On the left bank of the Seine, not far away from the Latin quarter, the Institut de France. This 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. M. De Broglie, you received the Nobel Prize in Physics in 1929 for your discovery of electron waves, which are nowadays named after you. Could you please explain the signification of these brand new waves, and how they deeply changed our conception of mechanics, and, in fact, created a new science: wave mechanics. 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, M. Einstein discovered that, in light, there are not only waves, as already well-known for long, but also particles making him able to explain the photoelectric effect, that was still unexplainable at this time. And, I had also studied a lot about the theory of relativity. This is after all these studies that I came up with the idea that one should extend to every concrete particles, and in particular to electrons, the idea that the particle is accompanied by a wave. This was deeply changing particle mechanics in that, up to now, we could use well known laws to calculate the trajectory of particles, whereas after the introduction of wave mechanics, the particle trajectory was now determined based on the trajectory of a wave so that some wave-like behaviour would now appear even in the case of particles like electrons. For example, if there are a lot of electrons associated to a same wave, it can happen, in some circumstances, that these electrons spread in a way which corresponds to diffraction phenomena which are well known for light, and this is actually the verification of the existence of these diffraction phenomena that provided the experimental confirmation of my ideas in 1927. 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. What I can tell on that, is that it was in 1923 that I wrote my first notes on wave mechanics and I then started again on this topic in a more elaborate way in my PhD thesis in 1924 But in 1913, during my military conscription, I already oriented my thoughts towards this topic in a very elementary way admittedly, because I was still very young but I might have been more efficient if serious circumstances did not change the course of my life during few years Indeed, during the first world war, I was mobilized 5 years at the Eiffel Tower where I dedicated myself to radioelectricity This might have delayed my works but it was, nevertheless, not totally useless for the development of my ideas because during these 5 years I had to study a lot about electromagnetic waves and electrons and to become familiar with them at a time when radio, which was still at an embryonic stage, was developing at a very fast pace because this is at this time that three-electrodes tubes were introduced for the reception and then the emission of electromagnetic waves so that I certainly learnt a lot about practical things, thought a lot about waves and electrons, and this may have prepared the works I accomplished during the later years. Is that 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, head of a prestigious research laboratory, important for the evolution of your ideas ? It is certainly true that I am, in a way, a pure theoretician I never did any experimental work myself, and it is always at my table, with a pen, if you want, that I worked on the theoretical questions but it is also true that during my youth, I had contacts with experimental groups not only at the Eiffel Tower, like I was just mentioning, where I was working in a laboratory but also at my brother's lab, where we were working on X-rays I did quite some work with him and he taught me quite a lot of things, and I also did some theoretical works myself on the experimental results obtained in my brother's lab so that, at this time, I certainly gathered knowledge that became very useful later for the development of my theoretical works. 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 ? Well, yes, it is very easy for me to answer it is certain that my works on wave mechanics were heavily inspired by the theory of relativity, and I actually regret that nowadays this origin got forgotten and that we forgot the relativistic considerations on which I was relying This is probably because my theory was indeed really relativistic that it grabbed Mr. 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 M. Einstein, asking for his advice and Einstein answered him by saying that it was extremely interesting, he even later published a note in the proceeding of the Berlin Academy of Science, in which he was mentioning my works and I think that this is what made people aware of them, in particular Mr. Schrödinger and others that continued my work in the same direction I don't know whether Mr. Elsasser was commisioned by Einstein to study this problem but I know that he published, almost at the same time, some works on the same topic, which brings me to the idea that it is indeed very likely So I think that the theory of relativity plays a major role, much bigger than most people usually think, in the basic ideas of wave mechanics and that, if one wants to really understand its origin, one has to come back to relativistic considerations. Your mechanics was mainly developed during the 30's, it is at that time that the importance of the uncertainty principle appeared This had major philosophical consequences by giving philosophers arguments against determinism. Do you think that these philosophical developments are interesting, either for philosophy itself, or for science ? Well, it is true that around 1927-1930, there was an evolution in our interpretation of wave mechanics that lead a great number of physicists to think that we mostly had to give up physical determinism and this is also around that time that Heisenberg exposed its uncertainty principle, according to which there are phenomena that we can't fully describe Philosophers naturally discussed this idea and, maybe imprudently, also generalized it But I think that today we have been to far in that direction and that in reality, the uncertainty principle is only an uncertainty principle that means that we don't know, we can't exactly know, things but this does not mean that they are undetermined, which is a much bigger statement As of today, my thoughts lead me to affirm that it would be quite advantageous to go back to much precise pictures because I think that science is always about creating causality links between phenomena and I think that this research of causality will always be extremely productive for science Obviously, affirming the absolute determinism of all phenomena may be too much I am careful and I don't want to state that but I think that bringing back the causality link is something which is very useful in science Einstein expressed the opinion that the probabilistic interpretation of your mechanics did not reach its ultimate development He said, while talking about the uncertainty principle: "I don't believe in this little god that plays with dices" Could you please give us your opinion on this philosophical issue ? Yes certainly, when the interpretation of wave mechanics and quantum mechanics were being developed, using uncertainties Einstein but also Schrödinger , that 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 adopted theories regarding very precise cases I personally thought a lot about these objections and I now think 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 if you want but not to an extent that forces us to believe that there is no causality. You wrote a lot of books, some on theory for specialists and some for a broader audience two finally on the applciations: the radar on the one hand and electron microscopy on the other hand What are in generals your ideas regarding the connection between teaching and research ? I indeed wrote a lot of books, I don't talk about the books for a general readership, but really scientific books that are almost exclusively lectures I gave during the 34 years I was a professor at the Faculté des Sciences de Paris at the exception of the one dedicated to electromagnetic wave guiding But these lectures where related to a broad variety of topics and two other books were dedicated to applications, like you mentioned, and it is obvious that I always considered teaching to be very important and I always wanted to write what I taught because I thought it was useful I hence think that teaching and research are strongly linked and, when one teaches, he learns how to precise his ideas and this helps when it comes to research However, there is a little drawback that I mentionned in some of my lectures which is that teaching forces you to be a little bit dogmatic, one can't say in front of its students: "well I think that it is this way but it may be the opposite also" because it may produce an effect that wouldn't really be excellent on the students so yes it makes you dogmatic and it can tricked a bit the mind because it makes you used to affirming certain things which are not really sure But, nonetheless, teaching and research are really useful things that, when a single man takes care of both of them, contribute a lot to make its research easier. Your students say that you are now very interested by the profound nature of light Could you please explain us what fascinates you in light and why this phenomenon that appears so simple and so accessible, is as interesting for you as strange particles from large collider are for others ? I mentioned earlier that my works on wave mechanics were based on Einstein's quantum theory of light in which he showed that, in light, there are at the same time waves and particles One shouldn't think that the current theory of light is completely understood despite this important progress because one should still understand how particles and waves are associated and I think that this problem is far from being solved, even using current formalisms Furthermore, this is based on this model that I built wave mechanics and this is why I came back to the study of light phenomena and of other particles which are connected with the light theory slightly neglecting, for the moment, the very interesting problems in the field of elementary particles, that I used to carefully follow with a lot of interest I think that there is still a lot to do in the theory of light there were actually new phenomena, recently discovered, most notably in the functioning of lasers and also the Brown and Twiss phenomenon, in which we compare the fluctuations of light at different points and all these questions, when we address them carefully, seem to be able to greatly help improving our understanding of light and our general ideas regarding the association of waves and particles.
