr. Elsasser, to study it. Could you please tell us the role that the theory of relativity plays in your conception of physics?
de Broglie: Yes, it is easy for me to answer. My works on wave mechanics were heavily inspired by the theory of relativity, and I actually regret that nowadays this origin has been forgotten, and people have forgotten the relativistic considerations on which I relied. This is probably because my theory was indeed very relativistic that it grabbed Einstein's attention. I submitted my PhD thesis to Paul Langevin, who was maybe a bit reluctant to such a new theory. He forwarded it to Einstein, asking for his advice, and Einstein replied by saying that it was extremely interesting. He even later published a note in the proceedings of the Berlin Academy of Science, in which he mentioned my works. I think this is what made people aware of them, including Schrödinger and others who continued my work in the same direction. I am not sure if Elsasser was commissioned by Einstein to study this problem, but I know that he published some works on the same topic almost at the same time, which makes it very likely. So, I believe that the theory of relativity plays a major role, much bigger than most people usually think, in the basic ideas of wave mechanics, and if one wants to understand its origin, one has to go back to relativistic considerations.
Grivlet: our wave mechanics was mainly developed during the 1930s, and it was during that time that the importance of the uncertainty principle became apparent. This had major philosophical consequences, as it gave philosophers arguments against determinism. Do you think that these philosophical developments are interesting, either for philosophy itself or for science?
de Broglie: Well, it is true that around 1927-1930, there was an evolution in our interpretation of wave mechanics that led a great number of physicists to think that we mostly had to give up physical determinism. This was also around the time that Heisenberg introduced his uncertainty principle, which states that there are phenomena that we can't fully describe. Philosophers naturally discussed this idea and, perhaps imprudently, also generalized it. However, I think that today we have gone too far in that direction, and in reality, the uncertainty principle is just that – a principle of uncertainty. This means that we don't know, we can't know exactly, things, but this does not mean that they are undetermined, which is a much stronger claim. As of today, my thoughts lead me to affirm that it would be quite advantageous to go back to much more precise pictures because I believe that science is always about creating causal links between phenomena, and I think that this search for causality will always be extremely productive for science. Of course, affirming the absolute determinism of all phenomena may be too much. I am careful and do not want to make such a claim, but I do believe that bringing back the idea of causality is something that would be very useful in science. Einstein expressed the opinion that the probabilistic interpretation of quantum mechanics did not reach its ultimate development. He said, while talking about the uncertainty principle: "I do not believe in this little god that plays with dice."
Grivlet: Could you please give us your opinion on this philosophical issue?
de Broglie: Certainly. When the interpretation of wave mechanics and quantum mechanics was being developed using uncertainties, Einstein, as well as Schrödinger, who were important founders of quantum theory, thought that the purely probabilistic interpretation was not exact. This is why Einstein said that God was not always playing dice when phenomena were taking place. Einstein and Schrödinger actually made very precise objections, and I think that they are not purely philosophical objections but objections to the adopted theories regarding very precise cases. Personally, I have thought a lot about these objections, and I now believe that they are very strong, and that we have to come back to a theory that will be way less profoundly probabilistic. It will introduce probabilities, a bit like it used to be the case for the kinetic theory of gases, but not to an extent that forces us to believe that there is no causality.
Grivlet: You have written many books, some on theory for specialists and others for a broader audience, and two on applications: radar and electron microscopy. What are your general thoughts on the connection between teaching and research?
de Broglie: I have indeed written a lot of books, not for general readership but scientific books that are exclusively compilations of lectures I gave during my 34-year tenure as a professor at the Faculté des Sciences de Paris, with the exception of one dedicated to electromagnetic wave guiding. These lectures covered a broad range of topics, and two of my books were dedicated to applications, as you mentioned. I have always considered teaching to be very important, and I always wanted to write down what I taught because I thought it was useful. Hence, I believe that teaching and research are strongly linked, and when someone teaches, they learn how to precisely articulate their ideas, which helps with research. However, there is a little drawback that I mentioned in some of my lectures - teaching can force you to be a little bit dogmatic. One can't say in front of their students, "I think it might be this way, but it could also be the opposite," because it may produce a negative effect on the students. Teaching can also trick the mind into thinking certain things are sure, even when they are not. Nonetheless, teaching and research are really useful things, and when one person takes care of both, it contributes a lot to making their research easier.
Grivlet: our students have mentioned that you are currently very interested in the profound nature of light. Could you please explain what fascinates you about light and why this seemingly simple and accessible phenomenon is as intriguing to you as strange particles from large colliders are to others?
de Broglie: I mentioned earlier that my work on wave mechanics was based on Einstein's quantum theory of light, in which he showed that light consists of both waves and particles. One should not think that the current theory of light is completely understood, despite this important progress, because we still need to understand how particles and waves are associated. I believe this problem is far from being solved, even using current formalisms. Furthermore, it was based on this model that I developed wave mechanics, and this is why I returned to the study of light phenomena and other particles that are connected with the theory of light, slightly neglecting, for the moment, the very interesting problems in the field of elementary particles that I used to carefully follow with great interest. I think there is still a lot to do in the theory of light. There were actually new phenomena recently discovered, notably in the functioning of lasers and the Brown and Twiss phenomenon, in which we compare the fluctuations of light at different points. All these questions, when carefully studied, seem to greatly improve our understanding of light and our general ideas regarding the association of waves and particles.